GEOLOGIC CONTROLS ON ARTIFICIAL RECHARGE THROUGH A THICK, HETEROGENEOUS UNSATURATED ZONE NEAR AN INTERMITTENT STREAM, WESTERN MOJAVE DESERT, SOUTHERN CALIFORNIA

Artificial recharge using imported water from the California Aqueduct is planned to replenish ground water pumped in excess of recharge along Oro Grande Wash near Victorville, Calif., about 100 miles east of Los Angeles. Previous studies showed that infiltration from a test pond to recharge aquifers underlying the Victorville fan is possible along the wash at locations where a regional caliche layer that impedes the downward movement of water is not present. The depth to water beneath the test pond was about 400 feet, and water from the pond infiltrated downward at rates as high as 3 feet/day. The site was selected because recharge at this site would increase water levels in nearby public supply wells that yield water from near the water table in highly permeable deposits of the ancestral Mojave River.

It is desirable to find other sites along the wash that also may be suitable for artificial recharge. Farther upslope on the fan, the unsaturated zone is thicker but coarse-grained; although infiltrating water may move downward more rapidly, highly permeable deposits of the ancestral Mojave River are not present at depth and no public supply wells are located in this area. Farther downslope on the fan, the unsaturated zone is thinner but finer grained, potentially impeding downward movement of water. Geologic sections prepared for this study suggest that as the ancestral Victorville fan was deposited, the downslope end of the fan was eroded and replaced by coarse-grained deposits from the ancestral Mojave River; alternatively, ancestral fan sediments may have been deposited on top of ancestral Mojave River deposits as the toe of the fan progressed eastward. As a result, highly permeable deposits may be present throughout much of the 300-foot-thick unsaturated zone near the present-day downslope part of the Victorville fan, and this area may be suitable for infiltration of artificial recharge from ponds. On the basis of these data, an additional pond was proposed. Cores, drill cuttings, geophysical logs, from an instrumented borehole will help better define the geology of the area.