Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota

U.S. Department of the Interior U.S. Geological Survey

By Cheryl A. Naus, Daniel G. Driscoll, and Janet M. Carter

Water-Resources Investigations Report 01-4129

Prepared in cooperation with the South Dakota Department of Environment and Natural Resources and the West Dakota Water Development District U.S. Department of the Interior Gale A. Norton, Secretary

U.S. Geological Survey Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey.

Rapid City, South Dakota: 2001

For additional information write to: District Chief U.S. Geological Survey 1608 Mt. View Road Rapid City, SD 57702 Copies of this report can be purchased from: U.S. Geological Survey Information Services Building 810 Box 25286, Federal Center Denver, CO 80225-0286 CONTENTS

Abstract...... 1 Introduction ...... 3 Purpose and Scope...... 3 Previous Investigations ...... 3 Acknowledgments ...... 4 Description of Study Area ...... 4 Geologic Setting ...... 4 Hydrologic Setting...... 10 Regional Setting ...... 10 Local Setting...... 12 Methods Used and Data Sets Considered...... 16 Geochemistry of Madison and Minnelusa Aquifers...... 16 Major-Ion Chemistry ...... 16 Distribution of Major Ions...... 17 Saturation State...... 20 Evolutionary Processes...... 24 Isotope Chemistry...... 26 Background Information and Isotopic Composition of Recharge Water ...... 26 Stable Isotopes of Oxygen and Hydrogen ...... 29 Tritium ...... 32 Conceptual Mixing Models ...... 36 Description of Models ...... 36 Limitations of Models ...... 40 Areal Flowpaths, Ages, and Mixing Conditions ...... 41 Isotope Distributions and General Considerations ...... 41 Stable Isotopes...... 41 Tritium ...... 46 Site-Specific Considerations...... 51 Rapid City Area...... 51 Headwater Springs...... 54 Northern Black Hills Area...... 55 Southern Black Hills Area...... 56 Regional Flowpaths ...... 56 Interactions Between Madison and Minnelusa Aquifers...... 60 Interactions at Well Pairs ...... 60 Hydraulic Considerations ...... 60 Geochemical Considerations ...... 61 Interactions at Artesian Springs...... 63 Hydraulic Considerations ...... 63 Geochemical Considerations ...... 65 Synopsis of Interaction Processes...... 67 Summary and Conclusions ...... 68 References ...... 72 Supplemental Information ...... 77

Contents III FIGURES 1. Map showing generalized outcrops of Madison Limestone, Minnelusa Formation, and outer extent of Inyan Kara Group within the study area for Black Hills Hydrology Study...... 5 2. Stratigraphic section for the Black Hills ...... 6 3. Map showing distribution of hydrogeologic units in the Black Hills area...... 7 4. Geologic cross section A-A′...... 9 5. Schematic showing hydrogeologic setting of the Black Hills area...... 10 6. Map showing general direction of ground-water flow in Paleozoic aquifers in the northern Great Plains...... 11 7. Map showing potentiometric surface of the Madison aquifer and locations of major artesian springs...... 14 8. Map showing potentiometric surface of the Minnelusa aquifer and locations of major artesian springs...... 15 9. Trilinear diagrams showing proportional concentrations of major ions in the Madison and Minnelusa aquifers ...... 17 10. Selected Stiff diagrams showing the distribution of major-ion chemistry in the Madison aquifer...... 18 11. Selected Stiff diagrams showing the distribution of major-ion chemistry in the Minnelusa aquifer...... 19 12. Graph showing relation between gypsum saturation index and dissolved sulfate in the Madison and Minnelusa aquifers ...... 23 13. Boxplots showing calcite and dolomite saturation indices for selected samples from the Madison and Minnelusa aquifers ...... 23 14-17. Graphs showing: 14. Modeled relations between calcium and magnesium concentrations, pH, and dissolved sulfate...... 25 15. Relations between dissolved calcium and magnesium concentrations and dissolved sulfate in the Madison and Minnelusa aquifers ...... 27 16. Relation between pH and dissolved sulfate in the Madison and Minnelusa aquifers...... 28 17. Relation between dissolved oxygen and distance from outcrop in the Madison and Minnelusa aquifers...... 28 18. Schematic showing fractionation of stable oxygen and hydrogen isotopes during rainout ...... 29 19. Graph showing relation between δ18O and δD in Black Hills samples in comparison to the Global Meteoric Water Line...... 30 20. Map showing generalized distribution of δ18O in surface water and ground water in near-recharge areas ...... 31 21. Graphs showing temporal variation of δ18O for selected sites ...... 33 22. Map showing cumulative tritium deposition on the continental United States, 1953-83, and location of selected collection sites ...... 34 23. Graph showing monthly tritium concentrations in precipitation at Ottawa, Canada ...... 34 24. Graph showing weighted annual tritium concentrations in precipitation at selected locations ...... 35 25. Schematic diagrams illustrating mixing models for age dating for various ground-water flow conditions ...... 37 26. Graph showing estimated tritium concentrations in precipitation for Black Hills area and decay curves for selected years ...... 39 27-32. Maps showing: 18 27. Distribution of δ O in selected Madison and Minnelusa wells and springs in the northern Black Hills area ...... 42 18 28. Distribution of δ O in selected Madison and Minnelusa wells and springs in the Rapid City area ...... 43 18 29. Distribution of δ O in selected Madison and Minnelusa wells and springs in the southern Black Hills area...... 44 30. Distribution of tritium for selected sites in the northern Black Hills...... 47 31. Distribution of tritium for selected sites in the Rapid City area ...... 48 32. Distribution of tritium for selected sites in the southern Black Hills area...... 49 33. Boxplots of tritium concentrations for selected ground-water and surface-water samples collected during 1990-98 in the Black Hills area...... 50 18 34. Map showing distribution of δ O in selected Madison wells and springs and generalized flowpaths in the Black Hills of South Dakota and Wyoming ...... 57 35. Selected Stiff diagrams showing the distribution of major-ion chemistry in selected well pairs and artesian springs in the Black Hills area...... 59 36. Graphs showing decay-curve families for time-delay mixing model ...... 79 37. Hydrographs of selected well pairs ...... 84

IV Contents TABLES 1. Saturation indices for selected samples from wells completed in the Madison and Minnelusa aquifers...... 21 2. Selected results of geochemical modeling...... 25 3. Selected data for observation well pairs ...... 45 4. Tritium data for selected sites having data for 2000...... 52 5. Generalized age estimates for headwater springs, derived using immediate-arrival mixing model...... 54 6. Selected hydraulic and geochemical information for major artesian springs...... 64 7. Selected site information and isotope data for sites used in report...... 87 8. Stable isotope data for sites with multiple samples...... 103 9. Weighted annual tritium concentrations in precipitation for Black Hills area ...... 114 10. Monthly estimated tritium concentrations in precipitation, monthly precipitation, and weighted annual tritium concentrations in precipitation for Black Hills area...... 115 11. Estimated tritium concentrations in precipitation for Black Hills area, adjusted for decay...... 117

CONVERSION FACTORS, ACRONYMS, AND ABBREVIATIONS

Multiply By To obtain acre 4,047 square meter acre 0.4047 hectare acre-foot per year (acre-ft/yr) 1,233 cubic meter per year acre-foot per year (acre-ft/yr) 0.001233 cubic hectometer per year cubic foot per second (ft3/s) 0.02832 cubic meter per second foot per year (ft/yr) 0.3048 meter per year gallon per minute (gal/min) 0.06309 liter per second inch 2.54 centimeter inch 25.4 millimeter inch per year (in/yr) 25.4 millimeter per year foot (ft) 0.3048 meter mile (mi) 1.609 kilometer

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = (1.8 × °C) + 32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C = (°F - 32) / 1.8

Water year: Water year is the 12-month period, October 1 through September 30, and is designated by the calendar year in which it ends. Thus, the water year ending September 30, 1998, is called the “1998 water year.”

Per mil (‰): A unit expressing the ratio of stable-isotopic abundances of an element in a sample to those of a standard material. Per mil units are equivalent to parts per thousand. Stable-isotopic ratios are computed as follows:

æöRsample() δX = ------– 1 × 1000, èøRs()tan dard where X is the heavier isotope and R is the ratio of the heavier, less abundant stable isotope to the lighter, stable isotope in a sample or standard.

The δ values for oxygen and hydrogen stable isotopic ratios discussed in this report are referenced to the following standard material:

Ratio (R) Standard identity and reference hydrogen-2:hydrogen-1 Vienna Standard Mean Ocean Water (VSMOW) oxygen-18:oxygen-16 Vienna Standard Mean Ocean Water (VSMOW)

Contents V Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota

By Cheryl A. Naus, Daniel G. Driscoll, and Janet M. Carter

ABSTRACT Isotopic information is used to evaluate ground-water flowpaths, ages, and mixing condi- The Madison and Minnelusa aquifers are tions for the Madison and Minnelusa aquifers. two of the most important aquifers in the Black Distinctive patterns exist in the distribution of Hills area because of utilization for water supplies stable isotopes of oxygen and hydrogen in precip- and important influences on surface-water itation for the Black Hills area, with isotopically resources resulting from large springs and stream- lighter precipitation generally occurring at higher flow-loss zones. Examination of geochemical elevations and latitudes. Distributions of δ18O in information provides a better understanding of the ground water are consistent with spatial patterns in 18 complex flow systems within these aquifers and recharge areas, with isotopically lighter δ O interactions between the aquifers. values in the Madison aquifer resulting from Major-ion chemistry in both aquifers is generally higher elevation recharge sources, dominated by calcium and bicarbonate near out- relative to the Minnelusa aquifer. crop areas, with basinward evolution towards Three conceptual models, which are simpli- various other water types. The most notable dif- fications of lumped-parameter models, are con- ferences in major-ion chemistry between the sidered for evaluation of mixing conditions and Madison and Minnelusa aquifers are in concentra- general ground-water ages. For a simple slug-flow tions of sulfate within the Minnelusa aquifer. model, which assumes no mixing, measured tri- Sulfate concentrations increase dramatically near tium concentrations in ground water can be related a transition zone where dissolution of anhydrite is through a first-order decay equation to estimated actively occurring. concentrations at the time of recharge. Two simplified mixing models that assume equal propor- Water chemistry for the Madison and tions of annual recharge over a range of years also Minnelusa aquifers is controlled by reactions are considered. An “immediate-arrival” model is among calcite, dolomite, and anhydrite. Satura- used to conceptually represent conditions in out- tion indices for gypsum, calcite, and dolomite for crop areas and a “time-delay” model is used for most samples in both the Madison and Minnelusa locations removed from outcrops, where delay aquifers are indicative of the occurrence of dedo- times for earliest arrival of ground water generally lomitization. Because water in the Madison would be expected. Because of limitations associ- aquifer remains undersaturated with respect to ated with estimating tritium input and gross sim- gypsum, even at the highest sulfate concentra- plifying assumptions of equal annual recharge and tions, upward leakage into the overlying thorough mixing conditions, the conceptual Minnelusa aquifer has potential to drive increased models are used only for general evaluation of dissolution of anhydrite in the Minnelusa mixing conditions and approximation of age Formation. ranges.

Abstract 1 Headwater springs, which are located in indicate deflection of regional flowpaths, with or near outcrop areas, have the highest tritium minor influence possible for several wells. The 18 concentrations, which is consistent with the δ O values for large springs along the southern immediate-arrival mixing model. Tritium concen- axis of the uplift essentially preclude regional trations for many wells are very low, or nondetect- influence, which also is supported by ion chemis- able, indicating general applicability of the time- try, and indicate potential recharge areas extending delay conceptual model for locations beyond out- along the entire southwestern flank of the uplift. crop areas, where artesian conditions generally Low, but detectable, tritium concentrations in occur. Concentrations for artesian springs gener- these springs along the southern axis confirm the ally are higher than for wells, which indicates influence of recharge from within the study area, generally shorter delay times resulting from pref- but indicate relatively long traveltimes. erential flowpaths that typically are associated Hydrographs for 9 of 13 well pairs are fairly with artesian springs. well separated and do not indicate direct hydraulic In the Rapid City area, a distinct division of connection between the Madison and Minnelusa isotopic values for the Madison aquifer corre- aquifers. Comparison of geochemical information sponds with distinguishing δ18O signatures for provides no evidence of extensive mixing nearby streams, where large streamflow recharge resulting from general, areal leakage between the occurs. Previous dye testing in this area docu- aquifers. mented rapid ground-water flow (timeframe of Aquifer interactions can occur at artesian weeks) from a streamflow loss zone to sites springs, which discharge about one-half of aver- located several miles away. These results are used age recharge to the Madison and Minnelusa aqui- to illustrate potential errors that may result from fers in the Black Hills area. Various investigators the simplified conceptualization of this complex have hypothesized that the Madison aquifer is the ground-water setting with dual-porosity hydraulic primary source for many artesian springs, based on characteristics. For Rapid City sites with time- geochemical modeling. The Madison aquifer is series data, minimal variability in δ18O values cor- inferred as the primary source for several springs responded with tritium data indicative of dominant where artesian conditions in the Minnelusa aquifer proportions of older water. Other sites showed are precluded by nearby outcrop sections. For response to temporal δ18O trends in streamflow many springs, quantifying relative contributions recharge, with tritium data indicating larger pro- from each aquifer is hampered by geochemical portions of modern recharge. Several large pro- similarities between the Madison and Minnelusa duction wells located near the isotopic transition aquifers, especially near recharge areas. For some zone had changes in δ18O values indicative of springs, high sulfate concentrations indicate changes in capture zones associated with recent Minnelusa influence, but may result from dissolu- production. tion of Minnelusa minerals by water from the Evaluation of major-ion and isotope data Madison aquifer. indicates that regional flowpaths for the Madison Generally higher hydraulic head in the aquifer are essentially deflected around the study Madison aquifer, in combination with gypsum area, with the possible exception of the south- undersaturation, is concluded to be a primary western and northwestern corners. Two wells just mechanism driving interactions with the north of the study area clearly show influence of Minnelusa aquifer, in areas where artesian condi- regional flow, and a well just within the study area tions exist in the Madison aquifer. Upward leak- shows possible influence. Large artesian springs age from the Madison aquifer probably contributes near the northern axis of the uplift show no to general dissolution of anhydrite deposits in the regional influence and are concluded to be Minnelusa aquifer and development of breccia recharged within the uplift area. Ion concentra- pipes, which enhances vertical hydraulic conduc- tions for wells west of the study area in Wyoming tivity. Breccia pipes are a likely mechanism for

2 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota upward movement of large quantities of water The Madison and Minnelusa aquifers are the primary through the Minnelusa aquifer at artesian spring focus of the Black Hills Hydrology Study. locations and many exposed breccia pipes of the Ground-water conditions in the Madison and upper Minnelusa Formation probably are the Minnelusa aquifers are extremely complex because of throats of abandoned artesian springs. Dissolution several factors. Water recharged in the uplifted Black processes are an important factor in a self- Hills area mixes with the regional flow system at lower perpetuating process associated with development elevations; however, interfaces and mixing conditions are not well defined. Both aquifers are potential of artesian springs and preferential flowpaths, sources for numerous large springs in the Black Hills which initially develop in locations with large area, and hydraulic connections are possible in other secondary porosity and associated hydraulic con- locations because of large secondary porosity and per- ductivity, with ongoing enhancement resulting meability in both aquifers. Ground-water flowpaths from dissolution activity. and velocities in both aquifers are influenced by aniso- Outward (downgradient) migration of the tropic and heterogeneous hydraulic properties caused artesian springs probably occurs as upgradient by secondary porosity resulting from fractures, faults, spring-discharge points are abandoned and new and dissolution activity. Karst features, including sink- ones are occupied, keeping pace with regional holes, collapse features, solution cavities, and caves, in erosion over geologic time. In response, hydraulic the Madison aquifer, and collapse breccia associated heads in the Madison and Minnelusa aquifers also with dissolution of interbedded evaporites in the Minnelusa aquifer, create the potential for the introduc- have declined over geologic time. Artesian tion and rapid transport of contaminants within the springflow and general leakage are concluded to Madison and Minnelusa aquifers. be important factors in governing water levels in Geochemistry is a useful tool for better under- the Madison and Minnelusa aquifers. Artesian standing the complex flow systems in the Madison and springs are especially important in acting as a Minnelusa aquifers. The major-ion chemistry of water relief mechanism that provides an upper limit for from these aquifers is an important water-quality con- hydraulic head, with springflow increasing in sideration. Isotope information provides insights response to increasing water levels. regarding recharge areas and traveltimes. Both major- ion and isotope information provide insights regarding flowpaths within the Madison and Minnelusa aquifers INTRODUCTION and potential interconnections between them.

The Madison and Minnelusa aquifers are two of the most important aquifers in the Black Hills area. Purpose and Scope These aquifers are used extensively for domestic, municipal, irrigation, and industrial uses and have a The purpose of this report is to present geochem- major influence on the surface-water resources because ical information for the Madison and Minnelusa aqui- of large spring discharges and large streamflow losses fers in the Black Hills area of South Dakota, with an that occur along many stream channels. emphasis on information relating to understanding the Population growth, resource development, and ground-water flow conditions in these aquifers. The periodic droughts have the potential to affect the quan- report includes discussions of major-ion and isotope tity, quality, and availability of water within the Black chemistry of the Madison and Minnelusa aquifers, pos- Hills area. Because of this concern, the Black Hills sible influences from regional flowpaths, and interac- Hydrology Study was initiated in 1990 to assess the tions that may occur between the two aquifers. quantity, quality, and distribution of surface and ground water in the Black Hills area of South Dakota (Driscoll, Previous Investigations 1992). This long-term study is a cooperative effort between the U.S. Geological Survey, the South Dakota Previous investigations have provided various Department of Environment and Natural Resources, information regarding the geochemistry of the Madison and the West Dakota Water Development District, and Minnelusa aquifers in the Black Hills area. The which represents various local and county cooperators. geochemical evolution of ground water in the Madison

Introduction 3 and Minnelusa aquifers has been studied by Bowles The Black Hills are a dome-shaped uplift about and Braddock (1963), Braddock and Bowles (1963), 125 miles long and 60 miles wide (Feldman and Heim- Back and others (1983), Busby and others (1983, 1991, lich, 1980). Land-surface elevations range from about 1995), Plummer and others (1990), and Naus (1999). 7,200 ft at the highest peaks to about 3,000 ft in the Potential source aquifers for springs in the Black Hills surrounding plains. The overall climate of the study area were investigated by Alexander and others (1988, area is continental, with generally low precipitation 1989), Whalen (1994), Klemp (1995), and Wenker amounts, hot summers, cold winters, and extreme vari- (1997). Browne (1992) studied water-quality charac- ations in both precipitation and temperatures (Johnson, teristics and geochemical differentiation of the Mad- 1933). Local climatic conditions are affected by topog- ison and Minnelusa aquifers near Rapid City. raphy, with generally lower temperatures and higher Applications of oxygen and hydrogen isotopes to the precipitation at the higher elevations. study of ground-water source areas, flowpaths, and The average annual precipitation for the study mixing in the Madison aquifer in the Black Hills area area (1931-98) is 18.61 inches, and has ranged from have been presented by Back and others (1983), Busby 10.22 inches for water year 1936 to 27.39 inches for and others (1983), Greene (1997, 1999), and Anderson water year 1995 (Driscoll, Hamade, and Kenner, 2000). and others (1999). The largest precipitation amounts typically occur in the northern Black Hills near Lead, where average annual precipitation exceeds 29 inches. Annual averages Acknowledgments (1931-98) for counties within the study area range from 16.35 inches for Fall River County to 23.11 inches for The authors acknowledge the efforts of the West Lawrence County. The average annual temperature is Dakota Water Development District for helping to 43.9°F (U.S. Department of Commerce, 1999) and develop and support the Black Hills Hydrology Study. ranges from 48.7°F at Hot Springs to approximately West Dakota’s coordination of various local and county 37°F near Deerfield Reservoir. Average annual evapo- cooperators has been a key element in making this ration generally exceeds average annual precipitation study possible. The authors also recognize the throughout the study area. Average pan evaporation for numerous local and county cooperators represented by April through October is about 30 inches at Pactola West Dakota, as well as the numerous private citizens Reservoir and about 50 inches at Oral. who have helped provide guidance and support for the Black Hills Hydrology Study. The South Dakota Department of Environment and Natural Resources has Geologic Setting provided support and extensive technical assistance to the study. David Parkhurst, U.S. Geological Survey, The oldest geologic units in the study area are the provided extensive guidance and numerous insights Precambrian metamorphic and igneous rocks (fig. 2), regarding this report. In addition, the authors acknow- which underlie the Paleozoic, Mesozoic, and Cenozoic ledge technical assistance from many faculty and rocks and sediments, except where exposed at the land students at the South Dakota School of Mines and surface. The Precambrian rocks range in age from Technology. about 1.7 to 2.5 billion years, and were eroded to a gentle undulating plain at the beginning of the Paleo- zoic era (Gries, 1996). The Paleozoic and Mesozoic DESCRIPTION OF STUDY AREA rocks were deposited as nearly horizontal beds and were later uplifted during the rise of the Black Hills The study area consists of the topographically during the Laramide orogeny. Uplift during the Lara- defined Black Hills and adjacent areas located in mide orogeny and related erosion exposed the Precam- western South Dakota (fig. 1). Outcrops of the brian rocks in the central core of the Black Hills with Madison Limestone and Minnelusa Formation, as well the Paleozoic and Mesozoic sedimentary rocks as the generalized outer extent of the Inyan Kara exposed in roughly concentric rings around the core. Group, which approximates the outer extent of the Deformation during the Laramide orogeny contributed Black Hills area, also are shown in figure 1. The study to the numerous fractures, folds, and other features area includes most of the larger communities in present throughout the Black Hills. Tertiary intrusive western South Dakota and contains about one-fifth of activity in the northern Black Hills (fig. 3) also con- the State’s population. tributed to rock fracturing.

4 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota o 104o 45' 103 30' C ro H w 44o45' Belle Fourche orse Creek Reservoir Owl Newell SOUTH BELLE Creek DAKOTA Creek Black F BELLE FOURCHE O Hills UR CHE RIVER

Hay Creek R

E BUTTE CO V ER R I MEADE CO REDWAT LAWRENCE CO Study Area Saint Creek Crow Onge

B Creek ea r reek 30' Gulch Spearfish C

reek C Whitewood Creek Bottom Creek e ls EXPLANATION Gulch a F Maurice Whitewood Butte Higgins OUTCROP OF MADISON STURGIS Tinton LIMESTONE (from Strobel 103o DEADWOOD 15' and others, 1999) Beaver Cr

h Lead s i f OUTCROP OF MINNELUSA r Bear Creek Tilford a

e r p Elk FORMATION (from Strobel S w C Meado Creek and others, 1999) Spearfish N Elk . 15' Little Boxelder F APPROXIMATE EXTENT OF o r Piedmont k Ellsworth R THE BLACK HILLS AREA, S. Fork Rapid Cr a Air Force p Base i Nemo d REPRESENTED BY C Blackhawk Creek r GENERALIZED OUTER rk Box Elder Fo Rochford s EXTENT OF INYAN KARA ad o N. Fork C Rapid h ast GROUP (modified from R le ID CITY Castle Cr RAP Strobel and others, 1999) ek Creek Rapid C C re Pactola re Reservoir PLATEAU ek Creek Castle V LIMESTONE Deerfield ictoria Creek S. Fork Spring o r Reservoir 44 C e Castl Sheridan Creek Creek Lake Hill City Keystone Mt. Rushmore National Spring Memorial Hayward n

o y PENNINGTON CO n Battle

a Hermosa C CUSTER CO

s Creek e l o B Grace French Creek CUSTER e idg Creek Cool 45' CUSTER Jewel Cave National Monument STATE French Fairburn

PARK

Canyon Creek anyon C Lame Beaver Pringle Wind Cave National Park Hell

WYOMING Creek Johnny Wind Creek Cave

SOUTH DAKOTA SOUTH Red Creek Buffalo Gap RIVER 30' Pass

FALL RIVER CO H n ot Brook yo Can HOT SPRINGS Minnekahta Fall R Oral

C H E Y E N N E

Edgemont Horsehead

Angostura o Creek Reservoir C 43 15' d reek oo w n o tt o Creek C Provo

Figure 1. Generalized outcrops of Madison Limestone, Minnelusa Formation, and outer extent of Inyan Kara Group within the study area for Black Hills Hydrology Study.

Description of Study Area 5 Massive to slabby sandstone. Coarse gray to buff cross-bedded conglomeratic sandstone, interbedded with buff, red, and gray clay, especially toward top. Local fine-grained limestone. Principal horizon of limestone lenses giving teepee buttes. Dark-gray shale containing scattered concretions. Widely scattered limestone masses, giving small teepee buttes. Black fissile shale with concretions. Impure slabby limestone. Weathers buff. Dark-gray calcareous shale, with thin Orman Lake limestone at base. Light colored clays with sandstone channel fillings and local limestone lenses. Includes rhyolite, latite, trachyte, and phonolite. Green to maroon shale. Thin sandstone. Massive fine-grained sandstone. Greenish-gray shale, thin limestone lenses. Glauconitic sandstone; red sandstone near middle. Red siltstone, gypsum, and limestone. Gray shale with scattered limestone concretions. Clay spur bentonite at base. Light-gray siliceous shale. Fish scales and thin layers of bentonite. Brown to light-yellow and white sandstone. Light-gray shale with numerous large concretions and sandy layers. Dark-gray shale. Red sandy shale, soft red sandstone and siltstone with gypsum thin limestone layers. Gypsum locally near the base. Thin to medium-bedded fine-grained, purplish-gray laminated limestone. Red shale and sandstone. Yellow to red cross-bedded sandstone, limestone, and anhydrite locally at top. Red shale with interbedded limestone and sandstone at base. Massive light-colored limestone. Dolomite in part. Cavernous upper Pink to buff limestone. Shale locally at base. Buff dolomite and limestone. Green shale with siltstone. Massive to thin-bedded buff purple sandstone. Greenish glauconitic shale, flaggy dolomite, and flat-pebble limestone conglomerate. Sandstone, with conglomerate locally at the base. Schist, slate, quartzite, and arkosic grit. Intruded by diorite, metamorphosed to amphibolite, and by granite pegmatite. Interbedded sandstone, limestone, dolomite, shale, and anhydrite. -- 0-25 0-45 25-65 0-235 0-150 0-500 0-300 0-220 0-225 0-150 30-60 80-300 Impure chalk and calcareous shale. 25-150 1 1 1 1 10-200 10-190 25-485 350-750 1 1 225-380 375-800 250-450 150-850 125-230 150-270 Dark-gray to black siliceous shale. 375-1,175 250-1,000 1 1 1 1,200-2,700 IN FEET THICKNESS South Dakota School of Mines and Technology (written commun., January 1994) NEWCASTLE SANDSTONE Turner Sandy Member Wall Creek Member Fuson Shale Minnewaste Limestone Chilson Member Goose Egg Equivalent Redwater Member Lak Member Hulett Member Stockade Beaver Mem. Canyon Spr Member MOWRY SHALE Modified from information furnished by the Department of Geology and Geological Engineering,

MUDDY

STRATIGRAPHIC UNIT DESCRIPTION FM

SANDSTONE LAKOTA

BELLE FOURCHE SHALE SKULL CREEK SHALE FALL RIVER FORMATION

PIERRE SHALE NIOBRARA FORMATION GROUP UNDIFFERENTIATED SANDS AND GRAVELSWHITE RIVER GROUP 0-50 Alluvial and colluvial materials. GREENHORN FORMATION SUNDANCE FORMATION GYPSUM SPRING FORMATION MINNEKAHTA LIMESTONE OPECHE SHALE MINNELUSA FORMATION MADISON (PAHASAPA) LIMESTONE ENGLEWOOD FORMATION WHITEWOOD (RED RIVER) FORMATION WINNIPEG FORMATION DEADWOOD FORMATION UNDIFFERENTIATED METAMORPHIC AND IGNEOUS ROCKS INTRUSIVE IGNEOUS ROCKS MORRISON FORMATION CARLILE SHALE SPEARFISH FORMATION UNKPAPA SS

GRANEROS GROUP GRANEROS INYAN KARA INYAN Ju Po Tw Ou Kik Tui R Kps pCu Pmk OCd T Ps MDm P Pm QTac FOR INTERVAL ABBREVIATION STRATIGRAPHIC PERMIAN TRIASSIC SYSTEM TERTIARY JURASSIC DEVONIAN CAMBRIAN ORDOVICIAN QUATERNARY CRETACEOUS MISSISSIPPIAN & TERTIARY (?)

PENNSYLVANIAN

Stratigraphic section for the Black Hills. AEZI EOOCCENOZOIC MESOZOIC PALEOZOIC PRECAMBRIAN ERATHEM Modified based on drill-hole data 1 Figure 2.

6 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota o 103o30' 104 45' EXPLANATION C ro H w Hydrogeologic Stratigraphic o Belle Fourche Indian orse 44 45' Cr Units Units Map Units Creek Reservoir Owl Newell BELLE Creek Unconsolidated QTac Alluvium and colluvium, units Creek undifferentiaed

F BELLE FOURCHE O White River aquifer Tw White River Group UR CHE RIVER Hay Creek Tertiary intrusive Tui R Undifferentiated intrusive units E BUTTE CO V igneous rocks ER R I MEADE CO REDWAT LAWRENCE CO Cretaceous- Kps sequence Pierre Shale to Skull Creek Shale, Saint Creek confining unit undifferentiated Crow Onge Bear Creek Inyan Kara aquifer Kik Inyan Kara Group Gulch Spearfish 30' reek C Jurassic-sequence Ju Morrison Formation to Sundance

Creek Whitewood semiconfining unit ns Creek Formation, undifferentiated gi g Bottom i

Creek e

H Spearfish confining s Gulch l TRPs Spearfish Formation a unit Maurice F Whitewood Butte STURGIS Tinton Minnekahta aquifer Pmk Minnekahta Limestone o DEADWOOD 15' 103 Beaver Opeche confining Cr Po Opeche Shale h Lead unit s i Bear f r Creek Tilford

e r Minnelusa aquifer PPm p Elk Minnelusa Formation S w C Meado Creek N Elk Madison aquifer MDme Madison (Pahasapa) Limestone 15' Spearfish Little F Boxelder o and Englewood Formation r Piedmont k R Ellsworth Ordovician-sequence a Air Force Ou Whitewood Formation and S. Fork Rapid Cr p i Base semiconfining unit d Nemo Winnipeg Formation C Creek r Blackhawk Deadwood aquifer OCd Deadwood Formation rk Box Elder Fo Rochford ds a Precambrian igneous o N Fork C Rapid pCu h ast and Undifferentiated metamorphic R le CITY Castle Cr RAPID A' metamorphic units and igneous rocks k Pactola Creek Rapid C ree Reservoir Cr PLATEAU e Creek e k Castle V

LIMESTONE i Creek A' Deerfield ctoria A LINE OF GEOLOGIC SECTION S. Fork Spring 44o r Reservoir C e FAULT--Dashed where approximated. A Castl Sheridan Creek Bar and ball on downthrown side. Creek Lake Hill City ANTICLINE--Showing trace of axial Keystone plane and direction of plunge. g Mt. Rushmore in National Dashed where approximated. r Hayward n p Memorial S o y PENNINGTON CO SYNCLINE--Showing trace of axial plane n Battle

a Hermosa C CUSTER CO and direction of plunge. Dashed Creek s where approximated. e l o Grace B reek French C MONOCLINE--Showing trace of axial CUSTER e plane. Dashed where approximated. idg Creek Cool 45' CUSTER DOME--Symbol size approximately pro- Jewel Cave portional to size of dome. Dome onument National M STATE French Fairburn asymmetry indicated by arrow length.

PARK

Canyon Creek anyon C L Beaver am Wind Cave e Pringle National Park

Hell

WYOMING Creek Johnny

Wind Creek ed

SOUTH DAKOTA SOUTH R Cave Creek Buffalo Gap RIVER 30' Pass

FALL RIVER CO H on ot Brook ny Ca HOT SPRINGS Minnekahta Fall R Oral

CHEYENNE

Edgemont Horsehead

Angostura o Creek Reservoir C 43 15' d reek oo w n o tt o Creek C Provo

Figure 3. Distribution of hydrogeologic units in the Black Hills area (modified from Strobel and others, 1999).

Description of Study Area 7 Surrounding the central core is a layered series of the deposition of the overlying formations (DeWitt and sedimentary rocks including outcrops of the Madison others, 1986). Because the surface of the Madison Limestone (also locally known as the Pahasapa Lime- Limestone was exposed to weathering and karstifica- stone) and the Minnelusa Formation (fig. 3). The bed- tion for millions of years, the formation is unconform- rock sedimentary units typically dip away from the ably overlain by the Pennsylvanian- and Permian-age uplifted Black Hills at angles that can approach or Minnelusa Formation. The Madison Limestone is exceed 15 to 20 degrees near the outcrops, and underlain by the Englewood Formation, which Gries decrease with distance from the uplift to less than (1996) included as an impure basal phase of the Mad- 1 degree (Carter and Redden, 1999a, 1999b, 1999c, ison Limestone but Fahrenbach (1995) described as a 1999d, 1999e) (fig. 4). Following are descriptions for separate unit from the Madison Limestone. The selected bedrock formations from the Deadwood Madison Limestone and equivalent units are regionally Formation through the Inyan Kara Group. extensive throughout the northern Great Plains area The oldest sedimentary unit in the study area is and pinch out in southern and eastern South Dakota. the Cambrian- and Ordovician-age Deadwood Forma- The Pennsylvanian- and Permian-age Minnelusa tion, which is composed primarily of brown to light- Formation consists mostly of yellow to red cross- gray glauconitic sandstone, shale, limestone, and local stratified sandstone, limestone, dolomite, and shale basal conglomerate (Strobel and others, 1999). These (Strobel and others, 1999). Anhydrite cements are sediments were deposited on top of a generally hori- prevalent in many layers within the subsurface, but zontal plain of Precambrian rocks in a coastal to near- generally have been removed by dissolution at or near shore environment (Gries, 1975). The thickness of the outcrop areas (DeWitt and others, 1986). Collapse Deadwood Formation increases from south to north in features filled with breccia (breccia pipes) occur within the study area and ranges from 0 to 500 ft (Carter and the upper part of the Minnelusa Formation in many Redden, 1999e). In the northern and central Black Hills, the Deadwood Formation is disconformably locations (Braddock, 1963; Long and others, 1999; overlain by Ordovician rocks, which include the Epstein, 2000). The thickness of the Minnelusa Forma- Whitewood and Winnipeg Formations. The Winnipeg tion in the study area increases from north to south in Formation is absent in the southern Black Hills, and the the study area and ranges from about 375 ft near Belle Whitewood Formation has eroded to the south and is Fourche to 1,175 ft near Edgemont (Carter and not present south of the approximate latitude of Nemo Redden, 1999c). The Minnelusa Formation was (DeWitt and others, 1986). In the southern Black Hills, deposited in a coastal environment, and dune structures the Deadwood Formation is unconformably overlain at the top of the formation may represent beach sedi- by the Devonian- and Mississippian-age Englewood ments (Gries, 1996). Along the northeastern flank of Formation because of the absence of the Ordovician the Black Hills, there is little anhydrite in the subsur- sequence. face due to a change in the depositional environment The Mississippian-age Madison Limestone is a (Carter and Redden, 1999c). On the south and south- massive, gray to buff limestone that is locally dolomitic west side of the study area, there is a considerable (Strobel and others, 1999). The Madison Limestone, increase in thickness of clastic units as well as a thick which was deposited as a marine carbonate, was section of anhydrite. The Minnelusa Formation dis- exposed at land surface for approximately 50 million conformably is overlain by the Permian-age Opeche years. During this period, significant erosion, soil Shale, which is overlain by the Minnekahta Limestone. development, and karstification occurred (Gries, The Minnelusa Formation and equivalent units are 1996). There are numerous caves and fractures within regionally extensive throughout the northern Great the upper part of the formation (Peter, 1985). The Plains area and pinch out in eastern South Dakota. thickness of the Madison Limestone increases from The Permian-age Minnekahta Limestone is a south to north in the study area and ranges from almost fine-grained, purple to gray laminated limestone zero in the southeast corner of the study area (Rahn, (Strobel and others, 1999), which ranges in thickness 1985) to 1,000 ft east of Belle Fourche (Carter and from 25 to 65 ft in the study area. The Minnekahta Redden, 1999d). Local variations in thickness are due Limestone is overlain by the Triassic- and Permian-age largely to the karst topography that developed before Spearfish Formation.

8 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota SEA 7,000 6,000 5,000 4,000 3,000 2,000 1,000 FEET LEVEL

A' QTac Kps R T Ps

QTac

Rapid Creek Rapid ed in figure 2.). Rapid City Rapid Modified from Strobel and others, 1999 Modified from Strobel and others, Kik Ju Pmk

Po Rapid Creek Rapid PPm

MDme

Rapid Creek Rapid OCd

Rapid Creek Rapid Rapid Creek Rapid MILES 8 6 10 KILOMETERS pCu 246810 2 210 4

210 South Fork Castle Creek Castle Fork South OCd pCu PPm MDme

Geologic cross section A-A' (Location of section is shown in figure 3. Abbreviations for stratigraphic intervals are explain stratigraphic for Abbreviations in figure 3. Geologic cross section A-A' (Location of is shown

SOUTH DAKOTA SOUTH

VERTICAL EXAGGERATION X5 EXAGGERATION VERTICAL WYOMING A SEA Figure 4. 7,000 6,000 5,000 4,000 3,000 2,000 1,000 FEET LEVEL

Description of Study Area 9 The Spearfish Formation is a red, silty shale with and local hydrologic settings. A fifth major aquifer interbedded red sandstone and siltstone, which ranges (Minnekahta) generally is used only locally, as are in thickness from about 375 to 800 ft (Strobel and aquifers in the metamorphic and igneous rocks within others, 1999). The Spearfish Formation contains mas- the central core area. sive gypsum throughout. The overlying Mesozoic-age units are composed primarily of shale, siltstone, and Regional Setting sandstone deposits, and include the Cretaceous-age Inyan Kara Group. The thickness of the Inyan Kara The Paleozoic aquifers underlie parts of Group ranges from about 135 to 900 ft in the study area Montana, North Dakota, South Dakota, Wyoming, and (Carter and Redden, 1999a). Canada (Downey, 1984). The Canadian part of the regional aquifer system is not described or shown in this report. For the description of the regional setting Hydrologic Setting (a large part of the northern Great Plains), it is conve- nient to use aquifer names of Downey and Dinwiddie The hydrologic setting of the Black Hills area is (1988) and Whitehead (1996). The Paleozoic aquifers schematically illustrated in figure 5, and the areal dis- include the Cambrian-Ordovician aquifer (or Dead- tribution of hydrogeologic units is shown in figure 3. wood aquifer in the Black Hills), Mississippian aquifer Four of the major bedrock aquifers in the Black Hills (or Madison aquifer in the Black Hills), and the Penn- area (Deadwood, Madison, Minnelusa, and Inyan Kara sylvanian aquifer (or Minnelusa aquifer in the Black aquifers) are regionally extensive and are discussed in Hills). The Paleozoic aquifers are recharged in outcrop the following sections in the context of both regional areas around major uplifts (fig. 6).

EXPLANATION MADISON AND MINNELUSA CONFINING UNIT AQUIFERS CAVE OTHER AQUIFERS BRECCIA PIPE

Water Potentiometric surface table of Madison aquifer

Alluvium

Artesian spring Precam and igneous rocks

D Inyan Kara G brian m eadw

M o Madison Limestone od innekahta etam roup Formation orphic Minnelusa Limestone

Formation

Dip of sedimentary rocks exaggerated Thicknesses not to scale

Figure 5. Schematic showing hydrogeologic setting of the Black Hills area. Figure also shows caves and breccia pipes, which contribute to secondary porosity in the Madison and Minnelusa aquifers. Breccia pipes may result from upward leakage from the Madison aquifer, creating conduits for artesian springs.

10 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota

Falls Sioux G

G Fargo

, 1988 and Whitehead, , 1988 and

Grand Forks Grand o EXPLANATION

100 Pierre

# Bismarck

RECHARGE AREA THE MISSISSIPPIAN (MADISON) DISCHARGE AREA FOR aquifers) (via adjacent and overlying AQUIFER THE CAMBRIAN-ORDOVICIAN DISCHARGE AREA FOR springs, (via adjacent aquifers, AQUIFER (DEADWOOD) and seeps) WITH DISSOLVED-SOLIDS OF WATER BODY THAN 100,000 GREATER CONCENTRATION MILLIGRAMS PER LITER AQUIFER--Dashed LIMIT OF CAMBRIAN-ORDOVICIAN located where approximately MOVEMENT DIRECTION OF GROUND-WATER

S O U T H D A K O T A T O K A D H T U O S

N O R T H D A K O T A T O K A D H T R O N Rapid City Rapid G Basin

illiston W

of ry

o a nd # ou Cheyenne

B 105 te a m xi ro pp A

Casper G G

Billings

W Y O M I N G

110 G

Great Falls

o Base modified from U.S. Geological Survey digital data, 1:2,000,000, 1972

43 M O N T A N A #

Butte MILES

elena G

H 100 100 KILOMETERS 50

o 50

115 General direction of ground-water flow in Paleozoic aquifers in the northern aquifers and Dinwiddie Great Plains (modified from Downey in Paleozoic flow direction of ground-water General 0 0

48 Figure 6. 1996).

Description of Study Area 11 The Cambrian-Ordovician (or Deadwood) area. Within the Paleozoic rock interval, aquifers in the aquifer consists of sandstones of Cambrian age (Dead- Deadwood Formation, Madison Limestone, Minnelusa wood Formation and equivalents) and limestones of Formation, and Minnekahta Limestone are used exten- Ordovician age. The Cambrian-Ordovician aquifer sively. These aquifers are collectively confined by the contains freshwater, with dissolved solids concentra- underlying Precambrian rocks (fig. 5) and the over- tions less than 1,000 mg/L (milligrams per liter), only lying Spearfish Formation (fig. 2). Individually, these in an area surrounding the Black Hills and in a small aquifers are separated by minor confining units or by area in north-central Wyoming (Whitehead, 1996). relatively impermeable layers within the individual The Mississippian (or Madison) aquifer is con- units. Extremely variable leakage can occur between tained within the limestones, siltstones, sandstones, these aquifers (Peter, 1985; Greene, 1993). and dolomite of the Madison Limestone and equivalent units. Generally, water in the Mississippian aquifer Artesian (confined) conditions generally exist is confined except in outcrop areas. Flow in the within the sedimentary aquifers where an upper con- Mississippian aquifer generally is from the recharge fining unit is present. Under artesian conditions, water areas to the northeast (fig. 6). Discharge from the in a well will rise above the top of the aquifer in which Mississippian aquifer occurs by upward leakage to the it is completed. Flowing wells will result when drilled lower Cretaceous aquifer in central South Dakota and in areas where the potentiometric surface is above the eastern flow to the Cambrian-Ordovician aquifer in land surface. Flowing wells and artesian springs that eastern North Dakota (Downey, 1984). Water in the originate from confined aquifers are common around Mississippian aquifer is fresh only in small areas near the periphery of the Black Hills. recharge areas and becomes slightly saline to saline as Numerous headwater springs from the Paleozoic it moves downgradient. The water is a brine with units on the western side of the study area (fig. 3) pro- dissolved solids concentrations greater than vide base flow for many streams. These streams flow 300,000 mg/L in the deep parts of the Williston basin across the central core of the Black Hills, and most (Whitehead, 1996). Black Hills streams lose all or part of their flow as The Pennsylvanian (or Minnelusa) aquifer is they cross outcrops of the Madison Limestone and comprised of sandstones and limestones of the Minnelusa Formation (Hortness and Driscoll, 1998). Minnelusa Formation and equivalent units of Pennsyl- Karst features of the Madison Limestone, including vanian age. Water in the Pennsylvanian aquifer moves sinkholes, collapse features, solution cavities, and from recharge areas to discharge areas in eastern South Dakota (Downey and Dinwiddie, 1988). Some water caves are responsible for the Madison aquifer’s discharges by upward leakage to the lower Cretaceous capacity to accept streamflow recharge. Large stream- aquifer (Swenson, 1968; Gott and others, 1974). flow losses also occur in many locations within the out- Several sandstone units compose the lower crop of the Minnelusa Formation (Hortness and Cretaceous aquifer, which generally is known as the Driscoll, 1998). Large artesian springs occur in many Inyan Kara aquifer in South Dakota and the Dakota locations downgradient from these loss zones, most aquifer in North Dakota. Generally, water in the lower commonly within or near the outcrop of the Spearfish Cretaceous aquifer is confined by several thick shale Formation, providing an important source of base flow layers except in aquifer outcrop areas around structural in many streams beyond the periphery of the Black uplifts, such as the Black Hills. Water in the lower Hills (Rahn and Gries, 1973; Miller and Driscoll, Cretaceous aquifer generally moves northeasterly from 1998). high-elevation recharge areas to discharge areas in Although the Precambrian basement rocks eastern North Dakota and South Dakota (Whitehead, generally have low permeability and form the lower 1996). Although the aquifer is widespread, it contains confining unit for the series of overlying aquifers, little freshwater. Much of the saline water is believed localized aquifers occur in many locations in the cen- to be from upward leakage of mineralized water from tral core of the Black Hills, where enhanced secondary the Paleozoic aquifers (Whitehead, 1996). permeability has resulted from weathering and fracturing. Where the Precambrian-rock aquifers are satu- Local Setting rated, water-table (unconfined) conditions generally Many of the sedimentary units in the Black Hills occur and land-surface topography can strongly control area contain aquifers, both within and beyond the study ground-water flow directions.

12 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota Overlying the Precambrian rocks is the Dead- sandstone and anhydrite in the upper portion. Shales in wood aquifer, which is contained within the Deadwood the lower portion of the Minnelusa Formation act as a Formation and is utilized primarily near its outcrop confining unit to the underlying Madison aquifer; area. Regionally, the Precambrian rocks act as a lower however, the extent of hydraulic separation is spatially confining unit to the Deadwood aquifer, and the variable and is not well defined, as discussed in a sub- Whitewood and Winnipeg Formations, where present, sequent section of this report. The Minnelusa aquifer act as overlying semiconfining units (Strobel and may have enhanced vertical hydraulic conductivity in others, 1999). Where the Whitewood and Winnipeg areas of collapse breccia (breccia pipes in fig. 5) asso- Formations are absent, the Deadwood aquifer is over- ciated with dissolution of interbedded evaporites (Long lain by the Englewood Formation, which Strobel and and others, 1999). Hayes (1999) concluded that others (1999) included as part of the Madison aquifer. upward leakage of relatively fresh water from the The Madison aquifer generally is considered to Madison aquifer is a probable agent for dissolution of consist primarily of the karstic upper part of the anhydrite within the Minnelusa Formation, which is a Madison Limestone, where numerous fractures and likely mechanism for development of breccia pipes solution openings provide extensive secondary within the Minnelusa Formation. Exposed breccia porosity. Strobel and others (1999) included the entire pipes, which are common in many areas, were hypoth- Madison Limestone and the Englewood Formation in esized to be conduits for abandoned springs, which their delineation of the Madison aquifer, which migrate outwards from the flanks of the Black Hills receives recharge from streamflow losses and precipi- over geologic time, keeping pace with regional erosion. tation on its outcrop. In this report, outcrops of the The Minnelusa aquifer receives recharge from Madison Limestone and Englewood Formation (fig. 3) streamflow losses and precipitation on its outcrop. are referred to as the outcrop of the Madison Limestone Streamflow losses to the Minnelusa aquifer generally for simplicity. Low-permeability layers in the lower are less than to the Madison aquifer (Carter, Driscoll, part of the Minnelusa Formation generally act as an and Hamade, 2001) because most of the flow is lost to upper confining unit to the Madison aquifer. However, the Madison aquifer before reaching the outcrop of the karst features in the top of the Madison Limestone may Minnelusa Formation. The Minnelusa aquifer is confined by the overlying Opeche Shale. contribute to reduced competency of the overlying confining unit in some locations. The potentiometric surface of the Minnelusa aquifer is shown in figure 8. Ground-water flow in the The potentiometric surface of the Madison Minnelusa aquifer in the study area generally follows aquifer is shown in figure 7. In many locations, the bedding dip, but may be affected by structural ground-water flow in the Madison aquifer follows the features. Regional ground-water flow from the west bedding dip, which generally is radially away from the may influence the potentiometric surface in the central core of the Black Hills. Ground-water flow- northern and southwestern parts of the study area. paths and velocities also are heavily influenced by The Minnekahta aquifer, which overlies the anisotropic and heterogeneous hydraulic properties of Opeche Shale, is contained within the Minnekahta the Madison aquifer. Flowpaths are not necessarily Limestone. The Minnekahta aquifer typically is very orthogonal to potentiometric contours because of permeable, but well yields are limited by the aquifer highly variable directional transmissivities and may be thickness. The overlying Spearfish Formation acts as a further influenced by vertical flow components confining unit to the Minnekahta aquifer. between the Madison and Minnelusa aquifers. Long Within the Mesozoic rock interval, the Inyan (2000) described anisotropy in the Madison aquifer in Kara aquifer is used extensively and various other the Rapid City area that causes ground-water flow to be aquifers are used locally to lesser degrees. The Inyan nearly parallel to the potentiometric contours in some Kara aquifer receives recharge primarily from precipi- cases. Regional ground-water flow from the west may tation on its outcrop and also may receive recharge influence the potentiometric surface in the northern and from leakage from the underlying Paleozoic aquifers southwestern parts of the study area. (Swenson, 1968; Gott and others, 1974). As much as The Minnelusa aquifer is contained within the 4,000 ft of Cretaceous strata (primarily shales) act as thin layers of sandstone, dolomite, and anhydrite in the the upper confining unit to aquifers in the Mesozoic lower portion of the Minnelusa Formation and within rock interval.

Description of Study Area 13 104o 45' 103o30' C ro H w 44o45' Belle Fourche orse EXPLANATION Creek Reservoir Owl Newell OUTCROP OF MADISON BELLE Creek Creek LIMESTONE (from Strobel

F and others, 1999) BELLE FOURCHE O UR CH RIVER Crow E 00 MADISON LIMESTONE PRESENT, Creek Hay Creek 34 0 R 3500 0 E BUTTE CO gage 7 BUT OVERLAIN BY SURFICIAL 3 3600 V ER R I MEADE CO Mirror REDWAT LAWRENCE CO DEPOSITS (from Carter and Lake Cox Old Spearfish Redden, 1999d) and Lake Hatchery Saint Creek McNenny Crow Higgins Gulch Onge B Creek MADISON LIMESTONE ABSENT Rearing ea Spring r reek Pond30' Gulch Spearfish C 3200 (from Carter and Redden, 1999d)

Creek Whitewood 3800 Creek 3000 4000 4200 Bottom POTENTIOMETRIC CONTOUR-- Creek e 4400 ls Gulch a Shows altitude at which water 4600 F

4800 3000 Maurice Whitewood Butte 5Higgins000 would have stood in tightly cased, 5200 STURGIS Tinton o nonpumping wells (modified from DEADWOOD 15' 103 Beaver Cr Strobel and others, 2000a).

h 5400 Lead 4800 s Contour interval 100, 200, or i f 5600 r 5800 Bear Creek Tilford a 4600 2800 e Elk 500 feet, where appropriate. r p 6000 Elk Elk Creek S C 260 6200 ow 4400 Springs Creek Mead 0 Dashed where inferred. Datum Creekgage Spearfish E N lk is sea level . 15' Little Boxelder F

r Piedmont 4200 k Ellsworth FAULT--Dashed where approxi- R 4400 3400 S. Fork Rapid Cr a 3600 Air Force p 4000 Base i Nemo mated. Bar and ball on down- d 3800

6400 C Blackhawk Creek 6600 r thrown side rk Box Elder Fo Rochford 6400s ANTICLINE--Showing trace of axial

ad 4 City

o . Fork 20 h N Ca Rapid R st Springs

le 0 plane and direction of plunge. Castle Cr RAPID CITY Cleghorn/ Dashed where approximated ek Pactola Creek Rapid C C re Jackson PLATEAU re Reservoir ek 3600 Creek Castle Springs SYNCLINE--Showing trace of axial

LIMESTONE V ic Creek 6600 Deerfield toria S. Fork Spring 2800 plane and direction of plunge. o r Reservoir 4000 44 C e Dashed where approximated 6400 Castl

6400 3000 Sheridan Creek 6200 Creek Lake MONOCLINE--Showing trace of 6000 Hill City 6600 Keystone axial plane. Dashed where 5500 Mt. Rushmore National approximated Spring Memorial Hayward n

o y PENNINGTON CO n Battle DOME--Symbol size approximately a Hermosa C CUSTER CO proportional to size of dome. s 5000 Creek e Battle l o B Grace Creek Dome asymmetry indicated by French Creek Spring 4500 CUSTER Grace arrow length e idg Creek Cool Coolidge 45' CUSTER Creek ARTESIAN SPRING Jewel Cave Spring National Monument STATE French Fairburn STREAMFLOW-GAGING STATION 4000 PARK FOR SPRING REACH Canyon 3600 Creek 3800 nyon 3400 3900a C Lam 3700 Beaver 3900 e Pringle Wind Cave National Park Hell 3200 3800

WYOMING Creek BeaverJohnny Wind Creek Creek Cave

SOUTH DAKOTA SOUTH Red Spring Creek Buffalo Gap RIVER 30' Pass Hot Brook Spring FALL RIVER CO H on ot Brook ny Evans Plunge Spring Ca HOT SPRINGS Minnekahta 3700 Fall Fall River gage R Oral 3600 C 0 H 0 E 35 Y Cool Spring E N N E Cascade Springs3400 Edgemont Horsehead

Angostura o Creek Reservoir C 43 15' d reek oo w n o tt o Creek C Provo

Figure 7. Potentiometric surface of the Madison aquifer and locations of major artesian springs.

14 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota 104o 45' 103o30' C ro H w o Belle Fourche orse EXPLANATION 44 45' 2 Creek Reservoir 800 Owl Newell OUTCROP OF MINNELUSA BELLE Creek Creek FORMATION (from Strobel

F and others, 1999) BELLE FOURCHE O Crow UR Creek CHE RIVER Hay Creek MINNELUSA FORMATION

gage R E BUTTE CO PRESENT, BUT OVERLAIN V Mirror ER R I MEADE CO REDWAT LAWRENCE CO BY SURFICIAL DEPOSITS Lake 3000 Old Spearfish 3200 and Cox 3600 (from Carter and Redden, 1999c) Hatchery Saint Creek McNenny Lake Crow Higgins Gulch Onge Rearing 3400 B Creek ea Spring MINNELUSA FORMATION Pond r reek Gulch Spearfish C 30'4200 3600 ABSENT (from Carter and 4000reek C Whitewood 3800 Creek Redden, 1999c) Bottom Creek e 4000 ls Gulch 5000 4400 a 3000 4600 F POTENTIOMETRIC CONTOUR-- Maurice4800 Whitewood Butte Higgins 4200 STURGIS Shows altitude at which water Tinton 4 0 o would have stood in tightly 0 3800 103 DEADWOOD 0 15' Beaver Cr cased, nonpumping wells 3600 h Lead s i (modified from Strobel and f 3000 r Bear Creek Tilford 6000 a Elk 2800 e r p Elk Elk Creek Creek others, 2000b). Contour S C w 3200 6200 Meado Springs gage interval 200 feet. Dashed where 6400 Creek Spearfish E 6400 N lk inferred. Datum is sea level . 15' Little Boxelder F

o r Piedmont k 3400 Ellsworth R FAULT--Dashed where approxi- 6600S. Fork Rapid Cr a Air Force p Base i Nemo d mated. Bar and ball on down- C Blackhawk Creek 6200 r thrown side rk Box Elder Fo Rochford s ad ANTICLINE--Showing trace of axial o N. Fork Rapid City h Cas R tle Springs plane and direction of plunge. Castle Cr RAPID CITY

k Creek Cleghorn/ Dashed where approximated e Pactola 0 Rapid C C re 0 Jackson PLATEAU re Reservoir 8 ek 3 Springs Creek Castle SYNCLINE--Showing trace of axial LIMESTONE V Deerfield ictoria Creek S. Fork Spring o r Reservoir 3800 plane and direction of plunge. 44 6800 C e 6600 Castl Dashed where approximated Sheridan Creek Creek 6400 Lake Hill City MONOCLINE--Showing trace of 6000 6200 0 Keystone axial plane. Dashed where 620 Mt. Rushmore 6000 National Spring Memorial Hayward approximated n

5800o y PENNINGTON CO n Battle a Hermosa DOME--Symbol size approximately

C 5600CUSTER CO

s Battle Creek proportional to size of dome. e l o 5400 Creek B Grace 5800 Dome asymmetry indicated by 6000 French Creek Spring CUSTER Grace e arrow length idg Creek Cool Coolidge 45' CUSTER Creek ARTESIAN SPRING Jewel Cave 5400 Spring 4400 National4800 Monument STATE

4200 French Fairburn STREAMFLOW-GAGING STATION 5200 4600 PARK 4000 3800 Canyon FOR SPRING REACH Creek 3800 5000 anyon C L Beaver am e Pringle Wind Cave National Park 3200 Hell 3400

4800 3000 WYOMING Creek BeaverJohnny 2800 Creek 4600 Wind Creek 2600 Cave SOUTH DAKOTA SOUTH Red 3600 Spring 4200 Creek 4400 Buffalo Gap RIVER 30' Pass3600

FALL RIVER CO H Hot Brook Spring on ot Brook ny Evans PlungeCa Spring HOT SPRINGS Minnekahta Fall Fall River gage R Oral

C H E Y Cool Spring E N N Cascade E Springs

Edgemont Horsehead

Angostura o Creek Reservoir C 43 15' d reek oo w n o tt 3400 o Creek C Provo

Figure 8. Potentiometric surface of the Minnelusa aquifer and locations of major artesian springs.

Description of Study Area 15 METHODS USED AND DATA SETS processes. Stable isotopes of oxygen and hydrogen are CONSIDERED used to evaluate ground-water recharge areas and flowpaths. Tritium, which is an unstable hydrogen isotope, Most of the data used in this study were collected is used for analysis of ground-water ages. as part of the U.S. Geological Survey Regional Aquifer System Analysis Program (Busby and others, 1983, 1991) or the Black Hills Hydrology Study, using appli- Major-Ion Chemistry cable methods and protocols. Data sets considered primarily include samples collected prior to the end of Major reactive minerals in the Madison and water year 1998, when data collection for the Black Minnelusa aquifers are calcite (CaCO3), dolomite Hills Hydrology Study was completed. However, (CaMg(CO3)2), and anhydrite (CaSO4). The inter- several isotope samples collected during water year actions of these minerals with water and dissolved 2000 also are considered. Additional chemical data for gases, to a large extent, defines the chemical composi- precipitation in Newcastle, Wyoming, were obtained tion of ground water in the two aquifers. An important from the National Atmospheric Deposition consequence of the presence of both calcite and anhy- Program/National Trends Network (1999). drite in the Madison and Minnelusa aquifers is the For geochemical analyses involving major ions, potential for precipitation of calcite by the common-ion a cation/anion balance within 10 percent was used as a effect, which can be represented as general sample selection criterion. For some selected - → 2- single constituents, all applicable data were used. For CaSO4 + 2HCO3 CaCO3 + SO4 + CO2 geochemical analyses based on pH or results of specia- (Langmuir, 1997). (1) tion calculations, only samples collected specifically for the Black Hills Hydrology Study were used because In partially dolomitized limestone, dolomite dissolu- these samples were collected using a closed-system tion also can lead to precipitation of calcite: device, thereby avoiding reaction with the atmosphere during sampling. Field measurement of pH values → 2+ 2- CaMg(CO3)2 CaCO3 + Mg + CO3 included temperature compensation to correct for the (Krauskopf and Bird, 1995). (2) effect of temperature on electrode response. For some samples used in speciation calcula- The term “dedolomite” was originally used to tions, an estimated temperature was calculated from a describe the replacement of dolomite by calcite during linear regression of temperature with distance from near-surface chemical weathering (von Morlot, 1847). outcrop area. The geochemical program PHREEQC Strictly speaking, “dedolomitization” describes the for- (Parkhurst and Appelo, 1999) was used for speciation mation of the mineral phase dedolomite, which is calculations and for conducting forward geochemical calcite pseudomorphous after dolomite. In this report, modeling to illustrate relations among chemical param- “dedolomitization” is used to describe a combination eters that would be expected for certain chemical of the above two reactions (eqs. 1 and 2), in which the reactions. irreversible dissolution of anhydrite in waters saturated Selected site information for all samples consid- with calcite and dolomite drives additional dissolution ered in this report is presented in table 7 in the Supple- of dolomite and concurrent precipitation of calcite mental Information section at the end of this report. (Back and others, 1983). The dedolomitization The same site numbers are used consistently reaction can be represented as: throughout the report. → CaSO4 + 1/2 CaMg(CO3)2 CaCO3 2+ 2+ 2- GEOCHEMISTRY OF MADISON AND + 1/2 Ca + 1/2 Mg + SO4 .(3) MINNELUSA AQUIFERS The extent to which dedolomitization occurs in the The major-ion and isotope chemistry of the Madison and Minnelusa aquifers can be illustrated Madison and Minnelusa aquifers are discussed in the through the major-ion chemistry and saturation indices following sections. The discussion of major-ion of the ground water as well as through identification of chemistry includes information regarding distribution trends in pH and calcium, magnesium, and sulfate of major ions, saturation state, and evolutionary concentrations.

16 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota Distribution of Major Ions dissolution. Ion chemistry is somewhat anomalous for site 188 (Rapid City area), which has high chloride The major-ion chemistry of water in the Madison with calcium and magnesium, rather than sodium. and Minnelusa aquifers can be illustrated with trilinear and Stiff (1951) diagrams, which show spatial varia- A trilinear diagram showing percentages of tions and possible trends in the major-ion chemistry of major ions in the Minnelusa aquifer is shown in the waters. In constructing Stiff diagrams, the average figure 9, and Stiff diagrams representative of the milliequivalent value was used where more than one major-ion chemistry in water from Minnelusa wells are sample existed for a site. For clarity, not all diagrams shown in figure 11. Figure 9 illustrates three main were plotted on the maps, but an effort was made to types of water in the Minnelusa aquifer: calcium mag- include diagrams representative of the water chemistry nesium bicarbonate type, calcium magnesium sulfate in a particular area. type, and calcium magnesium bicarbonate sulfate chlo- A trilinear diagram showing percentages of ride type. Figure 11 shows that water in the Minnelusa major ions in the Madison aquifer is shown in figure 9, aquifer generally evolves downgradient from a calcium and the distribution of major-ion chemistry in the magnesium bicarbonate type to a calcium magnesium Madison aquifer throughout the study area is shown in sulfate type due to dissolution of anhydrite. In the Hot figure 10. Figure 9 shows two main types of water in Springs area, ground water in the Minnelusa aquifer is the Madison aquifer: calcium magnesium bicarbonate characterized by higher concentrations of sodium and type and calcium sodium chloride sulfate type. chloride. Because chloride is conservative in water, it Figure 10 shows that water in the Madison aquifer is can be used to identify leakage between aquifers dominated by calcium, magnesium, and bicarbonate (Busby and others, 1995). The high chloride concen- ions throughout most of the study area. The high con- trations in this area in both the Madison and Minnelusa centrations of chloride, sulfate, and sodium in the aquifers could reflect hydraulic connection between the southwestern part of the study area relative to the aquifers. The dissolution of evaporite minerals and the rest of the study area probably reflect the presence of presence of more evolved ground water contributed by more evolved ground water and regional flow, or regional flow also are possible explanations for the greater amounts of evaporite minerals available for occurrence of this water type in the Minnelusa aquifer.

Number of samples = 127 Number of samples = 241

Calcium + Magnesium Calcium + Magnesium

80 80 0 0 8 8

60 60 0 0 6 6

40 40 0 0 4 4

20 20 0 0 2 2

Chloride + Fluoride + Sulfate Chloride + Fluoride + Sulfate

Sodium + Potassium Sodium + Potassium

20 20 0 0 80 2 80 80 2 80

40 Sulfate 40 Sulfate 0 0 60 4 60 60 4 60

60 60 0 0 6 6 Magnesium40 40 Magnesium40 40

80 80 0 Bicarbonate + Carbonate 0 20 8 20 20 8 Bicarbonate + Carbonate 20

80 60 40 20 80 60 40 20 0 0 0 0 0 0 0 0 2 4 6 8 2 4 6 8 Calcium Chloride + FluorideCalcium Chloride + Fluoride PERCENT OF REACTING VALUES PERCENT OF REACTING VALUES Madison Aquifer Minnelusa Aquifer

Figure 9. Trilinear diagrams showing proportional concentrations of major ions in the Madison and Minnelusa aquifers.

Major-Ion Chemistry 17 104o 45' 103o30' C EXPLANATION ro H w 44o45' Belle Fourche orse OUTCROP OF MADISON LIME- Creek Reservoir 4 Owl Newell STONE (from Strobel and others, BELLE Creek 6 Creek 1999) 8 F BELLE FOURCHE O UR MADISON LIMESTONE ABSENT CHE RIVER Hay Creek 12 R (from Carter and Redden, 1999d) E BUTTE CO V 47 ER R I MEADE CO REDWAT LAWRENCE CO STIFF DIAGRAM--

Saint Creek 39 20 Sodium + Potassium Chloride + Fluoride Crow 17 Onge

B Creek Calcium Bicarbonate + Carbonate e ar reek 36 Gulch Spearfish C Magnesium Sulfate 30' 59 100 10 reek 61 C 58 Whitewood Creek CONCENTRATION, IN MILLIEQUIVALENTS PER LITER Bottom Creek e ls Gulch a 82 F WELL COMPLETED IN THE Maurice83 Whitewood Butte Higgins 108 73 STURGIS Tinton MADISON AQUIFER--Number is 69 o site number from table 7 DEADWOOD 15' 103 Beaver Cr

278h Lead s SPRING--Number is site number i f 90 276 r Bear Creek Tilford a e 89 from table 7 r p Elk 85 S w C Meado 104 Creek Spearfish E N 100 lk . 15' Little Boxelder F 282 o r Piedmont k Ellsworth R S. Fork Rapid Cr a Air Force p 95 Base i Nemo d 126 138 C Blackhawk Creek 107 r 145 rk Box Elder Fo Rochford 108 110 284 s 106 283ad 143 o . Fork 124 h N Ca Rapid R stl 140 123 e C RAPID CITY 286 Castle r 117 188 ek Creek Rapid C C re Pactola PLATEAU re Reservoir 164 ek Creek Castle LIMESTONE V 172 182 Deerfield ictoria Creek 150 S. Fork 189 Spring o r Reservoir 191 44 C e 195 Castl Sheridan Creek Creek Lake Hill City Keystone Mt. Rushmore National Spring Memorial Hayward n 197 o y PENNINGTON CO n Battle

a Hermosa C CUSTER CO Creek s 203 e l o B Grace French Creek 201 CUSTER e idg Creek Cool 45' CUSTER 210 Jewel Cave National Monument 206 STATE French Fairburn

PARK

Canyon 220 Creek anyon C L 217 Beaver am e Pringle Wind Cave National Park Hell 215 221 222 WYOMING Creek Johnny

Wind Creek237 Cave 238

SOUTH DAKOTA SOUTH Red 231 233 Creek Buffalo Gap RIVER 30' Pass

FALL RIVER CO H on ot Brook ny 242 Ca 247 HOT SPRINGS Minnekahta Fall R Oral

C H 253 E Y E N N E 256 258 Edgemont 257 Horsehead Angostura o Creek Reservoir C 43 15' d reek oo w n o 260 tt o Creek C 262Provo261

Figure 10. Selected Stiff diagrams showing the distribution of major-ion chemistry in the Madison aquifer.

18 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota 104o 45' 103o30' C EXPLANATION ro H w 44o45' Belle Fourche orse OUTCROP OF MINNELUSA Creek Reservoir 48 3 Owl Newell FORMATION (from Strobel BELLE Creek Creek and others, 1999) 11 F BELLE FOURCHE O UR MINNELUSA FORMATION ABSENT CHE RIVER Hay Creek (from Carter and Redden, 1999c) 29 R 33 E 9 BUTTE CO V ER R I MEADE CO 44 REDWAT LAWRENCE CO ESTIMATED SULFATE CONCEN- 14 TRATIONS IN MINNELUSA 32 Creek 41 43 22 Saint Crow 15 Onge AQUIFER B Creek ea 16 r 35 reek 13 Gulch Spearfish52C Less than 250 milligrams per liter 30' 64 62 reek 57 Between 250 and 1,000 milligrams C Whitewood 53 Creek 49 276 Bottom Creek e 51 81 per liter 55 ls Gulch 54a 80 F Greater than 1,000 milligrams per Maurice Whitewood Butte Higgins STURGIS Tinton 76 72 liter 79 o DEADWOOD 68 15' 103 Beaver Cr STIFF DIAGRAM-- h Lead s 88 i f Sodium + Potassium Chloride + Fluoride r Bear Creek Tilford a

e r Calcium Bicarbonate + Carbonate p Elk 86 S w C Meado Magnesium Sulfate Creek Spearfish E 100 10 N 101 lk . 15' Little Boxelder F CONCENTRATION, IN MILLIEQUIVALENTS PER LITER o r Piedmont 98 k Ellsworth R 105 97 S. Fork Rapid Cr a Air Force p Base i Nemo d 139 WELL COMPLETED IN THE C Blackhawk Creek 204 r 129 MINNELUSA AQUIFER--Number rk Box Elder Fo Rochford is site number from table 7 ds 109 a 125 o N. Fork C Rapid 113 h ast R le ID CITY SPRING--Number is site number Castle Cr RAP 276 119 183 from table 7 ek Creek Rapid C C re Pactola 179 PLATEAU re Reservoir ek Creek Castle 155 LIMESTONE V 157 Deerfield ictoria Creek S. Fork Spring o r Reservoir 44 C 192 e Castl 194 Sheridan Creek Creek Lake Hill City Keystone Mt. Rushmore National Spring Memorial Hayward n o 199 y PENNINGTON CO n Battle

a Hermosa C CUSTER CO 198

s 200 Creek e l 287 o B Grace French Creek 202 CUSTER e idg204 Creek Cool 205 45' CUSTER 211 Jewel207 Cave 208 National Monument 212 209 STATE French Fairburn

288 PARK

Canyon 289 yon 213 Creek an C 218 Lam Beaver e Pringle Wind Cave 214 National Park 223 Hell 216 228 WYOMING Creek 219 236 Johnny Wind Creek 230 Cave SOUTH DAKOTA SOUTH Red 234 227 239 Creek 235 Buffalo Gap RIVER 30' Pass 244

FALL RIVER CO H 250 249 248 245on ot Brook ny Ca HOT SPRINGS Minnekahta243 Fall252 251 R Oral

C H E Y 254 E N N E

Edgemont Horsehead

Angostura o Creek Reservoir C 43 15' d reek oo w n o tt o Creek Base modified from U.S. Geological Survey digital data, 1:100,000 C Provo Rapid City, Office of City Engineer map, 1:18,000, 1996 Universal Transverse Mercator projection, zone 13 01020MILES Hat 01020KILOMETERS Figure 11. Selected Stiff diagrams showing the distribution of major-ion chemistry in the Minnelusa aquifer. Approximate location of anhydrite dissolution front showing transition between low and high sulfate concentrations also is shown.

Major-Ion Chemistry 19 Sulfate concentrations in the Madison aquifer are apparent in the Rapid City area and along the south- generally are low and increase slightly with increasing eastern flank of the uplift. In some areas, basinward distance from outcrop areas (fig. 10). In contrast, an deflections may correspond with structural features extremely sharp gradation in sulfate concentrations where enhanced hydraulic conductivity can occur; occurs in the Minnelusa aquifer, such as near sites 22, however, available data are insufficient for conclusive 29, 32, and 33 in the northern part of the study area. determinations. Sulfate concentrations in the Minnelusa aquifer are dependent on the amount of anhydrite present in the Saturation State Minnelusa Formation. Thick anhydrite beds were observed in cores from Minnelusa Formation drill Saturation indices measure departures from holes in the southern Black Hills (Braddock and thermodynamic equilibrium and can be used to develop Bowles, 1963; Brobst and Epstein, 1963). These thick hypotheses related to the reactivity of minerals in an anhydrite deposits are mostly absent in and near out- aquifer. The saturation index (SI) for a particular crops because of earlier removal by dissolution in the mineral generally indicates whether the ground water is subsurface (Epstein, 2000). undersaturated (SI < 0), at equilibrium (SI = 0), or Where dissolution of anhydrite currently is supersaturated (SI > 0) with respect to that particular taking place (Epstein, 2000), a transition zone from low mineral. If ground water is undersaturated with respect to high sulfate concentrations is postulated, as shown in to a mineral, as indicated by a negative SI, the ground figure 11. Sulfate concentrations less than 250 mg/L water would theoretically dissolve the mineral if delineate a zone in which anhydrite probably has been present. Conversely, if ground water is supersaturated largely removed by dissolution. The zone in which with respect to a mineral, then the mineral would theo- sulfate concentrations are between 250 and 1,000 mg/L retically precipitate from the ground water. There is marks the position of the “anhydrite dissolution front,” some uncertainty associated with the range in SI values an area of active removal of anhydrite by dissolution. that indicates equilibrium because of uncertainties in Downgradient from the anhydrite dissolution front, field-measured pH, laboratory-analyzed concentra- sulfate concentrations are greater than 1,000 mg/L, tions, and ionic strength and equilibrium constants which delineates a zone in which thick anhydrite beds involved in calculations of SI values (Langmuir, 1997). remain in the formation. A similar approach was pre- The geochemical program PHREEQC sented by Klemp (1995), who used specific conduc- (Parkhurst and Appelo, 1999) was used to calculate SI tance of ground-water samples from the Minnelusa values for 41 samples (35 wells) from the Madison aquifer to identify a line of dissolution of anhydrite. aquifer and 25 samples (24 wells) from the Minnelusa Brobst and Epstein (1963), Gott and others (1974), aquifer (table 1). The default thermodynamic data- Kyllonen and Peter (1987), and Epstein (2000) also base, phreeqc.dat (Parkhurst, 1995), provided the ther- presented models of this anhydrite dissolution front, modynamic data for calculations. Although anhydrite wherein the Minnelusa Formation is thinner near the is more common than gypsum in the Madison and outcrop area because of removal of the anhydrite and Minnelusa aquifers, gypsum (CaSO4•2H2O) is the thicker where the anhydrite still remains. Epstein more stable phase. Therefore, SIgypsum is used as an (2000) proposed that sinkholes in overlying formations indicator for both anhydrite and gypsum throughout the are caused by collapses in the Minnelusa Formation remainder of this report. because of removal of anhydrite, and that the sinkholes Calculated SI values show that water in the mark the zone of active dissolution of anhydrite. Madison aquifer is greatly undersaturated with respect Dissolution of anhydrite cements could increase to halite (NaCl) and, to a lesser extent, with respect to hydraulic conductivity and secondary porosity in the gypsum. These minerals should, if present, continue to Minnelusa aquifer; thus, low sulfate concentrations dissolve in the Madison aquifer. However, ground may indicate areas of greater hydraulic conductivity water in the Madison aquifer remains undersaturated and enhanced secondary porosity where anhydrite has with respect to gypsum even at the highest sulfate con- been removed from the aquifer. The anhydrite dissolu- centrations (fig. 12). Figure 13 shows that most waters tion front in the northern Black Hills generally is in the Madison aquifer are in equilibrium with respect located farther from outcrop areas than in other loca- to calcite and are slightly undersaturated with respect to tions, although basinward deflections of the front also dolomite.

20 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota

Table 1. Saturation indices for selected samples from wells completed in the Madison and Minnelusa aquifers

[SIcalcite, calcite saturation index; SIdolomite, dolomite saturation index; SIgypsum, gypsum saturation index; SIhalite, halite saturation index; e, estimated; --, no data]

pH Site Station identification Temperature (standard SI SI SI SI number number (degrees Celsius) calcite dolomite gypsum halite units) Madison Aquifer 6 444129103514801 29.4 7.2 0.0 0.0 -1.6 -10.2 36 443100104002001 11.3 7.4 .1 -.3 -1.3 -10.3 58 442802103544601 11.3 7.3 .1 -.1 -1.8 -7.7 59 442919103511601 12.3 7.4 -.1 -.4 -2.3 -9.6 61 442842103505501 7.8 7.6 -.1 -.4 -2.4 -9.4 83 442435103571101 e10.9 7.2 .0 -.4 -3.1 -10.5 85 441759103261202 11.5 7.5 -.1 -.4 -2.1 -10.6 95 441055103230501 13.8 7.4 .1 .2 -3.2 -11.0 100 441337103225002 12.5 7.3 -.1 -.3 -2.7 -10.5 106 440629103040901 47.5 6.9 -.2 -.3 -1.5 -9.9 112 440519103160701 20.6 7.7 .1 .2 -2.8 -10.2 116 440526103173001 13.6 7.7 .0 -.1 -2.7 -9.7 116 440526103173001 13.2 7.6 .0 -.2 -2.6 -9.2 121 440500103193601 13.8 7.6 .0 -.1 -2.5 -9.4 123 440612103152001 15.3 7.5 -.2 -.4 -2.7 -10.1 123 440612103152001 15.2 7.5 -.1 -.3 -2.6 -8.9 124 440655103140501 20.0 7.5 -.1 -.1 -2.7 -9.4 138 440931103141401 20.3 7.3 -.1 .0 -2.7 -10.4 145 440811103222201 16.8 7.3 -.1 -.1 -3.0 -10.2 161 440205103172001 13.7 7.6 -.1 -.5 -2.4 -8.9 164 440308103184601 7.0 7.6 -.2 -.8 -2.3 -9.0 168 440300103173501 e12.7 7.0 -.7 -1.7 -2.4 -9.1 168 440300103173501 18.4 7.7 .1 .0 -2.3 -8.9 169 440220103164001 15.2 7.7 .0 -.2 -2.3 -9.2 169 440220103164001 16.0 7.6 -.1 -.4 -2.4 -9.1 172 440342103160701 16.5 7.6 -.1 -.2 -2.3 -9.1 172 440342103160701 17.8 7.7 .1 .1 -2.0 -9.4 176 440310103173802 14.8 7.7 -.1 -.2 -2.0 -9.2 178 440338103173302 12.0 7.7 .0 -.2 -2.3 -9.3 182 440430103160202 15.5 7.7 .0 .0 -2.5 -9.6 185 440443103161301 16.2 7.7 .1 .0 -2.7 -9.7 186 440446103161701 e16.3 7.4 -.3 -.6 -2.7 -10.2 186 440446103161701 16.4 7.7 .1 .0 -2.7 -10.4 188 440427103131701 26.4 6.1 -.3 -.8 -2.1 -6.9

Major-Ion Chemistry 21 Table 1. Saturation indices for selected samples from wells completed in the Madison and Minnelusa aquifers—Continued

[SIcalcite, calcite saturation index; SIdolomite, dolomite saturation index; SIgypsum, gypsum saturation index; SIhalite, halite saturation index; e, estimated; --, no data]

pH Site Station identification Temperature (standard SI SI SI SI number number (degrees Celsius) calcite dolomite gypsum halite units) Madison Aquifer—Continued 195 435851103143501 13.3 7.5 -0.3 -0.8 -2.4 -8.7 201 434700104021401 10.1 8.1 .6 1.0 -2.7 -9.8 210 434350103201901 14.5 7.9 .2 .3 -2.4 -9.6 215 433517103534201 14.5 8.3 .6 .6 -1.4 -7.9 238 433115103251401 17.2 7.7 .1 .0 -2.4 -8.6 242 432548103414801 21.0 7.5 .2 .3 -2.0 -8.7 253 432136103321001 21.3 7.3 .2 .0 -1.0 -6.5 Minnelusa Aquifer 29 443515103513901 13.4 7.3 .4 .3 -.1 -9.8 35 443100104002002 10.4 7.4 .0 -.4 -1.7 -10.6 44 443320104004501 11.5 7.4 .1 -.2 -1.4 -10.3 48 443515103572501 18.3 7.1 .2 -.1 .0 -9.7 65 443100103543001 13.0 8.6 1.3 2.1 -2.5 -10.4 81 442545103343701 12.5 8.6 1.2 2.0 -2.2 -10.3 86 441759103261201 11.2 7.5 -.1 -.4 -2.7 -10.3 113 440528103155201 15.2 7.5 -.1 -.3 -2.4 -10.3 118 440544103180001 10.8 7.9 .2 .1 -2.3 -9.1 119 440516103194001 19.6 7.6 .2 .3 -2.4 -9.2 129 440818103180801 18.5 7.6 .2 .2 -2.5 -10.3 177 440310103173801 12.6 7.8 .1 .0 -2.3 -9.4 179 440338103173301 12.0 7.8 .2 .1 -2.4 -9.8 181 440414103164601 16.1 7.5 .1 .0 -1.6 -9.2 183 440430103160201 11.7 7.7 .0 -.2 -2.6 -- 183 440430103160201 14.2 7.7 .1 .1 -2.2 -9.6 187 440452103155301 14.8 7.7 .1 .2 -2.2 -- 192 435916103161802 12.4 7.2 -.4 -1.1 -2.5 -9.7 199 435018103155801 12.8 7.7 -.2 -.7 -2.7 -9.6 211 434350103201902 e11.5 7.7 .0 -.3 -2.1 -9.0 216 433517103534202 13.6 7.4 .3 .0 -.7 -8.7 227 433003103420701 13.8 7.5 .1 -.1 -2.5 -9.5 239 433115103251402 14.2 7.8 .1 .0 -2.3 -8.8 243 432548103414802 17.2 7.3 .2 .0 .0 -7.8 254 432127103325601 19.6 7.0 .2 .0 .0 -7.4

22 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota 1

Madison aquifer Minnelusa aquifer 0

-1

-2

GYPSUM SATURATION INDEX -3

-4 0.01 0.02 0.05 0.1 0.2 0.5 1 2 5 10 20 50 100 DISSOLVED SULFATE, IN MILLIMOLES PER LITER

Figure 12. Relation between gypsum saturation index and dissolved sulfate in the Madison and Minnelusa aquifers.

1.5 3 41 25 41 25

1.0 2 EXPLANATION 41 Number of samples Outlier data value more than 3 times the X 0.5 1 X interquartile range outside the quartile X Outlier data value less than or equal to 3 and more than 1.5 times the interquartile range outside the quartile 0 0 Data value less than or equal to 1.5 times the interquartile range outside the quartile 75th percentile X Median -0.5 -1 X 25th percentile CALCITE SATURATION INDEX CALCITE SATURATION DOLOMITE SATURATION INDEX DOLOMITE SATURATION

-1.0 -2 Madison Minnelusa Madison Minnelusa AQUIFER AQUIFER

Figure 13. Boxplots showing calcite and dolomite saturation indices for selected samples from the Madison and Minnelusa aquifers.

Major-Ion Chemistry 23 All samples from the Minnelusa aquifer are outcrops in the Madison and Minnelusa aquifers, indi- undersaturated with respect to halite, and most are cating that sulfate can be used as a measure of the reac- undersaturated with respect to gypsum. In some sam- tion progress. Variations in pH and concentrations of ples, SI values for gypsum approach zero, indicating calcium and magnesium as a function of dissolved near equilibrium with respect to gypsum at high sulfate sulfate can be used to evaluate chemical processes concentrations (fig. 12). Most samples in the Min- occurring in the aquifers. nelusa aquifer are near equilibrium to slightly super- Forward geochemical modeling was used in this saturated with respect to calcite and are saturated with study to illustrate trends in pH and calcium and magne- respect to dolomite (fig. 13). sium concentrations with increasing dissolution of For most samples in both the Madison and anhydrite. It is assumed in the modeling that the Minnelusa aquifers, SI values for gypsum, calcite, and aquifer contains calcite, dolomite, and anhydrite and is dolomite are indicative of the occurrence of dedolo- recharged by rainwater. Hypothetical recharge water mitization. Busby and others (1983) noted that condi- was created by defining water with a temperature of tions consistent with the dedolomitization process 15°C and a pH of 5.4, which was the average pH of generally include waters that are in equilibrium with rainwater at Newcastle, Wyoming, from August 1981 respect to calcite, undersaturated but approaching satu- through December 1998 (National Atmospheric Depo- ration with respect to dolomite, and undersaturated sition Program/National Trends Network, 1999). The with respect to gypsum. Samples that are undersatu- recharge water was allowed to equilibrate with calcite rated with respect to gypsum, calcite, or dolomite may and dolomite at a P of 10-2 bar, typical of values in CO2 be indicative of large hydraulic conductivity, especially the soil zone (Freeze and Cherry, 1979). Conceptually, in the Madison aquifer, where fast flow velocities and the recharge water then moved into a saturated zone short ground-water residence times in karst conduits containing calcite, dolomite, and anhydrite. The satu- may limit the extent of mineral dissolution. Undersat- ration index of dolomite was fixed at -0.2 to represent uration with respect to these minerals also could result slight undersaturation with respect to dolomite, and the from the influence of streamflow recharge for samples saturation index of calcite was fixed at 0 to represent collected near loss zones in outcrops of the Madison equilibrium conditions. These values are consistent Limestone and Minnelusa Formation. The contribu- with the dedolomitization process and reflect median tion of calcium from anhydrite dissolution could cause SI values in the Madison aquifer (fig. 13). Dissolution slight supersaturation with respect to calcite or dolo- of anhydrite occurred incrementally and was limited mite due to the common-ion effect. Samples with high not to exceed gypsum equilibrium. Reactions were SI values for calcite and dolomite may reflect errors in carried out at a constant temperature of 15°C, which is pH measurements, which are not uncommon primarily representative of average temperatures in the Madison because of instrumentation problems. All of the posi- and Minnelusa aquifers in the study area (table 1). tive outlier values in figure 13 are associated with pH For the simulated reactions occurring in the values greater than 8 (table 1), which may be of saturated zone, changes in modeled pH values and questionable validity. concentrations of calcium and magnesium, relative The saturation state with respect to gypsum of to increasing sulfate concentrations, are summarized water in the Madison and Minnelusa aquifers may pro- in table 2 and figure 14. Table 2 shows the pH; vide insight into mechanisms controlling hydraulic molality of calcium (mCa), magnesium (mMg), and connection between the aquifers. Because water in the sulfate (mSO4) in solution; number of moles of anhy- ∆ ∆ Madison aquifer remains undersaturated with respect drite ( manhydrite), calcite ( mcalcite), and dolomite ∆ to gypsum even at the highest sulfate concentrations ( mdolomite) dissolved or precipitated (negative values (fig. 12), upward leakage could drive increased disso- indicate precipitation); and saturation index for lution of anhydrite in the Minnelusa Formation, gypsum (SIgypsum) for each reaction step. Figure 14 especially where Minnelusa aquifer water is nearly shows how the modeled pH, calcium molality, and saturated with respect to gypsum. magnesium molality change with reaction progress. Calcite precipitated while dolomite dissolved, pH decreased, and concentrations of dissolved magnesium Evolutionary Processes and sulfate increased during reaction with anhydrite. As shown in figures 10 and 11, dissolved sulfate In the ninth model reaction step, the solution reached concentrations generally increase with distance from saturation with respect to gypsum.

24 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota Table 2. Selected results of geochemical modeling ∆ [°C, degrees Celsius; mCa, calcium molality; mMg, magnesium molality; mSO4, sulfate molality; manhydrite, number of moles of anhydrite dissolved or ∆ precipitated (negative values indicate precipitation); mcalcite, number of moles of calcite dissolved or precipitated (negative values indicate precipitation); ∆ mdolomite, number of moles of dolomite dissolved or precipitated (negative values indicate precipitation); SIgypsum, gypsum saturation index]

pH Temperature (standard mCa mMg mSO ∆m ∆m ∆m SI (°C) 4 anhydrite calcite dolomite gypsum units) 15 7.20 0.0029 0.0019 0.0025 0.0025 -0.0010 0.0006 -1.1316 15 7.11 .0044 .0028 .0050 .0050 -.0030 .0015 -.7708 15 7.05 .0059 .0038 .0075 .0075 -.0050 .0025 -.5555 15 7.01 .0074 .0047 .0100 .0100 -.0070 .0034 -.4020 15 6.98 .0089 .0057 .0125 .0125 -.0090 .0044 -.2830 15 6.95 .0104 .0066 .0150 .0150 -.0110 .0053 -.1859 15 6.92 .0119 .0076 .0175 .0175 -.0130 .0063 -.1041 15 6.90 .0135 .0085 .0200 .0200 -.0150 .0073 -.0334 15 6.89 .0143 .0090 .0213 .0225 -.0160 .0078 .0000 15 6.89 .0143 .0090 .0213 .0250 -.0160 .0078 .0000

16 7.3

14 7.2 12

10 7.1

7.0 6 pH, IN STANDARD UNITS

IN MILLIMOLES PER LITER 4 6.9

2 DISSOLVED CALCIUM OR MAGNESIUM,

0 6.8 2 4 6 8 10 12 14 16 18 20 22 DISSOLVED SULFATE, IN MILLIMOLES PER LITER

EXPLANATION CALCIUM MAGNESIUM pH

Figure 14. Modeled relations between calcium and magnesium concentrations, pH, and dissolved sulfate.

Major-Ion Chemistry 25 The forward modeling results indicate that the well oxygenated, even at considerable distances from incongruent dissolution of dolomite occurring in con- the outcrops. Much of the recharge to the aquifers is junction with anhydrite dissolution should result in from streamflow losses or through outcrop areas with increases in calcium and magnesium concentrations limited soil development. Thus, recharge has limited and a decrease in pH. Figure 15 shows the relation interaction with organic materials in the soil horizon between dissolved calcium and magnesium concentra- and consumption of oxygen is incomplete. Reduction tions and dissolved sulfate in the Madison and of sulfate, nitrate, and ferric iron minerals, methane Minnelusa aquifers. To show only data reflecting sig- fermentation, and anaerobic decay of organic matter, nificant reaction progress, only samples with sulfate therefore, are not likely in the Madison and Minnelusa concentrations greater than 1 millimole per liter are aquifers in the study area. included. The variations of dissolved calcium and magnesium concentrations with dissolved sulfate Isotope Chemistry shown in figure 15 for the Madison and Minnelusa aquifers are similar to those resulting from the This section presents information regarding the geochemical modeling shown in figure 14 in that both isotope chemistry of the Madison and Minnelusa aqui- calcium and magnesium concentrations increase with fers in the Black Hills area. Stable isotopes of oxygen increasing sulfate concentrations, with calcium (18O and 16O) and hydrogen (2H, deuterium; and 1H) increasing at a greater rate. However, the modeled are used to evaluate ground-water flowpaths, recharge calcium/magnesium ratio is lower than observed in the areas, and mixing conditions. The radioisotope tritium Madison and Minnelusa aquifers. The relative rates of (3H) provides additional information for evaluation of increase of calcium and magnesium concentrations are mixing conditions and ground-water ages. affected by the equilibrium constants (K) as well as Isotope data for selected sampling sites are pre- temperature. Comparison of actual calcium and mag- sented in tables 7 and 8 in the Supplemental Informa- nesium concentrations to model results indicates that if tion section. The majority of sampling sites are dedolomitization is occurring in both the Madison and ground-water sites, including wells (or caves) and Minnelusa aquifers, conditions in the aquifers (temper- springs. Spring samples are categorized as either head- ature, Kdolomite) are similar but not identical to those water springs (generally located upgradient of stream- that were modeled. flow loss zones) or downgradient springs (within or Figure 16 shows the relation between pH and downgradient of loss zones). Two of the downgradient dissolved sulfate in the Madison and Minnelusa springs (sites 291 and 299) are perched springs that are aquifers. In the Minnelusa aquifer, pH generally is located along stream channels where streamflow losses lower at high sulfate concentrations, which supports occur. The remainder of the downgradient springs are the occurrence of dedolomitization. In the Madison located downgradient of loss zones and are presumed aquifer, the data are consistent with dedolomitization, to be artesian springs. Data also are included for 14 but pH trends are limited by the extent of anhydrite surface-water sites, all of which are located immedi- dissolution. ately upgradient from loss zones where recharge occurs Other processes that could cause a decrease in to the Madison or Minnelusa aquifers. Data for three pH along a flowpath include nitrate reduction, denitri- meteorological sites also are provided. fication, and sulfate reduction. These reactions become important in environments in which the Background Information and Isotopic oxygen introduced by ground-water recharge has been Composition of Recharge Water depleted (Langmuir, 1997). Oxygen is supplied to ground water by the movement of air through unsatur- This section provides background information ated material above the water table as well as by regarding stable isotopes and tritium. Because of spa- recharge (Hem, 1992). This oxygen is consumed by tial variations in isotopic signatures for precipitation in reaction with organic materials and reduced inorganic the Black Hills area, stable isotopes can be used for minerals such as pyrite and siderite. Well-oxygenated identification of potential recharge areas and evalua- ground water may persist for long distances along a tion of ground-water flowpaths. Tritium is useful for flowpath if little reactive material is available. In both evaluation of ground-water ages because large tem- the Madison and Minnelusa aquifers, most samples poral variations of concentrations in precipitation have have dissolved oxygen concentrations greater than resulted from atmospheric testing of thermonuclear 2 mg/L (fig. 17), which indicates that the aquifers are bombs during the 1950’s and 1960’s.

26 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota MADISON AQUIFER 5

MINNELUSA AQUIFER 20

10 DISSOLVED CALCIUM OR MAGNESIUM, IN MILLIMOLES PER LITER

0 0 510152025 DISSOLVED SULFATE, IN MILLIMOLES PER LITER

EXPLANATION CALCIUM MAGNESIUM

Figure 15. Relations between dissolved calcium and magnesium concentrations and dissolved sulfate in the Madison and Minnelusa aquifers.

Isotope Chemistry 27 9.0

Madison aquifer Minnelusa aquifer 8.5

8.0

7.5

7.0 pH, IN STANDARD UNITS

6.5

6.0 0 2 4 6 8 101214161820 DISSOLVED SULFATE, IN MILLIMOLES PER LITER

Figure 16. Relation between pH and dissolved sulfate in the Madison and Minnelusa aquifers.

Madison aquifer 14 Minnelusa aquifer

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER 0 0 2 4 6 8 10 12 14 16 18 20 22 DISTANCE FROM OUTCROP, IN MILES

Figure 17. Relation between dissolved oxygen and distance from outcrop in the Madison and Minnelusa aquifers.

28 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota Stable Isotopes of Oxygen and Hydrogen A sample with a δ value of -20 ‰ is depleted by Stable isotopes, unlike radioisotopes, do not 20 parts per thousand (2 percent) in the heavier isotope decay by any known mechanisms. Stable isotopes of of the element relative to the standard. In this report, δ18 18 16 δ oxygen are ideal indicators of ground-water flowpaths O ( O/ O) and D (deuterium/hydrogen) values because the ratios of these isotopes in water are are reported in per mil relative to Vienna Standard affected by meteorological processes but generally not Mean Ocean Water (VSMOW) and are described as by interactions between minerals and ground water at lighter and heavier in relation to each other. The lighter temperatures less than about 100°C. values are more negative relative to the heavier values, Stable isotope values are given in “delta nota- which are less negative. tion,” which compares the ratio between heavy and Distinct isotopic signatures can result from iso- light isotopes of a sample to that of a reference stan- tope fractionation, which occurs during “rainout,” a dard. Delta values are expressed as a difference, in term used to describe the loss of water vapor from a parts per thousand, or per mil (‰), from value refer- cooling air mass as it passes from its oceanic source ence standard. For example, the oxygen isotope ratio over continents (fig. 18). As air masses rise to higher of a sample written in delta notation is: elevations, lower temperatures and the subsequent formation of precipitation cause fractionation to occur 18O/ 16O – 18O/ 16O within the cloud, and 18O and D are partitioned prefer- δ18 sample standard Osample = ------entially into the rain or snow. The heavy isotopes thus 18O/ 16O standard are distilled from the vapor, which is progressively 18 × 1,000 ‰ VSMOW. depleted in O and D (Clark and Fritz, 1997).

δ18Ο = -22‰ δD = -166‰

δ18Ο = -13‰ δD = -94‰ δ18Ο = -12‰ δD = -86‰ Rain

δ18Ο = -4‰ δD = -22‰ Mountain

Ocean δ18Ο = 0‰ δD = 0‰

Figure 18. Schematic showing fractionation of stable oxygen and hydrogen isotopes during rainout. Stable isotope values, which compare isotopic ratios relative to ocean water, are expressed in per mil (‰).

Isotope Chemistry 29 Data for Black Hills sites include both δ18O and precipitation that is isotopically heavier than in the δD values (table 7). For sites with more than one set of north (Back and others, 1983; Busby and others, 1983; 18 δ O and δD values, a mean value is listed in table 7, Greene, 1997). The resulting distribution of isotopes in and individual values are listed in table 8. A linear near-recharge areas of the Black Hills serves as a δ18 δ relation exists between O and D (fig. 19); thus, natural tracer for ground-water flowpaths. A general- subsequent discussions and illustrations generally refer ized distribution of δ18O values in near-recharge areas only to δ18O for simplicity. Most samples for the Black of the study area is presented in figure 20, which is Hills area fall along or slightly below Craig’s (1961) derived using samples from headwater springs with global meteoric water line. localized recharge areas, selected (Minnekahta) wells Precipitation in the northern Black Hills generally is isotopically lighter than in the south because of and caves in outcrop areas, and streams upstream from relatively high elevations and the influence of Pacific loss zones. The approximate centroids of drainage storms that are isotopically depleted due to rainout in basins were used to represent stream sampling loca- crossing the Rocky Mountains. The generally lower tions for contouring purposes; thus, gaging stations elevations in the southern Black Hills, combined with shown in figure 20 do not necessarily fall within the warm, moist weather patterns from the south, result in respective contours.

-80 # # -90 # # # # #### -100 # ## ) ## # ### # ## ‰ ##### # -110 # # #

# # -120 # #

D, IN PER MIL ( # ## δ -130 # #

-140

-150 -19 -18 -17 -16 -15 -14 -13 -12 -11 -10 δ18O, IN PER MIL (‰) EXPLANATION Global meteoric water line: Surface water - loss zone 18 δD = 8δ O + 10 # Well - Deadwood aquifer Downgradient springs Well - Madison aquifer Headwater springs - Deadwood aquifer Well - Minnekahta aquifer Headwater springs - Madison aquifer # Well - Minnelusa aquifer Headwater springs - Minnelusa aquifer

Figure 19. Relation between δ18O and δD in Black Hills samples in comparison to the global meteoric water line (Craig, 1961).

30 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota

104o 45' 103o30' C EXPLANATION ro H w 44o45' Belle Fourche orse OUTCROP OF MADISON Creek Reservoir Owl Newell LIMESTONE (from Strobel and BELLE Creek Creek others, 1999)

F BELLE FOURCHE O OUTCROP OF MINNELUSA UR CHE RIVER

Hay Creek FORMATION (from Strobel and R E BUTTE CO others, 1999) R I V ATE R MEADE CO 18 REDW LAWRENCE CO -15 LINE OF EQUAL δ O VALUE--

34 Saint Creek Interval 1 per mil Crow Onge -15.58B Creek ea CAVE--Numbers indicate site r reek Gulch Spearfish C 190 δ18 30' -13.33 number from table 7 and O reek C Whitewood Creek value in per mil 276 Bottom Creek e ls Gulch -16.91 a HEADWATER SPRING--Numbers 309 F 311 277 Maurice Whitewood Butte Higgins indicate site number from table 7 -16.96 -16.93 STURGIS -14.61 Tinton δ18 310 o and O value in per mil DEADWOOD 312 15' 103 Beaver -16.59 Cr -16.01 STREAMFLOW GAGING h Lead 84 s 313 i 318 f STATION--Numbers indicate r -14.63 Bear -16.24Creek Tilford a

e -12.59 r p Elk site number from table 7 and S 314 w C 277 M-15.74eado δ18 δ18 -14.61 Creek O value in per mil. The O Spearfish E 282 N 281 lk . 15' Little Boxelder values are contoured using the -16.56F -17.59 o 280 Piedmont -17 r k 279 Ellsworth approximate centroid of the R -13.96 S. Fork Rapid Cr a -14.06 Air Force p Base basin i Nemo d

C Creek -16 Blackhawk r -15 WELL COMPLETED IN THE rk Box Elder Fo Rochford 84 s 283 315 MINNEKAHTA AQUIFER-- ad o .-16.96Fork -14.63 h N Ca Rapid -14 -15.35 R stl Numbers indicate site number e C RAPID CITY Castle r from table 7 and δ18O value in ek Creek Rapid C C re Pactola 317 190 PLATEAU re Reservoir per mil ek Creek Castle -13.49 -13.33

LIMESTONE V Deerfield ictoria Creek S. Fork Spring o r Reservoir 44 C 285 e 318 Cas-16.41tl Sheridan -12.59 Creek Creek Lake Hill City Keystone Mt. Rushmore National Spring Memorial 319 Hayward n -13 o -11.51 y PENNINGTON CO n Battle

a Hermosa C CUSTER CO

s Creek e l 287 o B -14.60 -12 Grace 320 French Creek CUSTER -11.58 e idg Creek Cool 45' CUSTER 321 Jewel Cave -11.58 National Monument STATE French Fairburn

288 PARK

Canyon 289 -14.27 Creek anyon C Lame -13.41 Beaver Pringle Wind Cave National Park Hell 322 -12.28 WYOMING Creek 232 Johnny Wind Creek -12.25Cave

SOUTH DAKOTA SOUTH Red 240 Creek Buffalo Gap RIVER 30' Pass -12.44

FALL RIVER CO H on ot Brook ny Ca HOT SPRINGS Minnekahta Fall R Oral

C H E Y E N N E

Edgemont Horsehead

Angostura o Creek Reservoir C 43 15' d reek oo w n o tt o Creek C Provo

Figure 20. Generalized distribution of δ18O in surface water and ground water in near-recharge areas.

Isotope Chemistry 31 Many of the data points used to develop Tritium concentrations in precipitation have figure 20 are ground-water samples, which generally been collected at numerous locations around the world have small temporal variability in δ18O values relative (International Atomic Energy Agency, 1999). Loca- to surface-water samples. A perspective on temporal tions of collection sites in and near the United States variability in precipitation is presented in figure 21A, that are considered for this report are shown in δ18 which shows O values for selected loss-zone figure 22, which shows estimated cumulative deposi- streams and headwater springs. Some temporal vari- tion of tritium (weighted average concentration multi- ability in loss-zone streams (Spring Creek, Rapid plied by precipitation depth) for 1953-83 (Michel, Creek, and Boxelder Creek) is due to seasonal vari- 1989). Tritium concentrations are low in oceans; thus, ability in isotopic composition of precipitation. Data concentrations in precipitation generally are low in sets for Rhoads Fork and Castle Creek are somewhat limited, but indicate less variability because of mixing coastal areas but tend to increase in inland areas associated with ground-water storage. Thus, for the because of increasing stratospheric influence. wells and headwater springs shown in figure 20, Monthly tritium concentrations in precipitation variability in δ18O is assumed to be small and values are shown in figure 23 for Ottawa, Canada, which has are considered representative of average isotopic the longest available period of record. Three precipita- composition in near-recharge areas. tion samples collected in the Black Hills area during Temporal variability of δ18O is shown in the late 1990’s (table 7) also are shown. Figure 23 figures 21B and 21C for selected wells and down- shows that in addition to the long-term trends, there is gradient springs for which time-series data are avail- considerable seasonal variability in tritium concentra- able. Temporal variability for these sites is magnified tions, with highest concentrations generally in the somewhat by use of different Y-axis scales, but is much spring. smaller than for the surface-water samples (fig. 21A). Tritium concentrations in ground water can be Temporal variability for some sites is influenced by related through a first-order decay equation to esti- ground-water mixing conditions, as discussed in mated concentrations at the time of recharge. Because subsequent sections. recharge to the Madison and Minnelusa aquifers occurs from infiltration of precipitation on outcrops and from Tritium streamflow losses, tritium input from both sources Tritium (3H), which beta-decays to 3He with a must be considered. half-life of 12.43 years (Clark and Fritz, 1997), is A combination of methods has been used to esti- produced naturally in small concentrations by cosmic mate annual tritium concentrations for precipitation radiation in the stratosphere. Naturally occurring back- recharge for the Black Hills area. Annual estimates are ground concentrations of tritium in continental precip- presented in table 9 in the Supplemental Information itation are estimated to range from 1 to 20 TU (tritium section and are shown by dashed lines in figure 24, units), depending on location (Michel, 1989). One TU 3 18 which also shows the three actual samples for the Black is defined as one H atom per 10 atoms of hydrogen, Hills area. which is equivalent to 3.19 pCi/L (picocuries per liter) Estimates of tritium concentrations from Michel in water (International Atomic Energy Agency, 1981). Because of nuclear testing during the 1950’s and (1989) for 2- by 5-degree blocks were used for the 1960’s and a subsequent treaty limiting such tests, period 1953-83. These estimates are based on interpo- tritium concentrations in atmospheric water increased lations between stations with relatively long periods of sharply in 1953, peaked in 1963, and then declined. record. Michel (1989) extended station records, for Current sources of tritium, such as nuclear power periods prior to initiation of data collection, using production, contribute to atmospheric tritium concen- regression coefficients developed by the International trations that are slightly higher than background Atomic Energy Agency (1981) for correlations with concentrations prior to nuclear testing. long-term records for Ottawa, Canada.

32 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota -10 A -11 Rhoads Fork (site 283) x Castle Creek (site 285) Boxelder Creek (site 315) -12 Rapid Creek (site 316) Spring Creek (site 318)

-13

-14

-15

-16 x x xx x x xx -17

-18 -12.5 + + Black Hills Power & Light (site 112) + + + + Rapid City No. 6 (site 116) Westberry Trails - Madison (site 121) -13.0 x Rapid City No. 10 (site 123) ) Rapid City No. 8 (site 124)

‰ + Rapid City No. 9 (site 172) Rapid City No. 5 (site 185) + -13.5 City Springs (site 298)

-14.0 O, IN PER MIL ( 18

δ x xx x -14.5

B -15.0 -11.5

+ + + + x +++ -12.0 +++ + x x x xx x x xx x x xx

-12.5

-13.0 Highland Hills (site 149) Carriage Hills Main Well (site 165) Chapel Lane Madison (site 168) x -13.5 Rapid City No. 11 (site 169) Hart Ranch (site 195) + Pine Grove (site 196) C Cleghorn Springs (site 300) -14.0 1986 1988 1990 1992 1994 1996 1998 YEAR Figure 21. Temporal variation of δ18O for selected sites. Graph A shows selected loss-zone streams and headwater springs. Graphs B and C show selected wells and artesian springs.

Isotope Chemistry 33 6000 5000 Black Hills Portland Hydrology Study Area Bismarck Ottawa

7000

Lincoln Salt 6000 Lake Denver City 5000

2000 1000 3000 4000

3000 0 500 MILES 2000 0 500 KILOMETERS 1000 EXPLANATION 1000 LINE OF EQUAL TRITIUM DEPOSITION IN TRITIUM-UNIT-METERS

Figure 22. Cumulative tritium deposition on the continental United States, 1953-83, and location of selected collection sites (modified from Michel, 1989).

10,000 7,000 5,000 OTTAWA, CANADA 4,000 BLACK HILLS AREA 3,000 2,000

1,000 700 500 400 300 200

100 70 50 TRITIUM, IN TRITIUM UNITS 40 30 20

10 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 YEAR

Figure 23. Monthly tritium concentrations in precipitation at Ottawa, Canada. Samples collected in Black Hills area of South Dakota also are shown.

34 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota 10,000

5,000

2,000

1,000

500

200

100

2 1 TRITIUM CONCENTRATION, IN UNITS 1950 1960 1970 1980 1990 2000 YEAR

EXPLANATION WEIGHTED ANNUAL TRITIUM TRITIUM CONCENTRATION FOR CONCENTRATIONS SINGLE SAMPLES IN THE BLACK Bismarck, North Dakota HILLS AREA Portland, Oregon Ottawa, Ontario Lincoln, Nebraska Salt Lake City, Utah Denver, Colorado Michel (1989) estimates for Black Hills (1953-83) Regression estimates for Black Hills (1984-98)

Figure 24. Weighted annual tritium concentrations in precipitation at selected locations. Estimated input and samples for Black Hills area of South Dakota also are shown.

For 1984, monthly tritium concentrations in concentrations for Ottawa (fig. 24), which recently has precipitation for the Black Hills area were estimated by been affected by localized tritium sources (Bob Michel, averaging data for Bismarck, North Dakota, and U.S. Geological Survey, oral commun., 2000). For Lincoln, Nebraska. For 1985-98, estimates for the Bismarck, the resulting coefficients for a and b are Black Hills area were derived by averaging forward 0.483 and 0.732, respectively, and for Lincoln, the extensions of data for Bismarck and Lincoln, based on coefficients are 0.881 and 0.028. least-squares linear regression with monthly data for For 1984-98, annual weighted tritium concentra- Ottawa. The regression equation is of the form: tions were calculated from the reconstructed monthly TU = a TUOTTAWA + b; where: TU is tritium in data (table 9) and Black Hills precipitation records precipitation (for either Bismarck or Lincoln), and (Driscoll, Hamade, and Kenner, 2000) as follows: TU is tritium in precipitation at Ottawa. The OTTAWA 12 coefficients a and b are obtained from regression analysis using all available months of data from å PrecipiTUi January 1979 through July 1984 for Bismarck, and i = 1 TUweighted = ------(4) from January 1970 through May 1986 for Lincoln. 12 During these periods, annual concentrations for å Precipi Bismarck and Lincoln were consistently less than i = 1

Isotope Chemistry 35 where Precipi and TUi denote monthly precipitation Description of Models and tritium concentrations, respectively. Within the Madison aquifer, primary porosity Estimated monthly and weighted annual tritium and hydraulic conductivity generally are small and concentrations in precipitation for the Black Hills area hydraulic properties are dominated by fractures and for 1984-98 are presented in table 10 in the Supple- solution openings, which range in size from microfrac- mental Information section. Estimates for 1985-98 tures to massive caverns. In this dual-porosity system have larger uncertainty than for previous periods (Long, 2000), a large part of ground-water flow may because of discontinuation of data collection at occur in preferential flowpaths within openings, with Bismarck and Lincoln. All concentrations prior to smaller influence from the low-porosity matrix. A sig- 1953 are assumed equal to 15.0 TU. Actual back- nificant amount of ground-water storage, however, ground concentrations probably were slightly lower may occur in dead-end or poorly connected openings. (Bob Michel, U.S. Geological Survey, written Within some water-bearing layers of the Minnelusa commun., 2000); however, estimated input of 15 TU aquifer, relatively homogenous aquifer characteristics results in essentially negligible decayed tritium probably occur; however, extensive fracturing and concentrations for the primary sampling period solution activity also contribute to dual-porosity char- (1990-98). acteristics in many locations. Overall, the Minnelusa Additional inferences regarding recent tritium aquifer generally is heterogeneous across the Black concentrations in precipitation are available from Hills area. examination of tritium concentrations in samples col- Three conceptual models (fig. 25) are considered lected from 14 streams (table 7) where streamflow for general evaluation of mixing conditions and recharge occurs. These streams generally are relatively ground-water ages within the Madison and Minnelusa responsive to short-term precipitation influences aquifers. Given the large range of hydraulic character- (Miller and Driscoll, 1998); however, the range of istics within these aquifers, the simplified models tritium values indicates considerable variability in tri- cannot address all of the complex mixing and flow tium concentrations in precipitation and streamflow conditions that occur. Under the best conditions, none responses. The lowest concentrations are near 15 TU, of these simplified models would be expected to which probably is representative of the lower end of the exactly represent complicated ground-water flow con- range of average tritium concentrations in recent (since ditions that occur. Under the worst conditions, large about 1992) precipitation. The upper end of the range errors in estimated ages can occur. The conceptual for stream samples (25 to 40 TU) is considerably models do, however, provide a mechanism by which higher, however, than estimated concentrations in pre- finite numerical age estimates can be derived for water cipitation during the mid-1990's (fig. 24), which may samples. indicate slight underestimation of precipitation input Mathematically, the three conceptual models during this period or large seasonal variability. illustrated in figure 25 belong to a general group of The streams with higher tritium concentrations models known as lumped-parameter models (Yurtsever generally have larger influence from ground-water dis- and Payne, 1986), where the system description charge than streams with lower tritium concentrations. (relating tracer input to tracer output) is attained by This observation is consistent with the work of Rose assuming a system response function (transit-time (1993), who investigated tritium systematics of base distribution function). Models of this type often are flow and effects on tritium concentrations in streams. used to interpret radioisotope tracer data in ground- water studies (Katz and others, 1999; Manga, 1999). Conceptual Mixing Models Although this approach is simplistic in that the aquifer is assumed to be homogeneous (detailed spatial varia- Complex ground-water flow conditions within tions are not represented), it is appropriate when the the Madison and Minnelusa aquifers necessitate formu- available data are limited to temporal observations at lation of conceptual models for evaluation of mixing the input and output of the system (Yurtsever and conditions and ground-water ages. Within this section, Payne, 1986; Manga, 1999) and may be particularly three conceptual mixing models are described and useful for karst and fractured-rock systems (Zuber, limitations of the models are discussed. 1986).

36 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota 1998 1997 1996 1995 1994 1993 etc. Flow direction

A. Slug flow or pipe flow - negligible mixing with delayed arival

1998 Precipitation

1998 1997 Aquifer 1996 outcrop 90% in storage 1995 10% discharged 1994 1993 1992 Water table 1991 10% in storage 1990 90% discharged 1989 Spring

Confining unit Stream channel

B. Hypothetical water-table spring with maximum traveltime of 10 years - thorough mixing with immediate arrival

1998 Precipitation

Aquifer outcrop/recharge area

Well Potentiometric surface Water table

1998 Confining unit 1988 Aquifer 1978 1968 1958 1948 1938 Confining unit 1928

C. Well completed in artesian aquifer at considerable distance from recharge area - thorough mixing with delayed arrival

Figure 25. Schematic diagrams illustrating mixing models for age dating for various ground-water flow conditions.

Isotope Chemistry 37 For a lumped-parameter model, the relation Tritium concentrations remaining in ground between time-variable tracer input and output concen- water after decaying to a particular sample-collection trations (Zuber, 1986) can be expressed as: date can be computed for a given recharge year using the weighted annual tritium concentration (table 9) as

∞ the initial concentration in equation 6. Estimated tri- λ ′ tium concentrations in precipitation for recharge years ′ – t ′ ′ Cout()t = ò Cin()ett– g()t dt (5) through 1998 and decay-corrected tritium concentra- 0 tions for sample-collection dates ranging from 1978 through 1998 are included as table 11 in the Supple- ′ where, Cout()t and Cin()tt– are the output and input mental Information section. concentrations, t is the calendar time at which the out- Figure 26 shows estimated concentrations of ′ put is to be evaluated, t is the traveltime of the tracer, tritium in precipitation for the Black Hills area for λ ()′ is the radioactive decay constant, and g t is a 1940-98 and decay curves in 5-year increments system response, or weighting, function. The three between 1978 and 1998, which covers the primary models shown in figure 25 are represented mathemati- period for which samples are available. For slug-flow cally by using different system response functions in conditions, a recharge date (or range of dates) for any equation 5. sample can be estimated by selecting the appropriate Figure 25A depicts slug flow (often termed pipe decay curve for the year of sample collection (or inter- or piston flow), which is a common assumption in polating between curves) and identifying the point(s) tracer-based dating of ground water. In slug flow, a on the curve that are equal to the measured tritium con- given water front advances with a uniform velocity and centration. As an example, for a sample collected little dispersion or mixing. Existing water is com- during 1993 with a concentration of 5 TU, the esti- pletely, immiscibly displaced by the advancing water. mated recharge date for slug-flow conditions is about The slug-flow model often is applied to the percolation 1953. Two or more recharge dates are possible for of water through the unsaturated zone and also may be many samples. Numerous solutions are possible for a appropriate for isotropic, confined aquifers (Zuber, 1993 sample with a concentration of 20 TU. For this 1986; Gonfiantini and others, 1998) as well as uncon- example, recharge may have occurred anytime in the fined, surficial aquifers (Reilly and others, 1994). For range of about 1954-57 or 1980-93, considering the Madison and Minnelusa aquifers, a slug-flow within-year variability in tritium concentrations in pre- model could approximate ground-water flow condi- cipitation and uncertainty in estimating tritium input tions in dual-porosity settings if the dominant flow pro- for the Black Hills area. portions are in fractures and solution openings, with Figure 25B depicts a simplified mixing model minimal contributions from the low-porosity matrix. that generally is applicable for locations where water- For slug flow, the system response function is table conditions exist within outcrop areas, such as described by a mathematical function called the Dirac headwater springs. For this “immediate-arrival” delta function (sometimes called the unit impulse model, it is assumed that some percentage of the water function), δ()t – τ , where τ is the turnover time or recharged during a given year is discharged during that mean transit time of water (τ = volume of water in the same year; in other words, the minimum traveltime for system/volumetric flow rate through the system). In a portion of the water is zero. For this scenario, water this case, the traveltime of the tracer is equivalent to the recharged during a given year is mixed with equal turnover time; in all other cases, the two differ consid- proportions of water recharged during previous years. erably (Maloszewski and Zuber, 1982). Use of this For the hypothetical water-table spring with a max- system response function reduces equation 5 to: imum traveltime of about 10 years that is shown in figure 25B, 10 percent of the water recharged during the current year is discharged as springflow during that ′ –λt′ Cout()t = Cin()ett– (Zuber, 1986). (6) same year. The remaining 90 percent of the water discharged is composed of equal proportions of water Equation 6 describes the decrease in concentration due recharged during each of the previous 9 years. Thus, to decay during the time span t′ of a tracer that entered tritium composition for this scenario can be repre- the system at time ()tt– ′ and exited the system at sented as the average of decay-corrected tritium input time t . concentrations over the previous 10 years.

38 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota 10,000

5,000 TRITIUM INPUT 1978 2,000 1983 1988 1,000 1993 1998 500

200

100

2 1 TRITIUM CONCENTRATION, IN UNITS 1940 1950 1960 1970 1980 1990 2000 YEAR

Figure 26. Estimated tritium concentrations in precipitation for Black Hills area and decay curves for selected years. Decay curves depict decayed tritium concentrations for selected sampling years.

The mathematical expression for this immediate- (Cin()1998– 9 ). With the equal-weight system arrival mixing model utilizes a simple system response response function, equal proportions of water are con- function such that the observed output concentration is tributed each year; in this case the proportion is represented as a mix of equal proportions of decay- 1 ()------′ ′ = 10 percent. corrected tritium input concentrations for all years t max – t min between the minimum (0 years for immediate arrival) Figure 25C depicts a conceptual model that and maximum traveltimes: assumes a delay time before any recharge water reaches a discharge point. Use of this model requires a ′ minimum delay time of 1 year because annual input is t max – 1 1 –λt′ used for calculation purposes. This “time-delay” C ()t = --- å C ()ett– ′ (7) out T in mixing model is appropriate where an upper confining ′ t min unit is present and wells or springs are located some distance from outcrop areas, which is applicable for Equation 7 is identical to the example of a simple many locations around the periphery of the Black Hills, lumped-parameter model given by Zuber (1986, p. 12) especially where artesian conditions occur. For except that all input concentrations for the range of example, the hypothetical artesian well shown in years between the minimum and maximum traveltimes figure 25C withdraws a mixture of water that was are weighted equally in equation 7. Here, T is the recharged during a 50-year period from 1929 to 1978. mixing time period, computed as the difference The minimum traveltime (delay time) in this case is ()′ ()′ between the maximum t max and minimum t min 20 years; in other words, the earliest arrival of recharge traveltimes in the system. water is delayed by about 20 years before reaching the For the hypothetical water-table spring shown discharge point. The maximum traveltime is 70 years. in figure 25B, the summation is calculated using In this case, the summation (eq. 7) is calculated using decay-corrected tritium input concentrations for decay-corrected tritium input concentrations for recharge years 1998 (Cin()1998– 0 ) through 1989 recharge years 1978 (Cin()1998– 20 ) through 1929

Isotope Chemistry 39 (Cin()1998– 69 ). The equal-weight system response Estimates of tritium concentrations in precipita- function in this case results in a mix consisting of con- tion are subject to large potential error because of the ⁄ ()′ ′ tributions of 2 percent, (1 t max – t min ) for each sparsity of data-collection points near the Black Hills input year. area, especially since discontinuation of data collection Curves for estimating ages for both immediate- at Bismarck and Lincoln. Age estimates for recent arrival and time-delay mixing models (figs. 25B years have progressively increasing sensitivity to errors and C) are presented in figure 36 in the Supplemental in estimated tritium concentrations in precipitation Information section. Each graph includes a family of because atmospheric tritium has progressively curves depicting minimum traveltimes, or delay times, decreased since 1963. in 4-year increments. The 0-year delay curve in each The use of annual averages for tritium in precip- curve family is applicable for the immediate-arrival itation is another source of potential error because of mixing model (fig. 25B), and the other curves are the episodic nature of recharge; however, some within- applicable for time-delay mixing scenarios. Individual year proportioning is obtained because monthly con- curves in each family were constructed from output centrations are precipitation weighted. The immediate- concentrations calculated for ranges of input years and delayed-arrival mixing models are based on an using equation 7. Curve families are presented for implicit assumption of equal recharge each year; how- sampling years 1978 and 1989-97, which cover the ever, recharge may vary by as much as an order of majority of samples considered. Calculations should magnitude from one year to the next (Carter, Driscoll, theoretically be carried out for all possible traveltimes, from zero to infinity (Zuber, 1986); however, the and Hamade, 2001). curves are arbitrarily extended only to possible Variability of tritium concentrations in stream- recharge years through 1800. flow creates an additional complication for areas influ- Using a 1993 sample (fig. 36F) with a concentra- enced by streamflow recharge. The age of water in a tion of 30 TU as an example, if an immediate-arrival stream always lags that of precipitation, whether by mixing model is considered appropriate, the estimated minutes, days, or many years. The extent of this effect maximum traveltime is either 23 years (earliest is highly variable and depends on the lag time associ- recharge occurring about 1970) or 130 years (earliest ated with a particular stream, the degree of variability recharge occurring about 1863). For this example, no of tritium concentrations in the base-flow component solutions exist for time-delay mixing scenarios with of streamflow (Rose, 1993), and the proportion of delay times exceeding about 36 years, and only one ground-water recharge resulting from streamflow possible solution exists for delay times in the range of losses relative to direct precipitation on aquifer out- about 20 to 36 years. Two solutions are possible for crops. Although stream water is inherently older than each of the delay times up to about 20 years. precipitation, systematic influences on age dating probably are overshadowed, in most cases, by seasonal Limitations of Models variability in tritium input and by other sources of uncertainty. As discussed, the three simplified, conceptual Various hydrologic and hydraulic factors also models that are considered cannot address all of the complex mixing and flow conditions that occur within complicate the paramount assumption that samples the Madison and Minnelusa aquifers within the study contain a mix of equal proportions of water from area. The conceptual models do, however, provide a various years. Variability in recharge rates probably mechanism by which finite numerical age estimates causes temporal variability in ground-water traveltimes can be derived for water samples. Readers are cau- and flowpaths from recharge areas to discharge areas. tioned that accurate age dating is not possible because Variable residence times in the unsaturated zone in of large potential errors resulting from numerous limi- recharge areas and in confining units also may be com- tations associated with the simplified models, which plicating factors. For applications of the immediate- are intended only for general evaluation of mixing con- arrival model in unconfined headwater settings, the ditions and approximate age ranges. Sources of poten- effects of the shape of a contributing ground-water tial error are associated primarily with estimation of basin on the relative contributions of recharge from tritium concentrations in recharge and with assump- different parts of the basin also could seriously violate tions regarding the mixing models that are considered. this assumption.

40 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota The cumulative effects of all of the aforemen- figures 27, 28, and 29. Mean values are shown for sites tioned factors probably are small, relative to error with multiple samples. Figures 27 and 29 show the potential associated with the spatial variability in northern and southern Black Hills, respectively, and hydraulic properties of the aquifers. Dual-porosity figure 28 shows the Rapid City area. These figures hydraulic characteristics contribute to large variability generally include sites near or downgradient from the in ground-water flow and mixing conditions, which Madison and Minnelusa outcrops, which probably are cannot be generically accommodated by the conceptual influenced primarily by recharge originating from mixing models. Actual conditions may be more accu- within the study area. Sites considered representative rately represented as a combination of mixing models, with influence from slug-flow effects in fractures and of the isotopic composition of recharge in the study solution features. Even in cases where one conceptual area were presented earlier in figure 20 and are model may be appropriate, there are important limita- excluded from figures 27-29. tions. In settings where ground-water flow is predom- Distributions of stable isotopes generally are inantly in fractures and solution openings, which may consistent with spatial patterns in recharge areas be approximated by the slug-flow model, some mixing (fig. 20), with isotopically lighter precipitation of ground water occurs during convergence upon the generally occurring at higher elevations and latitudes. screened interval of wells (Reilly and others, 1994). A Effects of recharge elevation are examined in table 3, 18 limitation of the immediate- and delayed-arrival which shows that δ O values are lighter in the mixing models is the assumption of thorough ground- Madison aquifer than in the Minnelusa aquifer for 10 of water mixing. Specific examples and additional details 13 well pairs. An example is provided by sites 35 and regarding model limitations are provided in subsequent sections of this report. 36 (State Line wells, fig. 27), along with a co-located As discussed, two or more age estimates are Minnekahta well (site 34, fig. 20), which show the possible for many samples, depending on the tritium typical progression of isotopically lighter water concentration and the mixing model that is considered associated with progressively higher elevation most applicable. Multiple samples for any site improve recharge sources. The Madison aquifer also is influ- the ability to make credible age estimates; however, enced by preferentially larger volumes of isotopically effects of large well withdrawals at a site could change light streamflow recharge, relative to the Minnelusa the age characteristics between successive samples. aquifer. In some areas, effects of streamflow recharge on Areal Flowpaths, Ages, and Mixing Conditions isotopic composition can be identified. For example, δ18 Within this section, isotopic interpretations are the O values (by aquifer) for sites 35 and 36 (State used to evaluate areal flowpaths, ages, and mixing Line wells, fig. 27) are distinctly lighter than values for conditions for the Madison and Minnelusa aquifers in sites 57 and 58 (Tinton Road wells), which probably the Black Hills area. General considerations regarding results from the influence of streamflow recharge from isotope distributions are discussed first, after which Beaver Creek or Bear Gulch. The Tinton Road wells various site-specific considerations are addressed. probably are recharged primarily by precipitation on Potential influences of regional flow from the west are outcrops because no major streamflow loss zones are discussed in a subsequent section. nearby. Similarly, values for sites 85 and 86 (Tilford wells) are nearly identical (fig. 27) and are somewhat Isotope Distributions and General Considerations lighter than in nearby outcrop areas (fig. 20), where Distributions for stable isotopes and tritium are δ18O values are predominantly in the range of -14 to presented in this section. Various general consider- -15 ‰, indicating that these wells probably are heavily ations associated with the isotope distributions also are influenced by streamflow recharge from Elk Creek. In discussed. contrast, Madison and Minnelusa wells at Piedmont (sites 100 and 101) and springflow along Elk Creek Stable Isotopes (site 297) probably are influenced primarily by precip- 18 Distributions of δ O values in Madison and itation recharge on large outcrop areas southwest of Minnelusa wells and selected springs are shown in Piedmont.

Isotope Chemistry 41 104o 45' 103o30' Crow Indian Cr Horse 44o45' Belle Fourche Creek Reservoir Owl Newell Area BELLE 4 Creek Shown 6 Creek -17.07 -17.80 SOUTH DAKOTA F BELLE FOURCHE OU Study RCHE Area Creek Hay R RIVER

296 295 E BUTTE CO -16.67 -16.95 V TER R I MEADE CO REDWA LAWRENCE CO 294 48 29 -15.34 -17.46 -16.88 292 Saint Creek 43 Crow 44 -16.71 290 -17.43 -17.07 -16.36 Onge 293 Bear Creek 291 -17.19 35 -16.75 36 Gulch 59 30'-16.98 -17.28 Spearfish eek -16.65 Cr

s 61 Whitewood Creek n gi 57 58 -16.84 Creek g Bottom

i -15.90 -16.66Creek H 82 e Gulch s -16.13

l a Whitewood Butte Maurice F STURGIS 83 73 -17.19 -15.11 69 o DEADWOOD -15.04 15' 103 Beaver Cr Lead sh fi Bear 85 r -15.61 a Tilford e Elk p r 86 S -15.52 w C eado 297 M -13.79 Creek Spearfish N 101 Little 100 15' Boxelder -13.61 F -13.68 o r Piedmont k Ellsworth S. Fork Rapid Cr R Air Force a 95 p Base i Nemo -13.90 107 d Creek

C Blackhawk -14.13

r 106 rk Fo Rochford -14.15 s adN Fork Rapid Box Elder WYOMING o Ca h stle R Cr

Castle RAPID CITY SOUTH DAKOTA SOUTH k Pactola Creek Deerfield e e Reservoir 148 Rapid PLATEAU C r C Reservoir -12.55 Creek

LIMESTONE r ee Castle Vi k ctoria Creek 44o Base modified from U.S. Geological Survey digital data, 1:100,000 010MILES Rapid City, Office of City Engineer map, 1:18,000, 1996 Universal Transverse Mercator projection, zone 13 010KILOMETERS

EXPLANATION OUTCROP OF MADISON LIMESTONE (from Strobel and others, 1999) OUTCROP OF MINNELUSA FORMATION (from Strobel and others, 1999)

148 -12.55 WELL COMPLETED IN MADISON AQUIFER--Numbers indicate site number and δ18O value in per mil 101 WELL COMPLETED IN MINNELUSA AQUIFER--Numbers indicate site -13.61 number and δ18O value in per mil 297 δ18 -13.79 DOWNGRADIENT SPRING--Numbers indicate site number and O value in per mil

Figure 27. Distribution of δ18O in selected Madison and Minnelusa wells and springs in the northern Black Hills area. Sampling dates are through 1998, with mean values shown for sites with multiple samples.

42 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota 103o22'30" 20' 17'30" 15' 103o12'30"

o 44 10' 138 -13.45

MEADE COUNTY 145 126 129 130 PENNINGTPENNINGTONON COUNTY -13.25 -14.18 -13.40 -13.66

Creek 7'30" 124 oxelder -14.40 B 142 -14.43

123 -14.38

117 -14.35 City Springs 113 118 116 -13.95 -14.25 -14.28 112 119 Lime-14.30 298 187 -13.65 -14.37 121 186 -13.70 Creek 5' -13.94 -14.38Creek 185 180 -14.23 -13.69 183 179 -14.38 182 188 178 -13.90 -14.16 -14.40 -12.70 181 -13.83 Rapid Cleghorn/ 175 RAPID CITY Jackson -13.11 172 300 -12.77 Springs-12.92 Canyon 176 164 -12.35 -12.44 Lake 177 -12.90 167 168 299 -12.09 -12.05 -14.45 Tittle Creek 2'30" Rapid Springs 165 -12.13 169 -12.09

Creek

Victoria 154 -12.44

Area 151 149 Shown -12.26 -12.66 SOUTH DAKOTA 44o 193 Study -12.36 Area Spring Creek 192 195 -12.51 191 -12.78 -12.27

Base from U.S. Geological Survey digital line graph, 1:100,000, 0 1 2 MILES Rapid City, Office of City Engineer map, 1:18,000, 1996 EXPLANATION 0 12 KILOMETERS OUTCROP OF MADISON LIMESTONE (from Strobel and others, 1999) OUTCROP OF MINNELUSA FORMATION (from Strobel and others, 1999) 151 18 -12.26 WELL COMPLETED IN MADISON AQUIFER--Numbers indicate site number and δ O value in per mil 192 δ18 -12.51 WELL COMPLETED IN MINNELUSA AQUIFER--Numbers indicate site number and O value in per mil 299 DOWNGRADIENT SPRING--Numbers indicate site number and δ18O value in per mil -14.45 Figure 28. Distribution of δ18O in selected Madison and Minnelusa wells and springs in the Rapid City area. Sampling dates are through 1998, with mean values shown for sites with multiple samples.

Isotope Chemistry 43 o 15' 103 104o 45' 30' o 44 S. Fork r C Spring Castl e Sheridan Lake Hill City 196 Creek -11.89 Keystone Mt. Rushmore National 197 Hayward Memorial -12.06 o Battle y PENNINGTON CO 199 n -11.98 a Spo Hermosa C ka CUSTER CO n 301 e Creek Creek e race -11.62 l G o C Bear 201 B o -16.32 CUSTER o Gulch Creek French li d ge 302 202 -11.68 -15.88 210 45' CUSTER -11.75 Jewel Cave 211 -11.50 National Monument STATE Creek Fairburn PARK

Canyon

Canyon Beaver Lame Pringle Wind Cave National Park

WYOMING Hell 216 215 -14.21 -17.23

SOUTH DAKOTA SOUTH Creek Wind 237 Johnny Cave 238 -11.96 -11.93 Red 303 233 239 -14.10 Buffalo Creek 227 -12.13 -12.38 RIVER 30' Pass -14.27 Gap 304 Creek 305 FALL RIVER CO H -14.86 o on -16.71 Minnekahta t Brook ny Ca HOT SPRINGS 306 243 242 247 Fall -13.87 -14.88 -15.97 -15.43 Oral R 253 CHEYENNE 254 -15.15 -16.59 307 Study -15.23 308 Area -15.40 SOUTH DAKOTA Edgemont Horsehead Area Angostura Shown 43o15' Creek Reservoir Creek d oo w on tt 263 Creek o -17.09 C Provo

Hat 010MILES Base modified from U.S. Geological Survey digital data, 1:100,000 Rapid City, Office of City Engineer map, 1:18,000, 1996 010KILOMETERS Universal Transverse Mercator projection, zone 13

EXPLANATION OUTCROP OF MADISON LIMESTONE (from Strobel and others, 1999) OUTCROP OF MINNELUSA FORMATION (from Strobel and others, 1999)

Figure 29. Distribution of δ18O in selected Madison and Minnelusa wells and springs in the southern Black Hills area. Sampling dates are through 1998, with mean values shown for sites with multiple samples.

44 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota (mg/L) Chloride (mg/L) Sulfate lower Higher/ d >0.30 per mil for dissimilar. for mil per >0.30 d S/DS lower Higher/ L, picocuriesperliter; TU, tritium units;than; <, less --, nodata or not shown] Date pCi/L TU 1 S/DS O Tritium Hydraulic head 18 lighter δ Heavier/ -12.70-14.16 H-14.38 DS H L 09-28-93 S 75.9 S -- 23.8 -- H -- -- DS -- H -- -- 25 -- DS DS H 3.2 L 20 22 1.6 1.7 Per mil 2 2 2 O values for each well pair using criteria of <0.25 per mil for similar, between 0.25 and 0.30 per mil for somewhat similar, an similar, somewhat for mil per 0.30 and 0.25 between similar, for mil per <0.25 of criteria using pair well each for O values 18 δ Name Aquifer .pairs well for observation data Selected Comparison between between Comparison dates. sampling multiple for value Mean 1 2 3635 State Line58 State Line57 Road Tinton Madison85 Road Tinton Minnelusa86 Tilford Madison Tilford Minnelusa -17.28 -16.98 -16.66 Madison -15.90 L H Minnelusa L SS H SS -15.61 -15.52 DS 04-11-94 DS 04-11-94 L 09-08-98 61.0 H <1.0 09-08-98 30.4 S 78.4 19.1 S <.3 9.5 09-03-96 24.6 H 08-27-96 L 100.0 4.0 L H DS DS 31.3 H 1.3 DS DS L L H H L 150 73 SS SS 58 3.8 0.7 L H .5 30 1.2 8.4 38 .9 .5 Site 100101 Piedmont117 Piedmont118 City Quarry Madison178 City Quarry Minnelusa179 Canyon Lake Madison182 Canyon Lake Minnelusa -13.68 Madison 183 Park Sioux -13.61 Minnelusa191 Park Sioux -14.35 -14.25 L192 Gardens Reptile H Madison 201 Gardens Reptile Madison L -13.90 Minnelusa S H202 Canyon Boles S Minnelusa210 Canyon Boles S L Madison S 09-04-96 -12.78211 Airport CSP 09-04-96 Minnelusa -12.51215 Airport CSP <1.0 05-13-92 DS 05-12-92 3.0216 Canyon Madison Hell L -16.32 H 96.0 -15.88238 Canyon Minnelusa Hell 05-14-92 <.3 2.0239 Madison SS Ranch 7-11 .9 22.0 L SS 30.1 -11.75242 H Minnelusa Ranch 7-11 -11.50 L 07-29-96243 Jnct .6 Minnekahta Madison 07-30-96 H DS -17.23 6.9 H DS L Madison Minnekahta 15.0 Jnct Minnelusa -14.21 H DS 4.0 03-22-94 Minnelusa L DS 12-17-97 S H L SS L -11.93 4.7 H SS -12.38 L 9.0 -14.88 3.2 S 1.3 H -13.87 09-06-95 DS DS H 08-31-95 DS H L L <1.0 L 2.8 9.3 L 1.0 L 11-05-97 H 3.0 11-03-97 4.2 DS SS 20 DS DS 1.9 <.3 H <1.0 SS .7 L DS 03-31-94 H 27 1.0 22 04-05-94 .9 03-24-94 27 L <1.0 DS 09-11-95 L DS <.3 <1.0 .6 <1.0 L 6.0 H <1.0 H 1.4 20 <.3 S L 14 <.3 H <.3 S <.3 Same 1.5 -- DS DS 8.3 26 -- Same 1.5 -- L H 22 -- DS 1.1 DS DS 38 5.9 H DS L H 1.4 470 230 L 2.7 24 4.0 4.3 32 37 1,800 6.4 4.3 12 5.2 number Table 3 Table pCi/ liter; per milligrams mg/L, lighter. or L, lower heavier; or higher H, dissimilar; DS, similar; somewhat SS, similar; [S,

Isotope Chemistry 45 In the Rapid City area (fig. 28), recharge prob- Various observations can be made from exami- ably is dominated by large streamflow losses from nation of the tritium distributions (figs. 30-32). Large Boxelder, Rapid, and Spring Creeks, rather than from spatial variability in concentrations occurs near outcrop precipitation on the relatively small outcrop areas. In areas, which reflects large variability in mixing condi- western Rapid City, a distinct division of lighter δ18O tions and aquifer characteristics (heterogeneity). Most values to the north and heavier values to the south is of the wells that are far removed from outcrop areas apparent for the Madison aquifer along Rapid Creek. have low, or nondetectable (<0.3 TU) tritium concen- Areas south of Rapid Creek apparently are isotopically trations, which generally supports the use of the time- similar to Spring Creek (fig. 20) and sites north of delay mixing model for artesian areas. Concentrations Rapid Creek generally reflect isotopically lighter com- noted as <0.3 TU are equivalent to about <1.0 pCi/L, position similar to that of Rapid Creek and Boxelder which is the method reporting limit (MRL) for most of Creek. the laboratory analyses that have been performed. The 18 The δ O values for sites near Battle, Grace “2sigma” value is reported as 1.0 pCi/L for most Coolidge, and French Creeks (fig. 29) are isotopically samples with concentrations at or near the MRL heavier than for any other part of the Black Hills area. (table 7). The true concentration is expected to be Values in this area generally are in the range of about within 2 sigma of the reported concentration about -11.5 to -12.0 ‰ and are very similar to values for the 95 percent of the time (Bob Michel, U.S. Geological nearby streamflow loss zones. These values probably Survey, oral commun., 2000). Thus, based on esti- indicate a substantial influence from streamflow mated tritium concentrations in precipitation (table 11), recharge; however, information is not available for samples reported as <0.3 TU are assumed to be com- evaluation of isotopic composition in the outcrop areas. posed primarily of water recharged prior to initial Along the southern and southwestern flanks of the influence of nuclear testing in 1953 (pre-bomb water), uplift, δ18O values for most wells and springs (fig. 29) regardless of sampling date or mixing conditions are much lighter than estimated values for near- (figs. 26 and 36). recharge areas immediately nearby (fig. 20), indicating Samples with tritium concentrations that equal either recharge areas to the northwest or possible influ- or slightly exceed the MRL also are dominated by pre- ence of regional flow from the west, as discussed in bomb water, but probably are showing the presence of subsequent sections of this report. some proportion of modern tritium (tritium produced during 1953 or later). For concentrations between 0.3 and 1.0 TU, the detection of modern tritium is fairly Tritium certain, from an analytical standpoint, and would indi- Tritium data for Black Hills sites are presented in cate either: (1) the initial arrival of modern water for table 7. Most sites include data collected only through slug-flow conditions; or (2) at least some proportion of water year 1998. Additional tritium data collected modern water for all mixing conditions. during water year 2000 are included for six selected Given the uncertainty in estimation of tritium sites that are relevant to site-specific discussions of concentrations in recent precipitation (since about isotope geochemistry for the Rapid City area, which 1992) for the Black Hills area, concentrations as low as are presented in a subsequent section. 10 TU (fig. 23) may be possible for recently recharged Spatial distributions of tritium concentrations for water. Thus, for samples with tritium concentrations wells, headwater springs, and downgradient springs in between about 1 and 5 TU, dominant proportions of the Black Hills area are shown in figures 30, 31, pre-bomb water generally can be assumed. For con- and 32. The sites shown include a limited number of centrations greater than about 5 TU, it is difficult to samples available for the Deadwood and Minnekahta make generalizations because of numerous possible aquifers. For sites with more than one tritium sample, mixing scenarios; however, for all mixing conditions, the most recent concentration (through water year the probability of dominant proportions of modern 1998) is shown. water increases with increasing tritium concentrations.

46 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota 104o 45' 103o30' Crow Indian Cr Horse 44o45' Belle Fourche Creek Reservoir Owl Newell Area BELLE 6 Creek Shown <0.3 Creek SOUTH DAKOTA F O Study BELLE FOURCHE U RCHE Area Creek Hay R RIVER

296 E BUTTE CO V 294 19.1 48 ER R I 16.9 D W0.6AT LAWRENCE CO MEADE CO RE 29 295 0.9 21.0 292 Saint Creek 44 Crow293 43 13.4 290 Onge 19.4 20.7 11.4 28.1 Bear Creek 34 36 19.1 7.5 Gulch Spearfish59 30' 35 291 eek <0.3 19.7 Cr 24.0 Whitewood s 57 58 61 Creek n gi 24.6 9.5 27.3 Creek g Bottom i Creek

276 H 82 e Gulch 83 s 65.6 40.8 l

33.0 a Whitewood Butte

F 73 Maurice 0.3 STURGIS

DEADWOOD 15' 103o Beaver Cr Savoy Lead 85 86 sh 1.3 31.3 fi Bear r a 277 Tilford e 276.0 84 p r S C Elk 43.9 ow Mead 297 30.1 Creek Spearfish Little 282 N 15' Boxelder 100 101 31.3 F <0.3 0.9 o 280 r Piedmont k 281 53.3 53.3 Ellsworth S. Fork Rapid Cr R 279 95 Air Force a <0.3 p 40.8 Base i Nemo d Creek

C Blackhawk

rk 106 Fo 283 Rochford s 43.9 <0.3 ad o Fork Rapid Box Elder WYOMING h N Ca R st le C

Castle r RAPID CITY SOUTH DAKOTA SOUTH k Pactola Creek Deerfield C r e e Reservoir 148 Rapid PLATEAU C Reservoir <0.3 r Creek LIMESTONE ee Castle Vi k ctoria Creek 44o Base modified from U.S. Geological Survey digital data, 1:100,000 010MILES Rapid City, Office of City Engineer map, 1:18,000, 1996 Universal Transverse Mercator projection, zone 13 010KILOMETERS

EXPLANATION OUTCROP OF MADISON LIMESTONE (from Strobel and others, 1999) OUTCROP OF MINNELUSA FORMATION (from Strobel and others, 1999) 106 <0.3 WELL COMPLETED IN MADISON AQUIFER--Numbers indicate site number and tritium concentration in tritium units 86 31.3 WELL COMPLETED IN MINNELUSA AQUIFER--Numbers indicate site number and tritium concentration in tritium units 84 43.9 WELL COMPLETED IN MINNEKAHTA AQUIFER--Numbers indicate site number and tritium concentration in tritium units 281 53.3 HEADWATER SPRING--Numbers indicate site number and tritium concentration in tritium units 297 30.1 DOWNGRADIENT SPRING--Numbers indicate site number and tritium concentration in tritium units

Figure 30. Distribution of tritium for selected sites in the northern Black Hills area. Sampling dates are through 1998, with the most recent concentration shown for sites with multiple samples.

Isotope Chemistry 47 103o22'30" 20' 17'30" 15' 103o12'30"

o 44 10' 138 <0.3

MEADE COUNTY 129 PENNINGTPENNINGTONON COUNTY 126 <0.3 144 145 16.6 43.5 8.1 Creek 7'30" 124 Boxelder <0.3

123 4.7 117 26.3 City Springs 118 0.6 116 298 14.5 112 40.8 Lime 3.7 Creek 5' 121 Creek 187 31.0 186 <0.3 <0.3 188 185 <0.3 4.3 Rapid RAPID CITY Cleghorn/ 178 23.8 179 Jackson 6.9 172 Springs300 22.3 30.2 164 177 Canyon 176 25.7 20.1 Lake 2.2 299 168 28.8 27.5 Creek 2'30" Tittle Rapid Springs 165 169 22.9 16.0

Creek

Victoria

Area Shown 149 SOUTH 22.3 DAKOTA o 44 Study Area Spring Creek 192 191 195 1.3 4.7 21.6

Base from U.S. Geological Survey digital line graph, 1:100,000, 0 1 2 MILES and Office of City Engineer, Rapid City, 1:18,000, 1996 EXPLANATION 0 12 KILOMETERS OUTCROP OF MADISON LIMESTONE (from Strobel and others, 1999) OUTCROP OF MINNELUSA FORMATION (from Strobel and others, 1999) 195 21.6 WELL COMPLETED IN MADISON AQUIFER--Numbers indicate site number and tritium concentration in tritium units 192 1.3 WELL COMPLETED IN MINNELUSA AQUIFER--Numbers indicate site number and tritium concentration in tritium units 144 WELL COMPLETED IN DEADWOOD AQUIFER--Numbers indicate site number and tritium concentration in tritium units 43.5 299 DOWNGRADIENT SPRING--Numbers indicate site number and tritium concentration in tritium units 28.8 Figure 31. Distribution of tritium for selected sites in the Rapid City area. Sampling dates are through 1998, with the most recent concentration shown for sites with multiple samples.

48 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota o 15' 103 104o 45' 30' o 44 S. Fork r C Spring Castl e Sheridan Lake 196 Hill City 25.9 Creek Keystone Mt. Rushmore National Hayward Memorial 197 o 18.7 y PENNINGTON CO n Battle a Spo Hermosa

C k CUSTER CO an 301 199 e Creek 20.4 <0.3 Creek e 287 race l 18.5 G 201o C Bear B o 2.8 CUSTER o Gulch Creek French li d ge 302 202 17.2 1.0 45' 210 CUSTER <0.3 Jewel Cave 211 National Monument STATE 0.9 Creek Fairburn 289 PARK

8.2 288 Canyon 12.9

Canyon Beaver Lame Pringle Wind Cave 215 216 National Park WYOMING 0.6 Hell <0.3

SOUTH DAKOTA SOUTH Creek Wind 237 Johnny Cave 238 11.7Creek <0.3 Red 303 233 239 240 6.0 Creek <0.3 24.8 RIVER Pass 227 27.7 Buffalo Gap 30' <0.3 304 FALL RIVER CO 3.8 305 H o 0.6 Minnekahta242 243 t Brook nyon 241 Ca HOT SPRINGS <0.7 <0.3 <0.3 306 247 Fall 2.5 <0.3 Oral R

CHEYENNE 254 <0.3 253 <0.3 307 Study 2.5 308 Area 2.2 SOUTH DAKOTA Edgemont Horsehead Area Angostura Shown 43o15' Creek Reservoir Creek d oo w on tt o Creek C Provo

Hat 010MILES Base modified from U.S. Geological Survey digital data, 1:100,000 Rapid City, Office of City Engineer map, 1:18,000, 1996 010KILOMETERS Universal Transverse Mercator projection, zone 13 EXPLANATION OUTCROP OF MADISON LIMESTONE (from Strobel and others, 1999) OUTCROP OF MINNELUSA FORMATION (from Strobel and others, 1999)

242 <0.3 WELL COMPLETED IN MADISON AQUIFER--Numbers indicate site number and tritium concentration in tritium units 254 <0.3 WELL COMPLETED IN MINNELUSA AQUIFER--Numbers indicate site number and tritium concentration in tritium units 240 24.8 WELL COMPLETED IN MINNEKAHTA AQUIFER--Numbers indicate site number and tritium concentration in tritium units 288 12.9 HEADWATER SPRING--Numbers indicate site number and tritium concentration in tritium units 308 2.2 DOWNGRADIENT SPRING--Numbers indicate site number and tritium concentration in tritium units

Figure 32. Distribution of tritium for selected sites in the southern Black Hills area. Sampling dates are through 1998, with the most recent concentration shown for sites with multiple samples.

Isotope Chemistry 49 Boxplots showing the most recent tritium con- setting is conceptually inconsistent with the assump- centrations for 96 ground-water samples collected tion of thorough mixing; however, extensive mixing during 1990-98 from wells, headwater springs, and may be approximated by springs with large discharges, downgradient springs, and 12 samples from streams which can capture water from large recharge areas. upstream from loss zones are presented in figure 33. It can be demonstrated that effects of dilution These plots provide evidence that the mixing models from mixing have a major influence on tritium concen- illustrated in figure 25 have general applicability. The trations in the Madison and Minnelusa aquifers. For lower end of the range of tritium concentrations for slug-flow conditions, water recharged between about headwater springs is much higher than for the wells and 1962 and 1970 would have tritium concentrations of downgradient springs, which is consistent with the con- about 70 TU to as much as several hundred TU for all cept of an immediate-arrival mixing model in outcrop sample dates prior to 1998 (fig. 26). Tritium concen- areas for the Madison and Minnelusa aquifers. The trations for the 96 ground-water samples collected since 1990, however, are uniformly less than 70 TU lower end of the range of tritium concentrations for (fig. 33), indicating a conspicuous absence of values both wells and downgradient springs is near zero, reflecting slug-flow conditions for recharge during this which indicates that the time-delay mixing model period. Concentrations indicative of “pulses” con- generally is applicable for artesian conditions, where sisting of several consecutive years of recharge during an upper confining unit is present and recharge water this peak-tritium period also are absent from the sample must travel a substantial lateral distance before set. Dual-porosity flow conditions consisting of a reaching a discharge point. combination of near-slug flow scenarios (which may Delay times for wells generally are longer than include recently recharged water) and general mixing for downgradient springs, which probably tend to with older water, however, are entirely plausible and develop near preferential flowpaths that may be further are consistent with the range of sampled values enhanced by dissolution activity. This dual-porosity (fig. 33).

70 68 91912

EXPLANATION 50 68 Number of samples Maximum 90th percentile 40 75th percentile Median 30 25th percentile 10th percentile Minimum 20

10 TRITIUM CONCENTRATION, IN TRITIUM UNITS IN TRITIUM CONCENTRATION, 0 Wells Headwater Downgradient Streams springs springs upstream from loss zones SITE TYPES

Figure 33. Boxplots of tritium concentrations for selected ground-water and surface-water samples collected during 1990-98 in the Black Hills area.

50 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota Sophisticated methods are available to address As previously discussed, a distinct division of the numerous mixing possibilities in complex hydro- stable isotope values is apparent for the Madison logic settings. Combinations of lumped-parameter aquifer along Rapid Creek. Areas south of Rapid models have been used, for example, to represent set- Creek reflect isotopic composition similar to Spring tings where both slug-flow and mixed-reservoir effects Creek, which is isotopically heavier than Rapid Creek are important (Maloszewski and Zuber, 1982; Zuber, and Boxelder Creek (fig. 21). Sites north of Rapid 1986; Richter and others, 1993). Compartmental Creek generally reflect isotopically lighter composition mixing models (Yurtsever and Payne, 1986) and similar to that of Rapid Creek and Boxelder Creek. combinations of environmental tracer techniques with Isotopic composition of precipitation on outcrops in the numerical ground-water flow models (Reilly and Rapid City area is heavier than in Rapid and Boxelder others, 1994) also have been used to address compli- Creeks and averages about -13 to -14 ‰ (fig. 20). The 18 cated hydraulic settings. These methods, however, δ O values for several Madison wells north of Rapid require more extensive data sets than those that are City (sites 130, 138, and 145) reflect this heavier currently available. Future collection of time-series isotopic composition (fig. 28), which probably indi- tracer data in the Black Hills area would facilitate more cates larger influence from precipitation recharge. The sophisticated evaluations of mixing conditions and 18 δ O values for Minnelusa wells in the Rapid City area ground-water ages. The tritium data and conceptual show a general gradation from north to south of lighter models presented in this report, in conjunction with to heavier values, with no distinct division, which stable-isotope and major-ion data, can be used to draw various conclusions about flowpaths and mixing condi- probably indicates larger influence from precipitation recharge, than for the Madison aquifer in this area. tions in the Black Hills area, as discussed in subsequent 18 sections. The δ O values for the Madison wells south of Rapid Creek probably indicate a northerly flow compo- 18 Site-Specific Considerations nent for recharge in the Spring Creek area. The δ O values for springs along Rapid Creek, however, indi- Site-specific discussions of isotopic interpreta- cate flow from different sources. Cleghorn (site 300) tions regarding areal flowpaths, ages, and mixing con- and Jackson Springs are large springs with combined ditions for the Madison and Minnelusa aquifers in the 3 Black Hills area are presented in this section. The discharge in excess of 20 ft /s, which probably are Rapid City area is discussed first because detailed data recharged primarily by Spring Creek, Rapid Creek, and sets are available for this area. Discussions of head- precipitation on outcrop areas to the southwest and water springs and northern and southern Black Hills west (Anderson and others, 1999). Dye testing has areas also are included. Previous discussions of confirmed rapid movement of water from a loss zone in general considerations for stable isotopes included Boxelder Creek to City Springs (site 298) and several various interpretations regarding recharge areas and wells in northwestern Rapid City; however, detectable general flowpaths for several example sites along the dye concentrations have not been recovered in northern and northeastern flanks of the uplift; these Cleghorn Springs (Rahn, 1971; Greene, 1999). The 18 examples are not repeated herein. δ O value for Tittle Springs (site 299) is notably lighter than for precipitation in the outcrop area, which Rapid City Area probably indicates recharge from Rapid Creek. A large data set is available for the Rapid City Results of dye testing in the Rapid City area area for stable isotopes (fig. 28) and tritium (fig. 31), (Greene, 1999), in combination with tritium samples, with multiple samples available for many sites (tables 7 provides relatively definitive age-dating information and 8). Evaluation of flowpaths and mixing conditions for several sites. Dye was injected in the loss zone of for the Rapid City area is especially complicated Boxelder Creek (site 315) during August 1993. A tri- because: (1) recharge generally is dominated by tium concentration of 58.1 TU was measured in Box- streamflow losses, rather than precipitation on outcrop elder Creek one month later (table 7). Greene (1999) areas (Carter, Driscoll, and Hamade, 2001); and (2) reported dye recovery at several sampling locations, large and variable withdrawals from the Madison and including site 112 (Black Hills Power and Light) and Minnelusa aquifers have occurred. Municipal produc- site 123 (Rapid City No. 10), with dye arrival times of tion from the Madison aquifer has increased substan- 41 and 49 days, respectively, and tritium concentra- tially since about 1990 (Anderson and others, 1999). tions of 3.7 and 4.7 TU.

Isotope Chemistry 51 The conceptual mixing models, which are based tions. Unequal mixes of very recent and older water on an assumption of equal annual recharge proportions, again provide more plausible explanations. are unable to provide age estimates that are compatible Subsequent samples collected during 2000 from with dye results. Using a 0-year delay curve for 1993 Boxelder Creek, Rapid City No. 10, and City Springs (fig. 36F), which is consistent with prompt dye arrival, had tritium concentrations of 15.9, 7.3, and 25.7 TU, maximum traveltimes dating back hundreds of years respectively (table 4). The increased tritium concentra- are indicated for the sample concentrations of 3.7 and tion in the sample from Rapid City No. 10 probably 4.7 TU. Dye concentrations in the resulting mix would indicates an increased proportion of recent water (per- be much lower than reported by Greene (1999) and haps approaching 50 percent), which is attributed pri- probably would be undetectable. Unequal mixing of marily to effects of pumping during the interim years. about 5 to 10 percent very recent water (tritium con- The well was constructed during 1991 and produces centration of about 58 TU) with 90 to 95 percent about 1,800 gal/min (Anderson and others, 1999). The pre-bomb water (0 TU) would produce a viable mix. tritium concentration of the 2000 sample for City Intermediate-age water (with high tritium concentra- Springs is higher than the corresponding Boxelder tions) probably is not entirely absent; however, the pro- Creek sample, whereas the concentration of the 1993 portion contributed to the mix must be extremely small. sample was lower, which may be indicative of transient This scenario is a noteworthy example of unequal flow and mixing conditions associated with changing mixing conditions that can occur in a dual-porosity recharge and discharge conditions. system. Additional insights are obtained from examina- Greene (1999) also reported dye recovery at tion of temporal variability in stable isotope values for sites 116 (Rapid City No. 6), 121 (Westberry Trails), ground-water sites. Data for sites with multiple sam- and 298 (City Springs), with dye arrival times of 30, ples are listed in table 8 and selected sites are shown in 26, and 30 days, respectively, and tritium concentra- figure 21B, which was presented earlier. The four tions of 14.5, 31.0, and 40.8 TU. Dye also was recov- wells in which dye was recovered (Black Hills Power ered at site 117 (City Quarry Madison), which had a and Light, Rapid City Nos. 6 and 10, and Westberry tritium concentration of 26.3 TU; however, an arrival Trails) and City Springs show only minor variability in 18 time was not reported. Viable estimates of maximum δ O values. This dampened response to relatively 18 traveltime can be made for these samples using a 0- large variability in δ O values recharged by Boxelder year delay mixing model; however, the resulting mixes Creek is consistent with the traveltime analysis, which again are not compatible with reported dye concentra- indicates relatively small proportions of recent water.

Table 4. Tritium data for selected sites having data for 2000 [Data are included in table 7; --, no data available]

Tritium, in tritium units Site Name number 1991 1993 1996 1997 2000

123 Rapid City No. 10 -- 4.7 -- -- 7.3

169 Rapid City No. 11 10.7 16.6 16.0 -- 12.0

172 Rapid City No. 9 21.0 24.3 -- 22.3 18.2

195 Hart Ranch -- 23.5 21.6 -- 17.7

298 City Springs -- 40.8 -- -- 25.7

315 Boxelder Creek near Nemo -- 58.1 -- 26.8 15.9

52 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota 18 Cleghorn Springs (site 300) provides an example The δ O values for site 185 (Rapid City No. 5) of ground-water flow and mixing conditions that may show a trend towards progressively heavier water be reasonably represented by the time-delay mixing (fig. 21B), which is opposite to the trend in streamflow 18 model. The δ O values (fig. 21C) for 1986-95 have recharge. The single available tritium concentration of only minor variability (-12.80 to -12.95 ‰) and values 4.3 TU in 1993 indicates probable dominance by old 18 for 11 samples collected between January 1999 and water, which would tend to dampen δ O variability. A May 2000 range only from -13.09 to -13.19 ‰ (L.D. plausible explanation is increased proportions of isoto- Putnam, U.S. Geological Survey, written commun., pically heavier water from the south-southwest or from 2000). Thus, a delayed and dampened response to the the overlying Minnelusa aquifer (fig. 28), induced temporal trends in Spring and Rapid Creeks is indi- from recent production. Either source would be consis- cated (fig. 21A). Tritium concentrations in Cleghorn tent with aquifer test results (Greene, 1993) that Springs (table 7) from 1978 (182 TU) and 1993 showed strongly directional transmissivities and leaky (30.2 TU) indicate dominant proportions of water confinement at this site. Tritium concentrations of recharged within a timeframe of about 10 to 20 years <0.3 TU were measured in 1990 and 1993 from the (fig. 36, graphs A and F). Estimates of older age also Lime Creek observation well (site 186), which is are possible for each sample, but probably can be located about 700 ft away from Rapid City No. 5. 18 discounted because of the response of δ O values to Rapid City No. 5 produces about 1,700 gal/min, which recharge conditions. indicates that the well is located in a transmissive part of the aquifer. The low tritium concentrations in both Temporal changes in mixing conditions and age wells, however, may indicate that: (1) the matrix is not characteristics probably have occurred in the vicinity exchanging much water with preferential flowpaths; or of several high-production wells in the Rapid City area (2) preferential flowpaths are not well connected to that have been completed since about 1990 (Anderson modern recharge sources. and others, 1999). Potential changes for site 123 18 Variability in δ O values for site 195 (Hart (Rapid City No. 10) were discussed previously. An 18 Ranch) is the largest of any ground-water site for which initial (1991) δ O sample for site 172 (Rapid City temporal data are available (fig. 21C). Variability No. 9) is notably lighter than later samples, which during 1986-95 appears to be small; however, data for show negligible variability during subsequent years 1987-93 are absent. Values for 1996-98 are notably (table 8, fig. 21B). Tritium concentrations (table 4) lighter than for previous periods, which again is consis- increased from 21.0 to 24.3 TU (1991 to 1993), which tent with the trend for Spring Creek. Tritium concen- probably indicates a larger component of more modern trations decreased steadily for samples from 1993, water in an altered mix. Steadily decreasing tritium 1996, and 2000 (table 4), which can be indicative of concentrations measured in 1997 and 2000 may be long-term mixes. However, this mixing condition con- 18 indicative of progressive decay of a relatively long- tradicts the δ O data, which indicate a relatively large δ18 term mix, which is consistent with stabilized O response to recent recharge. values. The decay-curve families for 1993 and 1997 Tritium data for sites 168 and 176 (Chapel Lane (fig. 36, graphs F and J) show a variety of plausible age Madison wells), which are located within 1,000 ft of estimates. each other (fig. 31), provide an example of large differ- Tritium concentrations for site 169 (Rapid City ences in ground-water ages and mixing conditions No. 11) also increased between 1991 and 1993 resulting from aquifer heterogeneity. Low tritium con- (table 4), which may indicate a response to initial centrations for site 176 for 1991 and 1993 samples (1.9 production. Subsequent tritium samples for 1996 and 18 and 2.2 TU, respectively) indicate production prima- 2000 have smaller concentrations. The δ O values rily from the aquifer matrix. A much higher tritium (fig. 21C) trend towards progressively lighter values, concentration from 1993 (27.5 TU) for site 168 indi- which probably reflects a response to the Spring Creek cates production primarily from preferential flowpaths, 18 trend. The tritium decay-curve families for 1991, which is supported by variability in δ O values 1993, and 1996 (fig. 36, graphs D, F, and I) generally (fig. 21C). Another example of aquifer heterogeneity provide no solutions for mixes involving water recently is provided by the low tritium concentration for recharged (within several years), which is indicated by site 191 (Reptile Gardens Madison well) that is located 18 the δ O values. Thus, unequal mixing of recent and generally upgradient from site 195 (Hart Ranch), pre-bomb water is a likely scenario at this site. which has a much higher tritium concentration.

Isotope Chemistry 53 18 A low (undetectable) tritium value for site 124 distribution for δ O for the Black Hills area (fig. 20). 18 18 (Rapid City No. 8) is consistent with small δ O vari- Temporal variability in δ O values is small for head- ability (fig. 21B) and with large distance from the out- water springs (table 8), which supports the use of the 18 crop (fig. 31). The δ O values (fig. 21C) for site 149 immediate-arrival mixing model (fig. 25B) for general (Highland Hills) show large variability, which is con- evaluation of ground-water ages in this hydrologic set- sistent with relatively high and variable tritium concen- ting. The use of the immediate-arrival model is further 18 trations (table 4). The δ O values (fig. 21C) for supported by the distribution of tritium concentrations site 165 (Carriage Hills) show less variability than for headwater springs, relative to wells and down- Highland Hills, but trend more steadily towards lighter gradient springs, as discussed previously (fig. 33). All values, which may indicate more dampened response of the headwater springs are located within or near to recent recharge. The single tritium sample aquifer outcrops, with some recharge presumably (22.9 TU) for site 165 provides no information beyond occurring in the immediate proximity of springs. confirming the presence of modern water. For the Pine Generalized age estimates for headwater springs are Grove well (site 196), which is located just south of the presented in table 5. 18 Rapid City area (fig. 29), moderate variability in δ O The age estimates in table 5 are derived using the values (fig. 21C) is consistent with the tritium concen- 0-year delay curves (fig. 36) for the appropriate date of tration of 25.9 TU (fig. 32). sample collection. A mix of equal proportions from all years between the minimum (0 years for immediate arrival) and maximum traveltimes is assumed for this Headwater Springs mixing model. Two possible estimates for the max- The stable isotope values for headwater springs imum traveltime exist for most samples; thus, “young” were used in developing the map of generalized spatial and “old” estimates are provided for each sample.

Table 5. Generalized age estimates for headwater springs, derived using immediate-arrival mixing model1 [--, unnamed)

Maximum traveltimes1 (years) Tritium con- Minimum Site Spring Sample centration travel- Young estimate Old estimate number name date (tritium time1 Average Average units) (years) Year range2 Year range2 of range of range 276 Knight 08-04-95 40.8 0 1965-67 30 1900-20 80 277 Jones 01-01-78 276.0 0 1963-65 14 1948-54 27 279 -- 307-18-96 40.8 0 1964-66 31 1915-25 76 280 -- 08-08-95 53.3 0 1964-65 30 1930-40 60 281 JHD 09-13-95 53.3 0 1964-65 30 1930-40 60 282 Intake Gulch 08-03-95 31.3 0 1967-69 27 1875-95 110 283 Rhoads Fork 401-01-78 62.2 0 1972-78 3 1840-60 130 08-07-95 43.9 0 1965-66 30 1910-30 75 287 Barrel 07-25-95 18.5 0 1975-95 0-20 1800-30 160 288 Water Draw 09-14-95 12.9 0 (5)(5) pre 1800 >200 289 McKenna 09-14-95 8.2 0 (5)(5) pre 1700 >300 1For immediate arrival model, a mix of equal proportions from all years between minimum and maximum traveltimes is assumed. Two solutions for the maximum traveltime are possible for most samples. 2Range of years for maximum traveltimes. 3Exact sample date unknown (see table 7). 4Location for the 1978 sample is unknown and probably does not exactly coincide with the 1995 sampling location. Exact sample date also unknown (see table 7). 5No mixing solution available for younger estimate using immediate-arrival model; however, plausible solutions for unequal mixing are possible.

54 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota Two of the springs (Jones and Rhoads Fork) discharge of all artesian springs along the northern were sampled during 1978. The tritium concentration flank estimated as 90 ft3/s for 1987-96 (Carter, for Jones Spring (276 TU) is very high and shows a Driscoll, Hamade, and Jarrell, 2001). The potentio- substantial proportion of water recharged near the 1963 metric-surface maps for the Madison and Minnelusa peak (figs. 26 and 36A). The concentration for aquifers (figs. 7 and 8) indicate primary flow compo- Rhoads Fork (62.2 TU) is much lower, indicating nents to these springs from the south and west. The either: (1) dominance by recharge after 1972; or (2) possible contribution of regional flow components maximum traveltimes exceeding about 100 years. In from the west (fig. 6) is evaluated in a subsequent deriving the young estimate, it was necessary to section. assume a slightly higher tritium concentration (about A substantial component of recharge from the 80-100 TU) in order to obtain a viable solution. An Black Hills area is indicated for these artesian springs 18 additional (1995) sample for Rhoads Fork provides a by the δ O values (fig. 27), which are similar to values conflicting age estimate; however, the precise location in large recharge areas to the south and west (fig. 20), of the 1978 sample is not known and is presumed to be which may extend to the Limestone Plateau. With the 18 different. exception of site 294 (Mirror Lake), the δ O values The young estimates for many of the springs for these springs are very similar, which probably indi- indicate maximum traveltimes dating to the mid 1960’s cates generally thorough mixing conditions associated (just subsequent to the tritium peak of 1963). The old with large discharges and associated large recharge 18 estimates for most springs indicate maximum travel- areas. The δ O values for springs also are very similar times approaching or exceeding 100 years. Notable to those for Madison and Minnelusa wells in this area. exceptions are the last three springs listed in table 5, for Values for wells generally grade to slightly heavier which old estimates are much older than for other head- values in a southeasterly direction, reflecting more water springs. Viable solutions for young estimates for localized flowpaths (figs. 7 and 8) with isotopically these springs cannot be obtained using the immediate heavier composition (fig. 20). 18 arrival mixing model (fig. 36H); however, unequal The δ O value for site 294 (Mirror Lake) is sim- mixing conditions involving recently recharged water ilar to that for site 34 (fig. 20, State Line Minnekahta are plausible. well), which may indicate a recharge contribution from Continuous records of daily discharge for the Minnekahta aquifer or localized recharge from site 283 (Rhoads Fork) indicate dominance by ground- other low-elevation outcrop areas. The tritium concen- water discharge, with virtually no short-term vari- tration for site 294 (fig. 30) also is notably lower than ability in streamflow. Annual streamflow, however, for three other springs in close proximity, which also correlates strongly with 9-year moving average precip- may indicate a different recharge source. The rela- itation (Driscoll and Carter, in press). Flow character- tively high tritium concentrations for the artesian istics are similar for a gaging station downstream from springs do not necessarily preclude regional flow con- site 282 (Intake Gulch). The thickness of the Madison tributions, but indicate a substantial recharge contribu- Limestone near these two sites (Carter and Redden, tion from areas within the Black Hills area. 1999e) is insufficient to accommodate storage volumes The similarities in isotope values in the vicinity associated with the old estimates. Thus, the old of the large springs probably indicates generally estimates for Rhoads Fork and Intake Gulch are not uniform mixing conditions, which is consistent with considered plausible. the physical setting (major discharge area with large recharge area) and supports the general applicability of Northern Black Hills Area the time-delay mixing model for general evaluation of Several large artesian springs are located along ground-water ages. Tritium samples for two wells that the northern axis of the Black Hills uplift, all of which are located about one-quarter mile apart along Crow are presumed to originate primarily from the Madison Creek (fig. 30) provide a useful starting point for eval- and/or Minnelusa aquifers, based on the work of uation of ground-water ages. Site 43 (McNenny well Klemp (1995), who evaluated source aquifers using No. 1) was sampled in 1978 (11.4 TU) and site 44 geochemical modeling. These springs are a major (McNenny well No. 2) was sampled in 1994 (19.4 TU). discharge area for these aquifers, with cumulative Applying the time-delay mixing model for the earlier

Isotope Chemistry 55 sample (fig. 36A) indicates viable solutions only for for the nearby streamflow loss zones (fig. 20). delay times in the range of about 16 to 24 years. Con- Recharge in this area is dominated by streamflow sidering both samples, a common solution with a max- recharge (Carter, Driscoll, Hamade, and Jarrell, 2001) imum traveltime of about 175 years can be obtained because of larger drainage areas for contributing using a delay time of 16 years. Viable solutions with streams, relative to outcrop areas of the Madison and similar delay times and maximum traveltimes exist for Minnelusa Formations. Tritium concentrations for site 293 (McNenny Rearing Pond) and site 296, which artesian springs along Battle and Grace Coolidge consists of the cumulative discharge of numerous Creeks (sites 301 and 302) indicate younger water than springs along Crow Creek, as well as for site 295 (Cox for several nearby wells (fig. 32), which is consistent Lake), which is located about 1 mi farther east. The with mixing conditions associated with the develop- theoretical solutions for these samples are relatively ment of artesian springs along preferential flowpaths. 18 insensitive to changes in delay time, for delay times up The δ O values for sites in recharge areas to about 28 years. (fig. 20) along the southwestern flank of the uplift are Tritium samples from 1994 and 1997 are avail- isotopically lighter than for sites along the southeastern able (table 7) for sites 290 (Higgins Gulch) and 292 flank, which probably reflects larger influence from (Old Spearfish Hatchery) located northwest of storms of Pacific origin along the western flank. Spearfish (fig. 30). For site 290, the tritium concentra- Downgradient sites along the southwestern flank tions are 31.3 and 28.1 TU, respectively, for which (fig. 29) reflect this lighter isotopic composition, with 18 numerous combinations of delay times and residence δ O values that generally are even lighter than those in 18 times provide comparable solutions. Two δ O values nearby outcrop areas. This pattern indicates generally for the same years (-16.47 and -16.25 ‰, respectively) southeasterly flowpaths originating in higher elevation may be sufficiently different to indicate relatively areas to the north. This conclusion is supported by short-term response to transient recharge conditions. tritium concentrations for downgradient wells in this For site 292, a large difference in tritium concentra- area (fig. 32), which generally are dominated by pre- tions exists between the 1994 sample (26.0 TU) and the bomb water, indicating generally long traveltimes and 1997 sample (13.4 TU), strongly indicating short-term flowpaths. The possible influence of regional flow- response to transient recharge conditions. paths from the west is discussed in the following sec- A single 1978 tritium sample for site 82 tion. Complex flowpaths in the southern Black Hills (65.6 TU) is nearly identical to estimated concentra- also may be influenced by interactions between the tions in precipitation for that period (fig. 26). The Madison and Minnelusa aquifers, which also are immediate-arrival mixing model may be applicable discussed in subsequent sections. because this Madison well is located within the outcrop area. In contrast, tritium was not detected for site 95, which also is located within the Madison outcrop near REGIONAL FLOWPATHS Blackhawk. Tritium also was not detected for sites 6, 106, and 148, all of which are very remote from out- Regional flow characteristics (fig. 6) are an crop areas. Large variability in traveltimes is apparent important consideration in evaluating flowpaths, espe- for other wells along the northeastern flank. Substan- cially near the northern and southern axes of the uplift. tial influence of modern recharge is apparent for Generalized regional flowpaths for the Madison springflow along Elk Creek (site 297), which is aquifer near the study area, which are based on the consistent with general characteristics for artesian following discussions, are presented in figure 34, along δ18 springs. with O values for samples from selected artesian springs and Madison wells. The artesian springs shown are major discharge points for the Madison Southern Black Hills Area and/or Minnelusa aquifers and do not necessarily con- 18 The δ O values for sites near Battle, Grace sist entirely of flow from the Madison aquifer. Infor- Coolidge, and French Creeks are in the range of about mation is not available for evaluation of possible -11.5 to -12.0 ‰ (fig. 29) and are very similar to values regional influences for the Minnelusa aquifer.

56 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota 104o30'104o 2 103o30' 45o -19.66 1 -18.13 SOUTH DAKOTA Black R E WYOMING Hills IV R

o rse Study Crow Area Area BELLE Shown

Belle Fourche Creek Reservoir Owl Newell Creek 6 4 Creek -17.07 -17.80 FOURCHEHulett F BELLE FOURCHE O UR CHE Hay Creek 296 R RIVER E BUTTE CO -16.67 V ER R I MEADE CO REDWAT LAWRENCE CO ater 295 265 edw BELLE R 294 -17.85 Creek -16.95 Saint Creek -15.34 Crow Onge

Bear Creek ek e 293 reek r Gulch Spearfish C 44o30' C -17.19 reek C Whitewood 266 Creek Bottom Creek Sand e ood -17.49 ls G a u F hitew lch utte Sundance Maurice W B Higgins Inyan STURGIS Tinton o DEADWOOD 103

Cold Beaver Cr

h Lead s i r f ea Kara r B Creek Tilford a

e r

p Elk C S w Creek Springs Meado Creek Spearfish N Elk

. Little Boxelder F

o r Piedmont C k Ellsworth reek R S a Air Force . F p Base CROOK CO o i Nemo rk R d

a C pid Blackhawk C WESTON CO Cr r ree k 267 k Box Elder For Rochford s -18.18 ad 267 Upton o . For R h N k Ca ap R stle id -18.18 Castle Cr RAPID CITY

ek Creek Rapid C C re Pactola re Reservoir ek Creek Castle V Deerfield i Creek PLATEAU ctoria S. Fork Reservoir Spring 270 r o LIMESTONE 268 C 44 reek -17.75 tle -17.60 C Cas 271 Sheridan Creek Creek Lake 269 -17.45 Hill City -18.15 Keystone 274 Mt. Rushmore Spring National Hayward -17.66 n Memorial 273 o y PENNINGTON CO Newcastle n Ba r a ttle Hermosa -17.40e

C v CUSTER CO a e Creek s B 275 le 272 o Grace B -17.95 French Creek -17.60 CUSTER e idg Creek Cool Cr CUSTER Beaver Jewel Cave National Monument STATE French Fairburn

PARK Whoopup Canyon

Stockade Creek

Lam Canyon Beaver e Pringle Wind Cave ell National Park H

WYOMING Creek 303 J EY o CH E hn N C -14.10Creek n re Wind y

N ek Cave SOUTH DAKOTA SOUTH E Red Creek RIVER o Pass Buffalo Gap 43 30' NIOBRARA CO

FALL RIVER CO H n o yo t Brook an C HOT SPRINGS Minnekahta 306 Fall Oral R RIVE -15.43 C R H E Y E 308 N N E -15.40

Creek Edgemont Horsehead

Lance Angostura Creek Reservoir Creek d oo w n to t 263 reek o C C Provo -17.09 01020MILES

Hat 01020KILOMETERS Base modified from U.S. Geological Survey digital data, 1:100,000 Rapid City, Office of City Engineer map, 1:18,000, 1996 Universal Transverse Mercator projection, zone 13 EXPLANATION OUTCROP OF MADISON LIMESTONE (from GENERALIZED FLOWPATH Strobel and others, 1999; DeWitt and others, 263 WELL COMPLETED IN MADISON AQUIFER-- 1989) -17.09 Numbers indicate site number and δ18O value OUTCROP OF MINNELUSA FORMATION in per mil (from Strobel and others, 1999; DeWitt and 308 DOWNGRADIENT SPRING--Numbers indicate others, 1989) -15.40 site number and δ18O value in per mil

Figure 34. Distribution of δ18O in selected Madison wells and springs and generalized flowpaths, based on δ18O values, in the Black Hills of South Dakota and Wyoming.

Regional Flowpaths 57 18 The δ O values for sites 1 and 2 (north of the which also could indicate influence from regional flow. study area) are -18.13 ‰ and -19.66 ‰, respectively The reported sulfate concentration for site 265 (fig. 34), which are much lighter than any of the values (210 mg/L) is similar; however, sodium (3.7 mg/L) and in Black Hills recharge areas (fig. 20). Busby and chloride (2.6 mg/L) concentrations show little, if any, others (1983) and Plummer and others (1990) note influence of regional flow. Reported major-ion con- Madison wells near recharge areas in Wyoming near 18 centrations for all other sites in Wyoming are much the Bighorn Mountains with δ O values as light as lower and essentially preclude possible influence from -18.5 ‰ and near the Laramie Mountains as light as regional flow. -19.25 ‰. In addition, concentrations of sodium (45 and 36 mg/L, respectively), chloride (67 and 25 mg/L), For site 263 near Provo, sodium (200 mg/L) and and sulfate (1,700 and 1,600 mg/L) for sites 1 and 2 chloride (270 mg/L) concentrations are high and may (Busby and others, 1991) are approximately an order of indicate influence of regional flow or the presence of magnitude higher than for wells along the northwestern evaporites. The sulfate concentration (310 mg/L) is flank of the Black Hills (fig. 10) and are consistent with similar to some regional values reported by Busby and a regional flowpath trending northeasterly from the others (1991), but is lower than values for sites 1 and 2 Bighorn Mountains in Wyoming. This information north of the study area, which are influenced by provides strong evidence of a regional flow component regional flow. Thus, increasing constituent concentra- from the west, just north of the study area. tions resulting from basinward evolutionary processes 18 The δ O values for sites 4 and 6 (near Belle cannot be distinguished from a potential regional flow 18 Fourche) and large artesian springs on the northwestern component. The δ O value of -17.09 ‰ for site 263 flank of the uplift (sites 293- 296) are consistent with (fig. 34) does not preclude a regional flow component, values in Black Hills recharge areas (fig. 20). Major- but probably does preclude recharge originating south ion concentrations for site 6 (fig. 10) also are consistent of about Pennington County (fig. 20). with values near the uplift and show no influence from Sodium and chloride concentrations for large regional flowpaths. Major-ion concentrations for site 4 springs near the southern axis of the uplift (fig. 35) are δ18 are slightly higher than site 6, and the O value is sufficiently high to indicate possible influence of more similar to site 1 than site 6, indicating more regional flow. Anomalously high sodium and chloride evolved ground water at this well and perhaps a slight concentrations in this area (Whalen, 1994) are indi- regional influence. However, south of site 4, the cated by Stiff diagrams for various Madison and Madison aquifer probably is not influenced by regional Minnelusa wells (figs. 10 and 11). Chloride concentra- flow from the west. Relatively high calcium and tions also are comparable with those for several other sulfate concentrations for some of the springs (fig. 35) wells (table 3) that have no possible influence from are similar to many Minnelusa wells (fig. 11) and regional flow. Examples include the Tinton Road probably are influenced by dissolution of anhydrite in (site 36) and City Quarry (site 117) Madison wells. the Minnelusa aquifer, as discussed later. Relatively high tritium concentrations for the springs (fig. 30) High calcium and sulfate concentrations for sites 307 indicate substantial influence from modern recharge (Cool Spring), 308 (Cascade Springs), and other and support the conclusion that flow near the northern springs in this area probably are influenced by dissolu- flank of the uplift is dominated by Black Hills tion of anhydrite in the Minnelusa aquifer, as discussed δ18 recharge. in a subsequent section. The O values for large 18 The δ O values for sites 271- 274, just west of springs northeast of Provo (fig. 34) are much heavier the South Dakota/Wyoming border (fig. 34) are similar than for site 263 and for sites in Wyoming, which indi- to, but slightly lighter than, the estimated isotopic com- cates dominant recharge from within the study area, 18 position in nearby recharge areas (fig. 20). The δ O including influence of isotopically heavy recharge values for sites farther west are somewhat lighter and along the southwestern flank of the uplift, within provide an indication of possible influence from Custer County (fig. 20). Low, but detectable, tritium regional flow from the west. Busby and others (1991) concentrations in these springs (fig. 32) indicate rela- reported sodium, chloride, and sulfate concentrations tively long traveltimes, but confirm the influence of for site 275 of 11.0, 7.6, and 200 mg/L, respectively, recharge from within the study area.

58 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota o 104o 45' 103 30' C EXPLANATION ro H w 44o45' Belle Fourche orse OUTCROP OF MADISON Creek Reservoir Owl Newell LIMESTONE (from Strobel and BELLE Creek Creek others, 1999)

F BELLE FOURCHE O UR OUTCROP OF MINNELUSA CHE RIVER

Hay Creek FORMATION (from Strobel and R

296 295 E BUTTE CO V others, 1999) ER R I MEADE CO REDWAT LAWRENCE CO ESTIMATED SULFATE CONCEN- 292 Saint Creek Crow Onge TRATIONS IN MINNELUSA

B Creek ea 293 290 AQUIFER r ek 35 Spearfish re 30' 36 Gulch C Less than 250 milligrams per liter reek C Whitewood 58 Creek Between 250 and 1,000 milligrams 57 Bottom 294 Creek e ls per liter Gulch a F Maurice Whitewood Butte Higgins STURGIS Greater than 1,000 milligrams per Tinton o liter DEADWOOD 15' 103 Beaver Cr 85 h Lead STIFF DIAGRAM-- s i f r Bear Creek Tilford a Sodium + Potassium Chloride + Fluoride e 86 r p Elk S w C Calcium Bicarbonate + Carbonate Meado 297 Creek Magnesium Sulfate Spearfish N Elk . 15' Little Boxelder 100 10 F

o r Piedmont k CONCENTRATION, IN MILLIEQUIVALENTS PER LITER Ellsworth R S. Fork Rapid Cr a Air Force p 101 100 Base i Nemo d WELL COMPLETED IN THE C Blackhawk Creek r 117 MADISON AQUIFER--Number rk Box Elder Fo Rochford 117 s is site number on table 7 ad 298 o . Fork h N Ca Rapid R stl 118 e C RAPID CITY WELL COMPLETED IN THE Castle r 183 101 ek Creek Rapid MINNELUSA AQUIFER-- C C re Pactola PLATEAU re Reservoir ek 300 Creek Number is site number on Castle

LIMESTONE V Deerfield ictoria Creek table 7 S. Fork Spring o r Reservoir 182 44 C e ARTESIAN SPRING--Number is Castl 179 Sheridan 191 192 Creek 204 Creek Lake 178 site number on table 7 Hill City Keystone Mt. Rushmore National Spring Memorial Hayward n

o y PENNINGTON CO n Battle

a Hermosa C CUSTER CO 301

s Creek e l o B Grace 201 French Creek CUSTER e lidg 202 Creek Coo 302 45' CUSTER 210 Jewel Cave National Monument STATE 211 French Fairburn PARK

Canyon yon Creek an C Lame 215 Beaver Pringle Wind Cave National Park Hell 216 WYOMING Creek 238 Johnny Wind Creek Cave SOUTH DAKOTA SOUTH Red 303 Creek 239 Buffalo Gap RIVER 30' Pass 242 FALL RIVER CO H 304 on 305 ot Brook ny Ca HOT SPRINGS 243Minnekahta 306 Fall R Oral

C H E 307 Y E N N E 308 Edgemont Horsehead

Angostura o Creek Reservoir C 43 15' d reek oo w n o tt o Creek C Provo

Hat 01020MILES Base modified from U.S. Geological Survey digital data, 1:100,000 Rapid City, Office of City Engineer map, 1:18,000, 1996 Universal Transverse Mercator projection, zone 13 01020KILOMETERS Figure 35. Selected Stiff diagrams showing the distribution of major-ion chemistry in selected well pairs and artesian springs in the Black Hills area.

Regional Flowpaths 59 18 The δ O value of -14.10 ‰ for Beaver Creek connection is indicated in other areas. Hydraulic con- Spring (site 303) is much lighter than estimated values nection probably occurs at various artesian springs for nearby outcrop areas (fig. 20) and nearby wells around the periphery of the Black Hills, many of which (fig. 29), which indicates a possible flowpath are stratigraphically located within or slightly above extending from the general Hot Springs area. Such a the Minnelusa Formation. Previous investigators flowpath would include a substantial component of iso- (Rahn, 1971; Whalen, 1994; Klemp, 1995; Hayes, topically light water recharged west of the uplift axis 1999) have identified the Madison and Minnelusa and would require ground-water flow nearly parallel to aquifers as potential sources for many artesian springs. mapped potentiometric contours (figs. 7 and 8) in some Geologic conditions facilitate hydraulic connec- areas. This hypothesis is possible, given the hydraulic tion between the two aquifers. Confining layers in the characteristics of the Madison and Minnelusa aquifers lower portion of the Minnelusa Formation probably are (Long, 2000), a hydraulic gradient in that direction, and influenced by paleo-karst features such as caverns and the accuracy of potentiometric-surface mapping, which sinkholes in the upper Madison Limestone. Extensive is limited by sparsity of data points in the area (Strobel fracturing and solution activity have contributed to and others, 2000a, 2000b). A flowpath from the Hot extensive secondary porosity in both formations and Springs area to Beaver Creek Spring is supported by decreased competency of the confining layers. Poten- tial exists for downward leakage (from Minnelusa to three general factors, including: (1) a low tritium con- Madison) in recharge areas where the aquifers are centration for site 303 (fig. 32) indicating generally 18 unconfined (water-table conditions), and for leakage in long traveltimes; (2) the areal distribution of δ O either direction, depending on direction of hydraulic values (fig. 29) relative to recharge areas (fig. 20), indi- gradient, where artesian conditions exist. In this sec- cating generally southeasterly (nonorthogonal) flow- tion, potential interactions are evaluated through anal- paths along the southwestern flank; and (3) results of a ysis of hydraulic and geochemical information for well water-budget analysis (Carter, Driscoll, Hamade, and pairs and artesian springs. Jarrell, 2001), which indicates a substantial flow component to Beaver Creek Springs from west of the uplift axis. Interactions at Well Pairs Evaluation of all available geochemical information indicates that regional flowpaths for the Madison Potential interactions between the Madison and aquifer are essentially deflected around the study area, Minnelusa aquifers are evaluated through examination with the possible exception of the southwestern and of hydraulic and geochemical information for well northwestern corners. Sites 1 and 2 (fig. 34) north of pairs. This section primarily addresses the potential for the study area show definite influence from regional general, areal leakage between the aquifers. flowpaths; however, areas south of site 4 are dominated by local recharge and probably have negligible regional Hydraulic Considerations influence. Major-ion chemistry for sites in the southwestern corner of the study area (fig. 10) indicate Hydrographs for 13 well pairs (26 wells), for possible regional influence; however, north of this area which geochemical information is available, are pre- regional influence probably is minor or negligible. The sented as figure 37 in the Supplemental Information section. Locations of well pairs are shown in figure 35. potentiometric-surface map for the Madison aquifer A summary of hydraulic comparisons for these well (fig. 7) neither confirms nor precludes a regional pairs is included in table 3, which was presented previ- influence in these areas. ously. Hydrographs for 9 of the 13 well pairs are fairly INTERACTIONS BETWEEN MADISON well separated and do not indicate direct hydraulic AND MINNELUSA AQUIFERS connection. Hydrographs are nearly identical in hydraulic head and shape for the City Quarry wells Hydrographs for numerous well pairs indicate (fig. 37E) and the CSP Airport wells (fig. 37J). distinct hydraulic separation between the Madison and Hydrograph shapes for the Tilford wells (fig. 37C) are Minnelusa aquifers in many areas (Driscoll, Bradford, similar; however, hydraulic heads are slightly different. and Moran, 2000); however, possible hydraulic Hydrographs for the Reptile Gardens wells (fig. 37H)

60 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota have some similarities; however, the period of record is Geochemical Considerations insufficient to be definitive. In this section, geochemical information for Similar hydrographs could result from similar Madison/Minnelusa well pairs is examined for evi- hydraulic characteristics and recharge/discharge char- dence of leakage between the two aquifers. Geochem- acteristics, rather than hydraulic connection. Of the ical interpretations may be influenced by effects of well four well pairs with similar hydrographs (table 3), construction, especially for the Minnelusa aquifer, in hydraulic connection has been confirmed by aquifer which observation wells typically are completed only testing (Greene, 1993) only for the City Quarry wells in the top of the aquifer (Driscoll, Bradford, and (fig. 37E). These wells are located about one-half mile Moran, 2000). from City Springs, which emerge through the Min- Stiff diagrams (fig. 35) indicate major-ion chem- nelusa Formation, but have been shown through dye istry is very similar for many well pairs, especially near testing to originate from the Madison aquifer (Greene, outcrop areas. The most notable differences are in con- 1997, 1999). Absence of effective hydraulic connec- centrations of sulfate, which increase with increasing tion at some wells in the Rapid City area has been distance from recharge areas in the Minnelusa aquifer. indicated by aquifer testing (Greene, 1993), as exem- Comparisons of sulfate concentrations for well pairs plified by the lack of response for the Sioux Park are provided in table 3. δ18 Minnelusa well (fig. 37G). Aquifer testing for two The distribution of O values in the Black Hills area, including well pairs, was presented in production wells in the Spearfish area also indicated no 18 figures 27-29. Values for δ O for well pairs are sum- measurable hydraulic connection between the Madison marized in table 3, with lighter values in the Madison and Minnelusa aquifers for the wells tested (Greene aquifer for 10 of the 13 well pairs, which reflects and others, 1999). Given the wide variety of hydro- generally higher recharge elevations than for the graph shapes, the striking similarities for the CSP Minnelusa aquifer. Paired values also are categorized Airport wells (fig. 37J) and Tilford wells (fig. 37C) as similar, somewhat similar, or dissimilar, based on 18 probably result from hydraulic connection. the difference in δ O values, using criteria of less than Conversely, hydraulic connection does not nec- 0.25 ‰ for similar, greater than 0.30 ‰ for dissimilar, 18 essarily mean hydrographs will be similar. The Mad- and within this range for somewhat similar. The δ O ison and Minnelusa aquifers probably are hydraulically values are categorized as dissimilar for six pairs connected at Cleghorn and Jackson Springs, which (table 3), all of which also have dissimilar hydro- emerge through the Minnelusa Formation, but prob- graphs. ably originate primarily from the Madison aquifer, as All of the four pairs with similar or somewhat discussed later. The Canyon Lake wells (fig. 37F), similar hydrographs (sites 85/86, 117/118, 191/192, 18 which are located about one-fourth mile away, show no and 210/211) have δ O values that are similar or indication of connection, however. somewhat similar (table 3). These similarities are not necessarily indicative of extensive mixing because Hydraulic head is higher in the Madison aquifer 18 similarities in δ O values may result from similarities than in the Minnelusa aquifer for 10 of the 13 wells in recharge characteristics. Hydraulic connection pairs considered (table 3, fig. 37). One exception is the between the Madison and Minnelusa aquifers has been CSP Airport wells (fig. 37J), where heads are nearly confirmed at the City Quarry wells (sites 117/118), identical. The other exceptions are the Tinton Road however, by aquifer testing (Greene, 1993) and by dye wells (fig. 37B) and the Boles Canyon wells (fig. 37I), testing (Greene, 1999). Although hydraulic connection where water-table conditions occur in both aquifers. has been confirmed at the City Quarry wells, tritium Paired wells are measured in five other locations where values indicate much longer traveltimes for the Min- artesian conditions exist (Driscoll, Bradford, and nelusa aquifer. Moran, 2000); however, paired geochemical data are Hydrographs are dissimilar (table 3) for the not available. Hydraulic head in the Minnelusa aquifer Sioux Park wells (sites 182/183); however, the δ18O 18 consistently exceeds that of the Madison aquifer at only values are quite similar. Furthermore, the δ O value one of these locations. Thus, for areas near the out- for the Minnelusa well (-14.38 ‰) is notably lighter crops with artesian conditions, the greatest leakage than for several nearby Minnelusa wells (fig. 28) and potential generally is from the Madison aquifer to the closely resembles values for Madison wells to the Minnelusa aquifer. northwest. Hydraulic head in the Madison aquifer

Interactions Between Madison and Minnelusa Aquifers 61 generally exceeds that in the Minnelusa aquifer at the about 5 to 15 ft (fig. 37M); thus, leakage in either Sioux Park wells (fig. 37G); however, aquifer testing direction might be possible in upgradient locations, has indicated that direct hydraulic connection does not depending on the hydraulic gradient. The large differ- occur at this location. Thus, if the δ18O value for the ence in sulfate concentrations (table 3) probably pre- Sioux Park Minnelusa well (site 183) is influenced by cludes substantial leakage from the Minnelusa to the upward leakage from the Madison aquifer, the leakage Madison aquifer, however. probably occurs to the northwest, near where con- Tritium concentrations for well pairs also are firmed leakage occurs in the vicinity of the City Quarry summarized in table 3. Sample collection dates were wells (sites 117/118) and City Springs (site 298). comparable for most well pairs (table 7), with the Geochemical comparisons for wells along the exception of the Boles Canyon wells (sites 201/202), southwestern flank of the Black Hills provide useful which were collected several years apart. Tritium information regarding complex flowpaths in this area. concentrations cannot necessarily provide conclusive 18 The δ O value of -16.32 ‰ for the Boles Canyon information regarding aquifer mixing. An example is Madison well (site 201) is somewhat heavier than for the City Quarry wells (sites 117 and 118), where site 215 (fig. 29) and for wells to the northwest in hydraulic connection between the aquifers has been 18 Wyoming (fig. 34). The Madison aquifer is not fully confirmed by aquifer testing, and similar δ O values saturated at site 201 and may be influenced by down- indicate probable mixing. Without other information, ward leakage from the overlying Minnelusa aquifer, the low tritium concentration for the Minnelusa well where hydraulic head is about 500 ft higher (fig. 36I). (0.6 TU), relative to the Madison well (30.1 TU), The sulfate concentrations for sites 201 and 202 would indicate minimal mixing. With other informa- (table 3) do not preclude this possibility. tion, the tritium concentrations indicate that a substan- The sulfate concentration for the Hell Canyon tial traveltime occurs in the Minnelusa aquifer before Madison well (site 215) is higher than for most other arrival of any water originating as leakage from the Madison wells (table 3, fig. 10), which could indicate Madison aquifer. downward leakage from the Minnelusa aquifer. The The greatest potential for extensive mixing prob- hydraulic gradient between aquifers is adverse ably exists near the other three well pairs with similar (upward) at the well site (fig. 37K), but may be favor- hydrographs. Tritium concentrations, however, do not able at upgradient locations (such as Boles Canyon). provide additional insights regarding mixing at these 18 The δ O value for the Madison well (-17.23 ‰) is sites. For the Reptile Gardens wells (sites 191 and 192) much lighter than for the Minnelusa well (site 216), and the CSP Airport wells (sites 210 and 211), tritium however, which essentially precludes substantial influ- concentrations are low and do not provide useful infor- 18 ence from Minnelusa aquifer leakage. The δ O value mation. For the Tilford wells (sites 85 and 86), the also is lighter than for nearby recharge areas (fig. 20), hydraulic gradient is upward (Madison to Minnelusa) which could indicate a regional flow component. and small (fig. 37C). Tritium concentrations do not Overall major-ion chemistry (sodium sulfate) at this provide additional insights because the concentration site is somewhat distinct (fig. 10), however, and a chlo- for the Madison well (1.3 TU) is much lower than for ride deficiency (table 3) is not indicative of a regional the Minnelusa well (31.3 TU); thus, the only possible flow component. The high sodium and sulfate concen- effect from upward leakage would be undiscernible trations probably are explained by generally increasing dilution of concentrations in the Minnelusa aquifer. ion concentrations resulting from basinward evolu- Tritium concentrations for the State Line Madison tionary processes, which is consistent with the long (19.1 TU) and Minnelusa (<0.3 TU) wells (sites 35 18 flowpath indicated by the light δ O value. and 36) indicate extensive mixing can be ruled out, in 18 The δ O values for both Minnekahta Junction spite of a large hydraulic gradient from Madison to wells (site 242 and 243) are notably heavier than for Minnelusa. Of the four well pairs with similar hydro- other nearby sites (fig. 29) and probably reflect graphs, two pairs have notably different tritium con- recharge primarily near the southern end of the uplift. centrations (Tilford pair and City Quarry pair). Thus, 18 The relatively heavy δ O values essentially preclude extensive mixing near these two pairs probably does the influence of regional flow from the west at this not occur. Tritium concentrations for the other two location. The hydraulic head difference between the pairs (Reptile Gardens and CSP Airport) are very low Madison and Minnelusa aquifers at this site is only and do not provide conclusive information.

62 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota The overall conclusion from comparison of Minnelusa aquifers (figs. 7 and 8). Hydraulic head in hydraulic and geochemical information for well pairs is the Madison and Minnelusa aquifers at major spring that general leakage between the Madison and locations is estimated from potentiometric-surface Minnelusa aquifers probably does not result in areally maps and is summarized in table 6, along with approx- extensive mixing in most locations. Comparisons of imate land-surface elevations near the springs. Some hydrographs for well pairs indicates the possibility of of the springs considered in table 6 consist of a series hydraulic connection in 4 of 13 locations. Geochem- of springs along stream reaches. For these cases, the ical information provides indications of mixing only in elevations listed generally are for locations down- the general vicinity of City Springs in northwest Rapid stream from the individual discrete springs (sites 290, City. Geochemical information for other areas is 292, 296, 297, 301, 302, and 306). largely inconclusive and provides no definitive indications of extensive mixing. Interactions between the Several hydraulic possibilities exist for interac- two aquifers in the vicinity of artesian springs are tions between the Madison and Minnelusa aquifers at examined in the following section. spring locations, including: (1) water originates only from the Minnelusa aquifer, with no contribution from the underlying Madison aquifer; (2) water originates Interactions at Artesian Springs entirely from the Madison aquifer and passes through the Minnelusa Formation, with little interaction; Extensive interactions between the Madison and (3) water originates entirely from the Madison aquifer, Minnelusa aquifers may occur at artesian springs, part of which discharges at the surface and part of many of which have large discharges. Combined dis- which recharges the Minnelusa aquifer; and (4) water charge of all artesian springs within the Black Hills 3 originating from both aquifers contributes to spring- area is estimated as 189 ft /s for 1987-96, which repre- flow. For cases where the Madison aquifer contributes sents about 48 percent of average recharge to the to springflow, leakage to the Minnelusa aquifer could Madison and Minnelusa aquifers (Carter, Driscoll, consist of either focused leakage in the immediate Hamade, and Jarrell, 2001). Numerous investigators vicinity of the spring-discharge point or general have identified the Madison and Minnelusa aquifers as probable sources for artesian springs in the Black Hills, leakage in upgradient directions. based on hydraulic properties and geochemical charac- The Minnelusa aquifer probably can be dis- teristics. Extensive cavern development within the counted as a primary source for springs located where Madison aquifer creates potential for focused move- the Minnelusa Formation is exposed, which generally ment of large quantities of water. Breccia pipes within precludes artesian conditions in the Minnelusa aquifer. the Minnelusa Formation (Bowles and Braddock, One example is Cleghorn Springs (site 300) and 1963) were identified by Hayes (1999) as a pathway for Jackson Springs, which comprise a large spring com- vertical movement of water from the Madison aquifer. plex in western Rapid City (fig. 35) adjacent to an The Minnekahta aquifer also may be a contributing outcrop section of the Minnelusa Formation, where source in locations where the Minnekahta Limestone is several hundred feet of outcrop occurs in a cliff above present. The underlying Deadwood aquifer also cannot the springs. Hydraulic head in the Madison aquifer is be discounted as a possible source. considerably above land surface at this location Precise quantification of relative contributions (table 6); however, hydraulic head in the Minnelusa from source aquifers to individual springs is not neces- aquifer is approximately at land surface, as indicated sarily possible; however, geochemical information is by water levels in a pair of observation wells (fig. 37F, useful for evaluating interactions between the Madison sites 178 and 179) located about one-quarter mile east and Minnelusa aquifers at artesian springs. Hydraulic of the springs. The large and steady discharge of the information also must be considered before geochem- springs (Anderson and others, 1999) also indicates a ical information is addressed. dominant contribution from the Madison aquifer. The spring complex probably serves as a relief mechanism Hydraulic Considerations for controlling hydraulic head in the Minnelusa Locations of artesian springs are shown on aquifer; thus, a contribution to springflow from the potentiometric-surface maps of the Madison and Minnelusa aquifer cannot necessarily be excluded.

Interactions Between Madison and Minnelusa Aquifers 63 6.0 4.1 3.8 0.6 2.5 2.5 2.2 2.2 ------(TU) 31.3 28.1 26.0 13.4 20.7 16.9 21.0 19.1 30.1 40.8 30.2 20.4 17.2 182.0 Tritium O 18 ------δ -16.36 -16.71 -17.19 -15.34 -16.95 -16.67 -13.79 -14.37 -13.02 -11.62 -11.68 -14.10 -14.86 -16.71 -15.43 -15.23 -15.40 (per mil) (per Isotopes Date mean mean mean mean mean 09-26-94 10-10-97 09-26-94 08-21-97 09-28-94 09-28-94 09-27-94 08-30-96 09-05-96 06-26-93 01-01-78 09-27-93 09-06-96 09-10-96 04-25-94 04-21-94 10-03-94 01-01-78 04-21-94 09-11-96 09-12-96 10-19-95 09-12-96 1 1 1 1 1 2 2 ------2 3 studies , approximately, equal] ≈ Estimated spring 30% Minnelusa 30% Minnelusa 50% dissolved Minnelusa minerals dissolved Minnelusa minerals source from previous from source Mostly Madison 70% Madison, Mostly Madison 50% Madison, Mostly Madison with Madison Mostly Mostly Madison Mostly Madison with Madison Mostly less than; than; less , .8 .7 2.3 2.7 6.3 6.5 5.4 5.3 12 15 14 33 51 89 38 47 (mg/L) 110 Chloride 98 25 19 11 76 110 340 130 545 580 420 540 400 830 1,600 1,300 1,500 (mg/L) Sulfate 3,550 3,550 3,580 3,580 3,580 3,560 3,450 3,450 3,380 3,540 3,650 3,480 3,625 3,420 3,360 3,450 3,450 aquifer Minnelusa Hydraulic head Hydraulic (feet above MSL) 3,490 3,500 3,720 3,720 3,705 3,710 3,450 3,450 3,420 3,540 3,650 3,480 3,700 3,610 3,580 3,505 3,495 aquifer Madison 3,405 3,405 3,400 3,410 3,415 3,355 3,450 3,440 3,380 3,540 3,650 3,460 3,625 3,465 3,415 3,450 3,440 MSL) of land surface Elevation (feet above (feet above /s) 1 1 5 2 3 ≈5 ≈ ≈ ≈ ≈ <5 <5 0-5 5-10 0-20 1-10 0-20 mate 30-50 20-25 10-15 20-30 18-22 (ft Approxi- discharge Name Higgins Gulch Hatchery Old Spearfish RearingNcNenny Pond Lake Mirror Cox Lake Creek Crow Elk Creek Springs City Cleghorn Springs Creek Battle Grace CoolidgeCreek Spring Creek Beaver Hot Brook Spring Evans PlungeSpring River Fall Cool Spring Cascade Spring .springs artesian major for information geochemical and hydraulic Selected Estimatedby Klemp (1995). Estimated by Whalen (1994). Estimatedby Hayes (1999). 1 2 3 Site 290 292 293 294 295 296 297 298 300 301 302 303 304 305 306 307 308 /s, cubic feet per second; MSL, mean sea level; mg/L, milligrams per liter; TU, tritium units; -- no information; %, percent; < percent; %, information; no -- units; TU, tritium liter; per milligrams mg/L, level; sea mean MSL, second; per feet cubic /s, 3 number Table 6 Table [ft

64 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota A similar setting exists for Hot Brook Spring Cascade Springs (sites 293, 295, 296, 300, 303, 306, (site 304) located just northwest of Hot Springs 308, respectively). Hydraulic heads at these sites (fig. 35). This spring also is located within an outcrop generally are substantially above land surface for one section of the Minnelusa Formation, in an area where or both of the two aquifers. hydraulic head in the Madison aquifer is much higher In contrast, discharge is much more variable for than in the Minnelusa aquifer (table 6), as indicated by Elk Creek, City Springs, Battle Creek, and Grace water levels in a pair of observation wells in Hot Coolidge Creek (sites 297, 298, 301, and 302, respec- Springs (Driscoll, Bradford, and Moran, 2000). These tively). At these sites, mapped hydraulic heads in both wells are not included in figure 37 because geochem- aquifers are approximately coincident with land- ical data are not available for both wells. surface elevation; however, to some extent, land- All of the other artesian springs listed in table 6 surface elevations have been used to infer hydraulic heads for the two aquifers. This inference generally is occur in locations where the Minnelusa Formation is validated, however, by hydrographs for observation confined by overlying units and artesian conditions are wells in the vicinity of these springs (Driscoll, Brad- assumed for both the Madison and Minnelusa aquifers. ford, and Moran, 2000). An example is City Springs Mapped hydraulic heads at other artesian spring loca- (fig. 35, site 298), which is located about one-half mile tions generally are higher in the Madison aquifer than downgradient from the City Quarry wells (sites 117 the Minnelusa aquifer (table 6, figs. 7 and 8), with the and 118). The discharge of City Springs correlates exception of sites 290 and 292 (just northwest of strongly with hydraulic head, with flow occurring only Spearfish) where mapped hydraulic heads may not be when hydraulic head exceeds about 3,440 ft (fig. 37E). definitive because control points are sparse (Strobel The previous discussion supports a general con- and others, 2000a, 2000b). Hydraulic head is much clusion that artesian springs are a relief mechanism that higher in the Madison aquifer, relative to the Minnelusa provide somewhat of an upper limit for hydraulic head aquifer, for spring areas along and near Crow Creek in the Madison and Minnelusa aquifers. Artesian (sites 293-296) and in the Hot Springs area (sites 304- springflow increases in response to increasing recharge 306). Hydraulic head in the Madison aquifer is only and increasing water levels. Springflow responds rela- slightly higher than in the Minnelusa aquifer in the tively slowly in locations where hydraulic head is sub- vicinity of Cool Spring (site 307) and Cascade Springs stantially above land surface, with faster response in (site 308). locations where hydraulic head is near land surface. The Madison aquifer generally has larger potential for higher hydraulic head because recharge areas Geochemical Considerations occur at higher elevation than for the Minnelusa Previous investigators (Whalen, 1994; Klemp, aquifer. Higher hydraulic head in the Madison aquifer 1995; and Hayes, 1999) used geochemical modeling to could indicate higher potential for contributions to estimate contributions of the Madison and Minnelusa springflow, relative to contributions from the aquifers to selected springs. Their methods considered Minnelusa aquifer. An alternative line of reasoning major-ion chemistry and several isotopes that are may be plausible, however. Higher hydraulic head in influenced by rock/water interactions, including the Madison aquifer also indicates relatively competent carbon, sulfur, and strontium isotopes. Results are not confinement by the overlying Minnelusa Formation, necessarily definitive because of: (1) uncertainties in which could imply larger contributions from the selecting representative source and end-point waters Minnelusa aquifer. Thus, generalities regarding domi- along assumed flowpaths; (2) spatial variability in nant contributions to artesian springflow cannot be mineralogical characteristics within the two forma- inferred from comparisons of hydraulic head. tions; and (3) potential geochemical influences from Discharge rates for artesian springs (table 6) also interactions with other aquifers or confining units. provide insights regarding hydraulic considerations. Generalized results of previous modeling efforts for Flow variability is minimal for many artesian springs selected artesian springs are summarized in table 6. (Miller and Driscoll, 1998; Anderson and others, 1999; The Madison aquifer generally was identified as the U.S. Geological Survey, 2000), including McNenny primary source, with variable contributions from the Rearing Pond, Cox Lake, Crow Creek, Cleghorn Minnelusa aquifer, or chemical influences resulting Springs, Beaver Creek Springs, Fall River, and from residence time within the Minnelusa Formation.

Interactions Between Madison and Minnelusa Aquifers 65 Stiff diagrams for artesian springs were pre- The δ18O value of -16.67 ‰ for Crow Creek sented in figure 35. Sulfate concentrations, which can (fig. 27) reflects the general isotopic composition for be distinctly different for the Madison and Minnelusa the Madison and Minnelusa aquifers in this large dis- aquifers, are listed in table 6 for large artesian springs. charge area. Values for many other area springs are Springs with high sulfate concentrations have major- similar; however, the value for Mirror Lake (site 294) ion chemistry similar to that of many Minnelusa wells is notably heavier, as previously discussed. Modern (fig. 11), which generally indicates chemical influence tritium values for all springs in this large discharge from the Minnelusa aquifer (or from anhydrite dissolu- area support the conclusion of small or negligible influ- tion in overlying confining units). High sulfate con- ence of regional flow from the west in this area, as centrations, however, may result from dissolution of previously discussed. Minnelusa minerals by water from the Madison aquifer Three large spring discharge areas are located and do not necessarily indicate a contribution of water near the southern axis of the uplift (fig. 35), two of from the Minnelusa aquifer. Springs with low sulfate which consist of multiple springs. The discharge of concentrations have major-ion chemistry similar to that Cascade Springs (site 308) is much larger than Cool of many Madison and Minnelusa wells (figs. 10 Spring (site 307), and sulfate concentrations (1,500 and and 11) and are not necessarily indicative of a Madison 830 mg/L, respectively) are much different (table 6). aquifer source. The high sulfate concentrations resemble ion chemistry Large spatial variability in sulfate concentrations for Minnelusa wells and reflect influence from the sulfate dissolution front (fig. 35). Geochemical modeling occurs in the major artesian spring areas along the of Cascade Springs water by Hayes (1999) indicated a northern and southern axes of the Black Hills, which primary component of relatively fresh water from the probably results from differences in availability of Madison aquifer as a dissolution agent for anhydrite anhydrite within the Minnelusa Formation. Sulfate within the Minnelusa Formation. also could be derived from other potential sources, δ18 such as gypsum within the Spearfish Formation; how- The O values for Cascade (site 307) and Cool Springs (site 308) are notably lighter than for Madison ever, many springs with low sulfate concentrations and Minnelusa wells near the southern tip of the uplift issue from the Spearfish Formation. Source aquifers and notably heavier than values further north (figs. 29 cannot be determined from other major-ion constitu- and 34). This isotopic composition reflects a general ents. High calcium and magnesium concentrations mix from a large potential recharge area all along the could be contributed by either aquifer (figs. 10 and 11). western and southwestern flanks of the uplift, which High concentrations of sodium/potassium occur only may include contributions from the Minnelusa aquifer. in several Madison wells in the southwestern corner of This is consistent with tritium concentrations (fig. 32), the study area (fig. 10); however, concentrations in which indicate a small proportion of modern water downgradient springs are much smaller (fig. 35). mixed with a dominant proportion of pre-bomb water, The large springs in the northwestern part of the indicating generally long traveltimes. Upgradient study area have large spatial variability in major-ion wells all have even lower tritium concentrations, which chemistry (fig. 35). Sulfate concentrations among illustrates the enhancement of preferential flowpaths site 293 (McNenny Rearing Pond), site 294 (Mirror near artesian springs. The combination of geochemical Lake), site 295 (Cox Lake), and site 296 (Crow Creek) information generally excludes substantial contribu- range from 130 to 1,600 mg/L. The Madison aquifer tion from regional flow from the west, as previously was identified by Klemp (1995) as the primary source discussed. of McNenny Rearing Pond and Cox Lake (table 6), Three closely spaced springs in the Hot Springs both of which had low sulfate concentrations. A mix of area (sites 304, 305, and 306) have large spatial vari- water from the Madison and Minnelusa aquifers was ability in major-ion chemistry. The large flow (table 6) estimated for Mirror Lake (Klemp, 1995), which had of Fall River (site 306) includes the discharge of the highest sulfate concentration. The large discharge sites 304 and 305, as well as various other individual of Crow Creek (table 6), which has an intermediate springs (not shown), which have large spatial vari- sulfate concentration (580 mg/L), is comprised of the ability in ion chemistry (Alexander and others, 1989). cumulative flow of many artesian springs, including Sulfate concentrations for the three sites considered sites 293 and 294. range from 76 mg/L for site 304 (Hot Brook Spring) to

66 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota 540 mg/L for site 305 (Evans Plunge Spring). The more plausible, given the probable influence of prefer- Madison aquifer is the likely source of Hot Brook ential flowpaths associated with the nearby spring. Spring, based on hydraulic head (table 6) and physical setting, as previously discussed; this conclusion is consistent with Whalen’s (1994) determination. Based on Synopsis of Interaction Processes hydraulic head, the Madison aquifer also is the source of Evans Plunge Spring, but both aquifers could con- Consideration of both hydraulic and geochem- tribute to other springs along the Fall River. All three ical information is necessary to interpret complex inter- springs have relatively high sodium and chloride con- action processes between the Madison and Minnelusa centrations, which is consistent with localized condi- aquifers. A synopsis of interaction processes is pro- tions in the Madison and Minnelusa aquifers (Whalen, vided in this section. 1994). As previously discussed, various hydrogeologic Springs that consist predominantly of pre-bomb factors such as paleo-karst features, fracturing, and water comprise the flow of Fall River (site 306), which solution activity facilitate hydraulic connections has a tritium concentration of 2.5 TU. The youngest between the Madison and Minnelusa aquifers. The water is contributed by Hot Brook Spring (site 304, Madison aquifer has potential for higher hydraulic head than the Minnelusa aquifer because of higher ele- 3.8 TU), with Evans Plunge Spring (site 305) contrib- vation recharge areas. Hydrographs for paired obser- uting older water (0.6 TU). Water from the nearby Vets vation wells indicate that higher hydraulic head in the Home Madison well (site 247) is even older, with no Madison aquifer is maintained in most near-outcrop detectable tritium. areas where artesian conditions exist, which indicates As previously discussed, the recharge area for general competency of the confining layer between the Beaver Creek Springs (site 303, fig. 34) probably two aquifers. This is consistent with a previous conclu- extends to the west side of the uplift axis, based on the sion that general leakage between the Madison and δ18 O value, which is notably lighter than estimated Minnelusa aquifers probably does not result in areally values for nearby outcrop areas (fig. 20) and nearby extensive mixing. wells (fig. 29). Relative contributions from the Saturation indices indicate that the Madison Madison and Minnelusa aquifers cannot be determined aquifer is undersaturated with respect to gypsum, even δ18 from the O values; however, high sulfate concentra- at the highest sulfate concentrations. Thus, ongoing tions (table 6, fig. 35) indicate substantial influence dissolution of Minnelusa Formation minerals probably from the Minnelusa Formation (either flow contribu- occurs where the hydraulic gradient is favorable for tions or rock/water interactions). upward leakage from the Madison aquifer, especially The tritium concentration (6.0 TU) for Beaver in areas where the competency of the confining layer Creek Spring (fig. 32) is discernibly higher than for has been substantially decreased. other artesian springs to the southwest, presumably Hayes (1999) hypothesized that upward leakage from the influence of a localized recharge component from the Madison aquifer was contributing to ongoing from along the southeastern flank of the uplift. Tritium development of breccia pipes at Cascade Springs. concentrations for wells near the spring indicate a wide Hayes (1999) noted that breccia pipes commonly occur range of traveltimes. Tritium was not detected in the in the upper Minnelusa Formation, but very few have 7-11 Ranch Madison and Minnelusa wells (sites 238 been observed in the lower part of the formation. Net- and 239, respectively); however, some proportion of works of interbedded breccia layers and short, vertical modern water is apparent for other wells. A 1978 breccia dikes do occur in the lower Minnelusa Forma- sample (27.7 TU) for site 233 (Kaiser well) indicates tion, however, as schematically illustrated in figure 5. some modern water, presumably mixed with pre-bomb Paleo-karst features in the upper Madison Limestone water (fig. 36A). Site 237 (Streeter Ranch) has sam- have influenced the depositional environment of the ples of 10.6 TU from 1977 and 11.7 TU from 1997 overlying Minnelusa Formation. Deformation and (table 7). The time-delay mixing curves (fig. 36) fracturing associated with the subsequent uplift generally indicate theoretical delay times of 10 or more (Laramide Orogeny) also have contributed to years and mixing with substantial proportions of pre- decreased competency of the confining layer between bomb water. Unequal mixing conditions probably are the two aquifers.

Interactions Between Madison and Minnelusa Aquifers 67 Hayes (1999) further hypothesized that many investigators has identified the Madison aquifer as the exposed breccia pipes of the upper Minnelusa Forma- primary potential source for many springs. tion probably are the throats of abandoned artesian Upward leakage from the Madison aquifer is springs. An outward (downgradient) migration of concluded to be an important factor in development of artesian springs probably has occurred as upgradient artesian springs and probably contributes flow to most spring-discharge points are abandoned and new ones springs. This process is driven by the dissolution are occupied, keeping pace with regional erosion over potential of water from the Madison aquifer, which is geologic time (Hayes, 1999). In response, hydraulic undersaturated with respect to gypsum. Upward heads in the Madison and Minnelusa aquifers have leakage at artesian springs probably is a major factor in dissolution and transport of anhydrite cements, associ- declined over geologic time, as indicated by exposed ated with ongoing development of the anhydrite disso- breccia pipes located upgradient from Cascade Springs lution front in the Minnelusa aquifer. Although (Hayes, 1999). Further supporting evidence is pro- conclusive geochemical evidence of extensive mixing vided by Ford and others (1993), who concluded that is not apparent, general leakage from the Madison water-level declines of more than 300 ft have occurred aquifer probably contributes to ongoing dissolution of in the Madison aquifer during the last 350,000 years, anhydrite within the Minnelusa Formation. based on geochemical data for Wind Cave. Artesian spring development is further hypothe- Ground water discharging from the Madison sized to be a self-perpetuating process, with springs aquifer at artesian springs was referred to as “rejected initially developing in locations with large secondary recharge” by Huntoon (1985), who hypothesized that porosity and associated high hydraulic conductivity. recharge is rejected as transmissivity decreases with Development of preferential flowpaths in these areas distance from upgradient recharge areas. This hypoth- contributes to increased dissolution activity, which esis is consistent with decreasing potential for large continually contributes to increased enhancement of secondary porosity with increasing distance from the hydraulic conductivity. Similar development of prefer- uplift, which results from: (1) decreased deformation ential flowpaths also can occur in locations without and associated fracturing of rocks; and (2) decreasing artesian springs. potential for dissolution enhancement associated with Considering all available information, it is concluded that interactions between the Madison and increasing basinward concentrations of dissolved Minnelusa aquifers are an important factor governing constituents. the hydraulic behavior of the two aquifers. The Artesian springflow, which represents about exchange of water resulting from general, areal leakage 48 percent of average recharge to the Madison and probably is small, relative to that which occurs near Minnelusa aquifers (Carter, Driscoll, Hamade, and artesian springs; however, both processes probably Jarrell, 2001), is an important factor in controlling contribute to the control of hydraulic heads in the Black water levels in these aquifers. Artesian springs are Hills area. essentially a relief mechanism that provide an upper limit for hydraulic head. Springs located where hydraulic head is substantially above land surface SUMMARY AND CONCLUSIONS generally have relatively stable discharge. In locations where hydraulic head is near land surface, however, The Madison and Minnelusa aquifers are two of discharge characteristics generally are more variable, the most important aquifers in the Black Hills area with springflow increasing in response to increasing because of utilization for water supplies and important influences on surface-water resources resulting from water levels. large springs and streamflow-loss zones. Examination The Madison aquifer is identified as the primary of geochemical information provides a better under- source for several large artesian springs located within standing of the complex flow systems within these outcrop sections of the Minnelusa Formation, which aquifers and interactions between the aquifers. generally precludes artesian conditions within the Two main types of water exist within the Minnelusa aquifer. Precise quantification of relative Madison aquifer—calcium magnesium bicarbonate contributions from the Madison and Minnelusa aqui- type, which is expected in a carbonate aquifer; and cal- fers to artesian springs is complicated by numerous cium sodium chloride sulfate type, which is present factors; however, geochemical modeling by various mainly in the southwestern part of the study area. This

68 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota chemistry probably reflects the presence of more the Minnelusa aquifer, pH generally is lower at high evolved ground water and regional flow, or greater sulfate concentrations, which supports the occurrence amounts of evaporite minerals available for dissolu- of the dedolomitization reaction. In the Madison tion. aquifer, the data are consistent with dedolomitization, Three main types of water exist within the but pH trends are limited by the extent of anhydrite Minnelusa aquifer—calcium magnesium bicarbonate dissolution. type, calcium magnesium sulfate type, and calcium Ground water in both aquifers generally is well magnesium bicarbonate sulfate chloride type. Water in oxygenated at considerable distances from the outcrop the Minnelusa aquifer generally evolves downgradient areas because little organic material or reduced inor- from a calcium magnesium bicarbonate type to a ganic minerals are available for oxidation reactions. calcium magnesium sulfate type, due to dissolution of Reduction of sulfate, nitrate, and ferric iron minerals, anhydrite. In the southern part of the study area, methane fermentation, and anaerobic decay of organic ground water in the Minnelusa aquifer is characterized matter are therefore not likely in the Madison and by higher concentrations of sodium and chloride. The Minnelusa aquifers in the study area. high chloride concentrations in this area could reflect Isotopic information is used to evaluate ground- leakage between the aquifers, the dissolution of water flowpaths, ages, and mixing conditions for the evaporite minerals, or the presence of more evolved Madison and Minnelusa aquifers. Distinctive patterns ground water contributed by regional flow. exist in the distribution of stable isotopes of oxygen The most notable differences in major-ion chem- and hydrogen in precipitation for the Black Hills area, istry between the Madison and Minnelusa aquifers are with isotopically lighter precipitation generally occur- in concentrations of sulfate. Sulfate concentrations in ring at higher elevations and latitudes. A generalized the Minnelusa aquifer are dependent on the amount of distribution of δ18O (ratio of 18O/16O relative to a anhydrite present in the Minnelusa Formation. A tran- reference standard) for recharge areas is developed and sition zone where dissolution of anhydrite is actively used as a tracer for ground-water flowpaths. Distribu- occurring was inferred from lines of equal sulfate con- tions of δ18O in ground water are consistent with centrations in the range of 250 to 1,000 mg/L (milli- spatial patterns in recharge areas, with isotopically grams per liter). Upgradient from this zone, the 18 lighter δ O values in the Madison aquifer resulting anhydrite generally is not present because of earlier from generally higher elevation recharge sources, removal from the formation by dissolution. Down- relative to the Minnelusa aquifer. For some areas, gradient from this zone, thick anhydrite beds remain in dominant proportions of isotopically light streamflow the formation. recharge are identified, relative to recharge from Water chemistry for the Madison and Minnelusa aquifers is controlled by reactions among calcite, dolo- infiltration of precipitation on lower elevation outcrop mite, and anhydrite. Saturation indices for gypsum, areas. Both sources are important recharge mecha- calcite, and dolomite for most samples in both the nisms for the two aquifers. Madison and Minnelusa aquifers are indicative of the The radioisotope tritium is used to evaluate occurrence of dedolomitization. Because water in the mixing conditions and general ground-water ages. Madison aquifer remains undersaturated with respect Estimates of annual tritium concentrations in precipita- to gypsum, even at the highest sulfate concentrations, tion for the Black Hills are based on long-term records upward leakage has potential to drive increased disso- for Ottawa, Canada, and shorter records for Bismarck, lution of anhydrite in the Minnelusa Formation, espe- North Dakota, and Lincoln, Nebraska. cially where Minnelusa aquifer water is nearly Three conceptual models, which are simplifica- saturated with respect to gypsum. tions of lumped-parameter models, are considered for Forward geochemical modeling is used to illus- evaluation of ground-water flow and mixing condi- trate trends in pH and calcium and magnesium concen- tions. For a simple slug-flow model, which assumes no trations with increasing dissolution of anhydrite. mixing, tritium concentrations in ground water can be Comparison of actual calcium and magnesium concen- related through a first-order decay equation to esti- trations to model results indicates that if dedolomitiza- mated concentrations at the time of recharge. Two tion is occurring in both the Madison and Minnelusa simplified mixing models that assume equal propor- aquifers, conditions in the aquifers (temperature, Kdolo- tions of annual recharge over a range of years also are mite) are similar but not identical to modeled values. In considered. An “immediate-arrival” model is used to

Summary and Conclusions 69 conceptually represent conditions in outcrop areas and occurs from streamflow losses along Boxelder, Rapid, a “time-delay” model is used for locations removed and Spring Creeks, which have distinctively different 18 from outcrops, where delay times for earliest arrival of δ O signatures. A distinct division of lighter isotopic ground water generally would be expected. Decay values to the north and heavier values to the south curve families with incremental delay times are used occurs in the Madison aquifer along Rapid Creek. A for evaluating approximate age ranges for these less distinctive gradation occurs for the Minnelusa conceptual mixing applications. aquifer, which probably reflects larger influence from Limitations for use of the simplified, conceptual precipitation recharge. models include uncertainties in estimated tritium input Dye testing has confirmed rapid ground-water (which may include precipitation and streamflow flow (timeframe of weeks) from a loss zone in Box- recharge) and gross assumptions regarding equal elder Creek to City Springs and several wells located annual recharge and thorough mixing conditions. A several miles downgradient in northwestern Rapid major limitation results from highly heterogeneous City. Tritium concentrations were very low (about 4 to aquifer properties that commonly may be dominated by 5 tritium units) for two sites with prompt dye arrival, dual-porosity hydraulic characteristics, consisting of dramatically demonstrating the limitations of the con- high-porosity secondary openings within a low- ceptual mixing models, which were unable to provide porosity aquifer matrix. For this setting, a wide variety compatible age estimates. The combination of tritium of mixing conditions could occur, including relatively and dye data indicated unequal mixing of about 5 to old water from the aquifer matrix mixing with various 10 percent very recent water and 90 to 95 percent pre- and transient proportions of modern water flowing in bomb water, which illustrates the complex mixing near slug-flow conditions. Because of these limita- conditions that can occur in a dual-porosity system. δ18 tions, the conceptual models are used only for general Limited time-series O data for these sites showed evaluation of mixing conditions and approximation of minimal response to temporal variations in Boxelder age ranges. Creek, which is consistent with a dominant proportion of pre-bomb water. Several other sites with dye Despite limitations, the general applicability of recovery had higher tritium concentrations, reflecting arrival times associated with the conceptual models is 18 larger proportions of modern water, with δ O data for demonstrated by the distribution of tritium concentra- these sites showing larger response to temporal variations for several groupings of hydrogeologic situations. tions in Boxelder Creek. Headwater springs, which are located in or near out- 18 Time-series δ O and tritium data provide useful crop areas, have the highest tritium concentrations, information regarding mixing conditions and general which is consistent with the immediate-arrival mixing ages for numerous other sampling sites in the Rapid model. Tritium concentrations for many wells are very 18 City area. Sites with minimal variability in δ O values low, or nondetectable, indicating general applicability generally have tritium data indicative of dominant of the time-delay conceptual model for locations proportions of pre-bomb water, reflecting generally beyond outcrop areas, where artesian conditions gener- thorough mixing conditions. A number of sites, how- 18 ally occur. Concentrations for artesian springs gener- ever, showed response to temporal δ O trends in ally are higher than for wells, which indicates generally streamflow recharge, with associated tritium data shorter delay times resulting from preferential flow- generally indicating relatively large proportions of paths that typically are associated with artesian springs. modern recharge. Several large Madison production The general applicability of mixing models is sup- wells located near the Rapid Creek transition zone had 18 ported by the absence of extremely high tritium values, changes in δ O values indicative of changes in capture which would be indicative of near slug-flow condi- zones associated with recent production. tions, with recharge centered around periods of high Evaluation of major-ion and isotope data indi- tritium concentrations that occurred near the 1963 cates that regional flowpaths for the Madison aquifer tritium peak. Exceptions to this generality cannot be are essentially deflected around the study area, with the discounted, however. possible exception of the southwestern and north- Extensive isotopic data sets are available for western corners. Two wells just north of the study area evaluation of mixing conditions and general ground- clearly show influence of regional flow and a well just water ages in the Rapid City area, where large recharge within the study area shows possible influence. Large

70 Geochemistry of the Madison and Minnelusa Aquifers in the Black Hills Area, South Dakota artesian springs near the northern axis of the uplift minerals by water from the Madison aquifer. Various show no regional influence and are concluded to be investigators have hypothesized that the Madison recharged within the uplift area. aquifer is the primary source for many artesian springs, Major-ion concentrations for wells just west of based on geochemical modeling, which is consistent the study area in Wyoming indicate deflection of with generally higher hydraulic head in the Madison regional flowpaths; however, minor influence may be aquifer, relative to the Minnelusa aquifer. possible for the most westerly wells considered. High Generally higher hydraulic head in the Madison ion concentrations for several wells in the southwestern aquifer, in combination with gypsum undersaturation, corner of the study area indicate possible regional is concluded to be a primary mechanism driving inter- influence, but cannot necessarily be distinguished from actions with the Minnelusa aquifer, in areas where basinward increases in constituent concentrations. artesian conditions exist. Upward leakage from the North of this area, regional influence probably is minor Madison aquifer probably contributes to general disso- δ18 or negligible. The O values for large springs along lution of anhydrite deposits and development of the southern axis of the uplift essentially preclude breccia pipes in the Minnelusa aquifer. Breccia devel- regional influence, which is supported by ion chem- opment may be especially prevalent in locations where istry. Low, but detectable, tritium concentrations in the competency of intervening confining layers has these springs confirm the influence of recharge from been decreased by fracturing or by depositional influ- within the study area, but indicate relatively long ences in the Minnelusa Formation resulting from δ18 traveltimes. This is consistent with the O values, paleo-karstification of the Madison Limestone. which indicate potential recharge areas extending Development of breccia pipes probably contrib- along the entire southwestern flank of the uplift. The 18 utes to enhanced vertical hydraulic conductivity in the δ O value for Beaver Creek Spring is much lighter Minnelusa aquifer. Breccia pipes are a likely mecha- than estimated values for nearby outcrop areas and nism for upward movement of large quantities of water nearby wells, which indicates a possible flowpath through the Minnelusa aquifer at artesian spring loca- extending from the general Hot Springs area. This tions and many exposed breccia pipes of the upper flowpath to Beaver Creek Spring also is supported by a Minnelusa Formation probably are the throats of aban- low tritium value indicating generally long traveltimes. doned artesian springs. Dissolution processes are an Potential interactions between the Madison and important factor in a self-perpetuating process associ- Minnelusa aquifer are examined using hydraulic and ated with development of preferential flowpaths and geochemical information for well pairs and artesian artesian springs. Preferential flowpaths initially springs. Hydrographs for 9 of 13 well pairs are fairly develop in locations with large secondary porosity and well separated and do not indicate direct hydraulic associated hydraulic conductivity, with ongoing connection between the aquifers. Although some enhancement resulting from dissolution activity. exchange of water must occur in locations where hydraulic head differences occur, conclusive geochem- Outward (downgradient) migration of the arte- ical evidence of extensive mixing resulting from sian springs probably occurs as upgradient spring- general, areal leakage between the aquifers is not discharge points are abandoned and new ones are occu- apparent. pied, keeping pace with regional erosion over geologic The Madison aquifer has been positively identi- time. In response to this outward migration, hydraulic fied from dye testing/recovery as a source for City heads in the Madison and Minnelusa aquifers also have Springs and is concluded to be a primary source for declined over geologic time. several artesian springs (Cleghorn/Jackson and Hot Artesian springflow and general leakage are con- Brook Springs) where artesian conditions in the cluded to be important factors in governing hydraulic Minnelusa aquifer are precluded by nearby outcrop characteristics and water levels in the Madison and sections. Contributions from these aquifers cannot Minnelusa aquifers. Artesian springflow, which repre- necessarily be quantified for other artesian springs sents about one-half of average recharge to the because of geochemical similarities between the Madison and Minnelusa aquifers, is especially impor- Madison and Minnelusa aquifers. For some springs, tant. Artesian springs act as a relief mechanism that high sulfate concentrations indicate Minnelusa influ- provide an upper limit for hydraulic head, with spring- ence, but may result from dissolution of Minnelusa flow increasing in response to increasing water levels.

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SUPPLEMENTAL INFORMATION

GRAPH A - CURVES FOR 1978 SAMPLING DATE 10,000 0 year 5,000 4 year 8 year 2,000 12 year 16 year 1,000 20 year 24 year 500 28 year

200

100

2 TRITIUM CONCENTRATION, IN TRITIUM UNITS IN TRITIUM CONCENTRATION, 1 1800 1850 1900 1950 2000 YEAR

GRAPH B - CURVES FOR 1989 SAMPLING DATE 1,000 700 0 year 500 4 year 400 8 year 300 12 year 16 year 200 20 year 24 year 100 28 year 32 year 70 36 year 50 40 30 20