By Tracy Nishikawa, John A. Izbicki, Joseph A. Hevesi, Christina L. Stamos, and Peter Martin
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2004-5267—ONLINE ONLY
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Ground water historically has been the sole source of water supply for the community of Joshua Tree in the Joshua Tree ground-water subbasin of the Morongo ground-water basin in the southern Mojave Desert. The Joshua Basin Water District (JBWD) supplies water to the community from the underlying Joshua Tree ground-water subbasin. The JBWD is concerned with the long-term sustainability of the underlying aquifer. To help meet future demands, the JBWD plans to construct production wells in the adjacent Copper Mountain ground-water subbasin. As growth continues in the desert, there may be a need to import water to supplement the available ground-water resources. In order to manage the ground-water resources and to identify future mitigating measures, a thorough understanding of the ground-water system is needed.
The purpose of this study was threefold: (1) improve the understanding of the geohydrologic framework of the Joshua Tree and Copper Mountain ground-water subbasins, (2) determine the distribution and quantity of recharge using field and numerical techniques, and (3) develop a ground-water flow model that can be used to help manage the water resources of the region.
The geohydrologic framework was refined by collecting and interpreting water-level and water-quality data, geologic and electric logs, and gravity data. The water-bearing deposits in the Joshua Tree and Copper Mountain ground-water subbasins are Quarternary alluvial deposits and Tertiary sedimentary and volcanic deposits. The Quarternary alluvial deposits were divided into two aquifers (referred to as the "upper" and the "middle" alluvial aquifers), which are about 600 feet (ft) thick, and the Tertiary sedimentary and volcanic deposits were assigned to a single aquifer (referred to as the "lower" aquifer), which is as thick as 1,500 ft.
The ground-water quality of the Joshua Tree and Copper Mountain ground-water subbasins was defined by collecting 53 ground-water samples from 15 wells (10 in the Joshua Tree ground-water subbasin and 5 in the Copper Mountain ground-water subbasin) between 1980 and 2002 and analyzing the samples for major ions, nutrients, and selected trace elements. Selected samples also were analyzed for oxygen-18, deuterium, tritium, and carbon-14. The water-quality data indicated that dissolved solids and nitrate concentrations were below regulatory limits for potable water; however, fluoride concentrations in the lower aquifer exceeded regulatory limits. Arsenic concentrations and chromium concentrations were generally below regulatory limits; however, arsenic concentrations measured in water from wells perforated in the lower aquifer exceeded regulatory limits. The carbon-14 activities ranged from 2 to 72 percent modern carbon and are consistent with uncorrected ground-water ages (time since recharge) of about 32,300 to 2,700 years before present. The oxygen-18 and deuterium composition of water sampled from the upper aquifer is similar to the volume-weighted composition of present-day winter precipitation indicating that winter precipitation was the predominant source of ground-water recharge.
Field studies, conducted during water years 2001 through 2003 to determine the distribution and quantity of recharge, included installation of instrumented boreholes in selected washes and at a nearby control site. Core material and cuttings from the boreholes were analyzed for physical, chemical, and hydraulic properties. Instruments installed in the boreholes were monitored to measure changes in matric potential and temperature. Borehole data were supplemented with temperature data collected from access tubes installed at additional sites along study washes. Streambed hydraulic properties and the response of instruments to infiltration were measured using infiltrometers. Physical and geochemical data collected away from the stream channels show that direct infiltration of precipitation to depths below the root zone and subsequent ground-water recharge do not occur in the Joshua Tree area. The simulation of measured temperature data indicated that as much as 71 acre-feet per year (acre-ft/yr) of water infiltrated as a result of streamflow during the study period. Most infiltration was along stream reaches where upstream urbanization resulted in increased runoff.
Numerical simulations to determine the distribution and quantity of recharge included applying a distributed-parameter watershed model, INFILv3, to estimate spatially and temporally distributed recharge under 1950–2003 climate conditions for the area of the Joshua Tree and Copper Mountain ground-water subbasins and for the areas of the surface-water drainage basins upstream from the subbasins. The average annual simulated recharge in the Joshua Tree surface-water drainage basin is about 1,090 acre-ft/yr, which includes 158 acre-ft/yr in the Joshua Tree and Copper Mountain subbasins ground-water model area. The simulated total annual streamflow is 2 to 10 times greater than the measured total annual streamflow indicating that the recharge values estimated using the watershed model may be overestimated. Results from the watershed model indicated that recharge throughout the Joshua Tree surface-water drainage basin is strongly dependent on winter-season runoff generation during wetter than average periods and the subsequent infiltration of surface-water run-on routed to downstream locations.
Data collected during the study were used to develop and calibrate a ground-water flow model of the Joshua Tree and Copper Mountain ground-water subbasins. The simulation period of the ground-water flow model was 1958–2001. The ground-water flow model was developed using MODFLOW-2000. The model cell size was about 820 ft by 820 ft and was discretized vertically into three layers (the upper, middle, and lower aquifers). Recharge was simulated using the net infiltration estimates from the watershed model. The model was calibrated using a trial-and-error approach using water-level data collected between 1958 and 2001; however, the MODFLOW-2000 sensitivity process was used for the sensitivity analysis. In order to better match the measured data, little flow was allowed to cross the Pinto Mountain Fault, thereby compartmentalizing the Joshua Tree and Copper Mountain ground-water subbasins. The calibrated total natural inflow was about 207 acre-ft/yr, consisting of 123 acre-ft/yr of recharge and 84 acre-ft/yr of underflow from the adjacent Warren subbasin. The simulated value of recharge is very close to those values estimated using measured temperature differences (71 acre-ft/yr) and a distributed-parameter watershed model (158 acre-ft/yr). The cumulative volume of water pumped from the ground-water subbasins between 1958–2001 was 42,210 acre-feet (acre-ft); of this total pumpage, the model simulated that 99 percent (41,930 acre-ft) was removed from ground-water storage.
Summary of Major Findings
Geohydrology
Ground-Water Quality
Estimates of Natural Recharge
Ground-Water Flow Model
Abstract
Introduction
Description of Study Area
Purpose and Scope
Definition of the Hydrologic System
Climate Characteristics
Topography and Drainage Basin Characteristics
Surface Water
Ground-Water Hydrology
Geology
Stratigraphic Units
Depth to Basement Complex
Faults and Ground-Water Barriers
Definition of Aquifer System
Ground-Water Recharge and Discharge
Ground-Water Levels and Movement
Ground-Water Quality
Chemical Composition of Water from Wells
Selected Trace Elements
Vertical Variations in Ground-Water Quality
Isotopic Composition of Water from Wells
Oxygen-18 and Deuterium
Tritium and Carbon-14
Estimation of recharge
Estimating Recharge Using Borehole Instrumentation
Installation of Unsaturated-Zone Monitoring Sites
Instrumented Boreholes
Temperature Access Tubes
Analysis of Physical-Property Data
Analysis of Chloride Data
Analysis of Temperature and Matric-Potential Data
Continuous Data from Instrumented Sites
Temperature Data from Access Tubes
Analysis of Infiltrometer Tests
Estimation of Infiltration and Ground-Water Recharge
Summary of Recharge Estimates Based on Borehole Instrumentation
Simulated Recharge Using a Watershed Model
Watershed Model Description
Model Inputs
Daily-Climate Data
Digital Map Files and Attribute Tables
Topographic Parameters
Spatially Distributed Soil Parameters
Spatially Distributed Bedrock and Deep-Soil Parameters
Spatially Distributed Vegetation and Root-Zone Parameters
Model Coefficients
Model Outputs
Evaluation of Model Calibration (1950–2003)
1950–99 Simulated Results
Simulated 1950–99 Precipitation, Evapotranspiration, Snowfall, and Sublimation
Comparison of Simulated and Measured 1974–77 Precipitation at Joshua Tree 3 S
Simulated 1950–99 Surface-Water Runoff and Outflow
Comparison of Simulated and Measured Streamflow
Simulated 1950–99 Average Annual Recharge
2000–03 Simulated Infiltration/Recharge
Time-Series Results and Temporal Variability in Simulated Recharge
Annual Results for Water Years 1950–2003
Average Monthly Results and Seasonal Variability
Watershed-Model Limitations
Summary of Simulation Results Using the INFILv3 Watershed Model
Ground-Water Flow Model
Model Discretization
Spatial Discretization
Temporal Discretization
Model Boundaries
Subsurface Properties
Hydraulic Conductivity
Storage Coefficient and Specific Yield
Faults
Model Inflow
Model Outflow
Pumpage
Ground-Water Underflow
Model Calibration
Simulated Hydraulic Heads
Areal Distribution: Predevelopment and 2001
Simulated Hydrographs
Simulated Water Budget
Model Fit
Sensitivity Analysis
Ground-Water Flow Model Limitations
Summary and Conclusions
References Cited
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