Southern California Salt-Water Intrusion

CASA - A Multi-Faceted Effort

CASA is a multi-faceted task. USGS scientists are working in collaboration with other Federal and State agencies as well as regional, county, and local water managers to better understand the pathways of salt-water intrusion into fresh-water aquifers. USGS efforts involve both the Water Resources and Geologic disciplines. The latter includes the FOQUS-LA Project, a component of the USGS Earthquake Hazards Program, and CASA is a component of the CABRILLO Project (Southern California Bight Regional Investigations-Life, Land, and Ocean) that concerns coastal and marine geology. CASA spearheads studies of the seafloor and sub-seafloor marine geology to better understand fluid migration and constraints on the geologic modeling of ground-water flow.

Accomplishments and Current Work:

• Borings: CASA has completed three 425-m deep (1400 feet) continuous-recovery bore holes, and plans to integrate interpretations with onshore and offshore borehole geophysical logs in the near future.
• Geochemical Studies: CASA intends to correlate the chemistry of the existing chemical database and pore water extracted from the new boreholes with the chemistry and mineralogy of their associated solids.
• Seismic Lines: CASA has collected Geopulse, multi-channel minisparker, and sleeve-gun seismic-reflection data from the San Pedro mainland and LA and Long Beach harbors. Currently, interpretation of this seismic-reflection data is taking place.
• Modeling: CASA will continue developing a data model to integrate map, borehole, geophysical, and seismic data into a 3D-database system that is web accessible.

RELATED LINKS

What is Salt-Water Intrusion?

The diagram below illustrates the concept of salt-water intrusion into fresh-water aquifers. As fresh-water is pumped from a confined aquifer, an aquifer that is overlain by a confining layer, the pressure gradient forms a slight depression in the potentiometric surface (A). Increased pumping can cause an even greater cone of depression, and the gradient that formerly held salt-water at bay reverses, resulting in invasion of the aquifer (B). Coastal barriers can help to stem this invasion; these consist of sets of closely spaced injection wells that inject high-quality fresh water into the ground, creating a hydraulic pressure ridge (C). In the Los Angeles Basin, these barriers are not completely effective; salt water continues to leak through locally, affecting water quality. New studies indicate that the geology that controls flow paths is much more complex than recognized previously.

Understanding the potential pathways for salt-water movement is crucial for stemming the inflow of marine salt water. Part of CASA's success has been the interpretation of integrated geophysical, petrophysical, and surficial data collected off the Los Angeles and San Pedro continental shelves.

Glossary of Hydrological Terms

Aquifer. A body of rock or sediment that contains abundant fresh water in a network of connected pores (small intergranular spaces) or fractures. Aquifers generally are characterized by the amount of pore space that they contain (porosity) and the capacity for water to pass through interconnected pore networks (permeability). [Return to text passage]

Cone of Depression. The depressed shape of the water table around a well after active pumping. The water table adjacent to the well is drawn down by the water removal.

Confined Aquifer. An aquifer that is bounded above and below by impermeable layers of rock or sediment.

Confining Layer. A geologic unit which is relatively impermeable, such as clay or rock, and does not yield usable quantities of water. Confining layers, also referred to as aquitards, restrict the movement of ground water into and out of adjacent aquifers.

Potentiometric Surface. A pressure level in a confined aquifer, defined by the level to which water rises in wells.

Unconfined Aquifer. An aquifer that is not bounded by impermeable strata. It is simply the zone of saturation in water-bearing rock strata, with no impermeable overburden and recharge generally accomplished by water precolating down from above.

Water Table. The upper surface of groundwater; that contact zone between the zone of saturation and aeration in an unconfined aquifer.