Abstract: Sequence Stratigraphic Evaluation of Fresh Water Aquifers of the Dominguez Gap Region: Implications for Sea Water Intrusion into the Los Angeles Groundwater Basin

EHMAN, KENNETH, Skyline Ridge Inc., Sunnyvale, CA; RICHARD CRAMER, Groundworks Environmental, Inc., Shingle Springs, CA; WAYNE JACKSON, Los Angeles County Dep't of Public Works, Alhambra, CA

A hydrostratigraphic evaluation in the area of the Dominguez Gap Barrier Project (a seawater intrusion barrier) was completed to determine the geologically-optimum locations for 15 proposed fresh water injection wells. We redefined the hydrostratigraphic framework in the Dominguez Gap area, resulting in an improved understanding of the aquifer system and the seawater intrusion flow paths. This investigation focused on defining the seawater migration pathway from the Gaspur aquifer to the underlying deeper aquifers. The Gaspur is the main conduit for seawater intrusion. The underlying Gage aquifer (part of the Lakewood Formation) consists of up to five separate hydrostratigraphic units (HU). The primary migration pathway of seawater from the Gaspur to deeper aquifers occurs where the Gaspur channel has incised as deep as the Gage 3 HU. The migration of seawater-impacted groundwater continues downward from the Gage into the Lynwood where the unconformity at the base of the Gage 1 HU cuts down into the crest of an anticline occurring in the San Pedro Formation. This results in little or no aquitard between the Gage 1 HU and the underlying Lynwood aquifer along the axis of the anticline.

The area of mergence between the Gaspur and Gage aquifers (where the Gaspur truncates the Gage 3 HU) occurs inland (north) of Willow Street, but is seaward (south) of Spring Street. Therefore, injection along Spring Street would provide a better barrier to inland seawater migration in the Gage and Lynwood aquifers.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California