ARTIFICIAL RECHARGE THROUGH A THICK UNSATURATED ZONE, WESTERN MOJAVE DESERT, SOUTHERN CALIFORNIA

Ground-water levels in the regional aquifer in the western part of the Mojave Desert near Victorville, California, are declining as a result of pumping in excess of recharge. An experiment was done to determine the feasibility of infiltrating artificial-recharge water from ponds through the thick, heterogeneous unsaturated zone underlying this area. A site along Oro Grande Wash was selected to recharge highly permeable deposits near the water table, while avoiding impermeable caliche located away from the wash and fine-grained deposits associated with the distal part of the Victorville fan. About 470 acre-ft of water was infiltrated through a 1-acre pond between October 1, 2002, and September 30, 2003. Maximum infiltration rates were as great as 800 gal/min. Downward movement of artificial-recharge water to the water table about 400 ft below land surface was monitored at an instrumented borehole adjacent to the pond using heat-dissipation probes and advanced tensiometers emplaced at selected depths within the borehole, and electromagnetic logs collected through a 2-inch-diameter well installed within the borehole. Infiltrated water initially moved downward at a rate of 3 ft/d, although a small quantity of water moved in advance of the main wetting front at a rate as great as 6 ft/d. The downward rate of movement slowed to less than 0.3 ft/d by January 2003 as the wetting front spread laterally in the subsurface, and by July 2004 the wetting front was more than 300 ft below land surface. Chloride, nitrate, and other soluble salts were that were leached from the unsaturated zone moved downward with the recharge water. Simulation results from ground-water flow models predict that if 2,000 acre-ft of water is recharged annually for 20 years, water levels will rise more than 30 ft beneath the recharge site, benefiting more than 10 public supply wells.