Geohydrologic Study of the Central and West Coast Basins of Los Angeles County

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Situated on a semiarid coastal plain, the greater Los Angeles area relies significantly on groundwater for its water supply. Historical groundwater development of the Central and West Coast Basins caused a large decline in groundwater levels and seawater intrusion. These effects led to the adjudication of the basins in the early of 1960s and initiation of a series of groundwater management activities, including construction and operation of injection barriers, recharge of groundwater basins through spreading grounds, restriction of groundwater withdrawal in certain periods, and use of imported and reclaimed water in lieu of some pumping.

Groundwater currently meets almost 40% of water demand for 4 million people who live in the Central and West Coast Basins. To improve the scientific understanding and sustainable management of the basins, the U.S. Geological Survey (USGS), in cooperation with the Water Replenishment District of Southern California (WRDSC), has conducted multiple studies in the past 17 years. Most of the studies have been focused on the following 3 perspectives:

1. research drilling and data collection,
2. geohydrologic and geochemical analyses, and
3. model or database development and application.

The drilling and data collection have been extensive. As of July, 2016, the USGS has constructed 57 deep, multiple-well monitoring sites throughout the Central and West Coast Basins. Detailed hydrologic and water-quality data have been collected from these sites. An important aspect of the drilling in recent years has been the collection and interpretation of core samples and advanced geophysical logs from the boreholes of each drilling site. Using the new information, along with the recently acquired seismic reflection data, USGS scientists and co-workers have been able to develop a much more detailed understanding of the stratigraphy and structure of the Central and West Coast Basins.

Geohydrologic/geochemical analyses and modeling of recent years have been focused on the development of a sequence-stratigraphy-based, regional-scale groundwater flow model. The model also includes a recharge sub-model for watershed-wide, precipitation-runoff-infiltration simulation. Future plans for application of the regional groundwater flow model include:

1. assessment of alternative water-management strategies, such as conjunctive use and aquifer storage and recovery (ASR),
2. identification of potential flow paths and distribution of different sources of water (seawater, imported water, reclaimed water) after they enter into the basins,
3. simulation of contaminant movement by coupling the flow model with a transport module, and
4. evaluation of the impacts of urbanization on the groundwater system, such as an increase in pumpage, lining of rivers and reduction in pervious area, and introduction of new sources of water into the basins.

A web-based, integrated hydrogeological database is also in development for storage and access of various geological, hydrological and water chemistry data that were built into the regional groundwater flow model.