A foundation for future assessment and management of groundwater resources

Sequence stratigraphic models for the Pleistocene to Holocene sediments of the Los Angeles (LA) Basin will provide better understanding of
regional groundwater flow and have helped identify seawater intrusion pathways into important groundwater aquifers. 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. Seismic and sequence stratigraphy are now 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
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) hi-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, one of the
major groundwater supply aquifers, the Silverado Aquifer, when tied to the seismic reflection data has different ages depending on location.
This new understanding demonstrates the need for a re-evaluation of the Pleistocene to Holocene stratigraphy using existing seismic reflection
data integrated with well data from the LA Basin in order to understand the regional distribution of groundwater aquifers.