DEFINING AQUIFER ARCHITECTURE USING SEQUENCE STRATIGRAPHY IN THE LOS ANGELES BASIN, CALIFORNIA: A FOUNDATION FOR FUTURE ASSESSMENT AND MANAGEMENT OF GROUNDWATER RESOURCES

Groundwater provides more than one-third of the municipal water supply for the coastal Los Angeles Basin and defining the aquifer architecture is a high priority for ground-water managers. A new approach to understanding aquifer architecture is to use sequence stratigraphic techniques. Sequence stratigraphy, the state-of-the-art method for delineating reservoir geometry and continuity in the petroleum industry, is just now being incorporated into groundwater resource assessments and environmental investigations. By evaluating subsurface data using sequence stratigraphy, the geometry and distribution of aquifer and aquitard sediments are linked to the original depositional processes that formed the sediments. The first sequence stratigraphic groundwater investigation in the LA Basin was done in the mid 1990’s as part of a site-specific remedial investigation of a liquid hazardous waste disposal site in the Wilmington – Long Beach area. More recently, the U. S. Geological Survey (USGS), in cooperation with Los Angeles County Department of Public Works and the Water Replenishment District of Southern California (WRDSC) began a broader investigation of the Wilmington – Long Beach area with new exploratory research boreholes, marine seismic-reflection data, and hi-resolution gravity measurements. Sequence stratigraphy allows integration of the new borehole observations with structural and physical properties data from the geophysics, together with preexisting data from the area, to lead to defining new pathways for seawater intrusion into the freshwater coastal aquifers. The Wilmington – Long Beach region, however, is only a small part of the greater Los Angeles Basin, and a large scale, basin-wide sequence stratigraphic investigation is needed to address a wide array of groundwater resource and protection issues. This investigation would incorporate data collected from the regional network of mult-level groundwater monitoring wells that have been installed by the USGS and WRDSC. These new data need to be integrated into a sequence stratigraphic framework to effectively understand the complex 3D architecture of water-bearing sediment of the LA basin. A sequence stratigraphic framework will be critical in developing more detailed ground-water flow and transport models of the Los Angeles Basin.