Jill N. Densmore — Page 2

USGS2003

Simulation of Ground-Water Flow in the Irwin Basin Aquifer System, Fort Irwin National Training Center, California

Ground-water pumping in the Irwin Basin at Fort Irwin National Training Center, California resulted in water-level declines of about 30 feet from 1941 to 1996. Since 1992, artificial recharge from wastewater-effluent infiltration and irrigation-return flow has stabilized water levels, but there is concern that future water demands associated with expansion of the base may cause a resumption of water-level declines. To address these concerns, a ground-water flow model of the Irwin Basin was developed to help better understand the aquifer system, assess the long-term availability and quality of ground water, and evaluate ground-water conditions owing to current pumping and to plan for future water needs at the base. Historical data show that ground-water-level declines in the Irwin Basin between 1941 and 1996, caused the formation of a pumping depression near the pumped wells, and that recharge from the wastewater-treatment facility and disposal area caused the formation of a recharge mound. There have been two periods of water-level recovery in the Irwin Basin since the development of ground water in this basin; these periods coincide with a period of decreased pumpage from the basin and a period of increased recharge of water imported from the Bicycle Basin beginning in 1967 and from the Langford Basin beginning in 1992. Since 1992, artificial recharge has exceeded pumpage in the Irwin Basin and has stabilized water-level declines. A two-layer ground-water flow model was developed to help better understand the aquifer system, assess the long-term availability and quality of ground water, and evaluate ground-water conditions owing to current pumping and to plan for future water needs at the base. Boundary conditions, hydraulic conductivity, altitude of the bottom of the layers, vertical conductance, storage coefficient, recharge, and discharge were determined using existing geohydrologic data. Rates and distribution of recharge and discharge were determined from existing data and estimated when unavailable. Results of predictive simulations indicate that in 50 years, if artificial recharge continues to exceed pumpage in Irwin Basin, water levels could rise as much as 65 feet beneath the pumping depression, and as much as 10 feet in the wastewater-treatment facility and disposal area. Particle-tracking simulations were used to determine the pathlines and the traveltimes of water high in dissolved solids into the main pumping area. The pathlines of particles from two areas with high dissolved-solids concentrations show that in 50 years water from these areas almost reaches the nearest pumped well.

California↗

USGS2001

Use of InSAR to identify land-surface displacements caused by aquifer-system compaction in the Paso Robles area, San Luis Obispo County, California, March to August 1997

The population in San Luis Obispo County has grown steadily during the 1990s, and some land use has been converted from dry farming to grazing to irrigated vineyards and urban areas. Because insufficient surface-water supplies are available to meet the growing demand, ground-water pumpage has increased and the resulting water-level declines have raised concern that this water resource may become overstressed. One particular concern is whether the larger ground-water basins within the county function as large individual basins or whether subsurface structures divide these large basins into smaller subbasins, as differences in ground-water-level data suggest. Interferometric Synthetic Aperture Radar (InSAR) is used in locating land-surface displacement, which may indicate subsurface structures in ground-water basins by determining seasonal and historical land-surface changes. Owing to the high spatial detail of InSAR imagery, the InSAR-derived displacement maps can be used with ground-water-level data to reveal differential aquifer-system compaction related to the presence of geological structures or the distribution of compressible sediments that may define subbasin boundaries. This report describes InSAR displacement maps of the Paso Robles area of San Luis Obispo County and compares them to maps of seasonal changes in ground-water levels to detect the presence of aquifer-system compaction.

California↗

USGS2001

Use of InSAR to identify land-surface displacements caused by aquifer-system compaction in the Paso Robles area, San Luis Obispo County, California, March to August 1997

The population in San Luis Obispo County has grown steadily during the 1990s, and some land use has been converted from dry farming to grazing to irrigated vineyards and urban areas. Because insufficient surface-water supplies are available to meet the growing demand, ground-water pumpage has increased and the resulting water-level declines have raised concern that this water resource may become overstressed. One particular concern is whether the larger ground-water basins within the county function as large individual basins or whether subsurface structures divide these large basins into smaller subbasins, as differences in ground-water-level data suggest. Interferometric Synthetic Aperture Radar (InSAR) is used in locating land-surface displacement, which may indicate subsurface structures in ground-water basins by determining seasonal and historical land-surface changes. Owing to the high spatial detail of InSAR imagery, the InSAR-derived displacement maps can be used with ground-water-level data to reveal differential aquifer-system compaction related to the presence of geological structures or the distribution of compressible sediments that may define subbasin boundaries. This report describes InSAR displacement maps of the Paso Robles area of San Luis Obispo County and compares them to maps of seasonal changes in ground-water levels to detect the presence of aquifer-system compaction.

California↗

USGS2001

Evaluation of water-level recovery, 1996-97 to 1999-2000, and comparison of 1999-2000 and 1972-73 water levels in Goleta Central subbasin, Santa Barbara County, California

Ground-water levels were measured during January 1999-June 2000 to evaluate the rate of water-level recovery in the Goleta Central ground-water subbasin that has resulted from injection of about 2,225 acre-feet of surplus water for storage in the ground-water basin. Injection of surplus water was tabulated and compared with water-level rises since 1996 to evaluate the effectiveness of the recharge effort. Water levels have risen about 4 to 37 feet since 1996-97. A preliminary water budget was compiled to assess recharge and discharge in the basin, and it is estimated that total inflow exceeded total outflow during 1998-99 by about 2,844 to 7,518 acre-feet. In addition, water levels for 1999-2000 were compared with water levels for 1972-73 to determine if a 'drought buffer' exists. Water levels measured in two wells during January 1999-June 2000 exceeded January 1972-June 1973 levels. Water levels in the remaining wells measured during January 1999-June 2000 ranged from less than 1 foot to about 32 feet below 1972-73 water levels. In general, the largest water-level rise between 1996-97 and 1999-2000 was about 37 feet in the southeastern end of the basin; the rise was less than 4 feet in the western end of the basin and about 10 feet north of the Goleta Fault. Long-term hydrographs indicate that water levels have been recovering throughout the basin since the early 1990's.

California↗

USGS1997

Geohydrology and water quality of Marine Corps Logistics Base, Nebo and Yermo annexes, near Barstow, California

Because ground water is the only dependable source of water in the Barstow area, a thorough understanding of the relationship between the geology and hydrology of this area is needed to make informed ground-water management andremediation decisions. This report summarizes geologic and hydrologic studies done during 1992-95 at the Marine Corps Logistics Base, Nebo and Yermo Annexes, near Barstow, California. The geologic investigation dealt with the stratigraphy and geologic history of the area and determined the location of faults that cross the Marine Corps Logistics Base, Nebo Annex. Two of these faultscoincide with significant ground-water barriers. Geologic and hydrologic data collected for this study were used to define two main aquifer systems in this area. The Mojave River aquifer is contained within the sand and gravel of the Mojave River alluvium, and the regional aquifer lies in the bordering alluvial-fan deposits and older alluvium. Water-level data showed that recharge occurs exten sively in the Mojave River aquifer but occurs only in small areas of the regional aquifer. Dissolved- solids concentrations showed that ground-water degradation exists in the Mojave River aquifer near the Nebo Annex and extends at least 1 mile downgradient of the Nebo golf course in the younger Mojave River alluvium. Nitrogen concentrations show that more than one source is causing the observed degradation in the Mojave River aquifer. Oxygen-18, deuterium, tritium, andcarbon-14 data indicate that the Mojave River and regional aquifers have different sources of recharge and that recent recharge occurs in the Mojave River aquifer but is more limited in the regional aquifer.

Water-Resources Investigations Report↗

USGS1997

Ground-water hydrology and water quality of Irwin Basin at Fort Irwin National Training Center, California

Geohydrologic data were collected from Irwin Basin at Fort Irwin National Training Center in the Mojave Desert of southern California by the U.S. Geological Survey during 199296 to deter mine the quantity and quality of ground water available in this basin. In addition to data collected from existing wells and test holes, 17 monitoring sites were constructed in Irwin Basin to provide data on subsurface geology, ground-water levels, and ground-water quality. Eleven of these sites were multiple-well monitoring sites that were constructed to provide depth-dependent geohydrologic data in the aquifer system. The aquifer system of Irwin Basin, defined on the basis of hydrologic data collected from wells in Irwin Basin, consists of an upper and a lower aquifer. A 1994 water-table contour map shows that a cone of depression beneath Irwin Basin well field has developed as a result of ground-water development. Water-quality samples collected from Irwin Basin wells to determine potential sources of ground-water degradation indicate that water in three areas in the basin contains high nitrate and dissolved-solids concentrations. The stable isotopes of oxygen and hydrogen indicate that present-day precipitation is not a major source of recharge in this basin. Tritium and carbon-14 data indicate that most of the basin was recharged before 1953 and that this water may be more than 14,000 years old.

Calfornia↗

USGS1991

Use of stable isotopes, tritium, soluble salts, and redox-sensitive elements to distinguish ground water from irrigation water in the Salton Sea basin

Evaporative concentration of irrigation water diverted from the Colorado River to the Salton Sea basin for several decades has produced an overlying system (that includes drainwater and surface waters) whose composition is highly variable and differs from that of the shallow regional ground water beneath it. The role of hydrologic and geochemical processes in causing these differences (and the variability) is inferred from analyses of selected isotopes (3H, D, 18O, 15N, 34S) and elements (As, B, Br, Cl, Fe, N, Se). Selected element-to-Cl ratios establish the relative importance and location of the various processes. Isotopes of H O are used in estimating the relative contribution of leakage from an unlined canal and regional ground water to a nearby spring.

Conference Paper↗