--> Aquifers are not layer cakes: Defining aquifer architecture using seismic and sequence stratigraphy in the Los Angeles Basin, California

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Aquifers are not layer cakes: Defining aquifer architecture using seismic and sequence stratigraphy in the Los Angeles Basin, California


Because groundwater provides more than one-third of the municipal water supply for the coastal LA Basin, the aquifer architecture of this system is a high priority for groundwater managers. Sequence stratigraphic models for the Pleistocene to Holocene sediments of the Los Angeles (LA) Basin provide better understanding of regional groundwater flow and have helped identify seawater intrusion pathways into important groundwater aquifers. By evaluating subsurface data using sequence stratigraphy, the geometry and distribution of aquifer and aquitard sediments are defined, thus groundwater contaminant plumes and sea water intrusion pathways are better understood. The U.S. Geological Survey (USGS), in cooperation with Los Angeles County Department of Public Works and the Water Replenishment District of Southern California, undertook an investigation of the groundwater stratigraphy of the Wilmington – Long Beach area of the LA Basin. Sequence stratigraphic methods were used to integrate preexisting groundwater well data with: (1) new borehole observations, (2) structural and physical properties data derived from geophysical measurements, (3) high-resolution seismic reflection data obtained offshore of the present shoreline, and (4) vintage oil company exploration seismic reflection data from both onshore and offshore of the shoreline. These data were used to construct a series of Pleistocene to Holocene environment of deposition maps that show the overall progradation of sequences seaward over time. In addition, seawater intrusion pathways into coastal groundwater aquifers were identified and mapped. On a more regional scale, a vintage Texaco seismic reflection data set from the greater LA Basin was interpreted and integrated with a regional network of multi-level ground-water monitoring wells. The sequence stratigraphic correlation shows that many of the defined groundwater aquifers are not correlative. For example, the seismic stratigraphy of one of the major groundwater supply aquifers, the Silverado Aquifer, demonstrates that its age is dependent on location. The Silverado Aquifer is neither a correlative lithostratigraphic nor hydrostratigraphic unit. This new understanding shows the need for a re-evaluation of the Upper Pliocene to Holocene stratigraphy using existing seismic reflection data integrated with well data from the LA Basin to better understand the regional distribution of groundwater aquifers.