Overview:
Aquifer Science & Technology designed and implemented a geophysical investigation to define the subsurface characteristics and hydraulic properties of the aquifer system in the Oro Grande Wash area for the Mojave Water Agency (MWA) as part of a feasibility study for an artificial recharge project. Two electrical methods were utilized to collect the data, a high-resolution electrical resistivity survey to map the lateral and vertical grain size distribution of the upper 300 to 400 feet, and a time domain electromagnetic induction (TEM) survey to determine the grain size distribution to depths of up to approximately 1,000 feet.
Unusual Facts:
The Mojave Water Agency of southern California is in the process of evaluating their options for developing artificial recharge and water banking projects in their service area. A 2003 injection study indicated that the Oro Grande Wash area may be favorable for artificial recharge. When a deep boring found unexpected thick layers of clay, Mojave hired AST to conduct a geophysical survey of the wash and surrounding area. The survey consisted of high resolution electrical resistivity profiles to map the composition of the upper 400 feet of the subsurface and Time Domain Electromagnetic Induction (TEM) soundings to map the composition of the unconsolidated unit to over 1,000 feet.
The geophysical surveys showed that in much of the area, fine-grained material is present at the surface overlying a thick sequence of coarse-grained material. The fine-grained surface soils are thin or absent in some areas. Beneath this layer lies an extensive sequence of low-permeability clay. The modeled TEM resistivity data was used to construct maps of the elevation of the top of the lower clay formation, and of the depth to the top of the lower clay in the Oro Grande Wash survey area. This data will be used by the Mojave Water Agency to determine the most suitable locations for artificial recharge and water banking. Similar geophysical surveys were also conducted by AST for MWA in the Antelope Wash and Greentree area.
Additional Information
Presentations on this project were made at the National Ground Water Association Ground Water Summit in 2006 and the Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP) in 2006.