Geohydrologic Study of the Central and West Coast Basins of Los Angeles County
Since 1995, the U.S. Geological Survey (USGS), in cooperation with the Water Replenishment District of Southern California (WRDSC), has conducted an intensive study of the geohydrology and geochemistry of the Central and West Coast Basins of Los Angeles County. The major thrusts of the study over the past 12 years have been: 1) drilling and collecting detailed data from monitoring wells throughout the basins, 2) conducting regional geohydrologic and geochemical analyses of the basins, 3) developing and applying a computer simulation model of regional ground-water flow.
The data collection has been extensive. As of July, 2007, the USGS will have drilled 40 multi-completion monitoring wells throughout the study area. Detailed hydrologic and water-quality data have been collected from these wells. An important aspect of the drilling in recent years has been application of continuous coring techniques, in collaboration with scientists from the USGS Geologic Discipline (GD). Using new data collected from the monitoring wells, along with seismic data, USGS scientists and co-workers have been able to develop a much more detailed understanding of the sequence stratigraphy of the Central and West Coast Basins (Ehman, 2006; Edwards and others, 2007A,B; Ponti and others, 2007).
A focus of the geohydrologic/geochemical analyses and modeling over the past several years has been seawater intrusion in the West Coast Basin, with an emphasis on the Dominguez Gap area. This is the major regional ground-water quality issue. Approximately 15 million dollars are spent annually purchasing water for injection wells, which are aligned along three seawater barrier projects. Much of our data collection efforts have been directed toward this issue. We have looked at the geochemistry (Land and others, 2004), stratigraphy (Edwards and others, 2007A,B, Hanson and others, 2007; Ponti and others, 2007), and water management (Reichard and Johnson, 2005). Journal articles applying cross-sectional solute transport modeling using SUTRA (Nishikawa, lead author) and geostatistical analyses (Hanson, lead author) to seawater intrusion in the Dominguez Gap area are currently being prepared.
The USGS report by Reichard and others (2003) documents the regional flow model that the USGS developed cooperatively with WRDSC. The ground-water model has four layers with model cells that are ¼ mi2 (½ mi by ½ mi). The model grid is shown in fig.1. The model simulated historic ground-water conditions from 1971 through 2000. Since the report was completed, we have updated the model simulation period to extend through water year 2005. We have used the model to assess alternative management scenarios being considered by WRDSC. This has included linking the model with optimization methods. During the past two years, we have initiated initial conversion to MODFLOW-2000 and utilized visualization tools to more clearly present simulation results.
Reports
Edwards, B.D., Ehman, K.D., Ponti, D.J., Reichard, E.G., and Tinsley, J.C., 2007A, Chapter 6.4 - Stratigraphic controls on saltwater intrusion in the Dominguez Gap area of coastal Los Angeles: in Lee, H.J., and Normark, W.R., eds., Earth Science in the Urban Ocean: The Southern California Continental Borderland, Geological Society of America, Special Paper. 34 ms pages, 11 figures, 3 tables. Director's Approval 1/8/07.
Ehman, K.D., 2006, Sequence stratigraphy of Upper Pliocene to recent water-bearing sediments of the Los Angeles Basin, California, Report prepared for the Water Replenishment District of Southern California.
Edwards, B.D., Hanson, R.T., Reichard, E.G., and Johnson, T.A., 2007B, Chapter 6.1 - Characteristics of southern California coastal aquifer systems: in Lee, H.J., and Normark, W.R., eds., Earth Science in the Urban Ocean: The Southern California Continental Borderland, Geological Society of America, Special Paper. 40 pages, 13 figures, 4 tables. Director's Approval 1/8/07, in press
Hanson, R.T., Martin, Peter, Izbicki, J.A., Reichard, E.G., Land, M.T., and Edwards, B.D., 2007, Chapter 6.2 - Comparison of ground-water flow in southern California coastal aquifers: in Lee, H.J., and Normark, W.R., eds., Earth Science in the Urban Ocean: The Southern California Continental Borderland, Geological Society of America, Special Paper. 38 pages, 14 figures. Director's Approval: 12/26/06, in press
Hillhouse, J.W., Reichard, E.G., and Ponti, D.J., 2002, Probing the Los Angeles Basin –Insights into ground-water resources and earthquake hazards: U.S. Geological Survey Fact Sheet 086-02, 2 p.
Land, Michael, Everett, R.R., and Crawford, S.M., 2002, Geologic, hydrologic, and water-quality data from multiple-well monitoring sites in the Central and West Coast Basins, Los Angeles County, California, 1995-2000: U.S. Geological Survey Open-file Report 01-277, 178 p.
Land, Michael, Reichard, E.G., Crawford, S.M., Everett, R.R., Newhouse, M.W., and Williams, C.F., 2004, Ground-water quality of coastal aquifer systems in the West Coast Basin, Los Angeles County, California, 1999-2002: U.S. Geological Survey Scientific Investigations Report 2004-5067, 79 p.
Ponti, D.J., Ehman, K.D., Edwards, B.D., Tinsley, J.C., III, Hildenbrand, Thomas, Hillhouse, J.W., Hanson, R.T., McDougall, Kristen, Powell, C.L., II, Wan, Elmira, Land, Michael, Mahan, Shannon, and Sarna-Wojcicki, A. M., 2007, A 3-Dimensional Model of Water-Bearing Sequences in the Dominguez Gap Region, Long Beach, California: U.S. Geological Survey Open-File Report 2007-1013, 39 p.
Reichard, E.G. and Johnson, T. A., 2005, Assessment of regional management
strategies for controlling seawater intrusion: Journal of Water Resources Planning and Management, v. 131, no. 4, p. 280-291.
Reichard, E.G., Land, Michael, Crawford, S.M., Johnson, Tyler, Everett, R.R., Kulshan, T.V., Ponti, D.J., Halford, K.J., Johnson, T.A., Paybins, K.S., and Nishikawa, Tracy, 2003 Geohydrology, geochemistry, and ground-water simulation-optimization of the Central and West Coast Basins, Los Angeles, County, California: U.S. Geological Survey Water Resources Investigations Report 03-4065, 184 p.
Project Chief: Eric Reichard
Phone: 916-278-3000
Email: egreich@usgs.gov
