Abstract: Hydrogeology and Groundwater Level Fluctuations of Ojai Valley Groundwater Basin, Ventura County, California

Linda M. Jason, M. Ali Tabidian

Ojai Valley is a structural depression situated in the western Transverse Ranges of southern California. Synclinal folding of underlying Tertiary rocks and vertical displacements along the Santa Ana fault zone have allowed for an accumulation of sediments up to 600 ft thick within the valley during the Pleistocene and Holocene. These sediments, which mainly consist of unconsolidated to semiconsolidated alluvium and alluvial fan deposits, comprise the Ojai groundwater basin.

The saturated-zone deposits of the basin are undifferentiated and highly variable in thickness, grain-size distribution, storativity, and transmissivity. These variations are due to downwarping of the underlying bedrock, and past depositional activities. In general, the aquifer system is under unconfined conditions, except in the western end of the basin where a confining clay layer is present. Groundwater flows from the margins of the basin toward the southwest.

Seasonal groundwater level fluctuations ranging from 20 to 90 ft are commonly observed in wells throughout most of the basin. Fluctuations of up to 200 ft have been recorded when prolonged drought periods are followed by heavy rainfall events. Groundwater levels in the western part of the basin are relatively stable, fluctuating 10 ft or less annually. Wells in this area become "flowing wells" when groundwater levels in the basin are high. Variations in the hydrogeologic settings, the regional hydraulic gradient, the extent of groundwater pumpage, and the recharge from precipitation are the major influencing factors on the magnitude of the seasonal groundwater fluctuations in the basin.

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