Evaluating The Feasibility Of Artificial Recharge Sites Using High Resolution Electrical Methods Near Victorville, California

Artificial recharge and water augmentation are important aspects of many groundwater management programs. The subsurface must be understood in reasonable detail to design an efficient<br>and effective artificial recharge system. Often the cost to collect this data by drilling alone is prohibitively high. When properly applied, geophysical methods can provide subsurface information at a much lower cost. Geophysical data provides more complete coverage and can be used to target drilling efforts toward the most important areas. This presentation illustrates a recent project where geophysical methods were used to help a water agency design a testing program to develop an artificial recharge project. The Mojave Water Agency (MWA) was studying the feasibility of storing water in the unconsolidated material of the Oro Grande Wash due to its thick unsaturated zone and coarse-grained soils that allow for rapid infiltration, as well as its proximity to the California Aqueduct. A test boring installed in 2003 encountered an unexpected thick sequence of clay and silt above the water table. The presence of these fine-grained deposits could potentially impair the ability of the area to store and produce water and forced MWA to collect more data to test the conceptual model of the recharge system. A geophysical survey was used to collect subsurface information over an area of several square miles in an efficient and economic manner. Nine electrical resistivity resistivity profiles and 34 time domain electromagnetic soundings were collected to map the grain size of the soil to depths of over 1,000 feet. The results of the geophysical survey indicated that the subsurface geology was more<br>complex than originally expected. The deeper sediments are clay rich and poorly suited to recharge the aquifers used by the local municipal well fields. The shallow coarser grained sediments could be used to store water. However, the volume of water that could potentially be stored and recovered with a dedicated well field has yet to be determined. A modeling study of the recharge system is currently being conducted to determine if an economic system can be developed. Other areas are also under consideration.