DEVELOPING A 3-D SUBSURFACE MAP TO EVALUATE WATER AVAILABILITY IN CUYAMA VALLEY, CA

Cuyama Valley is an east-west trending valley in the southern part of the CA Coast Ranges. The valley is a structural trough filled with more than 1,200 feet of Holocene to Pliocene-age terrestrial deposits. Pliocene to Pleistocene-age thrust faults bound the valley on the north and south. Groundwater essentially is the sole source of water supply in the Cuyama Valley, and historically has been pumped from the Holocene age alluvium. Pumping has resulted in water-level declines of as much as 300 ft since the 1940s, which has dewatered the Holocene alluvium in much of the valley. Currently, most of the groundwater is being pumped from the underlying Pliocene deposits. A three-dimensional (3-D) subsurface map is being developed to better define the hydrogeologic properties of the valley-fill deposits and evaluate the long-term availability of groundwater in the valley.

Developing the 3-D subsurface map involves defining the areal and vertical extent of the Holocene and Pliocene deposits, defining the percentage of coarse-grained deposits by depth, and locating bounding and internal faults. The map is being constructed from available geologic maps, geophysical data, and lithologic and geophysical logs from oil and water wells. In addition, several research wells are being constructed as part of the project to provide detailed information on the hydrogeology of the valley at critical locations. Definition of unit contacts is challenging, given the similarity between the Holocene and Pliocene deposits. Lithologic and geophysical logs collected at the research wells will provide the basis for defining the unit contacts. Available lithologic logs and electric logs are being used to define the percentage of coarse-grained deposits and sorting variations within the valley-fill deposits; the spatial distribution of these properties is being interpolated within the stratigraphic boundaries of the 3-D subsurface map. Textural patterns in the deposits are controlled by a variety of factors including basin-wide sediment accumulation, incision related to tectonic uplift, Quaternary climate cyclicity, and the deposition associated with the axial Cuyama River.