Geohydrology, Geochemistry, and Ground-Water Simulation-Optimization of the Central and West Coast Basins, Los Angeles County, California

By Eric G. Reichard, Michael Land, Steven M. Crawford, Tyler Johnson, Rhett R. Everett, Trayle V. Kulshan, Daniel J. Ponti, Keith J. Halford, Theodore A. Johnson1, Katherine S. Paybins, and Tracy Nishikawa


Water–Resources Investigations Report 03-4065

Sacramento, California 2003

Prepared in cooperation with the

Complete accessible text of report (21.4 MB PDF)


Front cover for report (80 KB PDF)
Top photo: “The inner edge of the coastal plain west of Whittier, Cal.,” from Mendenhall (1905b).
Lower photos: "View of coastal plain looking northwest-to-west from the Puente Hills", photos taken by Michael Land, 2003.
Cover design by Laurel Rogers and Phil Contreras.
Helpful assistance was provided by the Whittier Historical Society.
Back cover for report (42 KB PDF)

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     Historical ground-water development of the Central and West Coast Basins in Los Angeles County, California through the first half of the 20th century caused large water-level declines and induced seawater intrusion. Because of this, the basins were adjudicated and numerous ground-water management activities were implemented, including increased water spreading, construction of injection barriers, increased delivery of imported water, and increased use of reclaimed water. In order to improve the scientific basis for these water management activities, an extensive data collection program was undertaken, geohydrological and geochemical analyses were conducted, and ground-water flow simulation and optimization models were developed.
     In this project, extensive hydraulic, geologic, and chemical data were collected from new multiple-well monitoring sites. On the basis of these data and data compiled and collected from existing wells, the regional geohydrologic framework was characterized. For the purposes of modeling, the three-dimensional aquifer system was divided into four aquifer systems—the Recent, Lakewood, Upper San Pedro, and Lower San Pedro aquifer systems. Most pumpage in the two basins is from the Upper San Pedro aquifer system.
     Assessment of the three-dimensional geochemical data provides insight into the sources of recharge and the movement and age of ground water in the study area. Major-ion data indicate the chemical character of water containing less than 500 mg/L dissolved solids generally grades from calcium-bicarbonate/sulfate to sodium bicarbonate. Sodium-chloride water, high in dissolved solids, is present in wells near the coast. Stable isotopes of oxygen and hydrogen provide information on sources of recharge to the basin, including imported water and water originating in the San Fernando Valley, San Gabriel Valley, and the coastal plain and surrounding hills. Tritium and carbon-14 data provide information on relative ground-water ages. Water with abundant tritium (greater than 8 tritium units) is found in and downgradient from the Montebello Forebay and near the seawater barrier projects, indicating recent recharge. Water with less than measurable tritium is present in, and downgradient from, the Los Angeles Forebay and in most wells in the West Coast Basin. Water from several deep wells was analyzed for carbon-14. Uncorrected estimates of age for these samples range from 600 to more than 20,000 years before present. Chemical and isotopic data are combined to evaluate changes in chemical character along flow paths emanating from the Montebello and Los Angeles Forebays.
     A four-layer ground-water flow model was developed to simulate steady-state ground-water conditions representative of those in 1971 and transient conditions for the period 1971–2000. Model results indicate increases in ground-water storage in all parts of the study area over the simulated thirty-year period. The model was used to develop a three-dimensional ground-water budget and to assess impacts of two alternative future (2001–25) ground-water development scenarios—one that assumes continued pumping at average current rates and a second that assumes increasing pumping from most wells in the Central Basin. The model simulates stable or slightly increasing water levels for the first scenario and declining water levels (25 to 50 ft in the Central Basin) in the second scenario. Model sensitivity to parameter values and to the assumed Orange County boundary condition was evaluated. Particle tracking was applied to simulate advective transport of water from the spreading ponds, the coastline, and the seawater injection barriers. Particle tracking results indicate that most flow within the Upper San Pedro aquifer system occurs within about 20 percent of the total aquifer system thickness and that virtually all water injected into the seawater barrier projects has flowed inland.
     The simulation model was linked with optimization to identify the least-cost strategies for improving hydraulic control of seawater intrusion in the West Coast Basin by means of increased injection and (or) in-lieu delivery of surface water. For the base-case optimization analysis, assuming constant ground-water demand, in-lieu delivery was determined to be most cost effective. Several sensitivity analyses were conducted with the optimization model. Raising the imposed average water-level constraint at the hydraulic-control locations resulted in non-linear increases in cost. Systematic varying of the relative costs of injection and in-lieu water yielded a trade-off curve between relative costs and injection/in-lieu amounts. Changing the assumed future scenario to one of increasing Central Basin pumpage caused a small (7-percent) increase in the computed costs of seawater intrusion control.





Purpose and Scope

Description of Study Area


Data Compilation and New Data Collection

Geologic Framework

Hydrogeologic Framework

Recent Aquifer System

Lakewood Aquifer System

Upper San Pedro Aquifer System

Lower San Pedro Aquifer System

Pico Unit

Analysis of Hydraulic Conductivities

Regional Ground-Water Flow System

Sources and Movement of Water

Ground-Water Development

Geochemical Analysis


Water-Quality Network

Construction and Well Selection

Data Collection and Purpose

Definition of Hydrologic Regions and Aquifer Systems

Ground-Water Quality

Dissolved Solids

General Chemical Character

Central Basin

West Coast Basin

Dissolved Chloride

Dissolved Oxygen

Dissolved Sulfate

Dissolved Manganese

Dissolved Iron

Isotopic Composition of Ground Water

Deuterium and Oxygen-18

Central Basin

West Coast Basin


Central Basin

West Coast Basin


Central Basin

West Coast Basin

Integrated Geochemical Analysis of the Regional Ground-Water Flow System

Development of a Ground-Water Simulation Model

Boundary Conditions

Model-Layer Elevations

Hydraulic Properties

Areal Recharge

Pumpage, Spreading, and Injection

Model Calibration

Model-Parameter Sensitivity

Analysis of Regional Ground-Water Budget with Ground-Water Simulation Model

Model Sensitivity to Orange County Boundary Condition

Model Limitations

Applications of the Ground-Water Simulation Model

Particle Tracking Analyses

Simulation of Future Water-Management Scenarios

Simulation-Optimization Analysis


References Cited

Appendix I. Geographic Information System

Appendix II. Well identification, Model Layer, and Aquifer-Systems information for U.S. Geological Survey Multiple-well Monitoring Sites, Los Angeles, California

Appendix III. Correlation between Specific Conductance and Dissolved-Solids Concentration

Appendix IV. Parameters used to generate model layer elevation surfaces

Appendix V: Estimation of mountain front recharge for 1970–71

Appendix VI. Hydrographs of simulated and measured water levels, 1971–2000

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