KISER, SUSAN CHANDLER, and LAURA M. BASELEY, WZI Inc., Bakersfield, CA

ABSTRACT: Hydrogeologic Facies Framework of Southern San Joaquin Valley, California

In the southern San Joaquin Valley, California, the potable groundwater is found in Pleistocene to Holocene aquifers that were deposited in an alluvial/fluvial fan (lacustrine) delta system. This system is often depicted as a series of simple, lateral facies changes (alluvial to fluvial deltaic to lacustrine) that are quite uniform over a large area. Understanding the relationship between these facies boundaries and the movement of the groundwater is critical in evaluating contamination plumes. Although the actual aquifer boundaries do not always occur at facies boundaries, facies mapping provides a technically rational way to understand the lithologic. geochemical, and hydrologic variations in the subsurface. However, in the southern San Joaquin Valley, there is evidence that tectoni s has influenced the sedimentation patterns. As a result, architecture of the aquifers records the constant struggle of a closed basin depositional system as it attempts to find equilibrium in a tectonically active region.

There are significant implications for resource management and protection. The prediction of contaminant movement in the subsurface may be in error if facies mapping is not integrated into the planning process, especially where structural development has caused local perturbation of facies. Unfortunately, this problem occurs through much of the southern San Joaquin Valley. At the scale of regional water-supply planning, the errors may be within acceptable limits, but in the case of site-specific groundwater remediation design and contamination monitoring, regional facies mapping is a requirement.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.