Hydrogeology of the Simi Valley hydrologic basin, Ventura County, California

The Simi Valley Hydrologic Basin is contained within a 50,260 ac watershed that is open only at its western discharge point. Within the watershed, groundwater is stored in both the alluvial deposits in the trough of the basin as well as the surrounding sedimentary and igneous rocks. Compared to the rocks, the alluvial deposits are generally more permeable, provide greater well yields, and therefore include the primary aquifer system within the basin. The alluvial sediments within the hydrologic basin restrict the groundwater to a mixture of unconfined, semi-confined, and confined conditions. In the rocks, the groundwater flows along bedding planes, fractures, and laminae, analogous to anisotropic flow in a stratified sedimentary environment. In this study, the dimensions and geometry of the watershed are assessed, the aquifer systems are characterized and delineated, and the hydrology of the basin is balanced. Findings from these efforts are applied to the original hydraulic models of the basin and compared to empirical observations. Early models relating to the primary aquifer system in the basin indicate that a confining layer separates an unconfined aquifer from a lower confined aquifer. This concept was later modified by workers that produced evidence to show that the confining layer is actually semi-confining in a regional scale, with some localized confinement. Prior to extensive groundwater development beginning in the 1920s, groundwater levels within portions of the valley were at or above the ground surface. These shallow water or artesian conditions resulted in the development of wetlands, pools, and flowing surface water. As groundwater exploitation continued, groundwater levels dropped, while urban development within the valley expanded. Following the completion of the State-operated water supply system in the early 1960s, the City of Simi Valley shifted its water use from groundwater to the State water supply. As a result, the groundwater levels in the basin recovered to their pre-development elevations, and artesian conditions began to recur. In response to the adverse affects caused by the shallow groundwater, the City of Simi Valley engaged in an aquifer dewatering program. This program focused on reducing the hydraulic pressure in the confined/semiconfined aquifer beneath the areas where artesian groundwater conditions exist. The results of the hydrologic basin balance and supporting information indicate that the groundwater within the basin moves most rapidly through the primary aquifer. The findings also imply that the groundwater stored in the adjacent rocks (secondary aquifer system) provides continued recharge to the primary aquifers. To mitigate the artesian conditions, morn water must be extracted from the pressurized aquifer than can be allowed to recharge. Interception of the groundwater, before it reaches the area of artesian conditions, may prove to be an economic alternative to pumping from within the zone of artesian influence.