Abstract: Dewatering of the Western Simi Valley Groundwater Basin, Ventura County, California

James M. Evensen, Jr., M. Ali Tabidian

Simi Valley is a broad, west-plunging synclinal basin, occupying approximately 27 mi² in eastern Ventura County. The floor of the basin is comprised of Pleistocene to Holocene alluvial deposits that are bound by Upper Cretaceous and Cenozoic sedimentary and volcanic rocks. The basin is transected along its northern flanks by the southwest-trending Simi-Santa Susana reverse fault.

Groundwater within the basin is stored mainly in the unconsolidated Pleistocene and Holocene sedimentary deposits, consisting of clay, silt, sand, and gravel. The aquifer system varies in thickness and is approximately 16 mi² in plan. Recharge to the aquifer system comes from rainfall within the watershed area and through underflow from the sedimentary rocks along the flanks of the basin. It is generally considered that the aquifer system displays confined, semiconfined, and unconfined conditions.

Due to a significant reduction in groundwater production coupled with increased import of state water since the mid 1900s, groundwater levels have recovered to predevelopment levels. As a result of the recovery, the city of Simi Valley began an aggressive dewatering program to overcome the effects of the elevated groundwater that impacts man-made structures. The dewatering project is comprised of five pumping wells to lower the groundwater elevation and six piezometer nests to monitor the effectiveness of the system. The purpose of this paper is to describe the progress of the dewatering program during the last five years and to discuss the alternatives for future management of the aquifer system.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994