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ltl nru eX' 1506-00055

Hydrogeology and Simulated Effects of Urban Development on Water Resources of Spanish Springs Valley, Washoe County, West-Central

U.S. GEOLOGICAL SURVEY

Water- Resou rces I nvestigations Repo ft 96-4297

Prepared in cooperation with the NEVADA DIVISION OF WATER RESOURCES

1506-00055 Northeastward photographic views of irrigated lands in southern part of Spanish Springs Valley in (A) 1967 and (B) 1996. Orr Ditch is in foreground. Trailer park seen at left in 1967 view is hidden by trees thirty years later. ln 1996, urban development in the form of a subdivision coexists with irrigated lands. Hydrogeology and Simulated Effects of Urban Development on Water Resources of Spanish Springs Valley, Washoe County, West-Central Nevada By David L. Berger, Wyn C. Ross, Carl E. Thodal, and Armando R. Robledo

U.S. GEOLOGICAL SURVEY

Wate r- Reso u rces I nvesti gatio ns Report 96- 4297

Prepared in cooperation with the NEVADA DIVISION OF WATER RESOURCES

Carson City, Nevada 1997 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY GORDON P. EATON, Director

Any use of trade names in this publication is for descriptive purposes only and does not constitute endorsement by the U.S. Government

For additional information Copies of this report can be write to: purchased from:

District Chief U.S. Geological Survey U.S. Geological Survey Branch of Information Services 333 West Nye Lane, Room 203 Box 25286 Carson City, NV 89706-0866 Denver. CO 80225-0286 email : [email protected] CONTENTS

Abstract......

Location and General Features of Study Area ...... 2

U.S. Geological Survey Site Designations ...... 4

Geologic Histor General Character of Hydrogeologic Units I 5

Areal Extent and Thickness ...... 2l Hydraulic Properties 2l 26 27 32 Sources and Movement of Ground Water ...... 34 Chemical Composition of Ground Water ...... 42 Ground-$/ater Recharge and Discharge ...... 42

Infltration

Evaluation of the Ground-Water Budget and Yield, 1994 ...... 50 5l 5l 52 Selection of Time Periods and Assigned Recharge and Discharge ...... 52 54 Initial Conditions and Aquifer Properties 58 Simulation of Transient Conditions, 1979-94 6l 6l Simulated Ground-Water Budget 65 68 Limitations of Flow Model 68 Simulation of Responses to Hypothetical Ground-Water Development Scenarios 69 69 70 70 References Cited 77

CONTENTS ill FIGURES

l-6. Maps of the Spanish Springs Valley area, west-central Nevada, showing: L Location and general features...... 3 2. Parceled land with public-water systems and domestic wells and general area of agricultural land use

3. Wells, precipitation stations, surface-water gages, and water-chemistry sampling sites ...... 7 4. Generalized geologic units, structural features, and seismic-refraction lines...... l6

6. Complete Bouguer gravity field...... 20 7 -9. Graphs showing profiles of measured and calculated total-intensity magnetic anomaly and complete Bouguer gravity anomaly, and generalized geologic section:

ll-12. Graphs showing: I l. Water levels for sites 72 and 73, November l993-September 1995..'...' 26 12. Annual precipitation and cumulative departure from average precipitation at Reno airport for 57-year period of record, 1938-94 28 13. Map showing precipitation stations and average annual precipitation 30 l4-16. Graphs showing: 14. Relation of average annual precipitation to altitude for 34 precipitation stations used in determining distribution of precipitation and relation of difference between predicted and measured precipiiation to predicted average precipitation 3l 15. Daily mean discharge of Orr Ditch and North Truckee Drain, May 1992-September 1994,and percent of average annual precipitation at Reno airport, 1992-94...... JJ | 6. Relation of deuterium to oxygen- I 8 in sampled ground water and imported surface water, Spanish Springs Valley and ...... '.... 34 17-19. Maps showing ground-water levels in basin fill:

20-21. Maps showing change in ground-water level in basin fill: 20. From December l992toMay 1994..... '..'..""""".'"" 39 21. From May 1994 to December 1994...... 40 22. Graph showing general chemical composition of sampled ground water...... '. 43 23. Sketch showing three-dimensional, schematic conceptualization of ground-water recharge and discharge

24. Map showing block-centered finite-difference grid used for ground-water-flow model 53 25-26. Graphs showing assigned monthly rates used as model input for transient simulations:

2i. Map showing cells with assigned ground-water pumpage for municipal, irrigation, and industrial use in annual stress periods representing 1979 and 1994 in model simulations...... 57 28-31. Maps showing: 28. Model cells in layer I used to simulate ground-water recharge in transient model ...... 59 29. Ground-water levels assumed to represent water-level conditions prior to large pumping stresses and used as initial heads in model simulations.' 60 30. Horizontal hydraulic conductivity in layer I used in calibrated transient simulations 62 31. Transmissivity in layer 2 used in calibrated transient simulations.. 63 lV Hydrogeology and Simulated Etfects of Urban Development on Water Resources ol Spanish Springs Valley' Nevada 32-33. Graphs showing: 32. Measured and simulated ground-water levels for selected cells, layer l, during transient simulations...... 64 33. Cumulative percentage of absolute difference between 27 measured and simulated hydraulic heads for monthly stress period representing December 1994 of model simulation. 65 34. Map showing simulated heads in layer I calculated from monthly stress period representing December 1994

3 5 . Cumulative change since I 979 in annual ground-water discharge, recharge, and storage from results of

36. Map showing cells with assigned ground-water pumpage for municipal, irrigation, and industrial use in hypothetical development scenarios 2 and 3 used in model simulations ...... 7 | 37-39. Maps showing model-computed water-level change at the end of 5, 10, and20 years since December 1994 for hypothetical development for simulation with and without flow in the Orr Ditch

40. Graphs showing simulated cumulative depletion in ground-water storage since December | 994 for hypothetical development scenarios l,2,and 3...... 75

TABLES

I . Site number, location, and type of data available for well, precipitation, and surface-water sites, Spanish Springs 8 2. Age,lithology, and generalhydrologic properties of principal hydrogeologic units...... l8 3. Physical properties for lithologic units in geologic section along geophysical profiles. 2l 29 5. Estimated average annual precipitation and ground-water recharge 32 6. Summary of flow for Orr Ditch and North Truckee Drain, 1976-94 JJ 7. Concentrations of chlorofluorocarbons and tritium in ground water and corresponding CFC-model recharge ages.... 4l 8. Concentration of chloride in ground water and precipitation at selected sites...... 46 9. Estimated disposition and budget of imported Truckee River water for 1994...... 47 10. Summary of population and estimated ground-water withdrawals, 1979-94...... 49 I l. Estimated and simulated components of ground-water recharge and discharge, 1994 conditions ...... 50 12. Summary of model-sensitivity simulations at end of 1994...... 68

CONTENTS CONVERSION FACTORS. VERTICAL DATUM, AND ABBREVIATED UNITS

Multiply By To obtain

acre 4,047 square meter acre-foot (acre-ft) 0.001233 cubic hectometer acre-foot per year (acre-fV^yr) 0.001233 cubic hectometer per year cubic foot per second (ft'/s) 0.02832 cubic meter per second foot (ft) 0.3048 meter foot per day (fVd) 0.3048 meter per day foot per mile (fVmi) 0. I 894 meter per kilometer foot squared per day (ft'ld) 0.09290 meter squared per day gallon per minute (gallmin) 0.06309 liter per second inch (in.) 25.40 millimeter mile (mi) 1.609 kilometer square mile (mi2) 2.590 square kilometer

Temperature: Degrees Celsius ("C) can be converted to d€grees Fahrenheit ('F) by using the formula 'F = [.8('C)]+32. Degrees Fahrenheit can be converted to degrees Celsius by using the formula'C = 0.556("F-32).

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929, formerly called "Sea-Level Datum of 1929"), which is derived from a general adjustment of the firsrorder leveling networks of the United States and Canada.

Metric water4uality and geophysical units: g/cm3 (gram per cubic centimeter) mGal (milliGal) mg/L (milligram per liter) pm (micrometer) pS/cm (microsiemens per centimeter at 25" C) mL (milliliter) permil (part per thousand) nT (nanotesla) pCi/L (picocurie per liter) pgll- (picogram per liter)

Vl Hydrogeology and Slmulated Effects of Urban Development on Water Resources of Spanish Springs Valley, Nevada Hydrogeology and Simulated Effects of Urban Development on Water Resources of Spanish Springs Valley, Washoe County, West-Central Nevada

by David L. Berger, wyn c. Ross, carl E. Thodal, and Armando R. Robledo

ABSTRACT that, although recharge from imported surface water prevented water-level declines in the inigated area, Land-use changes and increased ground-water declines from 20 to 60 feet may be expected through- withdrawals are expected as a result of increased out much of the modeled area after 20 years of sim- urban development in Spanish Springs Valley, about ulated pumping. In model simulations without 5 miles northeast of Reno, Nevada. Currently (1994), recharge from imported surface water, 8 percent imported Truckee River water used for irrigation to nearly 20 percent of the basin-fill aquifer would makes up more than 50 percent of the total source of be dewatered after 20 years of hypothetical ground- ground-water recharge to the basin-fill aquifer. The water withdrawal, and water levels long-term estimate of ground-water recharge from would decline more than 60 feet in the precipitation is about 770 to 830 acre-feet per year. southeastern part of the valley. Ground-water withdrawals have increased from about 500 acre-feet in1979 to nearly 2,600 acre-feet in 1994. An estimated 300,000 to 600,000 acre-feet INTRODUCTION of recoverable water is stored in the upper 330 feet of saturated basin fill. Augmented yield of the basin-fill Increasing population in the Reno-Sparks area of aquifer in Spanish Springs Valley, determined for west-central Nevada has resulted in urban development 1994 conditions, was estimated to be about 2,400 in outlying areas. Spanish Springs Valley is undergoing acre-feet, including about 1,400 acre-feet ofsecond- rapid growth because of its proximity to the Reno- ary recharge from imported Truckee River water. Sparks area. To accommodate this additional growth, Estimates of ground-water recharge and dis- land-use changes (from agricultural to suburban) and charge to the basin-fill aquifer were made 1994 for increased ground-water withdrawals are anticipated. conditions on the basis of field and empirical tech- For more than a century, flow from the Truckee River niques and evaluated using a mathematical flow has been imported to Spanish Springs Valley by way of model. The flow model constructed during this study the Orr Ditch for agriculture. Transmission losses from was calibrated principally in accordance with the the On Ditch and infiltration of applied irrigation water hydrologic data observed and collected during 1979- are the largest components of ground-water recharge to 94 because few predevelopment data were available. the basin-fill aquifer in the valley. As ground-water An estimated difference between ground-water withdrawals increase to support urban development recharge and discharge of 1,700 to 2,200 acre-feet and are accompanied with decreases in agricultural in 1994 suggest that an imbalance was created by land use, the potential for ground-water overdraft has ground-water withdrawals. become a concern. As a result, the U.S. Geological Sur- The probable response to land-use changes vey (USGS), in cooperation with the Nevada Division and increased ground-water withdrawals is evaluated of Water Resources, began a 5-year study in 1992 to by simulating three hypothetical development sce- improve the understanding of the water resources in narios. Simulated responses to the scenarios suggest Spanish Springs Valley.

ABSTRACT Purpose and Scope north. The drainage area of the valley floor is about 34 mi" including a small internally drained area in the The purposes of this report are to describe and north part of the valley. analyze the ground-water flow system in Spanish Spanish Springs Valley is bounded on the east by Springs Valley and to evaluate and refine estimates of the Pah Rah Range, whose highest summit, Spanish budget and yield of the basin-fill aquifer. The the water Springs Peak, is about 7,400 ft. Hungry Ridge and its ground-water budget and yield, estimated from field unnamed southern extension make up the western techniques, were determined for data and empirical boundary with summits approaching 6,000 ft. The The report also discusses 1994 hydrologic conditions. northem boundary separating Spanish Springs Valley of a ground-water flow model used to simu- the results from Warm Springs Valley is a narrow (less than 0.5 for a l6-year period late transient-flow conditions mi) topographic divide lying between bedrock out- (1979-94) and for several hypothetical development crops of the Hungry Ridge and the Pah Rah Range. The were designed to scenarios. The selected scenarios southem boundary is bedrock and includes a low allu- the removal of imported surface water and simulate vial divide where an irrigation ditch and drain enter and projected increases in ground-water withdrawal. In exit the study area. scope, the study includes Spanish Springs Valley and parts of the surrounding mountainous area. Hydrologic Agricultural land generally has been restricted to is data, including water levels, ground-water withdraw- the southern part of the valley that encompassed by Valley Ditch, als, precipitation, surface-water discharge, and water- the On Extension of the Spanish Springs (fig. chemistry analyses, were considered in this study. hereafter referred to as the On Ditch l). Water Geochemical techniques were used to determine diverted from the Truckee River for inigation has been (Nevada sources and relative ages of ground water. Geophysical delivered by way of the On Ditch since 1878 return Spanish methods were used to help define the geologic frame- State Journal, 1879). Irrigation flow in North Truckee Drain work and describe the general character ofthe hydro- Springs Valley is collected in the geologic units. and returned to the Truckee River downstream from the City of Sparks. Within the area encompassed by the On Work began in the spring of 1992. Principal work Ditch are several storage ponds where water from the included ( I reviewing published information elements ) ditch is impounded. Excluding the North Truckee well-construction, water-level, and and compiling Drain, no streams within the study area are perennial. pumpage data; (2) installing and maintaining two sur- face-water gages; (3) establishing a well-monitoring Spanish Springs Valley and Spanish Springs Peak network; (4) drilling several observation wells; (5) were named by Alces Blum after several springs on the installing nine precipitation stations and constructing valley floor; the springs were called Spanish Springs by an isohyetal map for the study area; (6) collecting early Mexican shepherds who wintered sheep there ground-water, precipitation, and surface-water samples (Carlson, 1974, p. 221). A land survey made on June for chemical analysis; and (7) constructing a ground- 24,1872, noted the spring area as being I chain long water flow model to test and refine the analysis of the (66 f0 and 50 links wide (33 ft), with numerous other ground-water system within the basin fill. small springs nearby (C.8. Hunter, written commun., 1980). Access to the study area from the Reno-Sparks metropolitan area, hereafter called the Truckee Mead- Location and General Features of Study Area ows, is provided by State Route 445 (Pyramid High- way), which passes through the south and north Spanish Springs Valley is a north-trending basin boundaries and continues north to Pvramid Lake. in west-central Nevada, about 5 mi northeast of Reno, and is tributary to the Truckee River (fig. I )' The study lFormal hydrographic areas in Nevada were delineated sys- area covers 80 mi2 including the Sp3nish Springs Val- tematically by the U.S. Geological Survey and Nevada Division of ley Hydrographic Arear and a 7 -mi' topographically Water Resources in the late l960's for scientific and administrative closed subbasin of the Tracy Segment Hydrographic purposes (Rush, 1968; Cardinalli and others, 1968). The official Area, hereafter called Dry Lakes. The valley floor is 3 hydrographic area names, numbers, and geographic boundaries to 4 mi wide, I I mi long, and the altitude ranges from continue to be used in Geological Survey scientific reports and 4,400 ft above sea level in the south to 4,600 ft in the Division of Water Resources administrative activities.

Hydrogeology and Simulated Etfects of Urban Development on Water Resources of Spanish Springs Valley, Nevada .t 19"45', 1 19'37',30',

39 37 30"

Base irom U S. Geo ogrcal Suruey dala. 1 r 1 00,000-scale, 1 980; O 1 2 3MILES Unrversal TrafsveTse Ny'ercalor projection, Zone 1 1 Shaded re iel base from 1:24.000-scale Digital Elevation l\,4odel |------# sun i iumrnal on lrom northwest al 30 degrees above horizon O 1 2 3KILOMETERS

Nevada EXPLANATION Study area ffi Pond

Basin boundary

Figure 1 . Location and general features of Spanish Springs Valley area, west-central Nevada. Dry Lakes, a subbasin of the Tracy Segment Hydrographic Area, is part of the study area.

INTRODUCTION Urban Development Previous Investigations

Many published and unpublished reports have Development in Spanish Springs Valley through been prepared on the hydrology and geology ofthe about 1960 was virtually non-existent, except for home- study area and vicinity. Descriptions of several reports the southern part the that were, steads within of valley pertaining to the hydrogeology of Spanish Springs and still are, associated with agriculture. Comparison Valley follow. among aerial photographs taken in 1956, 1977, and The first investigation, which described the gen- 1994, and parcel data, showed that general agricultural eral geology and hydrologic properties ofthe rocks and land use within the area serviced by the On Ditch for the the occurrence and quality of ground water, was made most part has remained unchanged, although some addi- by Robinson and Phoenix (1948). Wells within the val- tional acreage has been developed in the southwestern ley were inventoried and chemical analyses of ground- water samples from part. Growth and urban development proceeded at a selected wells were made. ln 1964, W.F. Guyton and Associates prepared a report for fairly slow pace until the late 1970's when a second Sierra Pacific Power Company to determine the feasi- mobile home park was established and the first of five bility of developing ground water to supply proposed subdivisions was started. Population within Spanish urban development in the northern part of Spanish Springs Valley has increased sharply from less than 800 Springs Valley. This study increased the general in 1979 to more than 4.000 in 1990 (U.S. Bureau ofthe knowledge of the distribution and properties of hydro- Census, written commun., 1995). Population in 1994 geologic units in the study area. Rush and Glancy was projected to be about 9,300 (Washoe County (1967), in appraising water resources of the Warm Department of Comprehensive Planning, written com- Springs-Lemmon Valley area, included a reconnais- They presented a mun., 1995). Most subdivisions have been constructed sance of Spanish Springs Valley. ground-water budget for Spanish Springs Valley under around the northern perimeter of the On Ditch, with natural conditions and conditions that incorporated the subdivisions in the southern part of the valley. smaller effects of imported surface water. Hadiaris (1988) generally In contrast, individual home sites are scattered describes the results of a two-dimensional mathemati- in the northern part ofthe basin, in and adjacent to the cal model used to simulate steady-state conditions of surrounding mountains. ground-water flow in Spanish Springs Valley. As of 1994, more than 3,000 houses associated with subdivisions had water supplied by a public utility; U.S. Geological Survey Site Designations however, nearly 1,000 of these are permitted to receive Each U.S. Geological Survey data-collection site water from outside the study area (Washoe County is assigned a unique identification on the basis of geo- Assessors Office, written conmun., 1995). Of the graphic location. Wells and precipitation and surface- public water supply, 1,600 had septic sys- houses with water sites are identified by a local (Nevada) system tems and about 1,400 were served by wastewater treat- and a standard latitude-longitude system. For conve- ment facilities outside the basin. Nearly 200 houses had nience, short site numbers (l to 89) also are used for domestic wells with septic systems. Land-use sumna- sites discussed in this report. ries as of l99l (Kennedy/Jenks/Chilton,1991, table 4- A local site designation is used in Nevada to iden- I ) indicate that 620 acres within the study area were tify a site by hydrographic area (Rush, 1968) and by the designated rural, 810 acres were suburban, and more official rectangular subdivision of the public lands ref- than 1,800 acres were agricultural. (Rural designations erenced to the Mount Diablo base line and meridian. Each site designation consists of four units. The first are defined as having I dwelling unit per 2.5 to l0 acres unit is the hydrographic area number. The second unit and suburban designations are defined as having I to 7 is the township, preceded by N to indicate location dwelling units per I -acre parcel.) More than 46,700 north of the base line. The third unit is the range, acres were classified as non-designated areas. Compar- preceded by E to indicate location east of the meridian. ison of land-use designations for 1979 and l99a $g2) The fourth unit consists of the section number and shows an increase in parceled land with municipal water letters designating the quarter section, quarter-quarter systems and domestic wells. section and so on (A, B, C, and D indicate the

Hydrogeology and Simulated Effects of Urban Development on Water Resources of Spanish Spilngs Valley, Nevada -xN =o- .9. oEv c O:U (5 -X! ao_ xo6 o,d E; I >o o-/ E o 96'.;E o) g Ni O)c l' 6C =Og; SE gE .=o o= oc E.3 32 OCD E> ;€ F E$E =90(E.= E*E 5(U .9(L a EHF bG' U 6.> >6 *atoc -- S (d9 IF t>lu (da96 l=IJ 6E "t 6lui E();io 8,i f' z EF '1- :o I O.q ruE -1_FN XE rrJ g -?i (U I o 50 olo o o .90 E o>\ ? ots ll ooEf tl .3 T Ec) ts 9A v> !o 4l KP r,o! nth EE -(d iEEO=tta 5= -00 6= 'i9oHbE AE .o(, Q(5 G'(!to -9d -oE EE tffit 3A=(5 9a>p 'Irbz aa 'FxEEB EE(U8 ;F E d)(,t Yda =(l) 9E F 3= - qtr E* JT- (L9 €v-C'O9 frE: e .,i E dd;j5N e3, .E'e LA

INTRODUCTION northeast, northwest, southwest, and southeast quar- hydrologic and geologic data and the application of sev- ters, respectively), followed by a number indicating the eral geochemical techniques and geophysical methods. sequence in which the site was recorded. For example, Mathematical models that describe the relation between site 85 N2l E20 26CDBDl is in Spanish Springs precipitation and altitude and that simulate ground- Valley (hydrographic area 85). It is the first site water flow in three dimensions also were used during recorded in the southeast quarter (D) of the northwest this study. General descriptions of the types of data quarter (B) of the southeast quarter (D) of the south- used, collection procedures, and data applications are west quarter (C) of section 26, Township 2l North, presented in the following paragraphs. Range 20 East, Mount Diablo base line and meridian. The standard site identification is based on the Collection of Hydrogeologic Data grid system of latitude and longitude. The number con- sists of l5 digits. The first six digits denote the degrees, Basic data collected consist of depth+o-water and minutes, and seconds of latitude; the next seven digits precipitation measurements; continuous measurements denote the degrees, minutes, and seconds of longitude; of surface-water discharge; and water-chemistry analy- and the last two digits (assigned sequentially) identify ses of ground-water, precipitation, and surface-water the sites within a l-second grid. For example, site samples. In addition, lithologic information was col- at 39"36'37" latitude and 393637119432901is lected during the installation of observation wells and 119"43'29" longitude, and it is the first site recorded from geologic mapping in the field. The site number, in that l-second grid. The assigned number is retained location, and type of data collected from sites used dur- as a perrnanent identifier even if a more precise latitude ing this study are listed in table l. Site numbers and are later determined. and longitude locations are shown in figure 3. Ground-water levels, precipitation, surface-water discharge, and general Acknowledgments water-chemistry data collected as part of this study were published by the U.S. Geological Survey in annual The authors express their appreciation to resi- water-data reports (Hess and others, 1993, p. 382-383, dents of Spanish Springs Valley for their cooperation p.444-447; Emett and others, 1994,p.387-390, p.552- in supplying data and permitting access to their wells. 567;Clary and others, 1995, p. 466-467,p.726-747; Further gratitude is expressed to the following individ- Bauer and others, 1996, p. 704-7 ll). observa- uals for allowing the installation of shallow Depth-to-water measurements were made at77 tion wells and collection of geophysical and hydrologic wells monthly from May 1992 through December 1994 David Kiley, data on their properties: Alan Oppio, and then quarterly through September 1995 (sites l-77, Grecian Iratcabal, and the Capurro family. Information table I ). These data were used to determine ground- Matthew Allen of the on the Orr Ditch was supplied by water flow directions and to characterize the effects of the Federal Orr Ditch Company and by the office of seasonal flows of imported surface water on the shal- were Water Master. Municipal water-supply data low ground-water system. Water levels in several wells furnished by Sky Ranch Utilities and by the Washoe County Department of Public Works, Utility Division. The drilling of several observation wells and collection EXPLANATION of borehole geophysical logs also were provided by the Washoe County Utility Division. Drillers' logs and lffi,tji,ffi Basln fill water-rights information were obtained from the consolldaled rock Nevada Division of Water Resources. I Basln boundary

Sites used in this study-Solid symbol METHODS USED IN THIS STUDY indicates water chemistry (except sites 52, 53, 75). Number corresponds to table 1 Several methods of investigation were used o76 Well (sites 1-77; site 77 is east of map area) during the course of this study to provide information o75 Well (chemistry only; sites 52, 53, and 75) needed to analyze the ground-water flow system and to evaluate the water budget and yield of the basin-fill o83 Precipitation station (siles 78-87) aquifer. These methods included the collection of t88 Surface-water gage site (sites 88 and 89)

Hydrogeology and Simulated Eflects of Urban Development on Water Resources of Spanish Springs Valley, Nevada 1 1 9"45', 1 19"37'30'

T. 22 N.

loe 07 oo T. 21 N. 19 610 \J'^tt ,Spanish Sprin Vallev ois df;,o' " ., 8 &d::?l'

39"37',30" 51 n Ps rf

T 20 N.

T. 19 N.

tructLe Meado*s 2Xu

R. 19 E. R.20 E R,21 E R.22E.

Bas€ from U.S. Gslogi€l Surusy data, O 1 2 3MILES Geology modified trom Bonham (1 969), 1 :100,000-scale, 1980: Universal Transversa Stewart (1980), Cordy (1985), and Mercator projection, Zon6 1 1 F--Lr---+------Bsll and Bonham (1987) O 1 2 3KILOMETERS

Figure 3. Wells, precipitation stations, surface-water gages, and water-chemistry sampling sites, Spanish Springs Valley, west-central Nevada. Site 77 is not shown because it is outside map area.

METHODS USED IN THIS STUDY Tabte 1. Site number, location, and type of data available for well, precipitation, and surface-water sites, Spanish Springs Valley area, west-central Nevada

[Available data: GL, geophysical logs; B precipitation; Q, dischargel QW, water chemistry: WL. ground-water levels. Data from Hessandothers(1993),WashoeCounty(1993),Emettandothers(1994),Claryandothers(1995),andBauerandothers(1996)l

U.S. Geological Survey site designations 1 Site Available Site name number Standard data Local identification (fis.3) identilicalion

Well sites I 84 N22 E2O 25DDCAI 394422119404901 Brown,D. WL 2 85 N2I E2O O2AAACI 394321119415101 O'Hair wL,QW 85 N2I E2O O2DDACI 394240119415001 SSP-9 WL,GL 4 85 N2I E2O IIDABAI 3942021194t5301 SSP-10 WL 5 85 N2I E2O I2DACDI 394154119405401 Wardrup WL 6 85 N21 E2O l3CAACI 3941091 l94l t50l ssP-7 WL 7 85 N2I E2O I4BDDAI 394n7r942t50t SSP-8 WL,GL 8 85 N2I E2O IsDABBI 394111119430301 Sha-Nevagravelpit WL 9 85 N2I E2O 22BADDI 394035119432901 Airport wL,QW l0 85 N2I E2O 23ADAAI 394032119414601 Donovan No. I WL ll 85 N2I E2O 23BCCDI 39402119424601 SSP-6 WL,GL t2 85 N2I E2O 24BDDAI 394023119412001 Donovan No. 2 wL,QW t3 85 N2I E2O 26CDBDI 393917119423601 AlyceTaylorSchool WL l4 85 N2I E2O 26DCCB2 393858119421301 SS-la WL l5 85 N2I E2O 2TCBDAI 39391711943440t SSP-5 WL,GL l6 85 N2I E2O 34CDADI 393813 I 19425301 Hawco monitoring WL l7 85 N2l 820 34DDCCl 393812119425701 Washoe Co. Utilities North WL t8 85 N2I E2O 35ADBDI 3938471 19415 l0l SS-3 WL l9 85 N2t E2035CCADl 393821I19423601 Golden West North WL 20 85 N2I E2O 35CCCDI 393812119424401 USGS-ParkObservation wL,QW 2l 85N2I E2O35DCBAI 3938191 19420501 55-6 wL,QW 22 85 N2l E2l ITCDCCI 394120119385601 Cole WL ZJ 85 N2I E2I 2OBBCBI 394040119392801 Cancilla WL .A 85 N2I E2I 3OABCCI 393944119400701 Armbruster WL 25 85 N2I E2I 3OCAAAI 393927119401301 Mack WL 26 85 N2l E21 3lADCCl 393839119394801 Sanders WL 27 85 N2I E2I 3IBABCI 393903119402001 Casale wL,QW 28 85 N2I E2I 3IBCBCl 393849119404001 Brown,W. wL,QW 29 85 N2r E2l 3rCACAl 393828119401601 May WL 30 85 N2l E2l 3lDDCCl 393812119394001 Colby wL,QW 3l 85 N2O E2O OIAADDl 393800119403601 Washoe Co. Countryside North WL tz 85 N2O E2O OICBABI 393743119413601 Huers WL 33 85 N2O E2O OIDABAI 3937431 l94l l50l Hon WL 34 85 N2O E2O OICACBl 393737119411501 Murray wL,QW 35 85 N2O E2O O2DBBAI 393743119420801 USGS-OppioObservation WL 36 85 N2O E2O O3ACDCI 393738119432101 Washoe Co. Utilities South WL JI 85 N2O E2O O3BCCCI 393744119435101 Falk WL 38 85 N2O E2O O3CDDCI 393720119432701 Springwood Utilities monitoring WL 39 85 N2O E2O O4BBADI 393804119443901 Sullivan wL,QW 40 85 N2O E2O O4CCBBI 393737119445201 Grube WL AI 85 N2O E2O O4DBDBI 393735119441701 Pace wL,QW qz 86 N2O E2O OSDBBC I 393643119453401 Kangas WL 43 85 N2O E2O IOAACB1 393713119430001 USGS-Lazy5aObservation wL,QW 44 85 N20 E20 I0AACB2 393713119430002 USGS-Lazy5bObservation WL 45 85 N2O E2O IOAACB3 393713119430003 USGS-Lazy5c Observation WL 46 85 N2O E2O IOCDABI 393637119432901 KileyNo. I WL 47 85 N2O E2O IODBBCI 393649119432301 KileyNo.2 WL 48 85 N2O E2O IODBBC2 393649119432302 Kiley No. 3 WL 49 85 N2O E2O l IBAAAI 393715119422801 Gaspari No. 2 WL 50 85 N2O E2O I IBDDAI 393655119421901 Gaspari No. I WL

Hydrogeology and Simulated Effects ot Urban Development on Water Resources of Spanish Springs Valley, Nevada Table 1. Site number, location, and type of data available for well, precipitation, and surface-water sites, Spanish Springs Valley area, west-central Nevad+Continued

U.S. Geological Survey I Site site designations number Site name Avallable (fig.3) Local identilication Standard data identification 51 85 N2O E2O I2AAAAI 3937t711940370t SSP-4 wL'Qwcr- 52 85 N2O E2O I2BDDAI 3937001l94ll20l Wingfield Ranch No. I Qw 53 85 N2O E2O I2CAABI 393651 I l94ll40l Wingfield Ranch No. 2 Qw 54 85 N20 E20 I2DBCCl 3936431194l0401 D'Anna wL,QW 55 85 N2O E2O I5DBBBI 393559119430801 KileyNo.4 WL 56 85 N2O E2O I5DBBA2 393559119431902 USGS-Kiley4aObservation wL,QW 57 85 N2O E2O I5DBBA3 393559119431903 USGS-KiL:y4bObservation wL,QW 58 85 N2O E2O I5DBBA4 393559119431904 USGS-Kiley4cObservation WL 59 85 N2O E2O 2IAAAAI 393527119435701 SpringcreekNorth WL 60 85 N2O E2O 21AABC1 393529119441601 BlueGem WL 6l 85 N2O E2O 2IADDAI 393515119435701 SpringcreekAbandoned wL,QW 62 85 N2O E2O 2IBDDAI 3935 l3 I 19443501 Bailey WL 63 85 N2O E2O Z2DBCAI 393459119432201 USGS-lratcabal2 Observation WL 64 85 N2O EzO22DCDCI 3934421 19431001 lratcabal No. I WL 65 85 N2O E2O 2TBBCAI 393434119435201 USGS-CapurroObservation WL 66 85 N2O E2O 2TCACAI 393405119433401 Bria wL,QW 67 85 N2O E2O ZTCCCBI 393400119435401 Brandsness WL 68 87 N2O E2O 34DABBI 3933221 19430801 Winners Corner Exxon WL 69 85 N2O E2I OTCBCBI 3936481 19403301 Sweger No. 2 WL 70 85 N2O E2I OTCCCCI 393631119403401 SwegerNo. I WL 7l 85 N20 E2l 0TDCCAl 393642119395601 TuckerNo.2 WL 72 85 N2O E2I O8BBCAI 3937t41 19393501 SSP-2 wL,QWGL 73 85 N2O E2I OSBBCA2 393714119393502 SSP-2a WL 74 85 N2O E2I I8DABDI 393558119395001 TuckerNo. I WL 75 85N20821 ISDAD I 3935521 19394501 Berry Qw 76 85 N2O E2I I8DADBI 393544119394701 Mills WL 2i7 83 N2O E22 2ICDDDI 39344611931 l70l Tracv WL Precipitation sites 78 84 N22 E2l 3lACABl 3944011 19395401 Bacon Rind Flat P 79 85 N2I E2I O9BACCI 394221119374901 CumowCanyon RQW 80 85 N2l 820 l5CCABl 394100119434801 Hungry Ridge North RQW 8l 85 N2I E2O 29DCDDI 393903119452501 Hungry Ridge South BQW 82 85 N2I E2I 34DBCDI 393903119423101 SpanishSpringsPeak P,QW 83 85 N2r E2035CCDAl 393824119363401 FireStation P 84 85 N2O E2O I4CDBBI 3935451 19423301 Msta P 85 85 N2O E2I 22BCBAI 393522119371901 DryLakes P 86 85 N2O E2O 21ACBCI 393515119443201 OasisTrailerPark P 87 85 N2O E2I 3ODAABI 393418119394101 CanoeHill BQW Surface-water sites3 88 85 N2O EzO 2TCACCI 3933581 19435 l0l On Ditch Q,QW 89 85 N2O EzO ?TCCADI 3934081 19433 l0l North Truckee Drain Q,QW I See report section titled "U.S. Geological Survey Site Designations."

2 Not rho*n in figure 3; outside map area. 3 USGS station numbers: 10348220,Orr Ditch at Spanish Springs Valley near Sparks, Nev.; 10348245, North Truckee Drain at Spanish Springs Road near Sparks, Nev.

METHODS USED IN THIS STUDY in adjacent basins were measured to provide water- Water-chemistry data were collected from 22 level information in areas of possible ground-water ground-water, 5 precipitation, and 2 surface-water movement from or to Spanish Springs Valley (sites l, sites. Ground-water and surface-water samples were 42,68, and77). analyzed for major ions, bromide, stable isotopes of hydrogen and oxygen, tritium, and selected species In cooperation with Washoe County Department of nitrogen and phosphorus. Samples from l9 ground- of Public Works, Utility Division, nine observation water sites were analyzed for chlorofluorocarbons wells were drilled in areas of the valley where geologic (CFC's). Precipitation samples were analyzed for and hydrologic information was sparse. The drilled selected species of phosphorus, bromide, and major depths ofthe observation wells (sites 3, 4, 6,7, I l, 15, anions. Results of the ground-water-chemistry analy- 51,72, and 73) ranged from 185 ft to 565 ft; differing ses, including the results from environmental tracer lengths of 2-inch slotted-steel pipe were installed for samples (stable isotopes, tritium, and CFC's), were sampling access. Lithologic and geophysical logs for used to support the conceptualization of the ground- these observation wells were described by Washoe water flow system determined from ground-water-level County ( 1993). In addition, several shallow holes (sites data. 20. 35. 43, 44, 45, 56, 57, 58,63, and 65) near the On Ditch and inigated fields were augered and 2-inch Field procedures for the collection of water sam- diameter pipe with 2-ft stainless steel screens was ples were similar to those described by U.S. Geological installed by the USGS. Survey (1977, chapters 1,2, and 5) and by Wood measured onsite Precipitation was measured monthly or as neces- (l 97 6). Water-quality variables included (l) temperature, by using a hand-held ther- sary at seven can-type collectors and quarterly at three to +0.5oC; (2) pH, by using a meter bulk-storage collectors (sites 78-87, table l). Each mometer, accurate with digital readout, accurate to +0.1 unit; (3) alkalinity collector contained a mixture of light oil and ethylene (reported as calcium carbonate, bicarbonate, and car- glycol to prevent freezingand evaporation. Most pre- by incremental-pH titration of 50-mL filtered cipitation sites were either on low hills or at higher alti- bonate), sample water with a field instrument, accurate to tudes in mountains that border the study area. Site 83, +l mg/L as CaCO3; (4) specific conductance, by using however, is on the valley floor (fig' 3) and is maintained a meter with digital readout, accurate to +l pS/cm; and by personnel at the Spanish Springs Valley Fire Station. (5) dissolved oxygen, by digital field instrument, accu- Precipitation data were used in developing a precipita- to 10.1 mg/L. Field instruments were calibrated tion-altitude relation for the study area. The period of rate before each use according to methods record for most precipitation data is luly 1992 through immediately described by Wood (1976). Except for alkalinity, September 1995. Bulk-precipitation (dry and wet fall) which is titrated, field measurements of surface-water collectors similar in design to those described by Lik- chemistry were made instream and measurements ens and others (1977 , p. l6) were installed at the pre- of ground-water chemistry were recorded using a flow- cipitation sites to collect samples for water-chemistry through cell. At each well, a minimum volume equiva- analyses. Results from the water-chemistry analysis lent to three times the volume of water in the well were used in a chloride-balance technique for estimat- allowed to discharge prior to sample collec- ing ground-water recharge from precipitation. casing was tion. Ground water was then pumped through the flow- stage-recording devices were used to Continuous through cell and field measurements of water tempera- on the basis of stage and channel compute discharge ture, electric conductivity, dissolved oxygen, and pH instantaneous or mean daily geometries where either were recorded to verify that these properties had stabi- be determined. Daily-mean discharge discharges could lized. Stable measurements were assumed to indicate (site 88) as it enters Spanish Springs of the Orr Ditch that the discharging ground water was representative of the North Truckee Drain (site 89) as it Valley and of water from the aquifer. Field-measurement techniques were computed from April 1992 to leaves the valley and instruments were quality-assured through annual miscellaneous discharge September 1995. In addition, participation in the U.S. Geological Survey National measurements using a standard USGS wading rod Field Quality Assurance program (Janzer, I 985)' and flow meter were made along the On Ditch as it traverses the valley floor. These measurements were Samples for determination of calcium, magne- made to estimate ground-water recharge by transmis- sium, sodium, potassium, sulfate, chloride, fluoride, sion losses from the On Ditch. silica, boron, bromide, orthophosphateo and selected

Hydrogeology and Simulated Elfects of Urban Development on Water Resources of Spanish Springs Valley' Nevada species of nitrogen (ammonia, nitrite, nitrate) were recharge water at the time when infiltrating water no passed through 0.45-pm pore-size capsule filters and longer is in contact with air. In general, pumps that collected in clean polyethylene bottles after rinsing might affect the pressure of sample water and contact with filtered sample water. Samples for cation analysis with most plastics, rubbers, greases, and oils were were collected in acid-washed polyethylene bottles and avoided. acidified with nitric acid to pH of about 2. Samples for determination of nitrogen and phosphorus were col- lected in opaque polyethylene bottles (to which mercu- Geochemical Techniques ric chloride was added as a biocide), immediately iced, and shipped to the laboratory within 5 days to minimize Water-chemistry characteristics, such as stable microbial degradation. Samples for determination of and radioactive isotopes, chloride ions, and CFC's, that major ions, boron, bromide, silica, and species of nitro- were determined as part of this study were used to eval- gen and phosphorus were shipped to the USGS uate infened recharge sources and volumes and National Water Quality Laboratory in Arvada, Colo., ground-water ages using documented geochemical for analyses (Fishman and Friedman, 1989). Quality- techniques described in this section. Appropriate data control procedures used in the laboratory are described for application of these geochemical techniques were by Friedman and Erdmann (1982) and Jones (1987). collected in Spanish Springs Valley during 1992-94. Unfiltered water was collected in 60-mL glass The stable isotopes evaluated were oxygen-18 bottles for stable-isotope analysis (oxygen- 18 relative l80/l60) relative to oxygen- l6'( and deuteriu- ltryO.o- to oxygen- l6 and deuterium relative to hydrogen- l ) gen-2) relative to hydrogen-l (D/tH). Each ratio is and in I -L high-density polyethylene bottles for tritium determined for a sampled water and is then related analysis. Polyseal caps were used on these bottles to mathematically to the comparable ratio for an intema- prevent evaporation of the sample. Stable-isotope anal- tional reference standard known as Vienna-Standard yses were done by the USGS isotope fractionation Mean Ocean Water (V-SMOW). By convention, the project in Reston, Va. Hydrogen-isotope ratios were computed results are expressed as delta oxygen- l8 determined using a hydrogen equilibration technique (6'oO) and delta deuterium (6D);the units of measure described by Coplen and others (1991) and oxygen- are parts per thousand (abbreviated "permil" or o/oo). isotope ratios were determined using the CO2 equili- The terms "isotopically heavier" and "isotopically bration technique described by Epstein and Mayeda lighter" are relative and are used for comparing the (1953). Samples collected for determination of tritium composition of water samples. A negative delta value (rH), the radioactive isotope of hydrogen, were ana- indicates that the sample water is isotopically lighter lyzedby the USGS Tritium Laboratory in Reston, Va. than the standard (that is, the sample has a smaller pro- Electrolytically enriched samples were analyzed by the portion of oxygen- l8 or deuterium relative to oxygen- liquid scintillation counting method (Thatcher and oth- l6 orhydrogen-l than the standard). Because SrdO and ers, 1977). 6D measure relative isotopic compositions of the two Samples for determination of chlorofl uorocarbon elements that constitute the water molecule (hydrogen concentrations were collected with an apparatus that and oxygen), they are ideal tracers of water movement excludes contact with air, and samples were flame- where the isotopic fractionation of both are usually sealed into borosilicate-glass ampules for transport and covariant. Isotopic fractionation results from physical, storage prior to analysis (Busenberg and Plummeq chemical, or biological processes, which cause the 1992,p.2257-2258). CFC analyses were done by the delta value of the stable isotopes of water to change. USGS Chlorofluorocarbon Laboratory in Reston, Va., For example, during the physical process of evapora- using a purge-and-trap gas chromatography procedure tion, 5'oO increases because 'oO is lighter and leaves with electron-capture detector (Busenberg and Plum- water at a faster rate than the heavier '60. Given two mer, 1992, p.2259-2260). Precautions were taken infened ground-water flowpaths composed of water regarding the type of pump used for ground-water sam- with different isotope compositions, the proportion of ple acquisition and types of material the sample water flow that each flowpath may have contributed to a might contact prior to its entry into the sampling appa- receiving flowpath can be evaluated on the basis of an ratus. These precautions were necessary because the isotopic mass balance. In addition, 6l80 and 5D com- use of CFC's as an age-dating tool is dependent on positions are useful for indicating ground-water source atmospheric concentrations and the temperature of areas.

METHODS USED IN THIS STUDY 11 Tritium is a radioactive isotope of hydrogen (5) atmospheric sources (both dry and wet fall; with an atomic mass of 3 and a half-life of 12.43 years Dettinger, 1989, p. 57). In Spanish Springs Valley, (Plummer and others, 1993, p.257). Tritium is pro- atmospheric sources of chloride are dominant and duced naturally at low levels in the atmosphere by runoff is negligible. Careful sample-site selection and interaction of cosmic rays with nitrogen and oxygen sample handling were used to avoid interference from (Drever, 1988, p. 379), but thermonuclear-weapons other sources. On the basis of the chloride-balance testing between 1952 and 1969 introduced amounts of method, the volume of recharge can be approximated tritium into the atmosphere that produced concentra- as follows: tions in precipitation as much as three orders of magni- R P(Clp/Clr) (1) tude greater than natural levels. This increased tritium = , serves as a marker of the timing of ther- concentration where, testing and thus provides a useful monuclear-weapons R is recharge, in acre-feet per year; indicator of the age (the time since the water has been P is total precipitation that falls in recharge- isolated from the atmosphere) of ground water. source area, in acre-feet per year; In 1993, mean levels of tritium in precipitation Cf is chloride concentration in bulk precipitation, were approaching the pretesting level of about in milligrams per liter; and 25 pCilL (Portland, Oreg., 16pCilL1, Albuquerque, C/, is chloride concentration of recharge water, in N. Mex., 37 pCilL; Robert L. Michel, U.S. Geological milligrams per liter. Survey, written commun., 1995). Tritium generated CFC's are relatively stable, volatile, organic com- prior to thermonuclear-weapons testing would have pounds that are present worldwide as a result of contin- decayed to levels of about I pCilL in 57 years. Samples uous releases to the atmosphere since the 1930's. No collected in 1993 from Spanish Springs Valley having natural sources of CFC's are known, which makes tritium concentrations less than I pCi/L are older than CFC's useful age-dating compounds for hydrologic 57 years; concentrations between I and l0 pCi/L rep- investigations (Plummer and others, 1993, p.268). resent ground water that is either a mixture of pre- and Dichlorodifl uoromethan e (CCI2F or CFC - I 2) was post-therrnonuclear weapons testing water or water 2 the first CFC compound manufactured. Production of that is about 38 to 57 years old; concentrations between trichlorofluoromethane (CCI3F or CFC-l l) began in l0 and 100 pCi/L represent water that is either a mix- the 1940's. These two compounds account for about 77 ture of pre- and post-thermonuclear weapons testing percent of the total CFC production. A third CFC com- water or water that is younger than 38 years; and con- pound--+richlorotrifl uoroethane (C2Cl3F3, or CFC- centrations greater than 100 pCi/L represent water I l3Fwent into production in the 1960's and accounts from 1958-59 or 1962-69 (Welch, 1994,p.16). for the remaining 23 percent of the global CFC market. The chloride-balance method was used in the All CFC's eventually are released to the atmosphere, present study as a reconnaissance effort to complement where they become partitioned into the hydrosphere by other techniques for estimating natural recharge. gas-liquid exchange equilibria. In 1990, atmospheric Because the concentration of chloride dissolved in volume fractions of CFC-12, CFC-ll, and CFC-ll3 ground water generally is not affected by sorption or were about 480, 285, and,77 parts per trillion, respec- ion-exchange processes, it has been used as a natural tively, and these levels have been increasing at an aver- tracer for estimating ground-water recharge from pre- age rate of about 3.7 percent annually (Plummer and cipitation, assuming that all sources of chloride are others, 1993, p.268). Low analytical reporting levels known (Dettinger, 1989; Claassen and others, 1989). (l pglL for CFC-12 and CFC-l l), relatively smooth, This method is based on the balance between total monotonic increase in atmospheric releases, resistance chloride concentration in bulk precipitation (dry and to degradation and transport retardation, and relative wet fall) that falls in recharge-source areas and chloride ease of reconstructing atmospheric concentrations concentration in ground water in and near recharge make CFC's useful as hydrologic tools for identifying areas. Potential chloride sources to ground water ground water that has recently received recharge and include (l) dissolution of evaporite minerals, (2) estimating the timing of recent ground-water recharge. weathering of nonevaporite minerals, (3) mixing Detection of as little as 0.01 percent modern water in with formation water, (4) anthropogenic sources a mixture with pre- 1940 ground water is possible (for example, road salt, wastewater leachate), and because of the low analytical reporting level.

Hydrogeology and Simulated Etlects ol Urban Development on Watel Resources ol Spanish Springs Valley, Nevada The presence of CFC-12 indicates that a portion of the of local industrial releases. Likewise, sewage effiuent sample water is composed of water that was recharged and (presumably) septic-system leachate can contain since 1940; CFC-l I indicates recharge since 1945; CFC concentrations that are orders of magnitude and CFC- I l3 indicates recharge as recent as 1966. greater than expected concentrations (Plummer and The age (time since infiltrating water recharged others, 1993, p. 272) due to domestic use of propellants an aquifer) of ground water may be estimated with a in aerosol cans. These sources of CFC's can result in precision of +l-2 years because the relatively smooth, CFC-model ages that are younger than the actual age of monotonic increase in atmospheric releases is known ground water. Relatively few CFC-discharging indus- (reconstructed from production records from 1940 to tries are based in northern Nevada and those that are 1975 and from atmospheric measurements since 1975; have a minimal effect on atmospheric levels. However, Busenberg and Plummer,1992). This "CFC model" of septic systems are relied on for disposal of domestic recharge age assumes that infiltrating water maintains waste and wastewater in some parts of Spanish Springs equilibrium with air in the unsaturated zone during Valley, representing a potential source of CFC intro- recharge-an application ofthe process ofgas solution, duction into ground water. based on Henry's law solubilities of CFC in water. The temperature at the base of the unsaturated zone is required to define the Henry's law solubility constant Geophysical Methods for nonstandard conditions. A recharge temperature of lOoC was assumed for deeper wells sampled for the In defining the ground-water flow regime of a Spanish Springs Valley investigation on the basis ofthe basin-fill aquifer system, the hydrogeologic framework long-term mean temperature for the weather station is described in terms of flow conditions at the bound- at the Reno-Tahoe International Airport (hereafter aries of the system, together with the hydrologic prop- referred to as the Reno airport). Shallow wells (less erties of the basin fill. Lateral boundaries lie along the than 45 ft below land surface) within irrigated areas periphery ofthe valley floor and the bedrock boundary were assigned a recharge temperature of l4oC to reflect lies beneath the basin fill. Because most wells within the temperature of infiltrating inigation water. The the study area do not penetrate through the basin fill to measured CFC concentration of a water sample can bedrock, geophysical methods were used to determine be converted to an equivalent air concentration at the thickness of the basin fill and to improve the knowl- appropriate recharge temperature by using the solubil- edge of the bedrock geometry. Geophysical methods ity of the compound. By comparing the equivalent air used for this study are gravity, magnetic, concentration to an atmospheric air-concentration and seismic refraction. Intrinsic differences in curve, the air concentration can be equated to a density, magnetic susceptibility, recharge date or age (Cook and others, 1995, p. 432). and seismic velocity are used by these Details of CFC-model age-dating are described by methods to define the contact between hydrogeologic Busenberg and Plummer (1992). units. Borehole-geophysical logs also were collected and used to characterizethe hydrogeologic units found Recent efforts to characterize transport properties during drilling of observation wells. of CFC's indicate thatCFC-12 was the most conserva- tive, CFC-l I shows degradation in an organic-rich Gravity and aeromagnetic data were evaluated by unsaturated zone and in anaerobic ground water, and qualitatively inspecting anomaly maps of the gravity CFC- I l3 shows retardation due to sorption (Cook and magnetic fields within the study area and by geo- and others, I 995, p. 432-433). Soil and aquifer charac- physical profile modeling. The anomaly map of the teristics in Spanish Springs Valley suggest that organic gravity data are the complete Bouguer anomaly, which content is relatively low, but some wells did have dis- have been corrected for latitude, elevation, and terrain. solved-oxygen concentrations of less than I mg/L. Rel- The magnetic data are presented as total magnetic ative agreement between CFC-model ages determined intensity or field strength. Anomalies within a gravity with CFC-12 and CFC-11 indicates that these com- or magnetic field are perturbations represented by pounds act conservatively. highs and lows in an otherwise uniform field. They pro- Water recharged in urban areas may contain con- vide information about the subsurface distribution and centrations of CFC's in excess of expected concentra- general geophysical character of hydrogeologic units. tions due to equilibrium with tropospheric air because Gravity data obtained during this study and in earlier

METHODS USED IN THIS STUDY 13 studies (Saltus, 1988) were used in combination with changes in relative grain size. These changes in grain aeromagnetic data (Hildenbrand and Kucks, 1988) to size may represent contacts between different hydro- create the anomaly maps. geologic units. Units composed of mostly coarse- grained deposits have a different response on the logs The gravity and aeromagnetic data also were used than fine-grained deposits. This allows a qualitative in an interactive geophysical profile modeling program estimate on the relative ability of different units to (Webring, 1985) that fits theoretical gravity and transmit water. Acoustic transit-time logs can be used magnetic responses from a geologic model to measured to provide information about lithology and porosity gravity and magnetic data. A mathematical representa- under a wide range of conditions. Transit times mea- tion of the geologic model includes locations of several sured by an acoustic log decrease with an increase in rock types as well as physical properties (bulk densities rock hardness and cementation. Caliper logs provide a and magnetic susceptibilities) of the rock. The geo- continuous measurement of diameter of uncased drill physical modeling during this study used three profiles, holes. Additional information on the application of across the northem, central, and southern parts of borehole geophysics to ground-water investigations is Spanish Springs Valley. The profiles were selected so presented by Keys (1990). that at least one well penetrating bedrock was inter- sected along the profile and used as a control point. Surficial geology at and near land surface and drill-hole Hydrologic Models information were used to constrain the model in terms of basin-fill thickness. Although the model results are To determine the distribution of precipitation in non-unique, additional constraints on the model were Spanish Springs Valley, precipitation data for 34 sta- provided by simultaneously using two geophysical tions, including 9 in the study area, were analyzed to methods--gravity and magnetics-that measure and develop a mathematical relation between precipitation respond to different properties ofthe subsurface geol- and altitude. Stations range in altitude from 3,900 ft to ogy. Once the adjusted modeled profiles coincided with 6,960 ft. Those stations outside the study area were the conceptualizationof the subsurface that was consis- selected because they lie in the rain shadow of the tent with available data, the profiles were used to guide Siena Nevada, as does Spanish Springs Valley. (Areas the development of a map showing basin-fill thickness. in a rain shadow lie on the lee side of a topographic Additional description of this type of gravity- and obstacle where the rainfall is noticeably less than on the magnetic-profile modeling is presented by Schaefer windward side.) The distribution of annual precipita- and Whitney (1992, p. 5). tion was used to estimate the potential for ground- water recharge from precipitation within the study area. Seismic refraction was used at two sites to pro- vide detailed information about the condition of the The U.S. Geological Survey modular finite- flow boundaries beneath the low alluvial divides at the difference model commonly called MODFLOW north and south ends of the valley. Determining the (McDonald and Harbaugh, 1988) was used to simulate thickness of basin fill beneath these areas aided in esti- ground-water flow through the basin-fill aquifer in mating the possible movement of ground water leaving Spanish Springs Valley. The digital model can simulate or entering the study area through basin fill. A third flow in three dimensions and was used to test the con- seismic refraction line was done to determine depth to ceptual model of ground-water flow developed during bedrock prior to installation of two observation wells this study and to estimate effects of hypothetical (sites 72 and 73). Additional information on the use of changes in land-use patterns and ground-water with- seismic refraction in water-resources investigations is drawal. General features of the ground-water flow presented by Haeni (1988). model of the basin-fill aquifer in Spanish Springs Valley are explained in the section "Simulation of Borehole-geophysical logs were collected in six Ground-Water Flow" observation wells drilled during this study (sites 3, 7, 1 l, 15, 51, and 72) and are presented in a report by Washoe County (1993). Borehole geophysics used in HYDROG EOLOGIC FRAMEWORK this study include caliper, natural-gamma, spontane- ous-potential, acoustic, and resistivity logs. In general, The geologic history of western Nevada and par- responses on natural-gamma, spontaneous-potential, ticularly the Spanish Springs Valley area is complex. and resistivity logs can be used, in part, to determine The area is situated in a transitional zone between the

14 Hydrogeology and Simulated Elfects of Urban Development on Water Resources of Spanish Springs Valley, Nevada Basin and Range and Sierra Nevada physiographic Valley is indicated by steep increases in basin-fill thick- provinces. Structural features associated with these ness near the contact between basin fill and consoli- provinces, in part, define the hydrogeologic framework dated rock along the west side of the study area. ofthe study area. Forpurposes ofthis report, five major Although extensional faulting is thought to have devel- geologic units identified in Spanish Springs Valley oped present-day topographic features, fault systems were divided into two general groups on the basis of associated with the Walker Lane also have created a their hydrogeologic properties: (1) consolidated rock, high degree of structural deformation. Faults that bor- which commonly have low porosity and permeability, der the Dry Lakes area and other faults in the southeast- except where fractured; and (2) basin fill, which gener- ern part ofthe study area that generally trend northeast ally has high porosity and transmits water readily. The (fig. 4) may be related to the Olinghouse Fault Zone, a areal distribution of the five geologic units is shown in left-lateral conjugate shear zone of the Walker Lane figure 4 and general hydrologic properties are pre- (Bell, 1981, p. 38). As a result of this combined defor- sented in table 2. mation, bedrock permeability has developed adjacent to and within highly fractured zones. In other places, Geologic History and Structural Setting faults may act as barriers to ground-water flow because ofcementation along fault surfaces within basin fill and Outcrops of rocks older than Mesozoic age are because of the offsetting of deposits with contrasting sparse in western Nevada and are not found within the permeabilities. study area. Consequently, geologic evidence from adjacent areas is used to describe the region during that General Character of Hydrogeologic Units time. Stewart (1980, p. 35-36) suggests that, during pre-Mesozoic time, western Nevada was the site of Fractured consolidated rock and the unconsoli- deep-water deposition with little tectonic activity. Dur- dated sediments that constitute the basin fill together ing most of the Mesozoic era, tectonic activity, which make up the ground-water reservoirs recognized within and thrustinE, may included compressional folding the study area. However, basin fill cunently forms the have been continuous. The oldest rocks exposed within principal water-bearing hydrogeologic unit. the study areaare metavolcanic and metasedimentary rocks of late to middle Mesozoic age (Triassic and Consolidated Rock Jurassic periods) that were regionally metamorphosed prior to intrusion by granitic plutons during the late Tertiary rocks make up most of the consolidated Mesozoic (Cretaceous period; Bonham, 1969, p. 42). rock exposed in the study area (fig. a); they are gener- A second period of deformation associated with exten- ally of volcanic origin and have intercalated lenses of sional faulting formed the present-day topographic sedimentary rocks. The Pah Rah Range consists almost features ofthe study area, including the structural entirely of Tertiary rocks, which can be characterized depressions underlying Spanish Springs Valley. This by high-amplitude magnetic anomalies (fig. 5). These period of deformation began during the middle to late rocks generally have little or no interstitial porosity, Tertiary period and has continued to the present (Bon- except where volcanic-flow units have abundant vesi- ham,1969,p.42). Tertiary volcanic rocks consisting of cles. Interbedded sedimentary rocks of the Tertiary unit several types of flows and tuffs interbedded with clastic consist mostly of fine-grained, partly consolidated sediments were extruded and deposited unconformably lacustrine deposits with low permeability, but may on the Mesozoic granitic and metamorphic rocks store a moderate amount of water. within the structural depressions. Most of the basin fill The complete Bouguer gravity field shown in fig- accumulated as a result of ongoing deposition of detri- ure 6 indicates that the volcanic rocks that compose tus from surrounding mountains bordering Spanish most of the Pah Rah Range are relatively less dense Springs Valley. (greater negative Bouguer values) than the intrusive The Spanish Springs Valley area lies between rocks that make up the western boundary and volcanic major tectonic structures of the Siena Nevada Frontal rocks ofthe southern boundary (fig. 4). This finding Fault Zone and the Walker Lane (Bell, 1981, p. 35). suggests that the northern and central parts ofthe Pah Range-front faulting associated with the formation of Rah Range are composed of a substantial amount of the structural depression beneath Spanish Springs low-density material. In addition, magnetic signatures

HYDROGEOLOGIC FRAMEWORK 15 , .t ir! tan tl l,!t lli tr rr :. I \..1. I I I !il r. I a' --jc -' B .rt !.1 rl a, '1r".-:\

nu Oircn

\= o :

\--Jo.ror Truckee Meadows ''t'ckee ''

Base trom U.S. Geological Survsy data, 0 1 2 3MILES Geology modified lrom Bonham (1969), 1 :100,00Gscale, 1980; Univereal Transveree Stewart (1980), Cordy (1985), and Memtor prciection, Zone 11 F-r--r-r---i------Bell and Bonham (1987) O 1 2 3KILOMETERS

Figure 4. Generalized geologic units, structural features, and seismic-refraction lines, Spanish Springs Valley, west-central Nevada.

16 Hydrogeology and Simulated Efiects of Urban Development on Water Resources of Spanish Springs Valley, Nevada of the Pah Rah Range indicate that the volcanic rocks As a result of structural deformation, secondary that compose the southern boundary may be a different permeability in the form of fractures has developed lithologic unit than those that compose the central and within the consolidated rock. In localized areas, con- northern parts. solidated rock may be capable of storing and transmit- The difference in geophysical responses of the ting large quantities of water. For example, wells in volcanic rocks can be seen on profiles shown in figures fractured volcanic rocks beneath the southeastern part 7 -9. On the basis of geophysical interpretations and of the valley reportedly have produced an average 300 information from drillers' logs, Tertiary rocks that to 400 gaUmin and as much as 1,500 gaVmin during a make up the Pah Rah Range were modeled as two sep- 5-day aquifer test (W.E. Nork, Inc., 1988a; site 70, fig. arate lithologic units (units 3 and 4 in figs. 7-9 and table 3 of this report). Anomalous spikes on the focused- 3) that extend beneath the eastern and southern parts of resistivity and acoustic logs collected by Washoe the valley. Lithologic unit 4, which represents volcanic County (1993, p. 5) at well7} suggest that the volcanic rock, was used to simulate measured magnetic and rock is highly fractured and that it may yield water in gravity highs on profiles B-B' and C-C centered at the interval between 180 and 315 ft. Below this inter- about the 4-mi mark along the profiles (figs. 8 and 9). val, the rock has considerably fewer fractures and pre- These measured highs are assumed to be caused by a sumably lower permeability. However, buried northern extension of volcanic rocks of the most con- solidated rock in the study area produce southern boundary and, as a result, are assumed to have about l0 to 40 gaVmin different geophysical properties than volcanic rocks and the probability of intercepting fractures represented by lithologic unit 3. The measured mag- within consolidated rock for developing high-yield netic and gravity lows along profile A-A'were simu- wells is low. lated by lithologic unit 3 (fig. 7). This interpretation of A relatively transmissive hydraulic connection two volcanic units is in agreement with the geologic between basin fill and volcanic rocks in the southeast- map of Bell and Bonham (1987), who differentiated the ern part of the basin was suggested by pumping effects Lousetown Formation of the Pah Rah Range from the on vertical gradients measured in nearby observation Alta Formation, which forms the southern boundary of wells. Hydrographs for November 1993 through Sep- the study area. Although detailed investigation of the tember 1994 from site72, which is a well screened in hydrologic character ofthese Tertiary volcanic rocks is fractured volcanic bedrock, and site 73, which is a well beyond the scope of this study, their hydrologic impor- screened within the overlying basin fill, are shown in tance at a regional scale may be significant. Cretaceous figure 11. These wells are next to each other about 0.5 intrusive rocks, which form the western and northern mi from the contact of the basin with rocks boundaries (fig. 4), and which were modeled to under- fill of the Pah Rah Range (fig. lie much of the study area, have virtually no interstitial 3).The upward vertical gradient measured between permeability. Thickness of the basin fill is presented in the two wells is interpreted as figure 10. ground-water recharge to the basin-fill aquifer gener- ated within the Pah Rah Range to the east. The gradi- ent, however, was reversed in response to ground- EXPLANATION water pumping in the springs area during the 1994 irri-

Younger alluvium (Holocene and Pleistocene) gation season (April through September). During 1995, no additional ground water was withdrawn in the Older alluvium (Pleistocene) springs area; consequently, the vertical gradient between sites 72 and73 returned upward. I Volcanic and sedimentary rocks (Tertlary) w Intrusive igneous rocks (Cretaceous) Basin Fill E Metavolcanic and metasedimentary rocks (Jurassic and Triassic) The structural depression occupied by Spanish Basin boundary Springs Valley is partly filled by interbedded deposits of sand, gravel, clay, and silt derived primarily from '---- Fault lines-Ball on downthrown side. Dashed adjacent where approximately located mountains. These deposits form the basin-fill aquifer, which is bounded and underlain by consoli- c- Selsmic-refraction line dated rock.

HYDROGEOLOGIC FRAMEWORK 17 o .c bF: ! ! o CL o CL sE c .9 EiaE Er o o It agggtgggEEaggiEigiie-9flesiEE: E a€; si$ (! p E E€!EegE* (! (! #gi o co z (to t caseOA a:rcrE$e i*EEaiH: FEEET EEi c o E !-q F^ q oP rB't E U'S a d'=;'i e E (Do35 9 b:- F F 9i:='a= g;r$=€i'g (U€-o0 EpF*, (Da af €g.g P: i'g rr E F I e E.9 ;= o 'j Et?;:sE: E5* i$sE$ oz (, :€ r H > ; E.e i -=a c Fj g;E g;{EE€tF EEtcE o *.EI:=E"E;#A PoEE EEg (,: e5 o L.; o €!E;EE€€ gEE g€aig 6E F€i€Fi;a€ a€EiE.€?€ _cg, ;H.E;: S16s rsj s::;: :( 'i >. >.'=- * = E q; .? (o6cbo E9g 9 F E F90- g!€g g€;E E E gB : E€ € .o: : € E6tr E iEE c6 =E= I! a oF -- cq 2- 9 E'H xe .F od E R'="" iER€E i iEd- o = ErEi;€Ei=EE.* sea (DE e G .= ;i (g o; s'a ao.Eflt= ?! *5'E - EiEIi€P€EEFEe =6xr 5E 6 3 l-u EEg -c ctt E 6 el Eb d -9 E FoE '66E o E I.e ,q 3 E'F g cv €€E!€E EEEgE EEg '= ,: = =g:8.Z 6H-:'o F E og cx= F:q+ v::g E; g,E E5" E}EE €{.e€ €; i qag E'*iEi''EiEgs*E o 6 g E€g E E ! E E i c; € s € €€s oo, E€gE3f Fgg .V

I b ,lza =c x 3 o F. OE .9 or.E E) !F o o d0) o o o0 FU a I C) bo >o o ? o s otV o o ..9 o ER o 60 6ta ct) I h o oF H# VE o tr o on(EA ,{.) (-) F=

18 Hydrogeology and Simulated Effects of Urban Development on Water Resources of Spanish Springs Valley, Nevada 1 1 9.45' 1 19.37',30',

;:,1^..$

j $ ,s\ !lj

i,'{ i lii r\fr-- \1\! I N/ wN\' "' \:i1.:::

\,..W\ 1\\\'q- :/.,t.1 ") ':.7:'i..:aiQ-;;--"-j ;22::---:::-:-,,-:..':ill* i,a.---* ffi)T 7'tr1(::-:', 1f'

Truckee Meadows

Base from U.S. Geological Surv6y data, O 1 2 3MILES Geology modified lrom Bonham (1969), '| :100,000-scale, 1980:Universal Transverse Stewarl ( 1 980), Cordy (1 985), and Mercator prcjection, Zone 1 1 F-r-Lr---+------Bsll and Bonham (1987) O -I 2 3KILOMETERS EXPLANATION

ffi.N Basln flll Llno ot egual magnstlc IntensltlFshows total-intensity magnetic lield of Earth relative rock I Consolidated to arbitrary datum (1,000 feet above land Basin boundary surface). Interval, in nanoleslas, is variable. Magnetic contouring by D.L. Berger, 1994; A-A' Location ol geologlc sectlon- based on data lrom Hildenbrand and See ligures 7, 8, and I Kucks (1988)

Figure 5. Total-intensity magnetic field, Spanish Springs Valley area, west-central Nevada.

HYDROGEOLOGIC FRAMEWORK 19 1 19"37',30',

u"#

39"37'30',

4Xuu Truckee Meadows

from Bonham (1969), Base trom U.S. G€ological Survey data, O 1 2 3MILES Geology modifi€d Stewart (1980), Cordy (1985), and 1 :'l 00,000-scale, 1 980;Universal Transverss Bell and Bonhaln ('1987) Mercator prcieclion, Zone 1 1 F--r-L-T--/------r O .I 2 3KILOMETERS

EXPLANATION

Basln flll /l:- Lines of equal Bougu€r gravlty anomaly- l?im -..1 Interval 1 millical, at density of 2.67 grams I Consolldated rock per cubic centimeter. Gravity contouring by D.L. Berger, 1994: based in part on data Basin boundary from Saltus (1988) A-A', T::?,il:3?:tn*oT"''*

Figure 6. Complete Bouguer gravity field, Spanish Springs Valley area, west-central Nevada'

20 Hydrogeology and Simulated Etlects of Urban Development on Waler Resources of Spanish Sprlngs Valley, Nevada Table 3. Physical properties for lithologic units in which transverses the northern part geologic section along geophysical profibs, Spanish of this feature, sug- Springs Valley area, west-central Nevada gest that the maximum thickness is at least 1,000 ft. In general, the basin fill is thickest beneath the western Denslty Magnetlc part of Spanish Springs Valley and thins toward the east. Body ilodeled (grams susceptlblllty Beneath the southem part of the valley, depth to bed- No. llthology percubic (centimeter{ram- centlmeter) second unlts) rock is less than 100 ft and becomes shallower toward the southem boundary. On the basis of data from an Proffle A-A'(ffgure 7) abandoned well in the Dry Lakes area, unsaturated I Granitic rocks 2.75 0.0009 basin fill is at least 350 ft thick and reportedly consists Basin fill l.9l .00001 2 almost entirely of clay. 3 Volcanic rocks 2.46 .0001 Proftle B-B'(ffgure 8) Hydraullc Properiles I Granitic rocks 2.71 0.0006 Two properties 2 Basin fill t.97 .00001 hydraulic used to express water- 3 Volcanic rocks 2.42 .0007 bearing characteristics ofaquifers are hydraulic con- 4 Volcanic rocks 2.75 .003 ductivity and storage. Hydraulic conductivity is a Profile C-C (figure 9) measure of the ability of an aquifer to transmit water. The term "storage coefficient" is used to describe the I Granitic rocks 2.71 0.001 2 Basin fill 1.90 .00001 water-storage capabilities of an aquifer. 3 Volcanic rocks 2.47 .0001 Typical values of hydraulic conductivity for Volcanic rocks 2.72 .003 4 coarse-grained deposits (sand and gravel) in other basins of Nevada range from 5 to more than 150 ff/d Area! Extent Thlckness and (Harrill, 1973, table 5; Berger, 1995, p. 2l). In fine- The areal extent of the basin-fill aquifer is approxi- grained deposits (clay, silt, and fine sand), the range mated by the contact between consolidated rock and is generally from 0.1 to 4 ft/d (Harrill, 1986, p. 10). basin fill along the periphery of the valley floor. Total sur- Hydraulic conductivity can be determined from esti- face area of basin fill is about 34 mi", almost 43 percent mates of transmissivity and aquifer thickness. Trans- of the total drainage area of Spanish Springs Valley. The missivity values determined from results of several basin fill is bounded on the east by the Pah Rah Range aquifer tests were used to calculate hydraulic conduc- and on the west by Hungry Ridge and its southern exten- tivities, which ranged from 0.5 to about 12 ftld (Shaw sion. On the north, an alluvial divide exists between Engineering, 1984, appendix; Guyton and Associates, Spanish Springs and Warm Springs Valleys. Results of a 1986, p. 6l; W.E. Nork,Inc., 1987, 1988b, 1989; seismic refraction survey (line A, fig.4) and drill-hole Washoe County, 1991). Data from these aquifer tests data indicate that this topographic divide is underlain by represent transmissivities of mixtures of coarse- and consolidated rock at shallow depths (less than 50 ft). At fine-grained deposits within the upper 330 ft of satu- the southern boundary ofthe study area, the saturated rated basin fill. basin fill in Spanish Springs Valley may be continuous Analysis of geophysical and lithologic logs and with saturated basin fill of Truckee Meadows. This grain-size distributions collected from some of the boundary which is not a topographic divide, is underlain observation wells drilled as part of this study (wells 7, by consolidated rock at depths of less than 20 ft. Basin fill ll, 15,51, and 72;Washoe County, 1993) provided also occupies the structurally controlled basins ofthe Dry qualitative estimates of the ability of the basin-fill aqui- Lakes area in the southeastern part ofthe study area. fer to transmit water. In general, basin fill that originates Wells drilled in Spanish Springs Valley range in in upland volcanic-rock tenains is predominantly com- depth from several tens of feet to more than 800 ft, with posed of finer grained deposits, resulting in an overall most completed in basin fill. Discrepancies in basin-fill lower hydraulic conductivity than basin fill derived thickness as reported on drillers' logs for several wells from granitic rocks, which is predominantly sands and limited the areal use of these logs as a tool to estimate gravels. Hydraulic conductivity beneath the deepest basin-fill thickness. The basin-fill-thickness map (fig. wells is unknown, but probably is lower than that of the l0), which was developed partly by geophysical meth- upper 330 ft due to additional compaction and indura- ods, indicates that basin fill is thickest along a northeast tion. The distribution of hydraulic conductivity within trending hough-like feature close to the mountain front the basin-fill aquifer was refined by using the ground- of Hungry Ridge. Results from profile A-A' (fi5.7\, water flow model.

HYDROGEOLOGIC FRAMEWORK A A' -1 00 I do ++++*+ iDdj . F5p -i.uzz=o Calculated *".^(), 361 9==- -400

-'t70 E -172 * .-- uJ> Measured -174

-176 ;;s< r.u -l -178 t4F>F>= Calculated i)z -1 80 =#=u(, -'t82

6,000

FJ TIJ I.IJ 5,000 r.! > tL uJ :< 4,000 HH tr>-) LlJ 3,000 F\J -J(D 2,000

1,000 1234 DISTANCE ALONG LINE OF SECTION, IN MILES

EXPLANATION

Well---Penetrates consolidated rock u nderlying basin fill 300 feet below land surface

Figure 7. Measured and calculated total-intensity magnetic anomaly and complete Bouguer gravity anomaly, and generalized geologic section A-A'showing distribution of lithologic units determined from geophysical modeling, Spanish Springs Valley, west-central Nevada. See figures 5 and 6 for section location and table 3 for physical properties of lithologic units.

22 Hydrogeology and Simulated Etfects ol Urban Development on Water Resources of Spanish Springs Valley, Nevada B B' 100

I cfo bdl Calculated F5-fi + zFo -1 00 -i.uz + + ++ =6'P1=- '\M"".rr"o

-'t70

E uJ> -172 J =l a

E=4 -1 76 F>= --Measured ulF> -178 + *'#= 86 -180 + -182

7,000

6,000 FJ [! Lu uJ> 5,000 rL tu

=i,,i [u 4,000 6@ f LrJ F> 3,000 JcOtro 2,000

1.000 234

DISTANCE ALONG LINE OF SECTION. IN MILES

EXPLANATION

Wdl-Penetrates consolidated rock underlying basin fill 600 feet below land surface

Figure 8. Measured and calculated total-intensity magnetic anomaly and complete Bouguer gravity anomaly, and generalized geologic section 8-B showing distribution of lithologic units determined from geophysical modeling, Spanish Springs Valley, west-central Nevada. See figures 5 and 6 for section location and table 3 for physical properties of lithologic units.

HYDROGEOLOGIC FBAMEWORK c c' 100 bdlLcio F5p t zFo -1 00 -.uz IY--carcurated ?===62 + +++ -300

-172 (r. uJ> -174 Measured Calculated -176 ++ ;E; *\,:.//- + r.r.t < -l -178 + F>= + 14F > + *3= -1 80 Eo -182 t++

-1 84

6,000 FJ UJ IU 5,000 IIJ > t! Lu :< 4.000 83 F>f LIJ 3,000 rCl 2,000

1,000 2345 DISTANCE ALONG LINE OF SECTION, IN MILES

EXPLANATION

WelF--Penetrates consolidated rock u nderlying basin fill (A) 460 feet and (B) 180 feet below land surface

Figure 9. Measured and calculated total-intensity magnetic anomaly and complete Bouguer gravity anomaly, and generalized geologic section C-C showing distribution of lithologic units determined from geophysical modeling, Spanish Springs Valley, west-central Nevada. See figures 5 and 6 for section location and table 3 for physical properties of lithologic units.

24 Hydrogeology and Simulated Effects of Urban Development on Waler Resources of Spanish Springs Valley' Nevada 1 19.45' 119.37'30"

Bas€ from U.S. Gmlogical Suryey data, O 1 2 3MILES Geology moditied trom Bonham (1969), 1:100,000-scale, 1980i Univarsal Transverse Stewart (1 980), Cordy (198s), and Mercator prcjetion, Zone 11 F__r-_Lr___i____J Bell and Bonham ('!987) O 1 2 3KILOMETERS

EXPLANATION ffi Bastntill Basin boundary Llne consotidatedrock of equal basin-fill thlckness- I -100- Interval 100 feet. Datum is land surface

Figure 10. Thickness of basin fill, Spanish Springs Valley, west-central Nevada.

HYDROGEOLOGIC FRAMEWORK 4,486.0

4,485.5 F t.U uJ l.L z 4,485.0

{9u.l 5 4,484.5 F:<-J

4,482.5 N D J FMAM J J ASOND J FMAM J J A S 1994 1995 WATER YEAR '1993-September Figure 11. Water levels for sites 72 and 73, November 1995, Spanish Springs Valley, west-central Nevada.

Storage coefficient is defined by Lohman (1972, coefficients were estimated by multiplying the thick- p. 8) as the volume of water an aquifer releases or takes ness of the deposits (in feet) by I x I 0-6, as suggested by into storage per unit surface area ofaquifer per unit Lohman (1972, p. 53). In the study area, thickness of change in head. The amount of ground water available saturated basin fill ranges from 0 to about 1,000 ft. If from storage in basin-fill aquifers depends on whether water yield is entirely from the expansion of stored the aquifer is under unconfined or confined conditions. water in the confined aquifer and none is from the com- Under unconfined conditions, the storage coefficient is paction of fine-grained material, the storage coefficient virtually equal to specific yield, which is the amount of for confined deposits that are 100 to 1,000 ft thick water released from storage by gravity drainage. Water would range from 0.0001 to 0.001. released from storage under confined conditions potentially recoverable water stored in basin depends on the elastic characteristics ofthe aquifer and The the expansion of water. Storage coefficients for con- fill can be estimated as the product of an area, a thick- fined aquifers are three to five orders of magnitude ness, and a specific yield. Thickness of saturated basin smaller than the specific yield of unconfined aquifers. fill was determined from data shown in figure l0 and Specific yield of basin fill ranges from about 30 depths to water measured in December 1994. The percent in well-sorted sands or gravel to less than 5 per- total amount of recoverable water stored in the basin- cent in compacted clay (Johnson, 1967). Estimates of fill aquifer in the upper 330 ft of saturated basin fill, specific yield in the upper 330 ft of saturated basin fill assuming a range of specific yield of l0 to 20 percent, (unconfined conditions) in Spanish Springs Valley is estimated to be between 300,000 and 600,000 acre-ft. average about I 3 percent and range from I 0 to about 20 percent on the basis ofbasin-fill textures reported from drillers' logs and aquifer tests (W.E. Nork, Inc., 1988a, HYDROLOGIC SETTING 1988b). Values of storage coefficient for confined Spanish Springs Valley lies at the extreme west- basin-fill aquifers are on the order of 0.005 or less (Fet- ern margin of the Great Basin desert region and within ter, 1994). Analysis of a short-term (980 minutes) aqui- rain shadow of the Sierra Nevada. These physio- fer test at a well near the center of the valley resulted in the to environ- a storage coefficient of 0.0005 (D.A. Mahin, Washoe graphic features create an arid semi-arid County, written comnun., 1984). For deeper parts of ment that dominates the hydrologic setting of the study the basin fill assumed to be confined, aquifer storage area.

Hydrogeology and Simulated Effects of Urban Development on Water Resources of Spanish Springs Valley, Nevada Climate and Vegetation downward slope indicates below-average precipita- tion. A trend of below-average precipitation is indi- The principal sources of moisture to the region cated beginning about 1984. Hydrologic data were are the Pacific Ocean and the Gulf of California. The collected as part of the current study during this trend most important source is the Pacific Ocean, which pro- of below-average precipitation, which may have some vides moisture from October to June; the Gulf of Cali- effect on water levels measured during this study. Pre- fornia produces most of the moisture from July to mid- cipitation at the Reno airport was 75 percent of the (Houghton, p. September 1969, 5; Brenner, 197 4). long-term average during 1992,92 percent during Moisture-laden air moving eastward from the Pacific 1993, and T2percent during 1994. Ocean cools and condenses as it rises over the Sierra Nevada, resulting in as much as 70 in. of precipitation The estimated distribution of precipitation in on the west slopes, with considerably less in valleys to Spanish Springs Valley, shown in figure 13, was devel- the east (Klieforth and others, 1983, p. 4). During sum- oped on the basis of a regression model between aver- mer months, tropical air from the Gulf of California age annual precipitation and altitude for 34 stations produces scattered but intense convective showers. The (table 4). Data for complete full-year periods were used sumrners are hot and dry with daytime temperatures to determine the average annual precipitation repre- occasionally exceeding 100"R and winters are cool, senting that period ofrecord for each station. A long- with temperatures sometimes falling below 0oF. term average was then estimated for each station from No long-term precipitation stations operate the 57-year period of record (1938-94) at the Reno- within the study area; however, precipitation data are airport station. The calculated linear regression equa- available for calendar years 1938-94 at the Reno air- tiono shown in figure l4A,was selected by evaluating graphs port about 5 mi south of the study area. The Reno air- ofresidual values versus predicted precipitation generated port is at about the same altitude as the south end of from several forms of the regression equa- Spanish Springs Valley. During this period, average tion. Using a linear relation, residual values, which are annual precipitation at the Reno airport was about 7.2 the difference befween measured and predicted precip- in. (fig. l2A),withamaximum of 13.2 in. (1983)anda itation, are scattered an approximately equal distance above and below zero minimum of I .6 in. (1947). Precipitation in 1993 and in figure l48. The residual graph 1994 was greater at the Reno airport than recorded at indicates that a linear model does not violate the the station on the valley floor in Spanish Springs Valley assumptions of normal regression analysis and thus, a (site 83, table 4). Weather data collected at the Reno linear regression is a valid representation ofthe precip- airport, however, were assumed to be fairly typical of itation-altitude relation derived from the 34 sites. The the study area. coefficient of determination indicates that about 62per- cent of the variance within the data can be explained Annual precipitation on the valley floor in Span- by the linear regression model. A longer or shorter record ish Springs Valley is generally less than 8 in. and may result in a slightly different regression annual evaporation is probably greater than 40 in. (Van equation. In addition, estimates average precipitation Denburgh and others, 1973, p.53). The surrounding of annual above 7,000 ft are increasingly uncertain mountains receive 9 to I I in. in an average year, and because this relation is not supported high-altitude more than l3 in. may fall in the higher altitudes of the by stations above 6,960 ft. Only about I acres near Pah Rah Range. The total precipitation estimated I Spanish Springs Peak in the Pah Rah Range are above ft. within the study area is about 36,000 acre-ft annually, 7,000 of which nearly 3,500 acre-ft falls within the topo- The pattem of natural vegetation in Spanish graphically closed Dry Lakes area in the southeast Springs Valley reflects the general long-term distribu- (table 5). tion of precipitation and average depth to water. The The greatest precipitation in the study area falls principal vegetation is referred to as northern desert from December through February, mainly in the form shrub (Lorain and Tueller, 1976, p.22) and is domi- of snow and freezing rain; minor amounts of precipita- nated by sagebrush (Artemisia tridentata) with local- tion accompany summer convective storms. Succes- ized areas of rabbitbrush (Chrysothamnus sp.), sive years with above- or below-average annual greasewood (Sarcobatus vermiculatus), shadscale precipitation are shown by the cumulative deparhrre (Atriplex confertiftlia)o and several other shrubs and from average in figure l2B. An upward slope to the grasses. In areas having altitudes of5,500 ft or higher, right indicates above-average precipitation and mixed woodlands ofjuniper (Juniperus utahensis) and

HYDROLOGIC SETTING Annual average precipitation is 7.2 inches lor 1938-94

Ifr10 C)z z8 I2 s6F o- O t4UJ TL

oTIJ t Lll <7

4Ftrk=0 tor.rJ i- iEEIU o-JAo- of,ur< ttr Z =<=z J l =l O

CALENDAR YEAR

Figure 12. Annual precipitation (A) and cumulative departure from average precipitation (B) at Reno airport lor S7-year period of record, 1938-94, west-central Nevada (data from National Climatic Center).

2g Hydrogeology and Simulated Effects of Urban Development on Water Resources ol Spanish Springs Valley, Nevada Table 4. Average annual precipitation at stations in and near Spanish Springs Valley, west-central Nevada

[Data from National Climatic Center, Harrill (1973), Joung and others (1983), Klieforth and others(1983), and H.E. Klieforth (Desert Research Institute, written commun., 1995).1

Averageannual Estimatedlong-telm Altitude Period of full- precipitation average annual Stationl (feetabove year record used for period of record precipitation2 sea level) (calendar years) (inches) (inches)

Churchill Canyon can No. 1l 6,960 1964-80 13.39 13. I Churchill Canyon can No. l0 6,400 I 964-80 13.3 I 13.0 Virginia City 6,340 r968-93 t4.25 l+.J

Churchill Canyon can No. l3 6,320 1964-77 1 1.03 10.9 Churchill Canyon can No. 12 6,020 1964-77 10.79 10.6 Churchill Canyon can No. 9 5,910 1964-80 12.60 12.3 Cumow Canyon, site 79 5,709 t993-94 37.9 9.6

Churchill Canyon can No. 5 5,700 1964-80 7.76 7.6 Churchill Canyon can No. 7 5,680 1964-77 8.78 8.6 Churchill Canyon can No. 8 5,550 l 964-80 10.68 10.4 Hungry Ridge South, site 8l 5,358 1993-94 37.8 9.5

Churchill Canyon can No. 6 5,330 l 964-80 7.82 7.6 Churchilt Canyon can No. l4 5,320 1964-77 8.82 8.7 36.8 Dry Lakes, site 85 5,282 t993-94 8.3 Churchill Canyon can No. l6 5,220 1964-77 7.58 /.) Churchill Canyon can No. l5 5,200 1964-77 7.10 7.0 Canoe Hill, site 87 5,200 t993-94 16.3 7.7

Churchill Canyon can No. 4 5,070 l 964-80 7.85 7.6 37.4 Bacon Rind Flat, site 78 { os, 1993-94 9.0

Stead 5,046 1952-66 I l.l0 8.4 3t Hungry Ridge North, site 80 4,954 1993-94 t 9.0

Churchill Canyon can No. 2 4,760 l 964-80 6.35 6.2 Vista, site 84 4,7 tO t993-94 36.4 7.8 Northwest Reno-HK 4,690 1973-94 9.t7 9.5

Carson City 4,651 1949-93 10.58 10.3

Churchill Canyon can No. I 4,620 l 964-80 6.84 6.7 Junction 395/47-8 4,590 1969-94 t0.62 10.7 36.5 Oasis Trailer Park, site 86 4,55l t993-94 7.9 Fire station, site 83 4,496 t993-94 34.1 5.0

Reno Airport 4,404 I 938-94 7.t9 I.Z

Churchill Canyon can No. 3 4,390 I 964-80 6.36 6.2 Femley 4,160 t94l-89 6.05 6.0 Sutcliff 3,980 1968-69, r971, 7.4r 6.3 1980-84,1987-90

Nixon 3,900 1929-47, t949 6.99 6.1 1952,1963-69

' Stations are listed in order ofdescending altitude. Bold site numbers can be used to locate stations in figures 3 and 13. 2 Adjusted to long-term averages on the basis ofcomparison with 57-year period ofrecord, 1938-94, at the Reno airport 3 Partly estimated.

HYDROLOGIC SETTING 29 Base from U.S. Geological Suruey data, O 1 2 3MILES Geology moditied from Bonham (1 969), (1980), (1985), and '! :1 00,000-scale, 1 980;Universal Transverse Stewart Cordy (1987) Mercator projection, Zone 1 1 F=-+--rl--_ Bell and Bonham O 1 2 3KILOMETERS

EXPLANATION

tffi Basinritl Basin boundary I consotidatedrock B Line of equal precipitation-lnterval 1 inch - 87- O Precipitation station and site number-See tables 1 and 4

Figure 13. Precipitation stations and average annual precipitation, Spanish Springs Valley, west-central Nevada.

Hydrogeology and Simulated Eftects of Urban Development on Water Resources ol Spanish Springs Valley' Nevada 16 a -lu 14 z(J z 2 't2 tr F oE LlJ 10 E. (L J P = (0.00241xA) - 3.71, where P is l predicted mean annual precipilation, z and A is station altitude (f = 0.62 and z n=34) (,r.u E u.,l

4l 3,000 4,000 5,000 6,000 7,000 8,000 ALTITUDE, IN FEET

a B -TJJ uzoo a--s= gou- lt I -1 a t!(rHN t! 6o- a zo

567891011121314

PREDICTED AVERAGE ANNUAL PRECIPITATION, IN INCHES

Figure 14. Relation of average annual precipitation to altilude for 34 precipitation stations used in determining distribution of precipitation (A)and relation of difference between predicted and measured precipitation (residual values) to predicted average precipitation (B/, Spanish Springs Valley, west-central Nevada.

HYDROLOGIC SETTING 31 Table 5. Estimated average annual precipitation and drained area on the valley floor in the northern part of ground-water recharge in Spanish Springs Valley area, the study area supports a shallow ephemeral pond that west-central Nevada probably has existed since long before settlement of the Average annual Average annual valley, based on archaeological evidence found near precipitation 2 recharge Precip- the pond. Currently, the pond is maintained by pumped itation Area Assumed ground water used in a nearby gravel-pit operation. The (acres) zonel Acre-feet3 percent Acre'feet On Ditch has delivered water from the Truckee River (inches) FeeI=^-. ;;;;;F (rounded) of ptec through the southern boundary ofthe study area since it"tio,lP Gouna"a) 1878. The Ditch is unlined throughout its 7-mi length Spanish Springs Valley in Spanish Springs Valley and has numerous take-out More than 12 240 l.l0 260 7 l8 gates to smaller ditches used for flood inigation and 8 to 12 29,130 .78 23,000 3 690 stock watering. The On Ditch terminates in a shallow Less than 8 17,450 .58 10,000 minor 0 pond that also receives discharge water from the adja- Subtotal 46,820 33.000 7t0 cent spring area. Several storage ponds, which cover Dry Lakes area about 140 acres, are within the area outlined by the On More than 12 330 1.08 360 25 Ditch (fig. l). The North Truckee Drain originates near 8to 12 3,780 .82 3,1 00 93 the center of the irrigated lands within the area encom- 4.110 3,500 t20 Subtotal passed by the On Ditch. The drain conveys unused irri- 50,930 36,000 gation water and, to a lesser extent, ground-water I Developed from precipitation-altitude relation as part ofthis study' discharge out of the study area to the Truckee River. 2 Area-weighted average precipitation within individual zones of Flow volumes of the On Ditch as it enters the more than 13 inches, 12 to 13 inches, I I to 12 inches, l0 to I I inches,9 to study area fluctuate annually depending on the avail- l0 inches, 8 to 9 inches, and less than 8 inches. 3 Rounded to two significant figures' ability of Truckee River water, which, in turn, depends on the amount of precipitation that falls in the Siena pinon pine (Pinus monphylla) are found in limited Nevada. Daily mean discharge for the On Ditch (site areas, mainly in the Pah Rah Range. With the onset 88) and the North Truckee Drain (site 89) forMay 1992 of delivered imported water for irrigation in the south- through December 1994 is shown in figure 15. Gener- em part ofthe valley, riparian-type vegetation has ally, delivery of inigation water begins in April and developed along several inigation ditches and numer- ends in late September for years with near-average con- ous shallow ponds. Currently (1994), meadow grasses ditions. Records kept by the Federal Water Master indi- and crop land cover about 980 acres ofthe total ditch- cate that some flow is maintained from October to inigated area (about 3,800 acres), and several hundred about February or March for stock watering. During head of cattl e graze within the area encompassed by the the course of this study, the On Ditch was completely On Ditch. Agricultural lands outside the influence of shut offby June for years 1992 and 1994 in response to the On Ditch are inigated solely with ground water' below-average precipitation (fig. l5). During 1993, The growing season is about 120 to 160 days. when conditions approached average (about 92 per-^ cent), flow remained at inigation levels (about 30 ft'ls and greater) until September, when it decreased to Surface-Water Flow SYstem stock-watering levels (less than about 3 ft'ls). The Surface water in Spanish Springs Valley consists North Truckee Drain fluctuates in direct response to the almost entirely of imported Truckee River water used availability of On Ditch water and the application of to support agriculture in the southem part of the valley' water for inigation. During periods of no flow in the However, several dry channels throughout the study Orr Ditch, the base flow of the North Truckee Drain area indicate that, occasionally, sufficient precipitation averages about 0.1 ft3ls and represents part ofthe falls to produce some runofffrom surrounding moun- ground-water discharge leaving the valley. tains. Annual potential runoffof 1,500 acre-ft was esti- For calendar years 1976 through 1984, average mated by Rush and Glancy (1967,p. 18) on the basis of surface-water inflow to the study area from the On stream-channel geometries; however, no streamflow Ditch was about 16,600 acre-ft annually (Federal Water greater than about 5 gallmin (0.01 ftrls) was observed Masteq written commun., 1994\.In 1985, flow in the during their brief period of fieldwork. An intemally On Ditch was reduced to comply with the Final

Hydrogeology and Simulated Elfects ol Urban Developmenl on Water Resources ol Spanish Springs Valley, Nevada 100 100 e06 zo F ()o 80F v)9t 10 (E 70 lrl (L "to-F UJ(r I.JJ 60 t.u 9P I cccc 91 50 9< co

0.01 M J J ASOND J FMAM J JASOND J FMAM J JASOND 1992 1993 1994 1995 WATER YEAR

Figure 15. Daily mean discharge of Orr Ditch and North Truckee Drain, May 1992-December 1994, Spanish Springs Valley, and percent of average annualprecipitation at Reno airport, 1992-94, west- central Nevada.

Federal Decree for the On Ditch Company (Hadiaris, Table 6. Summary of flow for Orr Ditch and North Truckee Drain, 1976-94, 1988). Consequently, annual inflow to the study area near Spanish Springs Valley, west-central Nevada for 1985 through 1994 averaged about 9,220 acre-ft. Surface outflow from the study area through the North [Values are acre-feet (rounded); -, ungaged] Truckee Drain for 1977 through 1984 averaged about North Truckee Difference (Jrr UIICn 9,430 acre-ft annually. No data are available for the Year'w^-- I (inflow minus i",rc dii North Truckee Drain for 1985 through April 1992. lsite,_Drain 89) outflow) For calendar years 1993 and 1994, outflow ofthe 1976 t0,620 North Truckee Drain was2,710 and2,370 acre-ft, r977 14,t40 8,1 50 5,990 1978 19,980 I 0,730 9,250 respectively. On the basis of available data, the average 1979 19,580 9,710 9,870 difference between surface-water inflow and outflow I 980 17,690 9,650 8,040 for 1977 through 1984 was 7,930 acre-ft/yr. This differ- l98l t7,940 6,640 I I,300 ence represents water imported from the Truckee River 1982 t6,440 8,350 8,090 that either was consumed in Spanish Springs Valley by 1983 17,050 I I,850 5,200 r 984 16,030 r 0,330 5,700 to the evapotranspiration or was recharged shallow r 985 t2,540 ground-water system. A summary of flow volumes for 1986 9,610 the Orr Ditch and North Truckee Drain is in table 6. 1987 9,980 1988 t0,320 Stable-isotope ratios of oxygen 16180; and 1989 n,260 hydrogen (6D) and the activity of tritium were used 1990 10,180 -- to characterize imported surface water. The isotope l99l 6,850 1992 4,350 composition of Truckee River water can be defined by 21993 l I,640 2,7 t0 8,930 an envelope (fig. l6) that encompasses data from 47 2ggq 5,510 2,370 3,140 samples collected at Farad, Calif., along the Truckee ' Data from Federal Water Master, except as indicated; River (Alan H. Welch, U.S. Geological Survey, written record for 1985-91 collected during irrigation season only. 2 commun., 1994). The mean global meteoric-water line Data collected as oart ofthis studv.

HYDROLOGIC SETTING 33 [6D = 8(6180) + l0;Craig, l96l] also is shown Sources and Movement of Ground Water (fig. l6) and represents the isotope composition of pre- cipitation. Average 6l80 and 6D of Truckee River sam- Virtually all ground water in Spanish Springs is derived from two sources: imported surface ples were -10.2 and -82 permil, respectively. Isotope Valley water and precipitation that falls within the drainage ratio of the Truckee River samples showed some vari- basin. Most ground water is found in saturated uncon- ability, partly due to seasonal evaporation and to tem- solidated and partly consolidated alluvium of the basin in upstream inflow. Samples from the poral changes fill under water-table and confined conditions. Ground On Ditch and North Truckee Drain collected at the water also is found in localized areas of highly frac- gaging stations (sites 88 and 89) plot near the edge tured rock, generally under confined conditions. of the envelope. The tritium activities for the Orr Ditch Ground water moves along paths of least resis- Truckee Drain samples were 17 and 15 and North tance, from areas ofhigh hydraulic head to areas oflow pCilL, respectively. Stable isotope compositions for hydraulic head. Water-level contourmaps, which show selected ground-water and surface-water sites in the lines of equal hydraulic head, were constructed from study area are presented by Emett and others (1994, water-level measurements collected in December p. 557) and Clary and others (1995,p.732)' l992,May lgg4,andDecember 1994 (figs. 17, 18, and

J

-60 o-ff z j ? -ro FUJ f, ollJ s -100 J ouJ

-120

-1 40

DELTA OXYGEN-18, IN PERMIL

EXPLANATION - Boundary of onvslopo sncompasslng data trom 47 Truckee Rlver samples-From Alan H Welch (U.S. Geological Survey, written commun.. 1994)

Slto and number (tablo 1 and tlgure 3|-Solid symbol indicates tritium activily above 1 picocurie per liter

O 12 Ground-water sampling site a 88 lmported surface-water sampling site in sampled groundwater and imported surface watoer,.SP?l':n Figure 16. Relation of deuterium to oxygen-18 , Sp-rings Valley and Truckee River, wesi-tentral Nevada. Equation for meteoric water line, 6D=8(6'oO)+10 (Craig, figure 3. t iet 1. lrlumOer corresponds to sample site listed in table 1 and location shown on

Hydrogeology and Simulated E lects of Urban Development on Water Resources of Spanish Springs Valley, Nevada 19, respectively). The water-levelcontours were con- generally beneath the area of irrigation. These declines structed from measurements in wells reported to be suggest that the ground-water mound that develops dur- screened within basin fill. For the most part, the ing the irrigation season dissipates with time. screened interval or the gravel pack is through the Ground-water recharge from precipitation takes entire saturated thickness penetrated by the well. This place in or adjacent to the mountains through weathered results in a hydraulic head that represents a composite or fractured bedrock or when intermittent runoff infil- water level not necessarily the same as the water table; trates dry channel deposits. Precipitation that falls on however, owing to the discontinuous nature of the the valley floor is considered to be a negligible source basin fill, the upper 200 to 300 ft probably acts as a sin- of recharge, although some recharge may be generated gle system. The December 1992 contour map (fig. l7) during extremely localized rain. In eastern and western refl ects ground-water conditions following several parts of the valley, ground water in basin fill generally years of below average precipitation and no flow in the flows toward the center of the basin, away from On Ditch since early June 1992. Water-level contours recharge-source areas in the mountains. The stable-iso- for May 1994 (fig. l8) show conditions following a tope composition of this water, sampled from several year ofnear-average precipitation (1993) and a subse- wells along edges of the basin and away from the effects quent extended period of flow in the On Ditch. Water- of recharge from the Orr Ditch, is closely grouped when level contours for December 1994 (fig. l9) show con- plotted on a graph of 6ruO versus 6D in figure l6 (sites ditions following a season of below-average precipita- 2,9,12,27,28,30,39,41, and 72). Samples from these tion and no flow in the On Ditch since June 1994. sites are isotopically lighter than Truckee River water Although the configuration of the shallow ground- and are assumed to be indicative of locally derived water surface in Spanish Springs Valley changed sea- recharge on the basis of comparison to the envelope sonally, the direction of ground-water flow generally defined by Truckee River water. Ages estimated from remained the same during these periods. In addition, a CFC data (table 7) collected at these sites (except site ground-water divide near the center of the valley is 72) indicate that the sampled ground water was more present during each time period. than 20 years old and probably more than 40 years old (Eurybiades Busenberg and L. Neil Plummer, U.S. During the inigation season, transmission losses Geological Survey, written cornmuns., 1993, 1995). along the On Ditch and infiltration of water applied The tritium activities in these waters were below the to fields result in the greatest source of recharge to analytical reporting limits (less than I pCi/L), indicat- the basin-fill aquifer system. Because of near-average ing aquifers sampled by these wells have not yet precipitation in 1993 (about 92 percent ofaverage, received recharge from precipitation that fell prior to from data collected at the Reno airport, fig. l5), the On about 1952. Tritium levels of precipitation prior to Ditch flowed almost continuously into Spanish Springs above-ground nuclear testing (1952) are assumed to be Valley from April 1993 through June 1994. Changes in similar to what present (1994) levels are approaching, ground-water levels shown in figures 20 and 2l illus- about 25 pCi/L (Welch,1994,p. l6). Radioactive decay trate the effects that near-average flow conditions have of 25 pCilL activity for about 60 years results in a tri- on the shallow ground-water system in the basin fill. tium level less than I pCilL. The absence of tritium Water levels beneath the inigated area rose 5 ft or more activiry in these waters and their relative positions on from December 1992 through May 1994 in response to the 5'60 and 6D plot in figure l6 support the conclusion recharge from imported surface water (fig. 20). Water that sampled gtound water from these peripheral wells levels in wells adjacent to but outside the inigated area probably originated as precipitation within the study also were affected by the On Ditch, with rises of I to 4 area. ft. The distribution of rises in water level defines the Recharge generated from about 3,500 acre-ft of extent of a ground-water mound that develops beneath annual precipitation estimated to fall within the topo- the southem part ofthe valley during periods offlow in graphically closed Dry Lakes area (table 5) may enter the On Ditch and subsequent application of inigation the basin fill of Spanish Springs Valley at depth through water. Declines in water levels for the same period fractures within the Pah Rah Range in the southeastern were measured generally in the northern part of the val- part ofthe study area. Stable-isotope composition (6180 ley in response to ground-water withdrawal with no and 6D) of sampled water from two flowing wells (sites substantial source of recharge. Between May and 52 and 53, fig. 3) near the spring area on the valley floor December l99a $g21), water-level declines were was isotopically lighter than Truckee River water.

HYDROLOGIC SETTING 35 1 19'45' 119'37'30"

39"37'30"

Base lrom U.S. Geological Suruey data, O 1 2 3MILES Gedogy modilied lrom Bonham (1969), 1:100,000-$ale, 1 980: Universal Transverse Stewart (1980), Cordy (1985), and Msrmtor prcjection, Zone l1 Bell and Bonham (1987) O 1 2 3KILOMETERS

EXPLANATION f-l Basinfill - Water-level contour, December 1992- Basin boundary -4,490- Shows altitude of water level in well Wil consottdated rock screened in basin fill. Contour interval a "" Well and site number- is 10 feet; dashed where approximately 4.397 Location of well used located. Datum is sea level as control ooint. For selected wells, water- level altitude is indicated

Figure 17. Ground-water levels in basin fill, December 1992, Spanish Springs Valley, west-central Nevada.

36 Hydrogeology and Simulated Effects of Urban Development on Water Resources of Spanish Springs Valley, Nevada 1 1 9"45' 119"37'30"

39"37'30"

Base trom U.S. Goologiml Suruey data, O 1 2 3MILES Geology moditied from Bonham (1969), 1 : l00,00Gscale, 1980;Universal Transverse Stewart (1980), Cordy (1985), and Mercator projection, Zone 1 1 F-r-Lr----rt------Bell and Bonham (1987) O 1 2 3KILOMETERS

EXPLANATION I f I Baslnflll Water-level contour, May 199tt- Basin boundary -4,4s0-- Shows altilude of water level in 68 n consottdatedrock well screened in basin fill. Contour a Well and site number- interval is l0 leet; dashed where 4.397 Location of well used approximately located. Datum is as control ooint. For sea level selected wells, water- level altitude is indicated

Figure 18. Ground-water levels in basin fill, May 1994, Spanish Springs Valley, west-central Nevada.

HYDROLOGIC SETTING 37 1 1 9'45' 1 19'37'30"

39"37'30"

Base from U.S. Geological Suruey data, O 1 2 3MILES Geology modified from Bonham (1969), 1:'100.000-scale. l980;Universal Transverse Stewart (1980), Cordy (1985), and (1 Mercator prcj4tion, Zone 11 # Bell and Bonham 987) O 1 2 3KILOMETERS

EXPLANATION

Water-level contour, December 199t1- Basin boundary r--l Basin fiff -4,490- - Shows altitude of water level in well 68 Consolidated rock screened in basin fill. Contour interval is a Well and site number- W well used 10 feet; dashed where approximately 4.397 Location of point. For located. Dalum is sea level as control selected wells, water- level altitude is indicated

Figure 19. Ground-water levels in basin fill, December 1994, Spanish Springs Valley, west-central Nevada.

38 Hydrogeology and Simulated Effects of Urban Development on Water Resources of Spanish Springs Valley, Nevada 'l 19.45' 1 19"37'30"

39'37'30"

Base lrom U.S. Geological Survey data, 0 1 2 3M|LES 'l:100,000-scale, 1980; Universal Transverse Mercator proiection, Zone 1 l |------r-----r_----r----- O 1 2 3KILOMETERS

EXPLANATION MTII Basln fill - - Llne of equal change in ground-water Basin boundary -+5 level, December 1992 to May 199rt- -;- I Consolldated rock Interval 5 feet; dashed where approximately O Well and slte numb€r- located. Negative change indicates decline, 0 Location where change positive change indicates rise in ground-water level was calculated. For selected wells, water- level change is indicated

Figure 20. Change in ground-water level in basin fill from December 1992 to May 1994, Spanish Springs Valley, west-central Nevada.

HYDROLOGIC SETTING 39 1 19"37'30"

39"37'30"

Bas€ trom U.S. Geological Survey data, O 1 2 3MILES Geology modilied from Bonham (1969), 1:100,000-scale, 1980; Universal TransveBe Stewart (1980), Cordy (1985), and Bell and Bonham (1987) Mercator projection, Zone 1 1 F-Lr---+------J O 1 2 3KILOMETERS

EXPLANATION f--*1 Basln fill - - Line of equal change in ground-watel Basin boundary -+5 level, May 1994 to December 199rS- feet; dashed where approximately 68 consolidatedrock Interval 5 Well and site number- ffi located. Negative change indicates decline, a 0 Location where change positive indicates rise change in ground-water level was calculated. For selected wells, water- level change is indicated

Figure 21. Change in ground-water level in basin fill from May 1994 to December 1994, Spanish Springs Valley, west- central Nevada.

40 Hydrogeology and Simulated Effects of Urban Development on Water Resources ol Spanish Springs Valley, Nevada Table 7. concentrations of chlorofluorocarbons (cFC's) and tritium in ground water and corresponding cFC-model recharge ages in spanish Springs Valley area, west-central Nevada

IAbbreviations and symbols: CFC- I I, trichlorofluoromethane (CCI3F); CFC- 12. dichlorodifluoromethane (CCI2F2); CFC- | | 3, trichlorotrifluoroethane (C2CL2F ); pelL. picogram per liter; pCi/L, picocurie per liter; --, not sampled or estimatedi <, below analytical reporting limits. Data from USGS Chlorofluorocarbon Laboratory and USGS National Water Quality Laboratoryl

CFc-model sampre cFc-11 cFc-12 CFC-113 "uT';,(fig.3) oate (F-11, pg/L) (F-12, pg/L) (F.113, pg/L) '":$'f" l;8,1il

2 8t19t93 6.6 I1.4

27 8^7t93 2.2 3.9

4l 8il6t93 t7 10. I

54 8t20193 t.l 1.7

indicates that part ofsarnple water was recharged since | 940, CFC- | I indicates recharge since | 945, and CFC- I l3 indicates recharge as recent as 1966.

This suggests that deep ground water is moving migration of near-surface ground water, analysis of the upward into the basin fill aquifer beneath the southeast- geochemical data suggests that ground water may leave em part of the study area possibly from the Dry Lakes the basin through fractured or weathered bedrock and area. These data also indicate that infiltration of through basin fill. imported surface water appears to be mainly in the Ground water also flows from the ground-water shallow basin fill. divide toward the northern part of the study area. Ground water flows southward out of Spanish Geochemical data from a municipal well (site 21,fig. Springs Valley through the basin fill and probably 3) screened in more than 120 ft of saturated basin fill through fractured bedrock to Truckee Meadows. suggest that the ground water is a mixture of local Water-level gradients to the south are about 16.5 ftlmi recharge and Truckee River water at least 2,000 ft north during the non-irrigation season (December) and about of the Orr Ditch. Using DD values of - I I 6 permil for l9 ftlmi during the inigation season (May). Stable- locally derived recharge and -82.1 permil for Truckee isotope data collected from a well screened in volcanic River water, ground water collected from the well is rock, locatedjust south ofthe southern boundary (site about 35 percent local recharge and 65 percent Truckee 66, fig. 3), indicate that the water is a mixture of locally River water. A modeled CFC recharge date for this derived recharge and Truckee River water. Although sample is 1983 (site 2l , table 7), adjusted for the rela- pumping from this well may facilitate downward tive composition of Truckee River water. In addition,

HYDROLOGIC SETTING 41 the sample contained a tritium activity of l0 pCi/L, wells in the southern part of the study area (sites 6l and implying that a component of the ground water was 66) are a sodium sulfate type water with dissolved- recharged since about 1958 (Welch ,1994, p. l6). These solids concentrations of 2,160 mg/L and 1,200 mglL, results all support the conclusion that imported Truc- respectively. Dissolved-solids concentrations for the kee River water moves northward from the On Ditch. remaining samples, except for those from site 20,range just In the northern part ofthe valley unaffected by from I 56 to 488 mglL. Water from site 20, north of recharge from the Orr Ditch, stable-isotope analyses the On Ditch near the center of the valley, is a calcium from ground water collected at sites 2,9,and l2 (fig. 3) sulfate type with a dissolved-solids concentration of indicate that these samples have no component of Truc- 2,680 mglL. kee River water. However, the 4,450-ft contour sug- Although water-quality determinations are not gests that water generally moves northwestward (figs. implicitly stated in the scope of work for the current | 8 and l9), apparently in response to ground-water study, a brief evaluation of the general ground-water withdrawals from a gravel-pit operation. In addition, quality as it relates to selected standards for beneficial water-level measurements in two observation wells use is warranted. Analyses of several constituents of (sites 3 and 4) in the extreme northem part of the valley concern (boron, chloride, dissolved solids, fluoride, indicate a northem flow direction, more than 200 ft nitrate as nitrogen, and sulfate) were obtained for use in higher than recently measured water levels in adjacent geochemical techniques. Nevada water-quality stan- Warm Springs Valley still farther to the north (Bauer dards for selected constituents were used as a basis for and others, 1996, p. 554). These data suggest that a comparing reported concentrations with concerns for hydraulic gradient exists from Spanish Springs Valley human consumption, aquatic life, irrigation, and water- to Warm Springs Valley. Seismic-refraction data indi- ing livestock. Primary standards for public water sys- cate that basin fill is relatively thin and unsaturated tems were not exceeded for fluoride and nitrate (as beneath the northern topographic divide (line A, fig. 3). nitrogen) in the sampled ground water. Secondary Consequently, for ground water to exit the study area to maximum standards for public water systems, which the north, it must flow through fractured bedrock. Lack are based on esthetic qualities, were exceeded for chlo- of water-level data in the northern part of the valley ride at site 20, for dissolved solids and sulfate at sites prior to this study precludes the determination of flow 20, 6 l, and 66, and for fl uoride at site 22. W ater- quality conditions before large ground-water withdrawals standards for support of aquatic life and for inigation began. However, results of a two-dimensional steady- purposes were exceeded for boron at site 2 l. Standards state flow model of conditions representing 1980 water for watering livestock were not exceeded in sampled levels (assumed to represent pre-pumping conditions), ground water. suggest that ground water moved northward toward Warm Springs Valley (Hadiaris, 1988). GROUND.WATER RECHARGE AND DISCHARGE Chemical Composition of Ground Water Components of ground-water recharge to and dis- Ground-water samples from22 wells within the charge from the basin-fill aquifer beneath Spanish study area (fig. 3) were analyzed for overall chemical Springs Valley under natural conditions include composition and are summarized in the trilinear dia- recharge from precipitation, discharge by evapotrans- gram in figure 22. Trllinear diagrams show the chemi- piration, and discharge by subsurface flow to adjacent cal character of water in terms of milliequivalent-per- basins. Under long-term natural conditions, the liter percentages of major dissolved constituents (Hem, ground-water system is in a state of dynamic equilib- 1985). Results of the chemical analyses for samples rium, in which inflow equals outflow, with no apprecia- collected as part of this study are presented by Emett ble net change in storage. Estimates of ground-water and others (1994,p. 557) and Clary and others (1995, p.732). Most ground water sampled in the study area is recharge and discharge under natural conditions were (1967,table20, p. 43). either a sodium bicarbonate (l I samples) or calcium developed by Rush and Glancy ground- bicarbonate (6 samples) type water. Samples assumed They suggested, on the basis of estimated total to represent locally derived recharge, based on stable- water discharge, that about 1,000 acre-fVyr represented isotope composition (fig. l6), generally fall within the inflow to and outflow from the ground-water sys- these two water types. Ground-water samples from tem in Spanish Springs Valley.

Hydrogeology and Simulated Etfects of Urban Development on Water Resources ol Spanish Springs Valley, Nevada /," 7s '"t ',,' ].t.0, .z

T I so6" I

ba

% a 4 !.s crs ^s CALCIUM CHLORIDE PERCENTAGES, ON BASIS OF MILLIEQUIVALENTS PER LITER

Figure 22. General chemical composition of sampled ground water, Spanish Springs Valley, west-central Nevada. Dashed lines and arrows show projection of values for a single sample (from site 20) from triangle to diamond that indicates water type.

In the current study, completely new estimates of secondary recharge from infiltration through septic recharge and discharge for natural conditions were not systems again altered the conditions of equilibrium. developed; instead, the existing ones were evaluated to A diagrammatic representation (fig. 23) illustrates see if any revisions were warranted on the basis of the current conceptualization of several components of information gained since 1967. Because the On Ditch recharge and discharge to the basin-fill aquifer system. has delivered Truckee River water to Spanish Springs These components were estimated during this study Valley for more than I l0 years, the ground-water sys- from field and empirical techniques. The ground- tem was probably in dynamic equilibrium with water- flow model, discussed in later sections of this recharge from the On Ditch and inigated fields prior to report, was used to help quantify those components that 1979. With increased urban development (beginning were either defined by a range of values or empirically about 1979), pumping for public water supply and estimated.

GROUND.WATER RECHARGE AND DISCHARGE b o ; o = og;,- j ,Ps -q) :569t Z:E a E o) 9;FE-O '=c o_ A E E o-cca FA dE e !E '= X ED- o(U ur b5 2 cE o 6v/2,* oo '= Ei g OF gE E ;i i5e uJ ;l o:-3 U)

= 'ac 6 (u 6 ^ 3 E € c ! o .= 3 CI _co .9.

(d- q)

(U -co (D

(D (o = f

c .F

.N f 6 o -

.E

o _co o j c(U 'o c c) = (D (D rt

orzc)0)

O=Llw J= ir3cDh

44 Hydrogeology and Simulated Effects of Urban Development on Water Resources of Spanish Springs Valley, Nevada Recharge from Precipitation the Reno airport in calendar year 1994, ground-water recharge from precipitation in the study area for the Rush and Glancy (1967, table 8), using a method same period was assumed to be aboutT2 percent of the (1949, described by Maxey and Eakin p. 40-41) and estimated average, or 600 acre-ft. Eakin and others ( 195 p. 79-80), ground- l, estimated Although the Maxey-Eakin method was not ini- precipitation water recharge from in Spanish Springs tially developed to estimate annual recharge from pre- Valley to be about 600 acre-fVyr. This method, called cipitation, annual estimates were made for incor- the Maxey-Eakin method, per- estimates recharge as a poration into the ground-water flow model. The results centage average annual precipitation of within speci- from model simulations using a constant annual fied altitude zones. The precipitation distribution of recharge were compared to those using recharge esti- used by Rush and Glancy (1967) was developed by mates that vary annually, as discussed in following sec- Hardman ( 1936) and resulted in an estimated total pre- tions of this report. cipitation of 30,000 acre-ft/yr (Rush and 1967, Glancy, Ground-water recharge also was estimated on the p.23). basis of chloride balance between precipitation and A revised precipitation map (fig. l3) was devel- ground water for comparison to the Maxey-Eakin oped during this study on the basis of a precipitation- method. On the basis of distribution of precipitation altitude relation constructed from selected stations that determined during this study (fig. l3), about 23,000 were assumed to represent conditions similar to those acre-ft/yr is estimated to fall in recharge-source areas in Spanish Springs Valley. About 33,000 aue-ftlyr (areas with greater than 8 in. of annual precipitation; (table 5) of precipitation was estimated from the more table 5). Rush and Glancy (1967, p. 23) estimated recent map (fig. l3). Although, for this precipitation- about 17,000 acre-ft of precipitation within recharge- altitude relation, precipitation was estimated to be source areas. Average chloride concentration in 24 somewhat greater at higher altitudes than those deter- samples of bulk precipitation collected from five sta- mined from the Hardman (1936) map, the general areal tions (sites 79,80,81, 82, and 87; table 8) is 0.38 mglL. distribution of precipitation is similar. Areas within Ground-water samples, collected from wells at the each I -in. precipitation zone were determined by periphery of the basin (sites 2, 9,12,27,28,30,39,41, superimposing the precipitation data shown in figure and72; fig. 3, table 8) and assumed to represent locally l3 onto 7 .5- and l5-minute topographic maps. The I - derived recharge, had an average chloride concentra- inch precipitation zones were then combined into three tion of about 13 m{L. Estimated ground-water zones (less than 8 in., 8 to 12 in., and greater than 12 recharge determined from the chloride-balance method in.) by using an area-weighted average. Areas within (eqn. l) is about 670 acre-ft/yr. Applying the same cri- particular precipitation zones, which differ somewhat teria to 3,500 acre-ft of precipitation estimated to fall from those presented by Rush and Glancy (1967,table annually in the Dry Lakes area results in an additional 8), reflect the revised precipitation map and the better 100 acre-ft/yr ofrecharge for a totalofTT0 acre-ff/yr. topographic resolution of more recent maps. Applying For 1994 conditions, assumed tobe72 percent of the Maxey-Eakin percentages used by Rush and average, 550 acre-ft of recharge is estimated on the Glancy to the volume of precipitation in each precipi- basis of the chloride-balance technique. tation zone (1967, table 8) results in an estimated aver- Although these estimates are slightly lower than age recharge of7l0 acre-ft/yr in Spanish Springs those estimated using Maxey-Eakin percentages, they Valley and 120 acre-ft/yr in the Dry Lakes area (table all indicate relatively low volumes of annual recharge 5). Rush and Glancy (1967) did not include the Dry and are considered to be reasonable. For purposes of Lakes subarea in their investigation. However, results estimating the ground-water budget, the ground-water of the current study suggest that recharge within the recharge component based on the Maxey-Eakin Dry Lakes area may provide recharge to basin fill in method was used. Spanish Springs Valley through systems of fractures within the volcanic rocks of the Pah Rah Range. An Infiltration of lmported Surface Water estimated 830 acre-ft/yr was used to represent the total average annual recharge from precipitation to the The disposition of surface water delivered by the basin-fill aquifer system in Spanish Springs Valley. On Ditch to Spanish Springs Valley was simplified into Because aboutT2 percent of normal precipitation fell at four components: (l) transmission losses directly from

GROUND'WATER RECHARGE AND DISCHARGE either recharged to ground-water Table 8. Concentration of chloride the shallow system or in ground water and precipitation at consumed by evapotranspiration. The estimated dispo- selected sites in Spanish Springs sition and budget of imported Truckee River water for Valley area, west central Nevada 1994 conditions are presented in table 9. Ground-water recharge from imported surface Dlssolved Site water takes place seasonally as transmission losses chlorlde number Date (mllllgrams from the On Ditch and as infiltration of water applied (fig.3) per llter) to inigated fields. To estimate transmission losses, a

Ground-Wrter Sites series of discharge measurements was made in April 1993 and May 1994 along the length of the Orr Ditch. 2 08-19-93 27 9 08-l 8-93 t2 Discharge through take-out gates used to provide water l2 08-l 8-93 ll for flood irrigation was measured or estimated between 27 08- l 7-93 9.0 each measurement of flow in the On Ditch. On the 28 08-r 7-93 8.4 basis of these measurements, a maximum of l0 acre-ft 30 08-l 7-93 9.4 per irrigation day was estimated to recharge the basin- 39 08- l 6-93 l3 4l 08- r 6-93 l8 fill aquifer as transmission losses directly from the On 72 0l-r3-94 t0 Ditch along its length. An inigation day is herein Precipitation Sitesl defined as a day with an average daily discharge of about 30 ftr/s or more as recorded at site 88 (fig. 3). 79 t2-lt-92 0.28 02-09-93 .08 From 1979 through 1984, the average number of irriga- 06-03-93 t.3 tion days was 170 per year. Because flows in the On 09-03-93 .45 Ditch were decreased in 1985 and considering the r 0- I 8-93 .24 effects of below average precipitation, the average 80 t2-11-92 .t7 number of inigation days for 1985 through 1994 was 02-l 8-93 .36 03-01-93 .17 decreased to about 124. During this study (1992-94), 06-03-93 .23 the number of inigation days was 58, 158, and 59, 09-08-93 .44 respectively. On the basis of the number of inigation I 0- I 8-93 .30 days in 1994, an estimated 590 acre-ft of transmission 8l 02-09-93 .t5 loss recharged ground water (table 9). 09-08-93 .39 I 0-l 8-93 .25 The volume of ground-water recharge beneath 03-01-93 .24 irrigated fields is assumed to be a percentage of the I 0-20-93 .57 amount of applied inigation water. In basins with sim- l0-20-93 .49 ilar inigation practices, about 40 percent of applied 87 t2-07-92 .35 inigation is assumed to infiltrate below the root zone t2-n-92 .20 and become ground water (Huxel and others,1966, 02-09-93 .07 06-02-93 .71 p.4l-42; Malmberg and Worts, 1966,p.40; Nichols, 09-03-93 .52 1979,p.22).The remaining 60 percent is lost by 09-03-93 .66 evapotranspiration processes. The amount of applied I 0- I 9-93 .43 irrigation water that potentially was available for ground-water recharge in Spanish Springs Valley was lsample results with same date at same site indicate analyses were made from two bulk assumed to be the difference between the total volume collectors at site. of imported surface water (5,500 acre-ft), as measured at site 88, and the sum of transmission losses, evapora- the On Ditch, (2) deep percolation of water to applied tion from surface-water bodies, and the overland flow fields as flood inigation, (3) direct evaporation of component entering the North Truckee Drain. The ponded surface water, and (4) plant and soil-moisture evaporation ofabout 460 acre-ft ofponded surface evapotranspiration requirements of applied irrigation water was estimated by applying an evaporation rate of water prior to ground-water recharge.In 1994, an about 40 inlyr (Van Denburgh and others, 1973,p.53) estimated 3,100 acre-ft of imported surface water was to 140 acres of surface-water bodies for 1994 (table 9).

46 Hydrogeology and Slmulated Eflects of Urban Developmenl on Water Resources ol Spanlsh Sprln$ Valley, Nevada Although flow in the North Truckee Drain responds water can be discharged by evaporation. Under natural nearly instantaneously to overland flow from flood ini- conditions, bare-soil evaporation in Spanish Springs gation (fig. l5), about 70 acre-ft in 1994 were estimated Valley probably took place in the area surrounding the to be contributed from ground water as base flow. Thus, springs, where the water table is near land surface. the overland flow component to the North Truckee Transpiration by phreatophytes has been documented Drain is estimated to be about 2,300 acre-ft in 1994. in other arid basins in Nevada to consume relatively Using this simple water budget, approximately 2,200 large quantities of ground water (Robinson, 1970;Har- acre-ft of imported Truckee River water was applied to rill, 1973, table 9;Berger, 1995, p. 35). Vegetation that the fields in the southem part of the study areain 1994. grows in areas where the water table or capillary fringe Assuming that about 40 percent infiltrated to the above the water table lies within reach of their roots are ground water, an estimated 860 acre-ft became ground- called phreatophytes (Meinzer, 1927, p. l). water recharge and nearly 1,300 acre-ft was lost to Estimates of ground-water discharge by evapo- evapotranspiration (table 9). On the basis of these transpiration under natural conditions (conditions estimates, the total volume of imported surface water without the On Ditch) were made by Rush and Glancy that was either recharged to the shallow ground-water (1967 , table l4). They estimated that about 1,900 acres system or consumed by evapotranspiration was 3,200 ofphreatophytes discharged nearly 900 acre-ft of acre-ft in 1994: this agrees well with the measured dif- ground water annually. On the basis of recent field ference between inflow and outflow at sites 88 and 89. observations, about 1,000 acres ofgreasewood, sage- The estimated total ground-water recharge from brush, and rabbitbrush lie along the outside of the On imported Truckee River water (transmission loss Ditch with a shrub density of about 20 percent, which plus infiltration of applied ditch water) was about suggests that a perennial source of water is available. 1,400 acre-ft 1994. in Direct estimates of evapotranspiration using microme- teorological methods were beyond the scope of this study. Estimates of ground-water discharge as a func- Discharge by Evapotranspi ration tion of shrub density, leaf-area index, and depth to water have been developed by Nichols (1993,1994). Prior to ground-water withdrawal for water sup- Assuming the 1,000-acre area has a leaf-area index of ply, evapotranspiration was the principal mechanism of about 2.5 (W.D. Nichols, U.S. Geological Survey, oral ground-water discharge from Spanish Springs Valley. commun., 1995) and using l5 ft as an average depth to Ground-water discharge by evapotranspiration water, 360 acre-ft of ground water is estimated to dis- includes direct losses from bare-soil evaporation and charge annually from areas just outside the Orr Ditch. transpiration from vegetation. In areas where the water Summing the previous estimate of 900 acre-ft (Rush table is only a few feet below land surface, ground and Glancy) with the additional 360 acre-ft results in a

Table 9. Estimated disposition and budget of imported Truckee River water for 1g94, Spanish Springs Valley area, west-central Nevada

[Values in acre-feet per year, rounded to two significant figures]

Measured quantity Estimated quantity

Inflow in Orr Ditch 5,500

Surface-water losses and consumption

Transmissionlossl.. 590 Infiltration of applied waterl . g60 Evaporation ofponded surface water . . 460 Evapotranspiration ofapplied water . . 1,300 Outflow in North Truckee Drain. . 2,400

Difference between measured inflow and outflow 3,100 Total estimated surface-water loss and consumption 3,200 I Transmission loss plus infiltration constitutes ground-water recharge from imported river water ( 1,400 acre-feet, rounded).

GROUND-WATER RECHARGE AND DISCHARGE total estimate of ground-water discharge by evapo- The steady-state model assumed no ground-water transpiration of about 1,300 acre-ft for 1994. The pumping. However, water levels used for calibration in ground-water flow rnodel was used to help quantify the northern part of the basin were measured in the ground-water discharge by evapotranspiration within mid-1980's and may have been affected by ground- the irrigated land area and transpiration from areas of water withdrawals in the northern part of the study phreatophytes that lie outside the perimeter of the On area. The potential for subsurface outflow toward Ditch. Warm Springs Valley was evaluated using the ground- water flow model discussed later in this report.

Subsurface Flow to Adiacent Basins Ground-Water Development Subsurface outflow from Spanish Springs Valley to the Truckee Meadows was estimated as 100-150 Spanish Springs Valley has been the site of agri- acre-ftlyr by Cohen andLoeltz(1964, p.23) and Rush cultural activity for many decades. Ground-water dis- and Glancy (1967, table l6). These investigators eval- charge from springs and shallow flowing wells was uated subsurface outflow through the basin fill and did used to inigate small areas of mostly grass near the not attempt to estimate flow volumes through fractured springs prior to construction of the On Ditch. With the bedrock. Although no attempt was made to directly importation of Truckee River water, the amount of estimate subsurface flow to the Truckee Meadows, developed and inigated land increased. Currently flow probably moves through fractured or weathered (1994),about 980 acres in the southern part ofthe study bedrock, as indicated by stable-isotope data. The rela- areaare serviced by the Orr Ditch irrigation system. In tive position of the sample from site 66 on the plot of addition, about 60 acres in two areas outside of the area 6180 versus 6D (fig. l6) suggests that the ground- encompassed by the Orr Ditch are irrigated with water sample is a mixture of locally derived water and ground water. Truckee River water. A mass-balance calculation, Ground-water pumping is seldom used to aug- using 6D values of - l l6 permil for locally derived ment inigation during periods when flow in the On water and -82. I permil for Truckee River water, indi- Ditch does not provide sufficient water for irrigation cates that the sample from site 66 is composed of about and stock-watering needs (Matthew Allen, Orr Ditch 24 percentTruckee River water and76 percent local Company, oral commun., 1996). During periods of recharge. Site 66 isjust south ofthe boundary between insufficient flow, minimal ground-water withdrawals Truckee Meadows and the study area and the well pen- are sometimes used for livestock watering; however, etrates more than 100 ft of bedrock (fig. 3). Conse- stock generally are moved to another valley and quently, subsurface outflow to Truckee Meadows may inigation is continued from ponded water supplies. be greater than previously estimated if ground water During this study, no ground-water pumping to supple- flows through bedrock that underlies the boundary and ment flow in the Orr Ditch was observed. As a result, through the saturated basin fill. the following estimates of ground-water withdrawals The potential for subsurface outflow to the north for inigation were determined only for active agricul- exists because water levels in Warm Springs Valley, tural lands outside the area serviced by the On Ditch. north of the study area, are200 ft lower than water lev- These areas include about 20 acres in the Spanish els in the northern part of Spanish Springs Valley. Sub- Springs Canyon area and about 35 acres in the northern surface flow may move northward through fractured part of the valley, west of Sugarloaf Peak (fig. I ). Dur- bedrock at depth along the base of Hungry Ridge. No ing1994, several acresjust east ofthe springs area, not attempt was made to directly estimate ground-water previously farmed, were irrigated as a result of exercis- flow northward from the study area because direct esti- ing a water right. Cunently, no records of inigation mates of flow through fractured bedrock require spe- pumping are maintained. Consequently, ground-water cialized techniques and because it is likely to be a small withdrawals for inigated areas not associated with the percent of the total basin discharge. During the calibra- On Ditch system were determined on the basis of tion of a steady-state flow model (Hadiaris, 1988), water-rights data, field observations, and discussions 280 acre-ft of subsurface outflow toward Warm with land owners. Summaries of active ground-water Springs Valley was needed for a favorable statistical rights were obtained from the Nevada Division of match between measured and simulated water levels. Water Resources (Kim Groenewold, written cofflmun.,

Hydrogeology and Simulated Eftects ol Urban Development on Water Resources of Spanish Springs Valley, Nevada 1995). Although the active ground-water rights within 1991, p. 4.2; Nevada Division of Water planning, 1992, the for basin irrigation purposes, as of 1994, were about p. 36; Washoe County Department of Comprehensive 2,300 acre-ft, only 720 acre-ft were estimated as Planning, written commun., 1995). Estimaied ground- pumped for inigation and applied on lands outside the water withdrawals for domestic use increased from 20_ service area of the Orr Ditch. Estimates of ground- 30 acre-ft in 1979 to more than I l0 acre-ft in 1994. A water withdrawals 197 for 9 through 1994 arepresented gravel-pit operation in the northern part of the study in table 10. area was the principal industrial user of ground water in the study Until 1983, agriculture was the principal user of area, having an estimated withdrawal of about 300 ground water, accounting for more than half of the total acre-ft in 1994. An estimated additional 20 withdrawn in the study area (table l0). As population acre-ft of ground water was pumped for other industrial uses. Total grew, withdrawals for public supply became the domi- ground-water withdrawals for all uses were nant use of ground water. Meter records of ground- estimated to be 2,600 acre-ft in 1994 (table l0). water discharge collected from several municipal water Part of the ground water withdrawn to inigate suppliers, including the Washoe County Utility Divi- fields and to support urban development is recycled as sion, were used in conjunction with water-rights data secondary recharge. The amount of applied irrigation to estimate that about I ,400 acre-ft of ground water water that infiltrates beyond the root zone and was withdrawn for municipal use in 1994. Annual recharges ground water, in part, depends on the appli- ground-water withdrawal s for non-municipal domestic cation method of inigation and depth to the watertable. purposes were determined from residential water-use For application methods of either sprinkler or drip sys- data and number of parcels with domestic wells. Esti- tems, the assumed percentage of pumped water con- mates of residential water use range from 0.6 to about sumed by crops is 65 percent and that lost by spray and 0. 8 acre-filyr per parcel (Kennedy/Jenks/Chilton, surface evaporation is l0 percent (Harrill, 1968,p.47).

Table 10. summary of population and estimated ground-water withdrawals, 1979-g4, Spanish Springs Valley area, west-central Nevaoa

IPopulation and withdrawal values rounded]

Ground.water withdrawals Year Populationl (acre.leet per year) lrrigation2 Municipal3 Domestic4 Industrials

t979 790 350 120 20-30 20 500 I 980 l,080 350 180 20-30 20 600 198 I l,380 350 230 30-40 20 600 t982 1,510 350 260 30-40 20 700 I 983 l,750 3s0 320 40-50 20 700 I 984 2,080 350 430 40-60 20 800 r 985 2,570 350 550 50-60 20 1,000 r 986 2,780 350 610 50-70 20 1,000 1987 2,930 350 &o 60-80 120 t,200 r 988 3,240 350 640 70-90 610 1,700 I 989 3,580 350 680 70- I 00 610 l,700 I 990 4,060 350 820 80- l 00 610 I,900 t991 4,760 350 960 80-ll0 610 2,000 1992 6,040 350 930 90-t l0 610 2,000 I 993 7,800 350 l,080 100-130 330 I,900 t994 9,320 720 I,400 I l0-150 320 2,600 I Estimates ofpopulation for 1979-90 from U.S. Bureau ofthe Census (written commun., 1995) and for l99l-94 from washoe county Department of comprehensive planning (written commun., 1995). 2 Ground-water withdrawals estimated from field observations and State water-rights data. 3 Dutu f.orn Washoe County Utility Department (written commun., 1995) and rnetered pumping records. 4 Based on 0.6 and 0.8 acre-foot per parcel. 5 Estimated from State water-rights data; includes estimates of commercial and constructron water use.

GROUND.WATER RECHARGE AND DISCHARGE The remaining 25 percent is assumed to be recycled discharge was estimated at 4,300 acre-ft. The imbal- back to the ground-water system. The percentage of ance between estimated recharge and discharge was applied inigation by flooding that recharges ground 1,700 acre-ft for 1994 conditions. water is 40 percent, similar to that assumed earlier for The average water-level decline calculated from areas that are flood inigated with water from the On December 1993 to December 1994 is about 2 ft. The Ditch. For this study, however, ground-water recharge volume of water represented by this decline can be from applied inigation was considered not to be signif- approximated by the product of the area of decline, the icant in areas not served by the On Ditch and where average water-level decline within this area, and an depth to water was 100 ft or greater. On the basis of estimated specific yield of the basin-fill aquifer where these considerations, recharge from applied inigation the decline occurred. The area of water-level declines of ground water was estimated to be about 170 acre-ft is about 8,500 acres. Applying specific yields of l0 and in 1994 for inigated lands outside the area encom- 20 percent, the estimated quantity of ground water passed by the Orr Ditch. removed from storage during 1994 was 1,700 to 3,400 acre-ft. The water-level declines and associated change The amount of ground water recycled from in storage support the empirically estimated imbalance municipal and domestic uses is assumed to be a per- for 1994 conditions. centage of the total withdrawals and differs with type The perennial yield a ground-water reservoir of use and with individual systems, including those of as natural maximum amount systems that remove wastewater to treatment facilities can be defined the of ground water that can be and consumed outside the study area. Water applied for outdoor uses withdrawn year an period of time (lawn and shrub watering) is mostly consumed by economically each for indefinite (Hanill, 1968, p. 56). The is called evapotranspiration and for this study was considered term commonly safe yield (Bear, 1979, p. Fetter, 1994, p.5 l8). Fet- not to be a significant contributor to ground water. l3; ter further defines safe as "the amount of naturally Secondary recharge from septic systems (indoor uses) vield is assumed to be 75 percent ofthe total amount ofwater ground- delivered during winter months, when outdoor water- Table 11. Estimated and simulated components of water recharge and discharge, 1994 conditions, Spanish water, ing is at a minimum. This monthly volume of Springs Valley area, west-central Nevada which is assumed to be constant, was then prorated for an entire year to arrive at an annual estimate of second- [Values in acre-feet per year, rounded to two significant figures] ary recharge. For 1994, secondaryrecharge from septic Based on . Easeo on systems was estimated at 450 acre-ft. This estimate of fle10 ano Component . Iransrenl secondary recharge reflects only that percentage of emDrtcal delivered water remaining within the basin. esilmates Recharge Precipitation 1600 580 Evaluation of the Ground-Water Budget and Infiltration of imported surface water I,400 1,300 Yield, 1994 Recycled irrigation water t70 170 Secondary recharge from septic systems 450 400 This section describes the ground-water budget Total recharge 2,600 2,400 and augmented yield under 1994 conditions in Spanish Discharge Springs Valley. The augmented yield represents the Evapotranspiration r ,300 I,800 natural yield of the basin fill plus the additional Subsurface outflow To south 2too-t5o r9o recharge from imported surface water. To north 3280 rio A summary of estimated ground-water recharge Ground-water withdrawal 2,600 2,400 and discharge components to and from the basin-fill Total discharge 4,300 4,600 aquifer system in Spanish Springs Valley forconditions lmbalance (discharge minus recharge) I,700 2,200 representing 1994 is presented in table I l. On the basis ' Estimated from Maxey-Eakin; represents 72 percent ofaverage of field and empirical techniques, total recharge from annual recharge. (1964, p.23) ( | all sources was estimatedat2,600 acre-ft, nearly 54 ' From Cohen and Loeltz and Rush and Glancy 967, p. 37). Estimate may be greater ifground water flows through fractured percent of which was estimated to be derived from bedrock and through saturated basin fill. 3 imported Truckee River water during 1994. Total From Hardiaris ( I 988).

Hydrogeology and Simulated Etfects of Urban Development on Water Resources of Spanlsh Sprlngs Valley, Nevada occurring ground water that can be withdrawn from an SIMULATION OF GROUND.WATER FLOW aquifer on a sustained basis, economically and legally, A mathematical flow model was developed to simu- without impairing the native ground-water quality or cre- late the effects of ground-water withdrawals on the ground- ating an undesirable effect such as environmental dam- water flow system in the basin-fill aquifer beneath Spanish age." Springs Valley. The model simulation starts in 1979 when Whenever water is initially withdrawn from a well, large increases in population and water demand began and the withdrawal creates an imbalance between ground- ends in 1994 with the then-current state of urban develop- water recharge and discharge, and water is removed from ment. The model was calibrated using estimated ground- storage in the aquifer. The imbalance continues and water withdrawals from 1979 to 1994 and the resultant ground-water levels decline in the aquifer until the with- water-level declines. The calibrated model was then used to drawals are balanced by a decrease in pre-pumping dis- simulate changes in water levels that reflect the probable charge or by an increase in recharge, or by a combination future response to selected ground-water development sce- of the two. Ground-water recharge can increase only narios. where surface water (a stream or lake) is directly con- The accuracy with which the flow model simulates an nected to the aquifer (no intervening unsaturated interval) actual ground-water flow system depends on how well the or where subsurface flow enters the aquifer from adjacent hydrologic processes of the system are understood and sim- aquifers. The magnitude of sustained ground-water with- ulated. The accuracy and distribution of input data used to processes the factors that drawals, therefore, depends on the maximum quantity of describe these are determining the model in simulating the actual system. A ground- pre-pumping discharge that can be captured for beneficial limit water flow model is not necessarily a unique representation use (Bredehoeft and others, 1982) and on the maximum of a flow system; however, by using reasonable hydraulic quantity of increased recharge. This maximum quantity is properties and boundary conditions, the flow model can dependent partly on the placement of wells with respect to closely simulate the natural flow system of the study area. areas of pre-pumping discharge and to areas where recharge can be increased, as considerable time may be required to capture the pre-pumping discharge or to Mathematical Basis increase recharge. The numerical technique used in this study to analyze The estimated perennial yield for Spanish Springs ground-water flow and yield of the basin-fill aquifer is a Valley for natural conditions, assuming that all the natural finite-difference ground-water flow model written by discharge can be captured, is about 1,000 acre-ft/yr (Rush McDonald and Harbaugh (1988). The model solves the and Glancy, 1967,table l2). three-dimensional equation of ground-water flow by using finite-difference approximations; the equation can be writ- Because surface water, some of which has been ten as follows: imported into Spanish Springs Valley since the late 1800's, the aquifer, the term augmented yield + 0 (KrrOh/ + recharges basin-fill 0 / x (K rr)h / 0x) / 0y 0y) is used herein to describe the total quantity of potentially A/ 6z (K 0z) I( = S 0t , (2) available ground water, assuming that all the pre-pumping rrAh/ - r}h/ captured. Augmented yield is defined as discharge can be where. the perennial yield plus salvable secondary recharge Kr* Kware hydraulic conductivities in the principal hori- resulting from the use of imported surface water (Harrill, zontal directions, in length per unit time; 1973,p.77-78). Thus, augmented yield of the basin-fill K," is hydraulic conductivity in the vertical direction, aquifer in Spanish Springs Valley is estimated to be about in length per unit time; 2,400 acre-ft, assuming that 1,400 acre-ft of ground-water ft is hydraulic head, in length; recharge was salvaged from infiltration of imported Truc- volumetric flux of recharge or discharge per kee River water in 1994.The estimated augmented yield of zis unit volume. in l/time: 2,400 acre-ft is about the same volume as the total ground- S" is storage, l/length; water pumping estimate. However, if the quantity of specific in imported surface water or its management changes, the l is time; and augmented yield for the basin-fill aquifer must be revised x, y, z are Cartesian coordinates aligned along the major to account for changes in ground-water recharge. axes of hydraulic conductivity.

SIMULATION OF GROUND-WATER FLOW The finite-difference method is used to obtain recharge and discharge were simulated in layer l. approximate solutions to the three-dimensional flow Layer 2, which functioned as a conduit for deep flow equation by replacing the continuous partial deriva- and as a reservoir of stored water. extends from the bot- tives with systems of simultaneous algebraic difference tom of layer I to the top of consolidated rock. Layer 2 equations. The difference equations are then solved in was simulated by the model as a confined aquifer, but terms of the unknown hydraulic head at discrete points, was allowed to convert to unconfined conditions if or nodes, and time. The time derivative of head, dh/dt, water levels dropped below the bottom of layer I due is approximated by the backward difference procedure to simulated ground-water withdrawal. Although con- (Remson and others, 1971,p.78). The approximation solidated rock underlying and adjacent to the basin-fill of the time derivative for each node is as follows: aquifer may store and transmit moderate to large amounts of water, the hydraulic properties of that sys- (h,- hl / a,t (3) tem are unknown. Inclusion of the consolidated-rock system into the ground-water flow model would add where, another level of complexity and uncertainty to the sim- ft,7 is hydraulic head at the beginning of a time ulation. Consequently, only model cells representing step, in length; saturated basin fill were used in this studv. ft7 is hydraulic head at the end of a time step, in length, which is unknown; and Selection of Time Periods and Assigned Recharge Ar is the time-step interval. and Discharge The strongly implicit procedure (McDonald and Harbaugh, 1988, p. 12- I is used to solve the system of ) The simulation of steady-state conditions in difference equations for each time step by iteration. Spanish Springs Valley was not attempted due to the Solution for each node is achieved when the head lack of predevelopment water-level data needed for change between each iteration is less than a specified steady-state calibration and the transient nature of value. The value specified for the model simulations ground-water recharge from imported surface water. for Spanish Springs Valley was 0.03 ft. Each node is Consequently, the flow model that numerically repre- centered in a model-grid cell that has dimensions ofx, y, andz. Hydraulic properties within each cell were sents the basin-fill aquifer system was calibrated under (1979-94). assumed to be homogeneous, so that the model-derived transient conditions for a l6-year period hydraulic head represents the average head over the Sixteen yearly stress periods were used to represent entire cell. each calendar year beginning January 1979 and ending December 1994. Final heads from the previous annual stress period were used as initial heads for the follow- General Features ol Flow Model ing annual stress period. Each simulated calendar year To translate the conceptual model of the hydro- was divided into l2 equal monthly stress periods to bet- logic system into a mathematical flow model and solve ter represent the effect of seasonal conditions. During the ground-water flow equation by finite differences, a each monthly stress period, all assigned stresses to the block-centered grid was superimposed over a map system were held constant. Monthly stress periods view of the study area (fi5.2q. The grid for the Span- were further subdivided into five time steps with each ish Springs Valley model contained 37 rows,28 col- step increasing geometrically according to a specified umns, and 2 layers that divided the saturated basin fill multiplier. The initial time step was 3.71 days and each into discrete three-dimensional model cells. Variable subsequent step was increased by |.25 times the dura- node spacing was used to provide higher resolution in tion of the preceding time step. Because the initial con- ground-water recharge and discharge related to areas of ditions were not derived from a steady-state of Truckee River water. Model-cell the importation simulation, model responses in early stress periods of size ranged from a minimum of 0.02mi2 (820 ft by 820 the transient simulation reflect not only specified ft) to a maximum of 0. I 0 mi' ( I ,640 ft by I ,640 ft). Of the 1,036 cells in each model layer,625 were active in layer I and282 were active in layer 2. The top 330 ftl 'The model was developed using metric units. Thus, 330 ft of saturated basin fill was generally unconfined and used in this study approximates 100 meters as used in the original was represented by layer l. Processes of ground-water model.

Hydrogeology and Simulated Eflects ol Urban Development on Water Resources ol Spanish Springs Valley, Nevada 1 1 9.45' 119'37'30"

39037'30u

30

35

o(D'tjji i*- 9,t3

Base from U.S. Goological Survey data, O 1 2 3MILES Geology modifisd from Bonham (1969), 1:1oo,(xx>scalo, 1980; Universal Transvorse Stewart (1980), Cordy (1985), and Mercator proieclion, Zone 1 1 F__r_LT__+-___r B€ll and Bonham (1987) O 1 2 SKILOMETERS

EXPLANATION

fffi? Bastnttrl Modol c€lls-Numbers on north side ol grid ttlodol boundary indicate columns: those on west side I conrotldatedrock indicate rows. Gell dimensions ditfer Ba3ln boundary I Cell where only layer'l is active inside model boundary I Cell where layers 1 and 2 are active inside model boundary I Head-d€p€ndenttlowboundary

Figure 24. Block-centered finite-difference grid used for ground-water-flow model of Spanish Springs Valley, west-central Nevada.

SIMULATION OF GROUND"WATER FLOW 53 stresses on the system, but also an adjustment of start- valley (fig.24), particularly in the northern and south- ing heads to boundary conditions and hydraulic proper- eastern parts of the model, because of numerical insta- ties. bility along the boundary during initial simulations (fig.2q. Ground-water recharge from septic systems asso- The modeled area was consequently perimeter ciated with municipal water supply, precipitation, and decreased in proportion to the reduced of the imported surface water were simulated in the model as model grid. assigned rates on the basis of either empirical estimates Flow conditions along the model boundary were or measured quantities. The transient nature of the simulated by no-flow and head-dependent flow bound- assigned values ofrecharge and discharge used in aries. Lateral and vertical boundaries between basin fill the model simulation is shown in figures 25 and26. and consolidated rock were specified as no-flow Recharge rates from precipitation and septic systems boundaries. Head-dependent flow boundaries were were assumed constant over a given annual stress used at the southern and northern part of the model to period, but varied between annual stress periods simulate subsurface flow between Truckee Meadows depending on estimates for that particular year. and Warm Springs Valley (fig.2q. Ground-water Because recharge from imported surface water is inflow and outflow conditions were simulated in layers seasonal as a function offlow in the Orr Ditch, recharge 1 and2 at head-dependent flow boundaries by extend- rates vary between monthly stress periods. Ground- ing external cells beyond the modeled region and spec- water discharge for domestic and municipal pumping ifying head values to those cells. A conductance term was simulated by assigning variable discharge rates for provides the link between the model and the external each monthly stress period on the basis of monthly cell and is a function ofthe cross-sectional area ofthe meter records and estimates of water use. The extrac- cell perpendicular to the ground-water flow and hori- tion point of a domestic well was assumed to be in the zontal hydraulic conductivity of the basin fill at the same area (cell) as recharge from the associated septic boundary. The specified head does not change in system; as a result, model values of domestic pumping response to flows across the boundary calculated by the input to the model were the net difference between dis- model. The specified external head used at the southern charge and recharge. Model input of recharge from sep- boundary was 4,397 ft above sea level, determined tic systems shown in figure 25 reflects only those from water-level measurements in the Truckee Mead- homes that were served by municipal water systems, ows. Saturated basin fill beneath the northern boundary where the point of extraction was different from the of the model is believed to be several hundred feet thick point of use. Irrigation withdrawals were simulated as because the northern boundary of the model is some- net pumping for lands outside the Orr Ditch inigation what south of the actual basin boundary. Although con- system. Model cells with assigned pumpage for munic- tinuity between saturated basin fill in Spanish Springs ipal, inigation, and industrial discharge greater than l0 Valley and saturated basin fill in Warm Springs Valley acre-ft for 1979 and 1994 are shown in figure 27. is unlikely, a deeper hydraulic connection, perhaps along fractures within consolidated bedrock, is possi- Boundary Conditions ble. To simulate this possible subsurface outflow con- dition, specified external heads used at the northem The ground-water-flow equation applied in the boundary range from 4 ,47 5 ft (cell: row 24, column22) model has an infinite number of solutions. To develop to 4,436 ft above sea level (cell: row l, column 18). a basin-specific model, information about conditions at Conductances used for the head-dependent flow the boundaries of the flow system and ground-water boundaries range from 180 ft'ld to 650 ft'ld. withdrawals within the flow system are required. Flow- boundary conditions are specified in the model on the Mountain front recharge from precipitation was basis of ground-water-flow concepts developed during simulated in model layer I as constant flow in active this study. Active model cells represent saturated basin cells along the east and west sides of the model grid fill; inactive cells represent less-permeable consoli- (fig. 28). The amount of recharge introduced to each dated rock and saturated basin fill less than about 30 ft cell depends on its position relative to a particular thick. The boundary between active and inactive cells recharge-source area. The distribution of recharge was represents the model boundary (fig.2q. The model determined for each recharge-source area on the basis boundary was moved inward, toward the center of the of the Maxey-Eakin method, as previously discussed.

Hydrogeology and Simulated Effects of Urban Development on Water Resources of Spanish Springs Valley, Nevada CALENDAR YEAR

1 1 1 979 981 983 1985 1987 1 989 1991 1 993 ut

LIJ A wu)l-F- Ed,6 *FCC*9> guJo-YOUJ LU)s ori! {odF'ff H ==E 10 -3=b=? (r(J tL

140 B .tzo A zi loo o$?: F;i _ln d t :gh 80 to+!! cc ru 60 ==H EEZ 40

20

uJ-cc c g5=2tL {u.tX f;?; u-ccd=|rUJ z= t- =@u *un+4FtJ.l !:(L< Yi>-=55 z ol,! 6 8 10 12 16

ELAPSED TIME, IN YEARS SINCE JANUARY 1979

Figure 25. Assigned monthly recharge rates (monthly stress period) used as model input for transient simulations (1979-94), Spanish Springs Valley, west-central Nevada. Ground-water recharge estimated for (4J municipal septic systems, (8,)precipitation, and (C/ infiltration of imported surface waler.

SIMULATION OF GROUND.WATER FLOW 55 CALENDAR YEAR

1979 1981 1 983 1985 1987 1989 199'l 1993 utF 2= A _g= .tlIE PGUJ kol>tr!; nH5 :-o h< 2z 0

ui- 100 E2 B o-O 80

ffer 60

=6t 40 ur=LilbrH (EA 20 r-{ 2z 0

T 60 ut-t- Rg PEcc HEH F=UJ =iI(/) * t! rrJ 6 ,rj ](E ooz< -z

T ^F oouz D s> F=fr < o-: E-rl. a=u-trsil urgLil 100 zcL:)()

= 68101214 16

ELAPSED TIME, IN YEARS SINCE JANUARY 1979

Figure 26. Assigned monthly discharge rates (monthly stress period) used as model input for transient siriulations (1979-94), Spanish Springs Valley, west-central Nevada. Ground-water discharge estimated tor (A) domestic pumpage, (8/ irrigation pumpage, (C/ industrial pumpage, and (D) municipal pumpage.

56 Hydrogeology and Simulated Effects of Urban Development on Water Resources of Spanish Springs Valley' Nevada 1 1 9.45' 119'37'30"

39'37'30"

Base lrom U.S. Geological Suruey data, 0 1 2 SM|LES Geology. modifi€d from Bonham (1 969), 1 :100,0oo-s€le, 1980: Universal Transvorse Stewart (1980), Cordy (1985), and Msrcator projection, Zons 1 1 F_]_Lr____/_r Bell and Bonham (1987) O 1 2 3KILOMETERS

EXPLANATION

t:-_l Basln till N Cell wlth asslgned pumpage greater Model boundary than 10 acre-fe€t for 1979 Consolldated rock Basln boundary I T Cell with assigned pumpage greater than 10 acre-feet for 1994

Figure 27. Cells with assigned ground-water pumpage for municipal, irrigation, and industrial use in annual stress periods respresenting 1979 and 1994 in model simulations, Spanish Springs Valley, west-central Nevada.

SIMULATION OF GROUND-WATER FLOW The Maxey-Eakin method uses long-term average pre- Recent work by W.D. Nichols (U.S. Geological cipitation to estimate long-term ground-water Survey, written commun., 1995) suggests that evapo- recharge. transpiration in areas where depth to water is less than In this study, recharge from precipitation was about 8 ft may begin as early as mid-May and continue simulated in two ways. In the first approach, the as late as early October. Evapotranspiration in the cur- recharge derived by using the Maxey-Eakin method rent study was simulated from June through September was assumed constant throughout the l6-year transient (about 120 days) during which time more than 60 per- simulation. In the second approach, an attempt was cent of the annual evapotranspiration may occur. In made to represent the annual variation in recharge that Spanish Springs Valley, evapotranspiration of ground may result from changing annual precipitation rates. To water is limited to the area encompassed by the On approximate the recharge rate for a specific year, the Ditch and along the outside of the On Ditch near the Maxey-Eakin recharge rates were multiplied by the central and southeast parts of the valley. Inside the On ratio of the precipitation for that year (measured at the Ditch where the depth to water is shallow, vegetation is Reno airport) to the long-term average precipitation. predominantly meadow grasses and alfalfa separated available for Spanish Springs Although no data are ,by large areas of bare soil. A maximum evapotranspi- Valley to support this linear relation between annual f ration rate of 0.018 fVd at land surface and an extinc- precipitation and annual ground-water recharge, mod- \tion depth of 9.8 ft were used to simulate eled hydraulic heads resulting from use ofthe second evapotranspiration inside the area of the Orr Ditch approach were slightly closer to observed heads than (W.D.Nichols, U.S. Geological Survey, written com- were those of the first aPProach. mun., 1995). Outside the On Ditch, where natural Ground-water recharge from imported Truckee shrubs dominate the vegetation, the maximum evapo- River water was simulated using flow records of the transpiration rate was determined with an equation that On Ditch. Transmission losses from the On Ditch uses plant density, leaf area index, and depth to water were entered as a constant recharge at a rate propor- (Nichols, 1994). Assuming that evapotranspiration is at tional to the number of irrigation days during each /amaximum when depth to ground water is 3.3 ft, the monthly stress period. Recharge rates were distributed / maximum evapotranspiration rate used in the model for along the Orr Ditch from discharge measurements col- { tne area outside the On Ditch was I .64x l0-3 ftld and lected during this study. Simulation of recharge from \ the extinction depth was 32.8 ft (W.D. Nichols, U.S. infiltration of applied inigation water using constant- Geological Survey, written commun., 1995). flow cells also was proportional to the number of ini- gation days during each monthly stress period. The Initial Conditions and Aquifer Properties spatial distribution of recharge from irrigation depended on each irrigator's allocation of On Ditch Before substantial urban development and water and the area to which that allocation was to be ground-water withdrawals (prior to applied (fig. 28). accompanying about 1979), the hydrologic system in Spanish Springs discharge by evapotranspiration Ground-water Valley may have been in a state of dynamic equilib- specified in model layer I as head-dependent flow was rium, fluctuating in response to the quantity of surface boundaries and was assigned to selected active cells water imported yearly into the valley. Long-term tem- corresponding to plant distribution based on field poral water-level data needed to describe the response observations. Evapotranspiration is simulated as a ofthe basin-fill aquifer to seasonal changes in recharge linear function with depth, and is computed from a are not available. Because of the uncertainty in equilib- maximum rate that is decreased linearly until the water rium conditions, steady-state conditions were not sim- level reaches the depth at which evapotranspiration is ulated. The distribution of ground-water levels shown assumed to cease. This depth is called the extinction sparse historical data depth. Ground-water recharge from imported surface in figure 29 was determined from water is seasonal, which causes ground-water levels (Robinson and Phoenix ,1948; Rush and Glancy, 1967) beneath inigated lands and near the On Ditch to fluc- and from a preliminary conceptualization of the flow tuate. Because ofthese fluctuations, annual ground- system in the basin fill. This distribution was assumed water discharge from evapotranspiration simulated in to represent water-level conditions prior to large pump- the transient model is not constant in time or space. ing stresses and was the basis for assignment of initial

Hydrogeology and Simulated Effects of Urban Development on Water Resources ol Spanish Springs Valley' Nevada 1 1 9'45' 1 19'37'30"

39"37'30"

n , Dil€n +^ t?-. r$ o o

sD_.

aX"d Truckee Meadows

Base from U.S. Geological Survey data, O 1 2 3MILES Geology modified from Bonham (1969), 1 :'100.000-scale. 1980: Universal Transverse Stewart (1980), Cordy (1985), and Mercator projection, Zone 1 1 Bell and Bonham (1987) O 1 2 3KILOMETERS

EXPLANATION | | Basin till T Cell where constant tlow of recharge lrom Model boundary precipitation is simulated ffi consotidatedrock Basin boundary I Cell where constant flow of recharge is used to simulate transmission loss from Orr Ditch Cell where variable flow is used to simulate I recharge from inliltration of applied water

Figure 28. Model cells in layer 1 used to simulate ground-water recharge in transient model, Spanish Springs Valley, west-central Nevada.

SIMULATION OF GROUND-WATER FLOW 59 1 19'45' 119.37'30"

39037'30u

otrry ?z) Sparks ,g )go o au' \ o-, Reno (.r 4dtx"u Truckee Meadows 39'30' -- -- Basetromu.S.ceotogicatsurueydata, 0 1 2 SMILES GeologymodifisdfromBonham(1969), 1:lOO,OOO-scale, 1980; Univsrsal Transvoe€ | I Stewart (1980), Co.dy (1985), and (1987) Mercator projection, Zon6 1 1 l----f---f--T---- Betl and Bonham O 1 2 3KILOMETERS

EXPLANATION ffi.q Bastn flll -4,440- water-lovel clntour-Shows altitude Model boundary I consordarod rock LTSll"J"li"#f ilt""ll't'|"' Basrn boundarv Figure 29. Ground-water levels assumed to represent water-level conditions prior to large pumping stresses (before 1979) and used as initial heads in model simulations, Spanish Springs Valley, west-central Nevada.

60 Hydrogeology and Slmulated Eftects of Urban Development on Water Resources of Spanish Sprlngs Valley' Nevada heads in the transient simulations. Initial heads for model are areally generalized. No attempt was made model layer2 were assumed to be the same as heads for during the calibration process to locally adjust hydrau- model layer 1. lic properties of cells to achieve a better head match. The horizontal hydraulic conductivity of layer I (fig. 30) was estimated from lithologic and geophysical Simulation of Transient Conditions, lg7g-94 logs, distribution of hydraulic heads, and limited aqui- fer-test results. No laterally continuous deposits were A transient simulation was done for 1979 through identified from lithologic logs and no trend in bulk 1994, a period ofvariations in natural recharge and dis- character of basin fill was apparent from available data. charge, changes in the quantity of imported surface However, in the northern and southeastern parts of the water, and increases in ground-water withdrawals. An study area, relatively flat hydraulic gradients suggest estimated 21,000 acre-ft of water was withdrawn from that hydraulic conductivity may be somewhat greater the basin-fill aquifer of Spanish Springs Valley from 1979 through than elsewhere in the study area. In addition, hydraulic 1994. heads in the Spanish Springs Canyon area (fig. 1) were Calibration higher relative to the heads several miles north, sug- and Results gesting that a banier of low permeability may inhibit The flow model was calibrated under transient ground-water flow. The banier may be caused by a conditions because no data are available for the period fault within the basin fill that trends northeastward (fig. before agricultural development began and few data 4). During model calibration, cells at the approximate are available prior to substantial ground-water with- location of the fault were assigned a lower hydraulic drawals. Model calibration consisted of uniformly conductivity than surrounding basin fill to simulate the adjusting hydraulic properties for large groups of cells observed water levels on either side of the presumed within layer I and rates of recharge from applied iniga- fault. Vertical hydraulic conductivities in the model tion water until simulated water-level trends matched were assumed to be I percent of horizontal conductiv- observed trends during the l6-year simulation. Hydro- ities (Freeze and Cherry, 1979). Owing to the vertical graphs of measured water levels for five wells (sites 12, discretization used in the model. simulated heads were 19,46,50, and 69) screened within the part of the aqui- fairly insensitive to changes in vertical hydraulic con- fer simulated by layer l, and simulated water levels for ductivity. The transmissivity of layer 2 in the model the corresponding model cells, are shown in figure 32. was estimated as the product of the thickness and These wells were selected because of their spatial dis- hydraulic conductivity (fig. 3l). The values used for tribution within the modeled area and their long period thickness of layer 2 were the total thickness of satu- of record. The simulated water-level trends are in gen- rated basin fill between the bottom of model layer I and eral agreement with measured trends. In particular, the the top of the consolidated rock (fig. l0). The hydraulic amplitudes of seasonal change and general trends of conductivity of layer 2 was assumed to be about 60 per- water-level decline were reproduced fairly well by the cent of the assigned hydraulic conductivity of layer I model. on the basis ofan increase offine-grained deposits Water-level measurements collected in December reported from wells that penetrated layer 2. 1994 from 27 wells were compared to model-derived heads calculated from the last monthly stress period of Although parts of the basin-fill aquifer simulated the simulation for cells corresponding to well loca- in layer I are locally confined or semi-confined, model tions. Calibration was discontinued when the sums of layer was assumed to be I unconfined. Layer I was the difference between simulated and measured heads assigned a uniform yield specific of 0.13. Layer 2 is (bias) were near zero, and the root mean squared error considered confined and each cell was assigned a stor- between simulated and measured heads was less than age coefficient by multiplying the thickness of the cell 3.3 ft. The absolute maximum difference between sim- (in feet) by I x 10-6, as suggested by Lohman (1972, ulated and measured heads for the 27 wells was about p. 53).A secondary storage coefficient of 0.1 was l0 ft. About 80 percent of these model-computed heads specified for layer 2 if overlying cells were dewatered were within 5 ft or less of the measured value (fig. 33). causing layer 2 to become unconfined. Hydraulic con- The 5-ft difference between measured and computed ductivity and storage coefficient distributions in the heads is only l0 percent of the total difference in water

SIMULATION OF GROUND.WATER FLOW 1 19"45' 119'37'30"

39'37'30"

6rOfton

sparks Xk (L ""' Reno aXui ^-o Truckee Meadows 39'30' Base from U.S. Geotogical Survey data, 0 1 2 3 MILES Gsology modified lrom Bonham (1969), t"""nu3ff';3""J"[H?L*i il::":i"Hffi'&J"%nuniv€rsarrransvers" F--'-'-----/-- O 1 2 SKILOMETERS

EXPLANATION

l$ffil Basin flll Hydraullc.conductivity for cells in Model boundary ln teet per dav f consordared rock I ::T:: Basrn boundarv I 1 tolessthan 10 I 1O to lessthan 50

Figure 30. Horizontal hydraulic conductivity in layer 1 used in calibrated transient simulations, Spanish Springs Valley, west-central Nevada.

62 Hydrogeology and Slmulated Effects of Urban Development on Water Resources of Spanish Sprlngs Valley' Nevada 119"37'30"

Base from U.S. Geological Survey data, 0 1 2 3M|LES Gsology modified lrom Eonham (1969), 1:100,000-scale, 1980: Universal TransveBe Stewart (1980), Cordy (1985), and Memtor prcjection, Zons 11 F--r-Lr---+------J Bsll and Bonham (1987) O 1 2 3KILOMETERS

EXPLANATION

ffTIII Basln till Transmisslvlty for cells in basin llodel boundary fill, in teet squared per day Consolldated rock Basln boundary w I Less than 1,000 I '1,000 to 2,000 T Greater than 2,000

Figure 31 . Transmissivity in layer 2 used in calibrated transient simulations, Spanish Springs Valley, west-central Nevada.

SIMULATION OF GROUND-WATER FLOW CALENDAR YEAR

1 979 1 981 1 983 1985 1987 1 989 1991 1993 4,480 Cell 5,23 (site 12) 4,470

4,460 Simulated 14^ 4,450 . a - Measured 4,440

4,490 Cell 13,14 (site 19) 4,480

4,470

J 4,460 n UJ ur J 4,450 IIJ U) tu 4,485 o Cell 27,9 (site 46) 4,475 F Lu lu t.|- 4,465 1 o .. o z t t . o o .- Y j t.t.l 4,455 llJ J (r 4,445 uJ 3 4,475 Cell 25,15 (site 50) 4,465

4,455

4,445

4,435

4,505 Cell 25,24 (site 69) 4,495

4,485

4,475

4,465 468101214 16 ELAPSED TIME, IN YEARS SINCE JANUARY 1979

Figure 32. Measured and simulated ground-water levels for selected cells, layer 1, during transient simulations, Spanish Springs Valley, west-central Nevada.

Hydrogeology and Simulated Eflects ol Urban Development on Water Resources of Spanish Spdngs Valley, Nevada levels in the model. The maximum altitude of the sim- Simulated Ground-Water Budget ulated water level was about 4,490 ft in the southeast- The calculated water budget for the final ern part of the model; in contrast, the minimum monthly stress period of the transient simulation was used to simulated altitude was less than 4.440 ft in the southern represent 1994 conditions. Ground-water budgets part (fig. 34). listed in table I I summarize the recharge to and dis- The distribution of simulated heads for the charge from the basin-fill aquifer system of Spanish monthly stress period representing December 1994 and Springs Valley for 1994. Figures based on field and empirical presented the location of 27 cells used for comparison are shown estimates are for comparison with the budget calculated by the model simulation. Most in figure 34. The configuration of the model-derived of the model results are in fairly good agreement with heads in general agrees with the distribution of mea- estimates based on field and empirical techniques sured heads for December 1994 shown in figure 19. (table I l). In particular, simulated total recharge and The general direction of ground-water flow simulated discharge are within l0 percent of the estimated values. in the model agrees with the conceptualization of the However, discharge of ground water by evapotranspi- flow system developed from data collected during this ration, as estimated by field methods, is 500 acre-ft less study. Ground water was simulated as flowing from the than that calculated by the transient model. Uncertain- recharge-source areas in the surrounding mountains ties in land-surface altitudes used in the model for com- toward the valley and a broad ground-water divide puting evapotranspiration may account for the difference. Subsurface flows simulated the model beneath the central part of the valley. From the divide, by are slightly greater to the south and more than 50 per- ground water was simulated as flowing north and south cent less to the north than flows estimated by other under gradients similar to those measured in December investigators (Cohen and Loeltz, 1964,p.23; Rush and t994. Glancy, 1967,table l6; Hadiaris, 1988, p. 85). The imbalance between recharge and discharge simulated U) ru 100 by the transient model for 1994 is about 2,200 acre-ft Q and is within the change in ground-water storage of frH tr9 1,700 to 3,400 acre-ft estimated from water-level declines measured between December 1993 and H$ eo December 1994. F>I,JJ O ?r Discharge from the basin-fill aquifer in Spanish ^oxt! Springs Valley increased between 1979 and,1994, g5 eo mostly due to ground-water pumping. ,.- @ The simulated 6fi cumulative changes from 1979 to 1994 in total dis- r.r.r E (9n charge, subsurface outfl ow, evapotranspiration, and tZ oo ground-water pumpage are shown in figure 351. The ()!llrJ O quantity of simulated evapotranspiration decreased below 1979 levels response increases o-

SIMULATION OF GROUND.WATER FLOW 't 1 1 9"45' 1 9'37'30"

39"37'30"

Base trom U.S. Geological Survsy data, O 1 2 3MILES Geology modified from Bonham (1 969), '1:100.000-scale, 1980; Universal Transverse Stewart (1980), Cordy (1985), and Bell and Bonham ('1987) Mercator proiection, Zone 1 1 F_r-L-r-_-r----r O 1 2 3KILOMETERS

EXPLANATION

Basin fill I Model cell in basin fill containing well Model boundary Consolidated rock Simulated water-level contour, Basin boundary ffi -4,440- December 19911-5hows altitude of water level. Contour interval, in feet, is variable. Datum is sea level

Figure 34. Simulated heads in layer 1 calculated from monthly stress period representing December 1994 of model simulation, Spanish Springs Valley, west-central Nevada.

66 Hydrogeology and Simulated Effects of Urban Development on Water Resources ol Spanish Springs Valley, Nevada 14,000

12,000 A Discharge

10,000

8,000

6,000

4,000

2,000

0

-2,000

-4,000

-6,000

F IIJ 4,000 Lu lJ- 2,000 f,l Recharge (ruJ o 0 z -2,000 (,ui z -4,000 - Total recharge o -6,000 I [! - mported surf ace-water F -8,000 fI -10,000 f= () -12,000

2,000

0

-2,000 C Storage (total)

-4,000

-6,000

-8,000

-10,000

-12,000

-14,000

1 980 1 982 1984 1986 1988 1990 1 992 1994 CALENDAR YEAR

Figure 35. Cumulative change since 1979 in annual ground-water (A) discharge, (8) recharge, and (C) storage from results of transient simulations, 1979-94, spanish springs valley, west-central Nevada

SIMULATION OF GROUND'WATER FLOW Sensitivity Analysis one-half. These changes in evapotranspiration had a predictable effect on the distribution of hydraulic head. Model sensitivity to the uncertainty in estimates Increasing maximum evapotranspiration rate resulted of hydraulic conductivity, specific yield, and the maxi- in lowering the average head (negative bias) and mum rate of evapotranspiration was evaluated by six decreasing the maximum rate raised the average head simulations using the calibrated transient model (table (positive bias). Changes in primary storage produced a l2). Each model property was changed from its cali- change similar in magnitude to the change produced by brated value to determine the effects on the differences variations in evapotranspiration. In addition, the rate of between measured and simulated heads for 27 model ground-water discharge to the North Truckee Drain cells (fig. 34) and flux rates for 1994 conditions at was affected most when the maximum rate was head-dependent boundaries. Only one modeled prop- decreased by one-half. erty was uniformly changed for a given simulation; the other properties remained constant. The modeled prop- Limitations of Flow Model erties were doubled or reduced by one-half in all the simulations. Changes in hydraulic conductivity were A numerical ground-water model is a simplifica- made only to the horizontal hydraulic conductivity. tion of a complex physical system. The model is based Because solution to the flow model is not unique, the on data that, to varying degrees, are uncertain. Errors in sensitivity analysis cannot be used to veriff the accu- the conceptual model of the system, hydraulic proper- racy of the model. However, the analysis can be used to ties, boundary conditions, initial conditions, and sys- test the response of the model to a range of hydrologic tem stresses are reflected in the model results. Some properties or conditions. data used in the model are estimated through the pro- As presented in table 12, changes in evapotrans- cess of inverse modeling, whereby values ofdependent piration had the grcatest effect on the final head distri- variables (such as hydraulic head) are known and inde- bution. The largest absolute difference between pendent variables are manipulated until agreement is simulated and measured heads was 93.7 ft when the reached between simulated and measured values of the maximum evapotranspiration rate was decreased by dependent variables. However, errors in model data

Table 12. Summary of model-sensitivity simulations at end of 1994 for Spanish Springs Valley area, west-central Nevada

Statlstical difference between Percenlage ol callbrated volume at head- measured and simulated head dependent boundarles (calibrated volume, in for 27 model cells (feet) acle-teet per year, in parentheses) Itlulti' Part of plication Property Subsurface outllow Ground- model tactor water varied Evapo- affected applied to Absorure discharge property n:l Bias 2 transpira- to North To To tion ffi:',HI #ilf south north Truckee Drain

Calibrated trrnsient simulrtion results rt end of 1994 None All layers 10.0 4.t +40.4 100 100 100 100 (le0) (r70) (1,800) (170)

Sensitivity analysis Hydraulic All layers 0.5 9.5 4.8 +63.0 87 68 l0l 65 conductivity 2 I1.9 5.5 4.2 ll9 ll5 99 t42 Primary stora&e All layers 0.5 ll.l 4.1 45.4 98 86 85 lll coefficient t 2 t2.l 5.1 +90.6 105 108 ll3 toz Maximum ET Layer I 0.5 ll.0 5.0 +93.7 t07 104 84 282 rate 2 9.3 4.2 -1.4 96 97 106 29

I Average ofsquared differences between measured and simulated heads (standard deviation). 2 Measurcment offeet to which simulated heads are above (positive) or below (negative) measured heads. 3 P.i.ury storage in layer I is specific yield and in layer 2 is confined storage coefficient'

68 Hydrogeology and Simulated Effects of Urban Development on Water Fesources of Spanish Springs Valley, Nevada can cancel each other, producing non-unique solutions Simulation ol Responses to Hypothetical to the inverse problem. If a ground-water model con- Grou nd-Water Development Scenarios tains these compensating erors, the model may suc- cessfully reproduce measured heads but fail to predict The hydrologic response of the basin-fill aquifer system to three future aquifer response when system stresses have sig- hypothetical ground-water develop- ment nificantly changed (Konikow and Bredehoeft, 1992, scenarios was evaluated using the l6-year transient p.77). Further, Oreskes and others (1994,p. 643) argue model. The hypothetical scenarios were developed from projected that numerical models ofnatural systems can be neither water use within Spanish Springs Valley verified nor validated, but only partly confirmed by and discussions with representatives from the Nevada agreement with observational data. Division of Water Resources and Washoe County. The flow model in this report is an approximation For each hypothetical scenario, the distribution of ground-water movement in the basin-fill aquifer in and quantity of ground-water withdrawal was speci- Spanish Springs Valley. Some model data were derived fied. To evaluate effects of changes in Truckee River from sparse records, from data ofuncertain quality, or importation and related recharge, each scenario was from empirical methods that are inherently uncertain simulated with and without flow in the On Ditch. (such as estimating recharge as a percentage of precip- Model-computed waterlevel changes referenced to itation). Other properties of the system were estimated December 1994 levels were determined at the end of without observation or measurement (for example 5-, l0-, and 2O-year periods to illustrate probable hydraulic properties of deep basin fill). Ground-water effects of ground-water withdrawal from results of flow in consolidated rock underlying and adjacent to the simulations. The final hydraulic head distribution basin fill is unqualified and may have significant effects shown in figure 34, which represents December 1994 on hydraulic characteristics of the modeled aquifer. conditions calculated from the l6-year transient model, The nature of the hydraulic connections at the northern was used as the initial head for each scenario. In all and southern boundaries of the study areaare poorly simulations, every residence was assumed to be con- understood, and hence the boundary conditions nected to a sewer system (no recharge from septic sys- employed in the model are uncertain. Land-surface tems), and the annual precipitation rate was assumed to altitudes also have an associated uncertaintv ofabout be constant and equal to the long-term average esti- l0 ft. mated by use of the Maxey-Eakin method. The hydrau- lic properties of the basin fill, boundary conditions, Although substantial information exists on some location and amount of domestic-well pumpage, and system stresses (for example, municipal pumping), distribution of evapotranspiration remained the same others, such as evapotranspiration rates and septic- as in the transient model. Daily flow records of the On system recharge, were estimated from values in the lit- Ditch representing near-average conditions also were erature. Agricultural pumpage was based on quantities used in the simulations. Ground-water pumpage was allocated to each user. Irrigation return flows and final specified in layer I (but not in layer 2) for each of the hydraulic conductivities were derived through inverse simulations. Stress periods, time-step methodology, modeling, a process that can produce non-unique solu- and the transient character ofassigned recharge and tions. discharge used in the l6-year transient model also were Identiffing the uncertainties, or the magnitude of applied in the development simulations. the uncertainties. in the above model data sets was not possible and that contributed to the lack of complete Selection of Scenarios agreement between simulated and measured hydraulic Hypothetical development scenario I was used to heads. For this reason, no attempt was made to improve represent continued ground-water pumping at 1994 the head match by locally adjusting hydraulic proper- rates (2,400 acre-ft/yr) and areal distribution. The loca- ties in the model. Instead, hydraulic properties were tion of model cells assigned pumpage greater than l0 kept general and were assumed to approximate the bulk acre-ft/yr in scenario I are shown in figure 27. Hypo- character of the basin fill. Similarly, no adjustments thetical development scenario 2 represents conditions were made to pumping rates or rates of recharge from resulting from projected maximum municipal pump- precipitation to improve head match. ing, as determined from water-rights data, while all

SIMULATION OF GROUIiITIWATER FLOW 69 other ground-water withdrawals remain at 1994 rates. After 20 years of simulated ground-water with- Municipal pumpage for scenario 2 is specified to be drawal under 1994 pumping conditions (scenario l), 3,000 acre-ff:/yr (out of about 4,000 acre-Nyr of total water-levels declined l0 to 20 ft in the entire northern basin withdrawal). Hypothetical development scenario half of the modeled area (fig. 37A).For the same sce- 3 was used to represent conditions if all available water nario, but without simulated ffow in the On Ditch (fig. rights were used within Spanish Springs Valley to sim- 378),water-level declines of l0 to 20 ft were indicated ulate the effects of maximum total-basin pumping by after only 5 years, and after 20 years, simulated water- all users. The assigned total-basin pumpage was 6,000 levels had declined between 20 and 60 ft in a large area acre-fVyr. The distribution of cells assigned pumpage in the north-central part of the modeled area. For con- greater than l0 acre-fl/yr in scenarios 2 and3 is shown ditions represented by scenario l, cumulative storage in figure 36. depletion increased about 5 percent (fig. 40) in the absence of recharge from imported surface water. Discussion of Results For model simulations that represent maximum municipal pumpage (hypothetical development sce- Simulation results of the three hypothetical devel- nario 2), much of the modeled area has simulated opment scenarios are presented in figures 37, 38, and water-level declines of 20 to 60 ft after 20 years of 39. Figure 40 shows the cumulative depletion in pumping and simulated flow in the Orr Ditch (fig. 381). ground-water storage within model layer I since The greatest decline (60 ft and greater) was in the December 1994, assuming a total storage of about southeastern part of the model area and was due to the 450,000 acre-ft. To meet part of the objectives of this location of simulated municipal wells. Without simu- study, the hypothetical scenarios were simulated with lated flow in the On Ditch, results of scenario 2 in the Ditch (scenarios la, 2a, and 3a) and flow On (fig. 388) indicate that nearly the entire modeled area in the Orr Ditch (scenarios I b, 2b, and 3b) without flow would have water-level declines of 20 to 60 ft and more to evaluate the effects of completely removing than 16 percent of the basin fill would be dewatered imported water. Results of the hypothetical scenarios (fig. a0l). demonstrate general basinwide trends and illustrate the Results of scenario 3 (ground-water withdrawals general responses. ln several simulations, some model for all available water rights) indicate that nearly the cells were completely dewatered. When a cell is dewa- entire modeled area would have water-level declines of tered, all recharge and discharge simulated in that cell 20 to 60 ft after 20 years. As in scenario 2, water-level cease for that annual stress period. For the cell in the declines of 60 ft and greater were simulated within the northeast (cell 5,23) and one along the west boundary southeastern part of the area after 20 years of pumping (cell 16,4), the initial saturated thickness was less than (fig. 39) and nearly 20 percent of the area of the mod- the simulated drawdown, causing these cells to go dry. eled aquifers were dewatered (fig. a0). The recharge and discharge assigned to these cells totaled about 6 acre-ff/yr and were insignificant. How- ever, a cell in the southeast (cell 5,23) was dewatered SUMMARY AND CONGLUSIONS after 20 years of simulated pumping because of the large pumping stress assigned to that cell (about 360 Infiltration of imported surface water for iniga- acre-ff/yr) and because the cell is within the area of low tion is the source of most ground-water recharge to the hydraulic conductivity used to simulate a possible fault basin-fill aquifer in Spanish Springs Valley. As urban zone. development increases, changes in land use from agri- Model-computed water levels in figures 371, cultural to suburban and resultant increases in ground- 381, and 391, which show simulated effects of contin- water withdrawal are anticipated. These changes may ual seasonal flow in the Orr Ditch under the different include reduction or complete elimination of surface- pumping scenarios, rose as expected in the south end water importation to the valley. Because of potential of the valley in response to recharge from imported ground-water overdraft associated with urban develop- water. Much of this rise can be attributed to the trend of ment, the U.S. Geological Survey, in cooperation with below-average conditions simulated in the last several the Nevada Division of Water Resources, made a 5- years of the transient model, as compared with average year study to evaluate and refine estimates of the water conditions used in the hypothetical development budget and yield of the basin-fill aquifer in Spanish scenarios. Springs Valley.

70 Hydrogeology and Simulated Etfects ol Urban Development on Water Resources of Spanish Springs Valley, Nevada 1 19"45' 119.37'30"

39037'30'

Nry

Sparks

a"*

Reno tt iv ef {o 4d{"" Truckee Meadows 39"30' qglghom U.S. Geologlcal Suruoydata, 0 1 2 3 MILES ceotogy modiffsdfrom Bonham (1969), 1:100,fl)Gscale, 1980; Udw6alTranw€rs€ | I | | Stewart (igSO),' Cordy (i98S), and Mercalor p(ieclion, Zone I I 1------f-----]-T-- BeI lnd Bonham (i Se4 O 1 2 sKILOMETERS

EXPLANATION

E Bastnfitt I Gelwtthasslgngdpumpagsgrcator todetboundary I conroildarcdrock HlJoac.-lb'tporyearln Ba3rnboundary N Cett wtth asslgnod prlmpaga groabl :B*$f;,**poryoarrn

Figue 36. Cells with assigned ground-water pumpage for municipal, inigation, and industrial use in hypothetical development scenarios 2 and 3 used in model simulations, Spanish Springs Valley, west-central Nevada.

SUililARY AND CONCLUSIONS .L:g@@ o- E 3 z o- $;r ;i r ie g g E IF o ;t tt c z *E c 5 E$ E E= E= o o- 3;E s f; () X uJ ; c(U o n I o I ll I aD c. coo(U eong Qz Eb6r v) =c DEi"= .96 ag= c.r Fa" esL= 9o -ol EJP xt- N -Eroc-c &, b.9 Hg ;aot .FE "''F. 35 cD io *{: ltJ O.= o F b5'o€ EOc-c 3E 56' (5E oc or(5 f;= €e (I)= d'= E3oc =.o Yl E(d9= a.E lIJt E? lo Y: o(,l EO sE N3 c) o) E: .9s,=|(U L.=

72 Hydrogeology and Simulated Effects of Urban Development on Water Resources of Spanish Spilngs Valley, Nevada z EI g, g () I ;sE ff ;s ,* ;g s o F ;i o z €E frE €c g * *: s ord (LI oo ;E E;€ i i '=(UOE X (U> lrJ co 8z :E I It M! 96 EC) o.to-.! 99 9*-_9 (I'= 'gJ A 9o Ul* Eg =o- R i_fi xb 9ro EE E5 N 9. Eo o=oP b) .Ec att .- oj ES == RE a E'= tre:(E (Uft ,rtE : b= = c# oi -o e: (t'v gE 6(U oE o ou) >L Se ba. i;O =s a Ee g{ +s ro 6HF_a >5E:d EE EE

II.Ea6

SUMMARY AND CONCLUSIONS .o J= P E s s E z OE I o(! F. isttii*;*tt;**EE z fiE fiE fiE fig .xzHd 5 ;E E B g Y- (L 3;E E eg X CTJ F t.lJ od coFI t t !ffi ! (')>l83 ll F9 (D(! E> oo,o_ s, o.= ab_ EU) @ E5 * EF !U =o u)* a:J -cg*- bi5 o)s6 '-c c) 3E ooE= o=RE (l))z ;io= l/)Q 3e>: (n IE u- cF !u d= o ,rtS

o=C(E otr FA6b z'9X.- 9-^ b'=' 6b i=i6 itEbo =s tr EE I,n fit o(U+E >PoL '6_ -o otEe:r5 o L-6O- .EF t! _o

74 Hydrogeology and Simulated Effects of Urban Development on Water Resources ol Spanish Springs Valley, Nevada CALENDAR YEAR

1 994 1996 2002 2004 2006 201 0 2012 2014 0r --'l 0

10,000 Depfetion with recharge tV ,,/ imported surface-water 20,000 Depletion without recharge by imported surlace-waler 30,000

A Scenario 1 40,000

(r 0 UJ

(r 10,000 llJ ul (, (L F E. I.IJ 20,0@ IIJ 5F IL -q) UJ (E (r 30,000 uJ () z 3 o (,ui 102 ol tr cc o (5 aF z TL z lu 150 F F I Scenario 2 u.l 3 (L z lJJ f o o llJ (,E z 0 ok z l o 10,000 f F uJ J t! c 20,000 o l! 5r- o z tIl TU 30,000 o E. F IU J (L l 40,000 l o 50,000

60,000

70,000

80,000

90,000 C Scenario 3

100,000 12 16 18

ELAPSED TIME, IN YEARS SINCE 1994

Figure 40. Simulated cumulative depletion in ground-water storage since December 1994 for hypothetical development: (A) scenario 1 , continued pumping at 1994 rates, (8) scenario 2, projected maximum municipal pumping rate, and (C) scenario 3, projected total-basin pumping rates of all available water rights, Spanish Springs Valley, west-central Nevada.

SUMMARY AND CONCLUSIONS Spanish Springs Valley is a north-trending struc- Total amount of recoverable water stored in the basin- tural basin in west-central Nevada, about 5 mi north- fill aquifer in the upper 330 ft is estimated to be between east of Reno, with a total area of about 80 mi2. Sub- 300,000 and 600,000 acre-ft. stantial urban development of this valley began in Annual precipitation on the valley floor is gener- the late 1970's. Population of less than 800 in 1979 ally less than 8 in. The surrounding mountains receive increased to more than 4,000 in 1990 and was pro- about 9 to I I in. annually and more than 13 in. may fall jected to be about 9,300 in 1994. More than 2,000 in the higher altitudes of the Pah Rah Range. The total homes receive water from municipal suppliers within precipitation estimated within the study area is about Spanish Springs Valley. An additional200 homes 36,000 acre-ff/yr, of which nearly 3,500 acre-ff/yr falls derive water from domestic wells. Toal ground-water within the topographically closed Dry Lakes area in the withdrawal from the basin in 1994 was an estimated southern part of the Pah Rah Range. Estimated ground- 2,600 acre-ft-about 2,000 acre-ft more than in 1979. water recharge from precipitation ranges from 770 acre- Secondary recharge from septic systems, estimated ff/yr using a chloride-balance method to 830 acre-ft/yr from water-use records for 1994, may be as much as using the Maxey-Eakin method. In 1994, recharge from 450 acre-ft. precipitation was estimated at about 600 acre-ft, or 72 percent of the long-term average estimated using the Several methods of investigation were used dur- Maxey-Eakin method. ing this study, including the collection of basic hydro- Surface-water resources in the basin consist logic and geologic data and the application of several almost entirely of imported Truckee River water con- geochemical techniques and geophysical methods. veyed by the On Ditch to support agriculture in the The distribution of precipitation within the study area southern part of the valley. During this study, the On was estimated from a relation between precipitation Ditch was completely shut offby Jun e in 1992 and 1994 and altitude. Estimates of ground-water recharge from in response to below-average precipitation. Flow in the precipitation were based on the distribution of precip- North Truckee Drain, which removes irrigation return itation. A flow model was developed to simulate the flow from Spanish Springs Valley to the Truckee River, movement of ground water and to evaluate the empir- fluctuates in direct response to the availability of Orr ical estimates of recharge and discharge. The cali- Ditch water and the application of water for irrigation. brated flow model was then used to estimate effects ln 1994, the difference between On Ditch inflow and of hypothetical changes in land use and ground-water North Truckee Drain outflow was about 3,100 acre-ft, development. which represents the amount of imported surface water The study area lies between major tectonic struc- consumed in Spanish Springs Valley. Of the 3,100 acre- tures of the Sierra Nevada Frontal Fault Zone and the ft of imported surface water consumed in 1994, about shear zone of the Walker Lane. Extensional faulting 1,400 acre-ft is estimated to have recharged the shallow is thought to have formed the present-day topographic basin fill as transmission loss from the On Ditch features, including the structural depression and infiltration of applied irrigation water. Direct underlying Spanish Springs Valley. Tertiary volcanic evaporation from ponded water and evapotranspiration rocks, which make up most of the consolidated rock prior to deep percolation make up the remaining con- exposed in the area, underlie much of the eastern part sumed imported water. of the valley. In the southeastem Part, a relatively Virnrally all ground water in Spanish Springs transmissive connection between basin fill and volca- Valley is derived from imported surface water and pre- nic rocks is suggested by water-level responses to cipitation that falls within the drainage basin. During ground-water pumping. Although detailed investiga- the irrigation season, Orr Ditch transmission losses and tion of the hydrologic characteristics of these Tertiary infiltration of water applied to fields raise shallow volcanic rocks was beyond the scope of this study, the ground-water levels more than 5 ft, creating a ground- volcanic rocks may be hydrologically significant at a water mound that dissipates with time after inigation regional scale. The structural depression underlying ceases. The movement of ground water is generally Spanish Springs Valley is filled in part by interbedded from the recharge-source areas in the mountains toward deposits of sand, gravel, silt, and clay with a maximum the valley and a ground-water divide that has developed thickness of about 1,000 ft as determined by geophys- beneath the central part of the valley. From the divide, ical methods. Hydraulic conductivity of the basin fill in ground water flows northward toward Warm Springs the upper 330 ft ranges from 0.5 ff/d to about L2 ftld. Valley and southward to the Truckee Meadows. Locally

Hydrogeology and Slmulated Etfects ol Urban Development on Water Resources ol Spanish Springs Valley' Nevada ground is derived water distinguished from imported development scenarios suggest that, although recharge Truckee River water on the basis of stable-isotope from imported surface water prevents water-level compositions and environmental age-dating tech- declines in the southern part ofthe study area,after 20 niques. Mass-balance calculations suggest that years ofpumping, water levels may decline between 20 percent imported Truckee River water makes up 65 of and 60 ft throughout much of the area. For model sim- the a discharge from well 2,000 ft north of the Orr ulations without recharge from imported water, 8 per- Ditch. cent to nearly 20 percent of the basin-fill aquifer was Estimates of ground-water recharge and dis- dewatered and water-level declines were more than 60 charge to the basin-fill aquifer for 1994 conditions were ft in the southeastem part of the valley. made using field and empirical techniques, and were evaluated using a two-layer mathematical flow model. REFERENCES CITED The flow model was calibrated under transient condi- tions using estimated ground-water withdrawals from Bauer, D.J., Foster, B.J., Joyner, J.D., and Swanson, R.A., 1979 through 1994 and the resultant water-level 1996, Water resources data, Nevada, water year 1995: changes. Model-computed ground-water recharge U.S. Geological Survey Water-Data Report NV-95-1, from imported surface water agreed well with the 734 p. empirical estimate of 1,400 acre-ft for 1994. Field- Bear, Jacob, 1979, Hydraulics of groundwater: New York, derived estimates of evapotranspiration, howeveq are McGraw-Hill,569 p. 500 acre-ft less than evapotranspiration simulated by Bell, J.W., 1981, Quaternary fault map of the Reno quadran- the flow model for 1994. Lack of water-level data in the gle [Nevada]: U.S. Geological Survey Open-File northern part of the valley precluded empirical esti- Report 8l-982,62 p. mates of subsurface flow to Warm Springs Valley. On Bell, J.W, and Bonham, H.F., Jr., 1987, Geologic map, Vista the basis of the current conceptualization of the flow quadrangle [Nevada]: Nevada Bureau of Mines and system, the calibrated flow model indicates that about Geology Urban Map Series, Vista Folio, Map 4Hg, 170 acre-ft of water may move northward toward scale l:24,000. Warm Springs Valley. In addition, model results indi- Bergeq D.L., 1995, Ground-water conditions and effects of cate that 190 acre-ft of subsurface flow may be entering mine dewatering in Desert Valley, Humboldt and Persh- the Truckee Meadows to the south. An imbalance ing Counties, northwestern Nevada, 1962-91 : U.S. between ground-water recharge and discharge ranging Geological Survey Water-Resources Investigations from 1,700 acre-ft to 2,200 acre-ft was estimated for Report 95-4119,79 p. 1994 conditions. Bonham, H.F., Jr., 1969, Geology and mineral resources of Washoe and Storey Counties, Nevada: The augmented-yield concept in this study is Nevada Bureau of Mines and Geology Bulletin 70,l4O p. defined as the perennial yield plus salvable secondary recharge resulting from use of imported surface water. Bredehoeft, J.D., Papadopulos, S.S., and Cooper, H.H., Jr., 1982, Groundwater-The water-budget myth Augmented yield of the basin-fill aquifer in Spanish in Scien- tific basis of water-resource management studies in Springs Valley, determined for 1994 conditions, was geophysics: Washington, D.C., National Academy estimated to be about 2,400 acre-ft, assuming that Press, p. 5l-57. 1,400 acre-ft of ground-water recharge was salvaged Brenner, I.S., 1974, A surge of maritime tropical air--{ulf from infiltration of imported Truckee River water. of California to the southwestern United States: Howeveq if the quantity of imported surface water or Monthly Weather Review, v. 102, p. 375-389. its management changes, the augmented yield for the Busenberg, Eurybiades, and Plummer, L.N., 1992, Use of basin-fill aquifer must be revised to account for chlorofluorocarbons (CCI3F and CCI2F2) as hydro- changes in ground-water recharge. logic tracers and age-dating tools'-:The alluvium and The hydrologic response of the basin-fill aquifer terrace system of central Oklahoma: Water Resources system to three hypothetical ground-water develop- Research, v. 28, p. 2257 -2283. ment scenarios was evaluated using the l6-year tran- Cardinalli, J.L., Roach, L.M., Rush, F.E., and Vasey, B.J., sient model. For each scenario, the distribution and 1968, State ofNevada hydrographic areas: Nevada quantity of ground-water withdrawal was changed and Division of Water Resources map, scale l:500,000. each scenario was simulated with and without flow in Carlson, H.S., 1974, Nevada place names-A geographical the On Ditch. Simulated responses to the hypothetical dictionary: Reno, University of Nevada Press, 282 p.

REFERENCES CITED Claassen, H.C., Reddy, M.M., andHalm, D.R., 1989, Use of Friedman, L.C., and Erdmann, D.E., 1982, Quality assur- the chloride ion in determining hydrologic-basin water ance practices for the chemical and biological analyses budgets-A 3-year case study in the San Juan Moun- of water and fluvial sediments: U.S. Geological Survey tains, Colorado, U.S.A: Joumal of Hydrology, v. 85, Techniques of Water-Resources Investigations, book 5, p.49-71. chap. A6, l8l p. Clary S.L., McClary D.R., Whitney, Rita, and Reeves, Hadiaris, A.K., 1988, Quantitative analysis of groundwater D,D., 1995, Water resources data, Nevada, water year flow in Spanish Springs Valley, Washoe County, 1994: U.S. Geological Survey Water-Data Report NV- Nevada: Reno, University of Nevada, unpublished 94-1,768 p. M.S. thesis, 202 p. Cohen, Philip, and Loeltz, O.J., 1964, Evaluation of hydro- Haeni, F.P., 1988, Application of seismic-refraction tech- geology and hydrogeochemistry of Truckee Meadows niques to hydrologic studies: U.S. Geological Survey area, Washoe County, Nevada: U.S. Geological Survey Techniques of Water-Resources Investigations, book 2, Water-Supply PaPer 1779-5,62 P. chap. D2, 86 p. Cook. P.G.. Solomon, D.K., Plummer, L'N, Busenberg, E., Hardman, George, 1936, Nevada precipitation and acreages and Schiff, S.L, I 995, Chlorofluorocarbons as tracers of of land by rainfall zones: Nevada University Agricul- groundwater transport processes in a shallow, silty sand tural Experiment Station report and map, l0 p. aquifer: Water Resources Research, v. 31, no. 3,p.425- Harrill, J.R., 1968, Hydrologic response to irrigation pump- 434. ing in Diamond Valley, Eureka and Elko Counties, and Chen, J., 1991, Improve- Coplen, T.B., Wildman, J.D., Nevada, 1950-65, with a section on Surface water by gaseous hydrogen-water equilibration ments in the R.D. Lamke: Nevada Department of Conservation and hydrogen isotope ratio analysis: Analyti- technique for Natural Resources, Water Resources Bulletin 35, 85 p. cal Chemistry v. 63, p. 910-912. Evaluation of the water resources of Lemmon G.E., 1985, Geologic map, Reno NE quadrangle Cordy, Valley, Washoe County, Nevada, with emphasis on Nevada Bureau of Mines and Geology Urban [Nevada]: effects of ground-water development to 197 I : Nevada Map Series, Reno Folio, Map 4Cg, scale l:24,000. -1973, Division of Water Resources, Bulletin 42, 130 p. Craig, Harmon, l96l Isotopic variations in meteoric waters: , Ground-water storage depletion in Pahrump Science, v. 133, P.1702-1703. Valley, Nevada-Califo rnia, | 9 62-7 5: U. S. Geological M.D., 1989, Reconnaissance estimate of natural Dettinger, Survey Water-Supply Paper 2279,53 p. recharge to desert basins in Nevada, U.S'A., by using -1986, Hem, J.D., 1985, Study and interpretation of chemical char- chloride-balance calculations: Journal of Hydrology' acteristics of natural water (3d ed.): U.S. Geological v. 106, p. 55-78. Survey Water-Supply Paper 2254,283 p. Dreveq J.1., 1988, The geochemistry of natural waters (2d Young, R.L., ed.): Englewood Cliffs, N.J., Prentice-Hall,437 p. Hess, D.L., Mello, K.A., Sexton, R.J., and 1993, Water resources data, Nevada, water yeat 1992: Eakin, T.8., Maxey, G.B., Robinson, T.W., Fredericks, J.C., U.S. Geological Survey Water-Data Report NV-92-l' and Loeltz. O.J., 1951, Contributions to the hydrology of eastern Nevada: Nevada State Engineeq Water 5ll P. and Kucks, R.P., 1988, Total intensity Resources Bulletin 12, l7 | P. Hildenbrand. T.G., anomaly map of Nevada: Nevada Bureau of Emett, D.C., Hutchinson, D.D., Jonson, N.A., and O'Hair, magnetic scale l:750,000. K.L.. 1994. Water resources data, Nevada, water year Mines and Geology Map 93A, 1993: U.S. Geological Survey Water-Data Report NV- Houghton, J.G., 1969, Characteristics of rainfall in the Great 93-1, 596 p. Basin: Universiry of Nevada, Desert Research Institute l80 Epstein, S., and Mayeda, T., 1953, Variation of content report, 205 p. of water from natural sources: Geochimica et Cosmo- Huxel, C.J., Jr., Parkes, J.E., and Everett, D.8., 1966, Effects chimica Acta, v. 4,p.213-224. of inigation development on the water supply of Quinn Fetter, C.W., 1994, Applied hydrology (3d ed.): New York, River Valley area, Nevada and Oregon, 1950-64: Macmillan Co.,69l P. Nevada Department of Conservation and Natural Fishman. M.J.. and Friedman, L.C., eds., 1989, Methods for Resources, Water Resources Bulletin 34, 80 p' determination of inorganic substances in water and flu- Janzer,V.J., 1985, The use of natural waters as U.S. Geolog- vial sediments: U.S. Geological Survey Techniques of ical Survey reference samples inTaylor, J.K., and Stan- Water-Resources Investigations, book 5, chap. lA, ley, T.W., eds., Quality assurance for environmental 545 p. measurements: Philadelphia, American Society for Freeze. R.A., and Cherry, J.A., 1979, Groundwater: Engle- Testing and Materials, Special Technical Testing Publi- wood Cliffs, N.J., Prentice-Hall, 604 p. cation 867, p. 319-333.

Hydrogeology and Simulated Effects of Urban Development on Water Resources of Spanish Springs Valley, Nevada Johnson, A.1., 1967,Specific yield--Compilation of specific Nichols, W.D., 1979, Simulation analysis of the unconfined yields for various materials: U.S. Geological Survey aquifer, Raft River geothermal area, Idaho-Utah: U.S. Water-Supply P aper 1662-D, 7 4 p. Geological Survey Water-Supply Paper 2060, 46 p. Jones, B.E., 1987, Quality control manual of the U.S. Geo- Estimating discharge of shallow ground water logical Survey National Water Quality Laboratory: U.S. by transpiration from greasewood in the northem Great Geological Survey Open-File Report 87-457, l7 p. -1993,Basin: Water Resources Research, v. 29, no. 8, p. 27 7 | - Kennedy/Jenks/Chilton, I 99 l, Water and wastewater facility 2778. plan for Spanish Springs Valley: Kennedy/Jenks/Chil- Ground-water discharge by phreatophyte ton Draft Report 907004.01, [prepared for Washoe shrubs in the Great Basin as related to depth to ground County Utility Divisionl unpaginated. -1994,water: Water Resources Research. v. 30. no. 12. Keys, W.S., 1990, Borehole geophysics applied to ground- p.3265-3274. water investigations: U.S. Geological Survey Tech- Nevada Division of Water Planning, 1992, Nevada water niques of Water-Resources Investigations, book 2, facts: Nevada Division of Water Planning, 79 p. chap. E2, 150 p. Oreskes, N., Shrader-Frechette, K., and Belitz, K., 1994, Klieforth, Hal, Albright, William, and Ashby, James, 1983, Verification, validation, and confirmation of numerical Measurement, tabulation, and analysis of rain and models in the earth sciences: Science, v. 263, p. 641- snowfall in the Truckee River Basin-Final report: 646. University ofNevada, Desert Research Institute report, Plummer, L.N., Michel, R.L., Thurman, E.M., and Glynn, 59 p. P.D., 1993, Environmental tracers for age dating young and Bredehoeft, J.D., 1992, Ground-water Konikow, L.F., ground water, in Alley, W.M., ed., Regional ground- validated, Celia, M.A., and others, models cannot be in water quality: New York, Van Nostrand Reinhold, part eds., Validation of geo-hydrological models, l: p.255-294. Advances in Water Resources, v. 15, p. 75-84. Remson, Irwin, Hornbergeq G.M., and Molz, F.J., 1971, Likens, G.E., Bormann, F.H., Pierce, R.S., Eaton, J.S., and Numerical methods in subsurface hydrology: New Johnson, N.M., 1977, Biogeochemistry of a forested York, Wiley-Interscience, 389 p. ecosystem: New York, Springer-Verlag, 146 p. Robinson, T.W., 1970, Evapotranspiration by woody Lohman, S.W., 1972, Ground-water hydraulics: U.S. Geo- phreatophytes in the Humboldt River Valley near Win- logical Survey Professional Paper 708, 70 p. nemucca, Nevada with a section on Soil moisture deter- Garwin, and Tueller, Paul, 1976, Vegetation map Lorain, minations by A.O. Waananen: U.S. Geological Survey area]: Nevada Bureau of Mines and Geology, [Reno Professional Paper 491 -D, 4l p. Urban Map Series, Reno Folio, Map 4Ae, scale Robinson, T.W., and Phoenix, D.A., 1948, Ground water in l:24,000. Spanish Spring and Sun Valleys, Washoe County, Malmberg, G.T., and Worts, G.F., Jr., 1966, The effects of Nevada: U.S. Geological Survey Open-File Report, 26 pumping on the hydrology of Kings RiverValley, Hum- p. boldt County, Nevada, 1957-64:. Nevada Department of 1968, Index to hydrographic areas in Nevada: Conservation and Natural Resource, Water Resources Rush, F.E., Resources. Information Bulletin 31, 53 p. Nevada Division of Water Report 6, 38 p. Maxey, G.B., and Eakin, T.E., 1949, Ground water in White River Valley, White Pine, Nye, and Lincoln Counties, Rush, F.8., and Glancy, P.A., 1967, Water-resources Nevada: Nevada State Engineer, Water-Resources Bul- appraisal of the Warm Springs-Lemmon Valley area, letin 8, 59 p. Washoe County, Nevada: Nevada Department of Con- servation and Natural Resources, Water Resources - McDonald, M.G., and Harbaugh, A.W., 1988, A modular Reconnaissance Report 43, 7 0 p. three-dimensional finite-difference ground-water flow model: U.S. Geological Survey Techniques of Water- Saltus, R.W, 1988, Nevada gravity data: Sioux Falls, S.D., Resources Investigations, book 6, chap. A1,586 p. EROS Data Centeq magnetic tape. Meinzer, O.E., 1927, Plants as indicators of ground water: Schaefer, D.H., and Whitney, Rita, 1992, Geologic frame- U.S. Geological Survey Water-Supply Paper 577,95 p. work and ground-water conditions in basin-fill aquifers National Climatic Center, l93l-82, Climatological data, ofthe Dayton Valley and Churchill Valley hydrographic annual summaries, Nevada, 1930-81: Asheville, N.C., areas, westem Nevada: U.S. Geological Survey Water- U.S. National Oceanic and Atmospheric Administra- Resources Investigation Report 9l-4072,12 p. tion, v.45 to v. 96, no. 13, (Nevada, published annu- Shaw Engineering, 1984, Desert Springs Water Co. system allY). evaluation report, Spanish Springs Valley, Washoe Nevada State Journal ,1879,The On Ditch: Reno, January 3, County, Nevada: Sparks, Nev., Shaw Engineering. p. 3. unpaginated.

REFERENCESCITED 79 Stewart, J.H., 1980, Geology of Nevada-Discussion to Welch, A.H., lgg4,Ground-water quality and geochemistry in accompany the geologic map ofNevada: Nevada Bureau Carson and Eagle Valleys, westem Nevada and eastern of Mines and Geology Special Publication 4,136 p. California: U.S. Geological Survey Open-File Thatcher, L.L., Janzer, V.J., and Edwards, K.W., 1977, Meth- Report 93-33,99 p. ods for determination of radioactive substances in water W.E. Nork, Inc., 1987, Exploration holes GWSA-A and and fluvial sediments: U.S. Geological Survey Tech- GWSA-B, drilling, construction, testing: Sparks, Nev., niques of Water-Resources Investigations, book 5, chap. W.E. Nork, Inc., report, T p. and appendix. A5,95 p. Aquifer stress test results for the "Big Well," U.S. Geological Survey, 1977, National handbook of recom- Spanish Springs Valley, Washoe County: Sparks, Nev., mended methods for water-data acquisition: Office of -1988a,W.E. Nork, Inc., report, project 88-439,21 p. and appen- Water Data Acquisition, l2 numbered chapters' dix. Van Denburgh, A.S., Lamke, R.D., and Hughes, J.L., 1973, 988b, Golden West/Sunset Associates production A brief water-resources appraisal of the Truckee River wells I and 2: Sparks, Nev., WE. Nork, Inc., report, 38 p. Basin, westem Nevada: Nevada Division of Water -l and appendix. Resources, Reconnaissance Report 57, 122 p' Aquifer stress test GWSA production well no. 2, Washoe County, 1991, Results of well construction and test July 16-22, 1989: Sparks, Nev., W.E. Nork, Inc., report, pumping, Springwood subdivision well, Desert Springs -1989,22 p. and appendix. Water system, Spanish Springs Valley, Nevada: Reno, William F. Guyton and Associates, 1964, Ground-water con- Washoe County Department of Public Works, Utility ditions in Spanish Springs Valley, Nevada: Austin, Tex., Division, unpaginated. William F. Guyton and Associates, l0 p. Spanish Springs Valley, monitoring well con- Study of ground-water availability in Honey struction, August-October 1993: Reno, Nev., Washoe Lake, Warm Springs, and Spanish Springs Valleys, -1993,County Department of Public Works, Utility Division, -1986,Washoe County, Nevada: Austin, Tex., William F. Guy- l9 p. and appendix. ton and Associates, 70 p. Webring, Michael, 1985, SAKI-A fortran program for gen- Wood, W.W., 1976, Guidelines for collection and field analy- eralized linear inversion of gravity and magnetic pro- sis of ground-water samples for selected unstable constit- fites: U.S. Geological Survey Open-File Report 86-122, uents: U.S. Geological Survey Techniques of Water- 3o p. Resources Investigations, book l, chap. D2,24 p.

Hydrogeology and Simulated Effects ol Urban Development on Waler Resources ol Spanish Springs Valley, Nevada CONTRIBUTING U.S. GEOLOGICAL SURVEY STAFF

COLLEAGUEREVIEWERS GIS SUPPORT

William D. Nichols, Hydrologist Rose L. Medina, Physical Scientist ScottN. Hamlin, Hydrologist

PUBLICATIONS SUPPORT

Robin L. Sweet, Technical Editor Shannon C. Demeo, Editorial Assistant Graham A. Patterson, Scientific Illustrator EARTH SCIENCE IN THE PUBLIC SERVICE

Sine 1879, fia U.S. Geologiel Swvey has !€en provtling nry, rcof,,rts,, aN intormation b f/p/p others wlp manqp, cbvelq, ald rrltect qtr NationS water, energy, mircnl, lad, aN biolqglical rc6,ourcf,ls. We heb frN netunl /asour6 rceded to bui6 tontrinow aN w$y sdenfrTb mdas|adirp nffi to tulp minimize or mitigate tlrre eft*B ot natural hazat& and tlle environmental danry criused by human arliviliec. Tlp raaiut/}sot ouretfor/rotouch thedaily liv*of ehnosfewryonc.