Hydrogeology of the Ojai groundwater basin: storativity and confinement, Ventura County, California

Located in western Ventura County, the Ojai Groundwater Basin underlies the intermontane lower Ojai Valley of the western Transverse Ranges geomorphic province of California. The basin is predominantly filled with Quaternary alluvial fan, floodplain and lacustrine deposits, which unconformably overlie older, folded and faulted sedimentary rocks of the Sespe, Vaqueros, and Rincon formations. Sand and gravel aquifer units appear to be thickest near the north and east portions of the basin (the alluvial fan heads) and thinnest to the south and west where lacustrine and floodplain deposits predominate as confining layers. The Ojai Groundwater Basin is managed by the Ojai Basin Groundwater Management Agency, a special district of Ventura County responsible for basin management. Several agency/governmental-funded and privately-funded studies, conducted over the past 100 years, have revealed variable conclusions with respect to the basin's aquifer system degree of confinement (confined, semi-confined or unconfined). For this study, and for the first time, a series of aquifer tests were conducted throughout the basin within a narrow (seven month) time frame to quantitatively determine the local aquifer system's degree of confinement and its characteristics (hydraulic conductivity and storage coefficient values). Six aquifer tests were independently designed, managed and implemented with the assistance of well owners, pumpers, and local water purveyors. Detailed water level monitoring was also conducted to corroborate findings from previous tests. Available data from two aquifer tests, conducted by others in 1961 and 1996, were also evaluated. In total, over 20 data sets were evaluated for this project. With respect to aquifer confinement conditions, it appears that water levels are imperative to the status of confined versus unconfined conditions observed in the basin both by aquifer testing and repo1ied historical artesian conditions for the basin. During this investigation, the results of only one of the six aquifer tests, which was conducted during low groundwater level conditions, revealed a storage coefficient value, reflective of unconfined conditions (0.02). Previously conducted aquifer test results for the same portion of the basin revealed confined conditions during a period of higher groundwater levels. The results of the other five aquifer tests, conducted during a season of relatively higher precipitation, revealed storage coefficient values, reflective of confined conditions. The results of this aquifer system characterization reveal that unconfined conditions (typified by a storage coefficient value of 0.02 at one tested area under low water level conditions) may only prevail in the areas such as alluvial fan head areas on the perimeter of the Ojai Valley to the east and n01ih. On the valley floor itself in general and western end of the basin in particular, the aquifer system is under confined conditions at the tested areas and times (storativity values ranging from 0.0001 to 0.000001).