NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCES
White Paper: A Summary of the Hydrogeology of the San Agustin Plains, New Mexico
Alex Rinehart Daniel J. Koning Stacy Timmons Open-File Report 615 November 2020 New Mexico Bureau of Geology and Mineral Resources
A Research Division of New Mexico Institute of Mining and Technology
Socorro, NM 87801 (575) 835-5490 www.geoinfo.nmt.edu White Paper: A Summary of the Hydrogeology of the San Agustin Plains, New Mexico
Alex Rinehart Daniel J. Koning Stacy Timmons
Open-File Report 615 November 2020
New Mexico Bureau of Geology and Mineral Resources PROJECT FUNDING
Funding for this project has been collaborative, with support from NM Bureau of Geology and Aquifer Mapping Program, Healy Foundation, Office of the State Engineer, USGS National Cooperative Geologic Mapping Program (STATEMAP), and private donations from community members in the San Agustin Plains.
DISCLAIMER
The reports and data provided here are intended to aid in the understanding of the geologic and hydrologic resources of New Mexico. However, there are limitations for all data, particularly when subsurface interpretation is performed, or when data are aggregated that may have been collected at different times, by different agencies or people, and for differ- ent purposes. The information and results provided are also dynamic and may change over time. Users of these data and interpretations should exercise caution, and site-specific conditions should always be verified. These materials are not to be used for legally binding decisions. Any opinions expressed do not necessarily reflect the official position of the New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, or the State of New Mexico.
Although every effort is made to present current and accurate information, data is provided without guarantee of any kind. The data are provided “as is,” and the New Mexico Bureau of Geology and Mineral Resources assumes no responsibility for errors or omissions. No warranty, expressed or implied, is made regarding the accuracy or utility of the data for general or scientific purposes. The user assumes the entire risk associated with the use of these data. The New Mexico Bureau of Geology and Mineral Resources shall not be held liable for any use or misuse of the data described and/or contained herein. The user bears all responsibility in determining whether these data are fit for the user’s intended use. The views and conclusions are those of the authors, and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the State of New Mexico.
Cover: A windmill in the Western Basin of the San Agustin Plains.—Photo by Melissa Brett HYDROGEOLOGY OF THE PLAINS OF SAN AGUSTIN
CONTENTS
Executive Summary ...... 1
Introduction ...... 4 Geology ...... 6 Wells and groundwater levels...... 7 Water quality and groundwater ages...... 10 Summary of conceptual model...... 14
References ...... 18 Figures 1. Overview map of the San Agustin Plains ...... 3 2. Conceptual figure summarizing geologic controls on groundwater ...... 5 3. Depiction of the three-dimensional structure of the San Agustin Plains...... 7 4. Histogram of water level change rates (ft/yr), with positive values...... 8 5. Water-table elevations for San Agustin Plains ...... 9 6. Map showing the total dissolved solids (parts per million)...... 11 7. Map of fluoride concentrations...... 12 8. Map of regions used to analyze precipitation and chloride...... 13 14 9. Map of apparent C age...... 15 10. Isotopic concentrations of groundwater compared to meteoric waters...... 16 11. Plot of δD (per mil) against apparent 14 C age (years) for San Agustin Plains...... 16 HYDROGEOLOGY OF THE PLAINS OF SAN AGUSTIN
EXECUTIVE SUMMARY
ver the last several years, the New Mexico Bureau of Geology and Mineral Resources O(NMBGMR), Aquifer Mapping Program, has been working on numerous research aspects of the geology and hydrology of the San Agustin Plains, New Mexico. This White Paper, Open-File Report 615, provides a detailed overview and summary of the results from this work, and will be followed by a comprehensive, peer-reviewed report on these results, with data, in the coming months. Major findings include:
• The San Agustin Plains consists of two distinct bedrock-floored, sediment-filled basins termed the East and West basins. Within each basin, groundwater flow is constrained by the underlying geologic structure. Of particular importance are areas where bedrock has been down-dropped along faults to form four distinct grabens: Horse Spring graben, North graben, C-N graben and White Lake graben. • The surrounding mountains are made of layered volcanic rocks and sedimentary rocks made of eroded material from volcanoes. The type of rock controls how water moves through the mountain blocks. Some of the volcanic layers are highly fractured and transmit water more easily. Other volcanic layers and the majority of the sedimentary rocks are low permeability, thus not allowing water to move readily through the rock. • The bedrock basin of the San Agustin Plains has been progressively filled with sediment over the past 25 million years. The sediment is mainly composed of sand and mud (silt-clay), with gravelly sand found near hills and mountains. The type of sediment deposited is a function of the paleo-environments of deposition. Piedmont deposits (including alluvial fans), near the edges of the basin, are composed of the coarsest sediment, whereas basin floor deposits (or alluvial flat) contain finer sediment, and sediment found in paleo-playa lake areas, typically near the middle of the basin floor, is the finest. • Because the structural grabens have been continuously filling with sediment containing notable but variable proportions of mud, the San Agustin Plains is highly susceptible to groundwater- withdrawal-driven subsidence. • Groundwater levels are nearly flat across most of the San Agustin Plains. The groundwater flow direction is generally west from the C-N graben through the West basin. At the southwestern end of the West basin, flow direction turns sharply south and is inferred to flow toward the Gila River watershed. • Groundwater levels have been slowly declining (less than 2 in/year) throughout the central part of the San Agustin Plains over the last 30 years. This effect is best documented with the NMBGMR measurements from 2007 to 2017. • The groundwater in the tributary valleys at the margins of the San Agustin Plains, and within the Plains, has low concentrations of total dissolved solids (TDS) in general, including low concentra- tions of arsenic and uranium (which are common naturally occurring water contaminants in New Mexico). Poorer water quality was found in some wells completed in tight bedrock and in wells under the playas in the western end of the basin. • Groundwater recharge enters the basin at depth through fractured volcanic rocks called tuffs, and through shallow alluvial aquifers in tributary valleys at the margins of the basin.
1 NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCES: OFR 615
• Recharge rate, defined as the ratio of water that enters the aquifer to amount of precipitation, is low (0.3 in/yr compared to 14.6 in/yr of precipitation), according to chloride mass-balance calculations. Recharge mainly originates in the mountain blocks and tributary valleys at the margins of the Plains. • Mountain block recharge into the aquifers hosted in volcanic rocks is likely structurally con- trolled and likely occurs less than 5 miles from the margin of the basin. Focused recharge occurs along the length of the tributary valleys during flow events; focused recharge is a mixture of recent waters and older waters. • Timescales for recharge are long. From the time that rain falls, and is focused through tributary valleys, it takes, on average, between 1,000 years and 4,000 years for that water to reach the aquifer. Water that percolates through the mountain-block fractured volcanic rocks (tuff) takes even longer to reach the aquifer, taking between 9,000 and 14,000 years. The majority of groundwater shows ages consistent with mountain block recharge, indicating that • Mountain block recharge is likely greater than focused recharge through tributary valleys. • It has taken 10,000 years for water to travel the five to ten miles from the mountain block into the basin aquifer. • The current amount of water use slightly exceeds the recharge rate of the basin, based on the following points: • The long time needed for water to move through the basin, as indicated by low water table gradients. • The relatively low flow rates of recharge, as indicated by long travel times over short distances. • The small proportion of water that become groundwater in the recharge area. • The slow lowering of the water table.
2 HYDROGEOLOGY OF THE PLAINS OF SAN AGUSTIN
180,000 220,000 260,000 300,000
N e w M e x i c o Datil Santa Fe" Mountains Gallinas Mountains " North Datil Albuquerque a e " 169 gra en ³ ± Crosby Datil East San Mountain Agustin Basin "Las Cruces Magdalena 3,780,000 N " ¡¢60 Magdalena Mountains I Mangas Very 12 T
Mountain ³ ± Horse Large S Array hite a e 34°N Mountain U gra en G 52 ³ ± A P 549 o ³ ± gra en in N C t a o A n f -N y R S McClure Hills o n o c F San k O s M Luera il 107 li ³ ± S orse Springs gra en Mateo Mountains g Mountain n a o N CATRON n I y West San Agustin Basin 163 SOCORROn n ³ ± o A a ny G C a u L C l A r r c a e h P l t a e
a 3,740,000
m B w e Tularosa Mountains it n o o Pelona h y s n W a Mountain a C C n te r a O Bar O e Y U e Mountain k A im E S T Monticello e A rs L o n h o Box P d y Upper il n Alamosa N S a i W C O lv Basin L Black Rangeer Cre 33°30'N L ek O 59 G ³ ± A la S I E R R A O 52 m / ³ ± o M s 0 9.5 20 Mi a Monticello C 3,700,000 r Placitas 25 Elephant e e Butte Winston k 0 9.5 20 Km Reservoir 108°30'W 108°W 107°30'W D Vents Selected caldera margins ! Proposed development wells
! Figure 1. Overview map of the San Agustin Plains. NMOSE permitted wells
3 NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCES: OFR 615
INTRODUCTION
he San Agustin Plains are a large, internally from windmills and solar wells, or from stock tanks Tdrained, hydrologically closed basin in west-central filled during spring runoff or summer monsoons. New Mexico, approximately 120 miles southwest of The USGS undertook geophysical surveys and water Albuquerque, NM and 60 miles west of Socorro, NM quality sampling that delineated four grabens (i.e., (Fig. 1). Low hills extending south of Datil (infor- areas of down-dropped bedrock in the subsurface, mally named the McClure Hills on Figure 1) separate corresponding with thickened basin-fill sediment the Plains into a west and an east topographic basin. aquifers), and found good water quality away from The Plains are sparsely populated, and the largest the playas in the Plains. Both studies indicated that settlement is the community of Datil, NM. Scattered, groundwater generally flowed southwesterly along the large, cattle ranches use the vast grasslands of the axis of the Plains and likely exited the southwestern Plains, and the Very Large Array (VLA) is found in end of the West basin by flowing southwards. Rates the eastern Plains. The San Agustin Plains are sur- of recharge, recharge pathways, the stability of water rounded by a series of ponderosa pine, pinon,˜ and levels, and the likely discharge pathways were not juniper covered mountains, the majority of which analyzed, though the authors indicated possibilities. rise 1,500 ft above the Plains. However, San Mateo The USGS study speculated that there was significant Peak, Mangas Peak and John Kerr Peak all reach up flow from the Eastern San Agustin Plains south into to 10,000 ft in elevation, towering above the 7,000 Alamosa Creek. ft-high Plains. Only two paved roads cut across the basin, US Highway 60 and State Route 12. The San Our study addressed several questions in an effort Agustin Plains groundwater table is above the water to better understand the water resources of the San tables in the surrounding basins that drain to the Agustin Plains: Rio Grande and Gila River, raising the possibility of natural flow of water from the Plains into • What is the subsurface bedrock structure of the these watersheds. San Agustin Plains and does it affect the storage For the last 13 years, the Plains have been the site and flow of groundwater? The East and West of a confrontation over water use and availability. topographic basins are underlain by four distinct In 2004, the Augustin Plains Ranch LLC submitted a grabens. These grabens do not always correspond permit application to the New Mexico Office of the to the topographic surface and are likely poorly State Engineer to pump 54,000 acre-feet of water per hydrologically connected at depth; the degree year—about 17% of the total water used for public of connection is difficult to assess due to the supply across New Mexico in 2015—from their chemical similarity of recharging waters property in the northwestern edge of the throughout the basin. San Agustin Plains (Fig. 1) The initial application • How stable are current water levels in the San and the three following applications were denied by Agustin Plains? Water levels are slowly declining the New Mexico Office of the State (NMOSE) and (<2 inches/year decline) throughout the basin. the courts. Current water use exceeds the current recharge Because of its sparse population and remoteness, rates, even along the edges of the basin where there has been little research on San Agustin Plains recharge and aquifer permeabilities are greatest. hydrogeology. Two previous studies include one in the • How is the groundwater recharged and how 1970s by Blodgett and Titus, and one by the USGS long does recharge take to enter the basin? in the early 1990s led by Robert Myers. Both studies Groundwater recharges the basin through noted the relatively flat groundwater table along relatively thin, fractured, but widespread volcanic the center of the basin. With few perennial springs, ash rocks (tuffs) in the mountain block; and as livestock, and wildlife in the region are often watered focused recharge in shallow alluvial aquifers in
4 HYDROGEOLOGY OF THE PLAINS OF SAN AGUSTIN
re e e e re e e e re e non-porous and low permea ility porous and low permea ility porous and permea le Recharge
ocu sed rec harge ( i 1-5 yr)
Basin fill
) Mountain block recharge (10-15 kyr
Tuff olcaniclastic deposits Basin fill Bedroc
ractured y cooling ractured y faulting
Figure 2. Summary conceptual figure of geologic controls on groundwater flow from the mountains surrounding the SanAgustin Plains into the basin itself. Includes depictions of porosity and conductive units, tight units, flowpaths and the length of time of each recharge flowpath.
5 NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCES: OFR 615
perched tributary valley aquifers. Mountain-block allows water to migrate, or be stored in “fractured” recharge takes between 9,000 and 14,000 years aquifers. However, a given tuff becomes thinner and to enter the basin, and focused recharge takes cooler farther away from its source caldera, resulting 1,000 to 4,000 years to enter the basin, depend- in less fracturing. The unfractured volcanic rocks ing on location. are barriers to groundwater flow—they often do not • Is the basin susceptible to groundwater-driven have connected pores to allow water flow (Fig. 2). subsidence? Yes. The geology of this basin is Volcaniclastic deposits (sedimentary rocks derived similar to that of the Mimbres basin in New from volcanoes) in the San Agustin Plains region Mexico, where earth fissures have opened due to typically are poorly sorted, strongly cemented, and pumping-induced subsidence. have a muddy matrix, resulting in low permeability for • Is the basin connected to the Rio Grande basin? groundwater flow (Fig. 2). Locally, the permeability Not on human time scales, if at all. The waters of volcaniclastic rocks has been further reduced by in Alamosa Creek to the south of the East Basin geothermal alteration, where sand grains are dissolved are chemically and isotopically distinct, and there and then re-precipitated as pore-clogging minerals. An is evidence of a barrier to flow due east from the exception to the generally low-permeability nature of East Basin is unlikely due to structural controls. volcaniclastic deposits are inter-layered, well-sorted sandstones originally deposited in wind-blown dune fields that periodically blanketed the western part of the San Agustin Plains region around 35 million Geology years ago. Beginning about 36 million years ago, tectonic The geology of the San Agustin Plains can be divided extension occurred in this area; extension rates into bedrock and basin fill components (Fig. 2). Both increased around 20–25 million years ago to form the are important in understanding the groundwater Rio Grande rift to the east and the Basin-and-Range to flow. The groundwater system mainly recharges in the the south and west. The San Agustin Plains region was bedrock of the surrounding mountains but eventually stretched between these two extensional regions, and flows into the weakly to moderately consolidated was very slowly (rates of < 0.3 inch/yr) pulled apart. sand, silt, clay, and gravel that make up the basin fill During this extension, four large crustal blocks of the (Fig. 2). The mountains around the San Agustin Plains basin gradually subsided alongside fault lines, forming are composed of volcanic rocks that are 38 to 24 what geologists call grabens. These fault-bounded million years old. These volcanic rocks include lavas grabens are important because they host thicker, more from ancient volcanic vents, tuffs formed by harden- permeable sedimentary basin-fill deposits. ing of volcanic ash and pumice, and volcaniclastic Interpretation of gravity data and resistivity rocks (sandstone, muddy sandstone, and conglomer- soundings (collected in previous studies) reveal that ate) eroded from the volcanic vents shortly after their underneath the relatively flat land surface of the San eruptions. Some volcanic eruptions resulted in several Agustin Plains are four separate grabens (Figs. 1 mile-wide, elliptical craters called calderas. Caldera and 3): the Horse-Springs graben, the C-N graben, eruptions commonly produced widespread tuffs that the North graben and the White Lake graben. thicken towards the source caldera, and within the As the faulting occurred and the grabens subsided, source calderas these tuffs can be several thousand they were also filling with sediment eroded from feet thick. Fracturing can accompany cooling of these the surrounding uplands. The sediment was largely tuffs, particularly if they were initially very hot and deposited in piedmont environments (sloping surfaces thick, forming extensive columnar structures extending from adjoining uplands to the centers of (Fig. 2). In the San Agustin Plains, fractured tuffs basins), alluvial flats (centers of basins that slope very include the Hell’s Mesa Tuff, the La Jencia Tuff gently towards playas), and playas (or lakes if water and the Vicks Peak Tuff. Fracturing associated with was retained perennially). This interpretation is based faulting may provide a "fast-path" from the surface off of a handful of well logs and cuttings from the into the fractured bedrock (Fig. 2). eastern San Agustin Plains, a deep core in the western The various volcanic rocks have physical char- San Agustin Plains and interpretation of modern acteristics that impart unique hydrologic properties. landforms within the San Agustin Plains that have In the tuffs, fracture networks can persist for tens existed for tens of thousands of years. Through erosion of miles. These fractures provide open space that and weathering of the volcanic and volcaniclastic rocks
6 HYDROGEOLOGY OF THE PLAINS OF SAN AGUSTIN
to Pie Town
est Basin
Datil Mou ntain untain s a Mo Luer Datil
ast Basin Alamosa Creek o Mountains VLA Mate San s tain Moun Gallinas
to Magdalena
Figure 3. Depiction of the three-dimensional structure of the top-of-bedrock in the the East and West basins also showing the Horse Springs graben (HSG), the C-N graben (CNG), the White Lake graben (WLG) and North graben (NG). View facing west. The dotted lines are faults.
of the mountains, the deposits are mixtures of gravel, end of the West basin and the lowest part of the sand and clay in the piedmont close to the mountain C-N graben. front and grade to deposits of finer sand, silt, and The growth and retraction of the old lakes are clay toward the center of the basin (in alluvial flats). testament to the effects of paleoclimate change. In In basins similar to the San Agustin Plains, there are addition, during the last Ice Age there were rock gla- often ribbon-like, higher-permeability channel sand ciers, or bodies of mixed rock and ice that persisted deposits embedded into low-permeability, clay-rich from decade to decade, in the San Mateo Mountains, sediment. Because the Plains have been continu- indicating a colder and wetter climate. Climate has ally deepening and filling, the sediments are at the been roughly constant and warm from 6,000 years maximum depth-of-burial, meaning that with further ago until today. These climate periods are important burial or increase of pumping of water from the because our groundwater age measurements span aquifer (water pressure pushes the solid grains apart, 20,000 years ago to 1,000 years, and if the climate so decreasing the water pressure would cause the has changed significantly over that time then this also grains to come together), we expect the San Agustin has impacts on recharge and groundwater movement. Plains sediments to consolidate and subside; this is a common phenomenon in other closed basins with similar bedrock surrounding them. Wells and Groundwater Levels During the last Ice Age, the western San Agustin Plains and the C-N graben were filled with a lake. Before the 1880s, the San Agustin Plains were largely The North graben and White Lake graben were not in Apache territory and did not have a European filled with a lake but instead had alluvial fans and population. After the cessation of the Apache Wars wetlands. The lake reached its maximum extent at the San Agustin Plains region was only sparsely 18,000 to 20,000 years ago and disappeared approxi- settled because of its remoteness and lack of reliable mately 10,000 to 6,000 years ago, after which there surface water. Initially the area was mostly used for were intermittent playas that filled during the wetter sheep ranching, but during the latest 19th century seasons. These playas are located in the western through the early 20th century, cattle ranching
7 NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCES: OFR 615 became dominant. Small capacity (10–100 gallons per minute) wells were completed for domestic and livestock use. In the late 20th century through today, 50 a few high capacity (800 gpm) wells have been completed to irrigate the small sections (less than 1 40 square mile) of farmland in the basin. Most wells have been completed in basin-fill sediment Water levels used in the study by Blodgett and 30 Titus, and the USGS study, provide reliable one-to- Num er of wells (-) two year snapshots of depth to water. However, the 0 low gradient of the groundwater table in the San Agustin Plains makes such short-period snapshots subject to artifacts such as drawdown from pumping. 10 Since 2007, the New Mexico Bureau of Geology conducted annual winter water-level measurements to 0 capture the non-growing or non-pumping season. We -7.5 -5 - .5 0 .5 5 7.5 have analyzed the data collected between 2007 and Rate of increase of water ta le depth (ft year) 2017, focusing on the median water levels and the average changes in water level. Figure 4. Histogram of water-level change rates (ft/yr), with positive When we compare water levels from 2007 and values indicating an increasing depth-to-water below ground surface 2017 in each well, more the 80% of wells show little (lowering of groundwater table), between 2007 and 2017. change (less than 2 in/year; Fig. 4). However, 2/3 of those wells with little change show a slight (< 2 in/ graben appears essentially flat. There is less obvious year) but persistent lowering of water levels over 10 correspondence in the White Lake graben, where years. These stable, but declining, water levels appear groundwater table appears to be flat or slopes slightly throughout the San Agustin Plains. Tributary valleys to the west. The C-N graben has a clear amphitheater- show more variability than wells in the basin, but still like shape, with flow focused from the south, north show a slight downward trend. Depths to water range and east towards the small break in bedrock between from nearly 500 ft at the edge of the North graben, the C-N graben and the Horse Springs graben. In the to 30 ft underneath the playas in the West graben. Horse Springs graben, there is a general flow to the A gradual shallowing of groundwater is observed southwest with water draining into the basin from between the C-N graben, where water levels are all flanks. The water table shallows until it reaches approximately 200 ft below ground surface, to the the playas in the Horse Springs graben, where it is West basin playas. approximately 30–50 ft below the ground surface. Water table elevation maps show that ground- From the playa deposits, there is a drop in water water is flowing generally to the southwest (Fig. 5). table elevations to the south. This shows that the San Given the slow rates of change and the generally Agustin Plains may be draining slowly to the south. low-sloping water table, we have used the median This same pattern has been observed in every study of water-table elevation measurement from between the San Agustin Plains. 2007 and 2017 for our analysis. This was done to Even gentle slopes in water-table elevations avoid spurious estimates, such as some of those seen show that there is connection between grabens; year-to-year and in previous work, and to provide moreover, groundwater flow rates are proportional confidence in the water table elevation maps given the to the water-table gradient. The gradient from the spatial paucity of data—wells are far apart so more C-N graben to the center of the West basin playa is measurements are needed to be more confident in approximately 2.5 ft/mile, or half of the slope of the the estimate. Rio Grande from Cochiti Pueblo, NM to Belen, NM. The water table elevation contours were drawn Thus, we infer that groundwater flow within the San independent of the structural geologic contours Agustin Plains is very slow. Looking south from the (i.e., top-of-bedrock contours) used to make the San Agustin Plains into the upper Alamosa Creek, geologic model of Figure 3. The boundaries between there is a 500 ft drop in the water table over a mile. different grabens are apparent in the water table Sharp drops such as this exist only where there is a elevation contours. The water table within the North
8 HYDROGEOLOGY OF THE PLAINS OF SAN AGUSTIN
DDD D 180,000 205,000 230,000 255,000 280,000 305,000 D Water Table Elevation since 2007
Gallinas D Datil 6799 Mountains (feet above mean sea level)
6796 3,800,000 D Mountains 7163 8092 6800 7906 D 6799 / 7764 6795 5,000 5,500 6800 169 6800 ³ ±
7574 5,500 ,000 7468 6795 East San 7645 7107 6806 34°10'N Agustin Basin 6800 ,000 ,500 Datil 6801 D " 6 6802 Crosby 6799 800 6800 D 7163 7183 Mountain 7401 ,500 7,000 7324 6801 Magdalena 7295 7223 s n i a t n u o M a n e l " a d g a M 6799 7032 6805 6970 6799 7,000 7,500 6798 "Very D 7008 6768 ¡¢60 7335 7033 Large 3,775,000 D 6802 D Mangas 7008 6798 Array 7,500 8,000 6796 6797 6801 Mountain 6769 6771 6801 6799 D 6773 6797 6799 549 8,000 8,500 Horse 6790 ³ ± D D 6765 6770 D Mountain 6787 6761 6 D 77 6 6933 6763 0 7 7480 D 8 6794 52 ³ ± 6872 6760 0 6795 D I N Alamosa ree 50 ft contour 12 T 6789 6788 ³ ± 6759 6791 D 6849 S 6758 6780 U D 6759 107 G 6765 ³ ± 50 ft contour 6751 s n i a t n u o M o e t a M n a S A 6765 D 6471 6739 N SA 6760 10 ft contour 6754 F 6789 6790 S O 6787 D 3,750,000 D AIN k D PL 6757 6790 e Luera 6 00 e 8 r West San 6750 6751 D Selected Caldera Margins Mountain C M i 6740 a ll Agustin Basin s ig o D a
m n 33°45'N 30 7 a G 6 163 7113 l ³ ± 6854 u 6735 A 6733 lc 7045 6740 h Vents Tularosa Mountains 6730 D Pelona D MountainD U D A 6409 E 6462 T 6322 O Bar O A D P L 6263 D Mountain N 6391 D D D 6360 L LO 7099 6259 3,725,000 D O 6230 G 6389 6499 O 6489 M 6644 6496 5940 5810 D 7058 6363 6897 6454 D 6496 6905 D 6466 D A 7229 la 59 m 5480 ³ ± 52
³ ± o s D 6536 a 5327 D D C D r ¨ e "
D e 3,700,000 D k 5216 Monticello 5187 25 0 5 10 15 Mi Black Range Winston " 5185 33°20'N 0 5 10 15 Km 108°20'W 107°55'W 107°30'W D 107°5'W D Figure 5. Water-table elevations for San Agustin Plains and surrounding basins based on median water levels measured between 2007 and 2017. Mapped volcanic vents and selected caldera margins are shown.
9 NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCES: OFR 615 nearly complete barrier to flow. Thus, we interpret a north, rather than northward transport of fluoride groundwater barrier in the topographically low area from waters recharged through volcanic sources. between the C-N graben and upper Alamosa Creek. Instead, the elevated F on the southern edge of the Such a barrier would coincide with thin, unsaturated Plains indicates northward groundwater flow into basin fill and the presence of intersecting caldera the Plains. margins. The initial high degree of fracturing expected We estimated the recharge rate, or the rate of con- along these intersecting margins may have promoted version of rainfall and streamflow into groundwater, upwelling of hot fluids and precipitation of using a chloride mass balance that uses the modern pore-filling minerals, such as quartz or opal, precipitation rate and chloride deposition rate to find which reduced permeability. an chloride concentration of initial infiltrated water, and the chloride concentration in the groundwater, which is assumed to have been concentrated by Water Quality and Groundwater Ages evapotranspiration. We found a paleo-recharge rate of about 0.3 in/yr, using a precipitation rate of 14.6 The overall water quality in the San Agustin Plains in/yr [i.e., the rate between 2000 and 2018 (Fig. 8)]. is good. It has low total dissolved solids (Fig. 6) and The full range is from <0.05 in/yr to approximately specific conductivity, neutral pH, few wells with 4 in/yr. The higher values are found either in warm constituents at or above EPA drinking water guide- wells or in areas of intense focused recharge (i.e., near lines, and generally moderate temperatures. Some arroyo bottoms). These rates estimate the amount wells near the graben boundaries have warm (>75°F) of water converted from rainfall to groundwater, temperatures, indicating deep, warmer flow exiting assuming that the remainder is evaporated, thereby from the mountain block. Waters are carbonate rich concentrating chloride. They do not tell us the rate of and range from calcium rich to sodium rich, with transport of recharge into the basin. younger waters having higher proportions of calcium To understand how long it takes for recharge and older waters having higher proportions of to enter the basin, we sampled for and analyzed the sodium. Overall, however, the quality of groundwater stable isotopic composition and groundwater age of a in the San Agustin Plains is remarkably uniform, with subset of the wells. We found that groundwater in the outliers existing near the playa deposits in the San Agustin Plains falls into three groups: very old, western basin and in wells that are completed in stagnant waters in bedrock; groundwater between warm bedrock. 9,000 and 18,000 years old according to carbon-14 Previous workers hypothesized significant flow age dating; and groundwater between 1,000 and leaving the basin under the mountains on the central 6,000 years old according to carbon-14 age dating southern rim of the West basin. We find this generally (Fig. 9). A handful of samples exhibit ages that unlikely based on the composition of groundwater, demonstrate mixing of older water and younger water specifically with respect to fluoride (F). The argument in the basin. Groundwater at the outlet of tributary hinges on a generally decreasing trend of F moving valleys have the younger ages (e.g., the canyon east north from those mountains into the basin (Fig. 7). of Datil), while groundwater near the edge of moun- The southern rim of the San Agustin Plains hosts the tains and close to faults is older. large calderas and more recent, smaller shield volca- Isotopically, all of the sampled groundwater falls noes; further south are the large caldera complexes within the range expected for rain and snowfall. that form the Gila Wilderness. These volcanic rocks There are no outliers from the local meteoric waters along the southern rim of the basin contain relatively (rain and snow). This implies both that waters have high concentrations of F, which is highly soluble. not significantly evaporated and that they have not Groundwater recharge through these rocks will have undergone intensive water-rock interactions (i.e., an elevated concentration of F compared to recharge moved through and dissolved local bedrock). Modern away from volcanic sources—such as recharge along winter and spring—when most of the snowfall the northern edge of the Plains. We observe distinctly occurs—waters generally have δD less than 69 per higher concentrations of F along the southern edge of mil. Most groundwater in the San Agustin Plains falls the Plains compared to the groundwaters along the below this, showing that recharge largely occurred northern edge of the Plains. If there was significant during the cold seasons (Fig. 10). The lack of any southward flow, we would expect low concentrations evaporative trends, which appear as branching trends, of fluoride, as observed entering the Plains from the with lower slopes, off of the main meteoric water
10
HYDROGEOLOGY OF THE PLAINS OF SAN AGUSTIN
3,750,000 3,725,000 3,700,000 3,800,000 3,775,000 D " " D D D 25 D D 107 D 60 107°20'W D 280,000 169
S a n M a t e o M o untains ! ( 307 549 ! ( 192 207 ! ( 196 ! (
G a l l i n a s M ountains D D ! ( 531 487 535 ! ( D ! ( 231 ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( 518 ! ( ! ( 165 230 440 197 211 ! ( 220 ! ( ! ( ! ( ! ( 235 ! ( 255,000 469 ! ( ! ( 339 154 ! ( ! ( 267 219 166 52 241 156 181 ! ( D 253 152 ! ( ! ( 255 107°40'W 228 ! ( ! ( 1407 ! ( ! ( ! ( ! ( 253 359 ! ( 199 295 175 251 337 ! ( 450 249 210 180
! ( ! (
! (
319 N
Black Range Black
Datil I ! ( " "
! ( 192 299
T The recommendation for drinking water is less than 500 mg/L. 168 ! ( Hills McClure
! ( D 210 ! ( S ! (
269
59 a tD i l ntains u o M 315
U 234
! ( Luera 572
G Mountain 230,000 A 163
! ( Crosby N Mountain 196
! ( A D
! ( 108°W 271 S D D
328
! ( F ! ( D D
265
D O 207 Pelona ! ( Mountain
! ( S 441
183 N
205,000 I Agustin Plains and surrounding regions.
D
D A
! ( L 458
D Mangas P D
Mountain D ! ( D O BarO Mountain 195 D 108°20'W 12 D 180,000 D D D D D D D D D D D 10 Mi D D D D D D D D . D D D 108°40'W D D D D D 155,000 ) 0 0 10 Km D D D D D D D ppm D D D D D
D D D D D Vents Calderamargin 152–200 201–300 301–400 401–500 501–600 601–1407 D D D D Solids( D D D ! ( ! ( ! ( ! ( ! ( ! ( D Total DissolvedTotal D D
D
/
33°40'N 33°20'N 34°N 34°20'N D D D D D D D Figure 6. Map showing the total dissolved solids (parts per million) for San D D D
11
NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCES: OFR 615
3,800,000 3,725,000 3,775,000 3,750,000 D 3,700,000 " " D D D 25 D 107 D 169 D 60 107°20'W D 280,000 0.67 ! ( San Mateo Mountains 549 0.23 ! ( 0.5 0.45 ! ( 0.6 ! ( D D ! ( 2.8 Gallinas Mountains 2.8 0.23 0.33 0.6 ! ( D ! ( 0.6 1.6 ! ( ! ( ! ( 0.63 0.36 2.6 2.6 ! ( ! ( ! ( 0.72 ! ( 1.3 ! ( ! ( ! ( ! ( ! ( ! ( 0.34 0.72 0.22 0.3 ! ( 1.2 0.2 ! ( 2.2 ! ( ! ( 0.47 ! ( ! ( 2.2 ! ( 255,000 ! ( ! ( ! ( 52 ! ( 0.21 0.21 0.54 0.28 0.14 0.28 0.52 ! ( D ! ( ! ( 107°40'W ! ( 0.5 0.88 ! ( 0.22 ! ( ! ( ! ( ! ( ! ( 0.3 0.25 ! ( 1.4 0.6 1.4
0.22
! ( ! (
! ( 0.89
N
! ( Range Black " Datil
0.23 I " ! (
0.18
! ( T
0.67 Hills McClure ! (
1.2 D ! ( ! ( 0.62 S
59 ! (
U Datil Mountains Luera
1.4
Mountain G
230,000 163 A ! ( 0.52
Crosby Mountain N ! (
0.31 A D ! ( 108°W
0.23 D S 1.46 D
! ( F ! ( 0.26 D D D
3.6 O Pelona ! ( Mountain ! (
0.83
S N
205,000 I 2.29 D D
! ( A L D
Mangas 0.9 P D D Mountain ! ( D 12 O Bar O Mountain D 108°20'W D 180,000 D D D D D D D D D D D 10 Mi D D D D D D D D D D D 108°40'W D D D D D 155,000 0 0 10 Km D D D D D D D
D D D D D D
D D D D Vents 0.14–0.35 0.35–0.7 0.7–1.0 1.0–2.0 2.0–3.6 Caldera margin Map of fluoride concentrations (parts per million) in the San Agustin Plains. Map of fluoride concentrations (parts per million) in the San D D D D D D D ! ( ! ( ! ( ! ( ! ( D D D Fluoride (ppm)Fluoride D
/
Figure 7. 34°N 33°40'N 33°20'N 34°20'N D D D D D D D D D D
12
HYDROGEOLOGY OF THE PLAINS OF SAN AGUSTIN
3,800,000 3,775,000 3,750,000 3,725,000 D 3,700,000 " " D D D 25 D 107 D 169 D 60 107°20'W D 280,000
San Mateo Mountains 549 D
D Gallinas Mountains D 255,000 52
D
107°40'W
N
I
T
Datil Range Black "
"
S
McClure Hills McClure
D Datil Mountains Datil
U
G 59 A Luera 230,000
Mountain 163 N Crosby
Mountain A S D 108°W D
D F D
D D O 12
Pelona S
Mountain
N 205,000 I
D A
D L
D P Mangas D O BarO Mountain
Mountain D D D 108°20'W D 180,000 D D D D D D D D D D D 10 Mi D D D D D D D D D D D 108°40'W D D D D D 155,000 0 10 Km 0 D D D D D D D
D D D D D D
D D D D 0.06–0.4 0.4–1.6 1.6–3.2 3.2–5 Caldera margin Vents S.A.Plains Rim San Mateo Mountains D D D D D D D D D D Mean Recharge (in/yr )
D
/
34°N 33°40'N 33°20'N 34°20'N Figure 8. Map of regions recharge and estimated rates (in/yr) groundwater in the basin. D D D D D D D D D D
13 NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCES: OFR 615 line, means that groundwater was recharged quickly 8–10. Additionally, we can say with confidence that and deeply, avoiding evaporation, and that once the additional recharge is entering the basin-fill aquifer water is in the basin it is too deep to evaporate to the through gradual groundwater flow from the base surface. If recharge was occurring through the basin of geologically young alluvium in tributary valleys. floor, we would assume that it would be subjected This water is a mixture of ages, with some likely very to evaporation in the subsurface during infiltration, recent. But, the average age of the water recharging leading to a evaporative trend in the groundwater iso- the basin via tributary valley groundwater is between topes. We do not observe this, which further supports 1,000 and 4,000 years old. This implies that ground- that recharge only occurs as focused and mountain water transport times are long even in the block recharge and not as distributed recharge in the tributary valleys. plains themselves. The distinct groundwater age populations Older waters within the San Agustin Plains are (Figs. 9 and 11) allow us to say with confidence that generally ‘lighter’ isotopically, indicating they are there are two significant recharge pathways into the sourced more from snowfall and colder temperatures San Agustin Plains basin fill aquifer: mountain block (Fig. 11). There is a clear, linear relationship between recharge through fractured volcanic rocks requiring groundwater age and isotopic composition of waters 10,000 or more years; and focused recharge transit- older than 5,000 years old. This is consistent with a ing through valley-fill alluvial aquifers and then into warming climate from approximately 18,000 years the basin fill (at the mouths of the valleys) that takes ago to 5,000 years ago (Fig. 11). The trend is caused up to 4,000 years. by gradual warming from the Last Glacial Maximum To examine the existence of a connection between (18,000 years ago) to the so-called Altithermal Event San Agustin Plains and the perennial upper Alamosa 7,000 to 5,000 years ago. Evaporation would hide Creek, we also sampled wells and springs in the this trend, as would extensive three-dimensional upper Alamosa Creek. The general chemistry of mixing of water or complicated mixing of old and these samples, with the exception of the Ojo Caliente young waters. Groundwater ages younger than 5,000 Warm Springs (11.1 kyr orange circle on Figure 9, years old are unreliable but are likely ‘young’; they east of the Black Range), overlapped very closely are also relatively constant, reflecting the largely with the younger waters of the San Agustin Plains constant climate over the last 5,000 years. That we (Fig. 6). Both the groundwater age dating (Fig. 9) can see these trends, which are consistent with ancient and isotopic data (Fig. 10) revealed that the waters climates, show that waters are not extensively mixing in upper Alamosa Creek are in general much younger during flow into the basin, or during flow within and generally isotopically heavier than those in the the basin. San Agustin Plains: thus, the groundwater in Alamosa The isotopic and groundwater age data has Creek is distinct from the groundwater under the serious implications for how we understand the flow San Agustin Plains. We find no reason to believe system. In other extensional basins in New Mexico, significant flow comes from the San Agustin Plains like the Albuquerque basin, old waters dilute younger into Alamosa Creek. Geologically, it is also unlikely waters, creating a possible complicated age signal. In that significant flow goes to the east from the White the San Agustin Plains, however, it appears that the Lake graben. If a significant amount of water did flow 14C ages reflect the actual age of the water, rather than in that direction, we would expect to see groundwater a mean age of mostly young water diluted with some gradients shifting there and springs on the slope older water. The lack of mixing of waters implies that down to the east away from the basin. flow is occurring within individual, largely horizontal, relatively thin aquifers, with little communication or mixing between different parts of aquifers. Summary of Conceptual Model Our conceptual model of the hydrogeology of the region is as follows: recharge entered the The geologic structure under the San Agustin Plains fractured volcanic rocks at about the time of the includes four distinct grabens that likely have poor end of the last Ice Age (10,000 to 18,000 years ago), hydrologic connection, but are connected to similar flowing out through the mountain block and into recharge pathways. Overall, the water table is gradu- the basin (Fig. 2). Alternatively, we could argue that ally declining, indicating that the current water use, the current groundwater is old lake water, but this as small as it is, is not balanced by recharge. This would bear an evaporative signature in the isotopes is consistent with the long (1,000 to 4,000 years that would degrade the linear trends seen in Figures for tributary alluvial aquifers and a total range of
14
HYDROGEOLOGY OF THE PLAINS OF SAN AGUSTIN
3,800,000 3,725,000 3,775,000 3,750,000 D 3,700,000 " " D D D 25 D 107 D 169 D 107°20'W 60 D 280,000 5,390 ! ( San Mateo Mountains 549 2,800 10,130 ! ( 17,350 ! ( D 000 D ! ( Gallinas Mountains 11,070 2,740 ! ( D 17, 10,060 4,360 13,730 ! ( ! ( ! ( ! ( ! ( 100 ! ( 4,770 22,900 255,000 ! ( ! ( 10,700 52 10,650 ! ( D 107°40'W ! ( 2,010 4,070 1,140 6,310 ! ( ! ( ! ( ! ( 2,520 ! ( 13,800 10,730 19,020 ! ( ! ( ! (
" 580 " Range Black Datil 1,020
! ( McClure Hills McClure ! ( D ! ( ! ( 59
8,200 Datil Mountains Luera Mountain 12,540 230,000 10,680 163 ! ( 3,140 Crosby Mountain ! ( 6,350 D ! ( 108°W D 8,770 D ! ( 9,330 ! ( D D D 12 Pelona 18,490 Mountain ! ( 205,000 4,780 D D ! ( D C corrections. 5,350 D
Mangas 13 D Mountain ! ( D O Bar O Mountain D 108°20'W D 180,000 D D D D D D D D D D D 10 Mi D D D D D D D D C age (years) of groundwater using δ 14 D D D 108°40'W D D D D D 155,000 0 10 Km 0 D D D D
D D D
D D D D D D Vents Caldera margin D D D D apparen age (yrs) 100–1,100 1,100–3,100 3,100–5,400 5,400–8,200 8,200–11,100 11,100–13,800 13,800–22,900 D D C t D D D ! ( ! ( ! ( ! ( ! ( ! ( ! ( D D D 14 D D
D
/
34°N 33°40'N 33°20'N 34°20'N Figure 9. Map of apparent D D D D D D D D D D
15 NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCES: OFR 615
-45 -50
-50 - 0 -55
- 0 -70 D (per mil) - 5 -80 -70
D (per mil) -75 -90 0 5,000 10,000 15,000 0,000 -80 Age (year apparent 14 )
-85 14 San Agustin Plains, ells Figure 11. Plot of δD (per mil) against apparent C age (years) for San Tularosa, ells Agustin Plains only. The 14C age shows the average age of the water, -90 Alamosa, ells Alamosa, Springs and the δD reflects the temperature the water was recharged at— -95 inter Spring M Summer Autumn M colder temperatures go to more negative δD values. Groundwater ages less than 5,000 years old are unreliable because of mixing but still likely -100 "young." Major climate eras in the last 20,000 years include the Last -13 -1 -11 -10 -9 -8 -7 - Glacial Maximum (coldest temperatures) at ~18,000 years ago followed 18O (per mil) by a period of slow warming, the cold Younger Dryas event at ~10,000 years ago followed by a period of warming until a maximum tempera- Figure 10. Isotopic concentrations of groundwater compared to ture was reached at ~6,000 years ago during the Altithermal Event. meteoric waters in the San Agustin Plains and upper Alamosa Creek. After the Altithermal Event, climate has remained relatively constant. Meteoric waters (precipitation) is represented by the line on this graph, as the local meteoric water line (LMWL), which was collected during Winter (1 Jan to 1 Mar), Spring (1 Mar to 1 Jun), Summer (1 Jun to 1 Sept) and Fall (1 Sept to 1 Jan). ages 9,000 to 18,000 for mountain-block volcanic of the basin-fill aquifer in the San Agustin Plains is aquifers; Figure 2) transit times over short (5–10 known to be mud-rich in the middle of the North gra- miles) distances into the basin-fill aquifer. Recharge ben and near modern-day playas, and this sediment rates are low (<0.1 in to 0.5 in) based on chloride- is at its maximum depth-of-burial. This configuration mass balance modeling. Carbon dating indicates that or set of circumstances makes these areas of the basin well over 80% of the groundwater in the basin was highly susceptible for groundwater-related subsidence recharged during the melting of a large snowpack during large scale pumping. The water quality in the and ice in rock glaciers at the end of the last Ice Age. San Agustin Plains is excellent, with very fresh water In the basin and in the recharge zone, flow is largely common throughout the basin except near the playas unmixed between young and old, and the majority and in wells in relatively tight bedrock. We believe, of the waters near the edge of the San Agustin Plains consistent with observations of previous workers, indicate water from the last Ice Age. Where there that groundwater is exiting the basin at its extreme are large tributary valleys, such as the canyon near southwestern corner, where groundwater elevations Datil, connecting to the valley basin, we see younger decrease to the south. It is key to note that ground- waters; these waters are rapidly diluted by the older, water levels are slowly declining now even for these mountain-block recharge waters. In general, flow small fluxes, so one could expect that further rates through the basin are very slow, consistent with declines would occur with increased pumping rates the low water table elevation slopes. The sediment or water use.
16 HYDROGEOLOGY OF THE PLAINS OF SAN AGUSTIN
ACKNOWLEDGMENTS
We thank the well and land owners of the San Agustin Plains and along Alamosa Creek for granting access. Particularly, we are grateful for the leadership and knowledge of NM State Representative Gail Armstrong and Catron County Commissioner Anita Hand. Several engaged local community members—including Eileen Dodds, Linn Kennedy, James Hall, Carol and Ray Pittman and Roy Farr—helped us gain access to wells and land, and informed our understanding of historical land-use. We thank the San Augustin Plains LLC and John Shomaker and Associates for their donation of well cuttings and well logs, two critical sources of data in a data-poor region.
Hydrologic field data collection over the years was conducted by numerous staff of NMBGMR, we particularly wish to acknowledge the work of Cathie Eisen, Trevor Kludt, Scott Christenson and Melissa Brett. Photography, cartography, technical illustration and layout were performed by Melissa Brett, Stephanie Chavez, Mark Mansell, and Brigitte Felix. Copy editing was done by Belinda Harrison. This work benefited from conversations with Jeffrey Pepin, Fred Phillips, Geoffrey Rawling, Shari Kelley, Ron Broadhead, Jack Oviatt, Bruce Allen, John Hawley, Ginger McLemore, and David Love. Water and isotopic analyses were conducted at the NMBGMR Analytical Chemistry Laboratory.
17 REFERENCES
Blodgett, D.D., and F. B. Titus. 1973, Hydrogeology of the San Augustin Plains, New Mexico. Independent study submitted for M.S. degree to NMIMT, Open-File Report 51, New Mexico Bureau of Mines and Mineral Resources, Socorro, New Mexico. Myers, R.G., Everheart, J.T., Wilson, C.A., 1994, Geohydrology of the San Agustin Basin, Alamosa Creek Basin upstream from Monticello Box, and upper Gila Basin in parts of Catron, Socorro, and Sierra Counties, New Mexico. Water-Resources Investigations Report 94-4125, U.S. Geological Survey, Albuquerque, New Mexico.
New Mexico Bureau of Geology and Mineral Resources
A Research Division of New Mexico Institute of Mining and Technology
Socorro, NM 87801 (575) 835-5490 www.geoinfo.nmt.edu