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
U.S. GEOLOGICAL SURVEY
Water–Resources Investigations Report 03-4065
Sacramento, California 2003
Prepared in cooperation with the
WATER REPLENISHMENT DISTRICT OF SOUTHERN CALIFORNIA
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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Abstract
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.
CONTENTS
Abstract
Introduction
Background
Purpose and Scope
Description of Study Area
Acknowledgments
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
Introduction
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
Tritium
Central Basin
West Coast Basin
Carbon-14
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
Summary
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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Water Resources of California
