Description
The Ocotillo-Coyote Wells basin is an arid desert region in southwestern Imperial County, California. The basin is underlain by Tertiary marine and non-marine sedimentary deposits overlain by a variable thickness of Recent and older alluvium. Residents in the basin are wholly dependent on ground water for their water supply. The Elsinore fault transects the basin from northwest to southeast. Fault movement has displaced Tertiary deposits toward the surface east of faulting and allowed thicker accumulation of alluvium to the west. In general, ground water encountered in the Tertiary deposits to the east is saline. Ground water in the alluvium to the west is fresh potable water. The Elsinore fault is considered to be a barrier which divides fresh and saline water. The basin was studied in an attempt to better define the affects faulting has on the aquifer and to delineate the boundary between fresh and saline water. In addition, the origin of the saline ground water was investigated. Measurement and contouring of water levels indicate that ground water moves generally to the southeast and increases in gradient east of the Elsinore fault. The contouring shows a ground water barrier due to faulting at one location, but was not conclusive at other locations. Changes in gradient are also present west of suspected faulting. Pump test results confirm that the general change in the water table gradient is due to the difference in transmissivity between the unconsolidated alluvial deposits and the Tertiary sediments. Chemical testing of ground water samples indicate that the saline ground water is connate. Connate ground water is also present west of the suspected Elsinore fault barrier. Electrical resistivity surveying reveals distinct contrasts between the saline aquifer conditions in east and the fresh aquifer to the west. The survey also indicates a fresh-saline ground water boundary west of postulated trace of the Elsinore fault. It is concluded that the structural complexities observed at the surface (faulting and steeply dipping folds) are also present beneath the cover of alluvium. The deformation is reflected in the aquifer conditions studied. The connate saline ground water is directly associated with the Tertiary sedimentary deposits. Division of the fresh and saline ground water along the postulated trace of the Elsinore fault is not reliable. The stratigraphic deformation within the basin has created very complex aquifer conditions both laterally and with depth. A singular fault break and ground water barrier is not indicated.
