Artificial Recharge through a Thick, Heterogeneous Unsaturated Zone near an Intermittent Stream in the Western Part of the Mojave Desert, California

By John A. Izbicki and Christina L. Stamos
U.S. Geological Survey, 5735 Kearny Villa Road, Suite O, San Diego, California 92123

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Abstract

Population in the western part of the Mojave Desert, near Victorville, has increased from 90,000 in 1980 to more than 300,000 in 1999 (Ronald Rector, High Desert Economic Development Agency, oral communication, 1999). Ground-water pumping has increased with population resulting in declining water levels: artificial recharge may help mitigate the declines. The Victor Valley Water District is planning an experiment to test the feasibility of using surface spreading to artificially recharge the regional aquifer near the channel of Oro Grande Wash, about 90 miles northeast of Los Angeles. The regional aquifer is composed of alluvial deposits more than 1,000 feet thick that consist of partly consolidated sand, silt, and gravel weathered from the San Gabriel Mountains. Natural recharge is limited to the infiltration of stormflow in washes and averages about 10,700 acre-ft/yr (Stamos and others, 2001). In comparison, natural recharge to the floodplain aquifer along the Mojave River, also from infiltration of stormflows, averaged about 46,000 acre-ft/yr during 1995-99, and about 25,000 acre-ft/yr during 1931-90 (Stamos and others, 2001). Ground-water discharge, primarily from pumping, greatly exceeded recharge in the area, and averaged about 74,000 acre-ft between 1995 and 1999 (Stamos and others, 2001). Due to the disparity between pumping and recharge, water levels in this area of the regional aquifer have declined more than 1 ft/yr between 1995 and 2000 (Stamos and others, 2001) and water rights in the basin have been the subject of recent adjudication.

Artificial recharge will be by infiltration of water from a pond located near the channel of Oro Grande Wash, an intermittent stream that flows for brief periods after storms. The site was selected because it is near public supply wells, small only amounts of natural recharge occur in the wash, and thick, impermeable caliche deposits that underlie much of the area are not present beneath the wash (Izbicki and others, 2000). In addition, highly permeable sand and gravels deposited by the ancestral Mojave River, are present near the water table beneath the proposed recharge site. The high permeability of these deposits will facilitate the recovery of recharged water by nearby wells. At this site, artificial recharge water must move to the water table through about a 430-ft thick heterogeneous unsaturated zone.

Downward movement of recharge water will be measured at a highly instrumented borehole equipped with heat dissipation probes and advanced tensiometers used to measure changes in matric potential in the unsaturated zone, and a pressure transducer that measures changes in water table levels in the water-table. The well also will serve as an access port for electromagnetic geophysical tools used to measure the movement of the recharge water between instruments. Samples collected from suction-cup lysimeters will monitor changes in the concentration of chloride, nitrate, arsenic, chromium, and other constituents as recharge water infiltrates through the thick unsaturated zone beneath the site. Heat dissipation probes, advanced tensiometers and pressure transducers will be linked to the web for remote monitoring during the recharge experiment.

A calibrated ground-water flow model developed by the U.S. Geological Survey (Stamos and others, 2001) was used to simulate effects of artificial recharge at selected sites during a simulated 20-year drought. Model simulations show that if 8,000 acre-ft of artificial recharge is applied annually for the 20-year period at existing facilities located in the floodplain aquifer along the Mojave River (10 mi to the east) the water-level may rise rise less than 10 ft in the regional aquifer beneath the study site. In contrast, if 2,000 acre-feet of artificial recharge is applied annually over the same 20-year period at the site in Oro Grande Wash, water levels may rise as much as 30 ft in the regional aquifer beneath the site; if 8,000 acre-ft of water is applied annually water levels may rise as much as 90 ft.

References Cited

Izbicki, J.A., Radyk, John, and Michel, R.L., 2000, Water movement through a thick unsaturated zone underlying an intermittent stream in the western Mojave Desert, southern California, USA. Journal of Hydrology, Vol. 238, pp. 194-217.

Stamos, C.L., Martin, Peter, Nishikawa, Tracy, and Cox, B.F., 2001, Simulation of ground-water flow in the Mojave River basin, California, U.S. Geological Survey Water-Resources Investigation Report 01-4002, 129 p.

In George R. Aiken and Eve L. Kuniansky, editors, 2002, U.S. Geological Survey Artificial Recharge Workshop Proceedings, Sacramento, California, April 2-4, 2002: USGS Open-File Report 02-89

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