Geology topics

Allen H. Christensen

Publications and source records attributed to Allen H. Christensen.

23 records · Page 2Linked to original sources

USGS1999

Generalized water-table and water-level data at the US Air Force plant 42 and vicinity, Palmdale, California, March-April, 1997

The U.S. Air Force Plant 42 (Plant 42) which is in the Antelope Valley about 1.5 miles northeast of Palmdale and 3 miles southeast of Lancaster in Los Angeles County. Historically, ground water has been the primary source of water owing, in large part, to the scarcity of surface water in the region. Since 1972, supplemental surface water has been imported from the California Water Project to help meet the demand for water. Despite the importation of surface water, ground-water withdrawal for both municipal and agricultural uses is affecting ground-water levels in the vicinity of Plant 42. To better understand the effects of ground-water withdrawal on ground-water levels and movement in the area, the U.S. Geological Survey (USGS), in cooperation with the U.S. Air Force, constructed a generalized water-table-contour map of the aquifer system underlying Plant 42 and the surrounding area.

Water-Resources Investigations Report↗

USGS1998

Geohydrology of the Winchester Subbasin, Riverside County, California

The 20-square-mile Winchester structural subbasin is an alluvium-filled paleocanyon that is as much as 900 feet deep. The alluvial aquifer is composed of detrital material that generally ranges in size from clay to fine gravel; the fine and coarse materials are mixed in some places and inter- bedded in others. The apparent lenticularity of fine- and coarse-grained materials and differing water quality with depth indicate that the aquifer is partly or locally confined. A ground-water divide exists east of the town of Winchester. West of the divide, ground water moves toward and into the South Perris and the Menifee subbasins. East of the divide, ground water moves toward and into the Hemet subbasin. The components of flow direction in the Winchester?Hemet subbasins border area are complex: along the border, some water moves from the southwest corner of the Hemet subbasin into the Winchester subbasin and then eastward subparallel to the border before moving back into the Hemet subbasin. The direction of ground-water movement between the Winchester and Hemet subbasins, and the position of the ground-water divide in the central part of the Winchester subbasin, have changed with time. Prior to about 1974, ground water moved both eastward from the divide and westward from the Hemet subbasin toward a local depression of the water table caused by pumping in the eastern part of the Winchester subbasin. Comparison of spring 1970 and spring 1993 ground-water levels indicates a net rise of as much as 150 feet in the east end of the Winchester subbasin. For this same period, water levels rose about 3 to 20 feet in the western and central parts of the subbasin. Ground-water chemistry in the Winchester subbasin and adjacent subbasins varies areally and vertically. In general, sodium, calcium, chloride, and sulfate are dominant ions. Water quality is generally poor: dissolved-solids concentration exceeded 2,000 milligrams per liter throughout much of the subbasin and was highest west of the town of Winchester. Eastward along the subbasin axis (toward the Hemet subbasin), the dissolved-solids concentration decreases and the pH increases (generally greater than 7.0). Samples from two multiple-well monitoring sites at the west and east ends of the subbasin indicate that the best quality water (dissolved-solids concentrations of 395 and 483 milligrams per liter) is from the deepest wells (perforated near the alluvium- bedrock contact). Samples from the deeper wells in the eastern part of the Winchester subbasin are similar in water type to a sample from a well in the western part of the Hemet subbasin, which suggests that the water may have flowed from the Hemet subbasin; alternatively, the chemistry may reflect the influence of good-quality water flowing from the fractured bedrock basement to the alluvium in the eastern part of the Winchester subbasin. In addition, the potential problem of poor-quality water moving from the Winchester subbasin into the Hemet subbasin may not exist at all depths; fair- to good-quality water may be present below a depth of about 450 feet. Dissolved-solids concentrations in the southwest part of the Hemet subbasin ranged from about 900 milligrams per liter at well 5S/1W-19Q1 about one-quarter mile north of the Winchester?Hemet subbasin boundary to about 3,500 milligrams per liter at well 5S/2W-24C2 near the bedrock outcrops southeast of the Lakeview Mountains. High dissolved-solids concentration in the vicinity of well 5S/2W-24C2 most likely is a result of dissolution of constituents from the aquifer matrix, evaporative processes, and agricultural practices that occur in that vicinity rather than a result of flow from the Winchester subbasin. Aquifer-test results indicate that the transmissivity is about 950 feet squared per day in the eastern part of the Winchester subbasin near the boundary with the Hemet subbasin and about 72 feet squared per day in the western part of the subbasin near the boundary with th

California↗

USGS1997

Regional water table (1996) and water-level changes in the Mojave River, the Morongo, and the Fort Irwin ground-water basins, San Bernardino County, California

The Mojave River, the Morongo, and the Fort Irwin ground-water basins lie in the southwestern part of the Mojave Desert Region of southern California. These basins supply ground water to local water districts, military bases, and private wells. The rapid growth in population in these basins, which is due, in part, to their proximity toLos Angeles, has increased the demand for water and, therefore, the need to understand the Mojave ground-water systems. Ground-water conditions for the Mojave River, the Morongo, and the Fort Irwin ground-water basins for 1996 and areas with significant changes in water levels are identified in this report. Water-level data were compiled for 632 wells in the study area during January-September 1996 to define the water- table surface and direction of ground-watermovement. These data were used to construct the water-table map included in this report. Also shown on the map are 31 hydrographs that show long-term water-level changes in the study area. Short-term water-level changes were determined and a water- level change map was made by comparing 1996 ground-water conditions to 1990-94 conditions in the Mojave ground-water basin and to 1994 conditions in the Morongo and the Fort Irwin ground-water basins. In general, ground-water levels and the direction of ground-water movement in the regional aquifer have not changed significantly since previously published maps (1995). However, the short-term water level did change at specific locations in all three ground-water basins. Water levels in the Mojave River ground-water basin had a maximum rise during the period 1992-96 of 52 feet and a maximum decline of 28. Water levels in the Morongo ground-water basin had a maximum rise of 66 feet and a maximum decline of 57 feet. The Fort Irwin ground-water basins, however, had relatively little change in water level with a maximum rise of 6 feet and a maximum decline of 8 feet. Hydrographs in the regional aquifer systemindicate a decline or, in some areas, no change in the water table during the period of record. Water levels in the shallow alluvial aquifer, generally within 1 mile of the Mojave River, fluctuate in response to streamflow. Ground-water levels rise during wet periods, when floodflows in the Mojave River recharge the shallow alluvial aquifer.

California↗

USGS1996

Well-construction, water-quality, and water-level data, and pond-infiltration estimates, for three ground-water subbasins, Riverside County, California

Reclaimed water in the Eastern Municipal Water District of Riverside County,California, is used within the service area for agricultural irrigation.Owing to the seasonal demand for reclaimed water, storage/infiltration ponds were constructed in the Winchester, Menifee, and south Perris subbasins.Reclaimed water infiltrates from these ponds and enters the groundwater system. Little is known of the effects of the reclaimed water on groundwater quality. In cooperation with the Eastern MunicipalWater District, the U.S. Geological Survey began a study in 1995 to determine the quantity and fate of reclaimed water percolating from these storage ponds. Data compiled during the first phase of this study are presented in this report. Field reconnaissance of the Winchester, Menifee, and south Perris subbasins indicated the existence of many wells. Wellconstruction data for 115 of these wells were tabulated. Available historical waterquality and waterlevel data for 178 wells in the subbasins also were tabulated. In addition, water levels in 86 wells were measured during the spring and autumn of 1995. On the basis of these data, waterlevel contour lines were drawn and the direction of groundwater flow was determined.Three lithologic sections through the subbasins were constructed from drillers' logs of 26 wells.

Water-Resources Investigations Report↗

USGS1995

Geohydrology, water quality, and nitrogen geochemistry in the saturated and unsaturated zones beneath various land uses, Riverside and San Bernardino counties, California, 1991-93

The U.S. Geological Survey, in cooperation with the Eastern Municipal Water District, the Metropolitan Water District of Southern California, and the Orange County Water District, has completed a detailed study of the Hemet groundwater basin. The quantity of ground water stored in the basin in August 1992 is estimated to be 327,000 acre-feet. Dissolved-solids concentration ranged from 380 to 700 mg/L (milligrams per liter), except in small areas where the concentration exceeded 1,000 mg/L. Nitrate concentrations exceeded the U.S. Environmental Protection Agency Maximum Contaminant Level (MCL) of 10 mg/L nitrate (as nitrogen) in the southeastern part of the basin, in the Domenigoni Valley area, and beneath a dairy in the Diamond Valley area. Seven sites representing selected land uses-- residential, turf grass irrigated with reclaimed water, citrus grove, irrigated farm, poultry farm, and dairy (two sites)--were selected for detailed study of nitrogen geochemistry in the unsaturated zone. For all land uses, nitrate was the dominant nitrogen species in the unsaturated zone. Although nitrate was seasonally present in the shallow unsaturated zone beneath the residential site, it was absent at moderate depths, suggesting negligible migration of nitrate from the surface at this time. Microbial denitrification probably is occurring in the shallow unsaturated zone. High nitrate concentrations in the deep unsaturated zone (greater than 100 ft) suggest either significantly higher nitrate loading at some time in the past, or lateral movement of nitrate at depth. Nitrate also is seasonally present in the shallow unsaturated zone beneath the reclaimed-water site, and (in contrast with the residential site), nitrate is perennially present in the deeper unsaturated zone. Microbial denitrification in the unsaturated zone and in the capillary fringe above the water table decreases the concentrations of nitrate in pore water to below the MCL before reaching the water table. Pore water in the unsaturated zone beneath the citrus grove site contains very high concentrations of nitrate. Even though there are zones of microbial denitrification, nitrate seems to be migrating downward to the water table. The presence of a shallow perched-water zone beneath the irrigated-farm site prevents the vertical movement of nitrate from the surface to the regional water table. Above the perched zone, nitrate concentrations in the unsaturated zone are variable, ranging from below the MCL to four times the MCL. Periodically, nitrate is flushed from the shallow unsaturated zone to the perched-water zone. The unsaturated zone pore-moisture quality could not be adequately addressed because of the very dry conditions in the unsaturated zone beneath the poultry-farm site. Surficial clay deposits prevent water from percolating downward. At the two dairy sites, nitrate loading in pore water at the surface was very high, as great as 7,000 mg/L. Microbial denitrification in the unsaturated zone causes such concentrations to decrease rapidly with depth. At a depth of 20 ft, nitrate concentration was less than 100 mg/L. In areas where the depth to water is less than 20 ft, nitrate loading to ground water can be very high, whereas in areas where depth to water is greater than 100 ft, most of the nitrate is microbially removed before reaching the water table.

California↗