USGS

Ground-Water Quality of Coastal Aquifer Systems in the West Coast Basin, Los Angeles County, California, 1999-2002

By Michael Land, Eric G. Reichard, Steven M. Crawford,Rhett R. Everett,Mark W. Newhouse, and Colin F. Williams

 

U.S. GEOLOGICAL SURVEY

Scientific Investigations Report 2004-5067—ONLINE ONLY

 

Sacramento, California 2004

In cooperation with the

Water Replenishment District of Southern California




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Abstract

     The extensive use of ground water throughout the Central and West Coast Basins of Los Angeles County during the first half of the 20th century resulted in declining water levels, widespread seawater intrusion, and deterioration of water quality along most reaches of the coast. In order to control seawater intrusion in the West Coast Basin, freshwater is injected into a series of wells at two seawater barrier projects. In order to better understand the processes of seawater intrusion and the efficiency of current barrier operation, data were collected from multiple-well monitoring sites installed by the U.S. Geological Survey, from local observation wells, and from production wells. The occurrence and areal extent of native, saline, and recently injected ground water near the coast were defined through the collection and analysis of inorganic and isotopic water-quality data and geophysical logs.

     Most water in the West Coast Basin with a dissolved-solids concentration less than 500 milligrams per liter generally has a sodium-bicarbonate to sodium/calcium-bicarbonate character. Water with a dissolved-solids concentration greater than 1,000 milligrams per liter also contains variable amounts of calcium and sodium, but chloride is predominant. Most of these high-dissolved-solids wells are perforated in the Upper aquifer systems; several have dissolved-chloride values near that of seawater. Elevated chloride concentrations were measured at many wells in both the Upper and Lower aquifer systems inland from the barrier projects. Although water levels have increased in many wells over the last 30 years, some of the wells do not show a corresponding decrease in dissolved chloride.

     A detailed assessment of saline ground water was provided by examining the ratios of chloride to bromide, iodide, and boron. Seawater-freshwater mixing lines were constructed using all three ratios. These ion ratios also identify water affected by mixing with injected imported water and oil-field brine water.

     Isotopic data -oxygen-18, deuterium, strontium-87, boron-11, tritium, and carbon-14-also were collected. The stable isotopes oxygen-18 and deuterium were used to distinguish between isotopically heavier water that originated in the Los Angeles Forebay, isotopically lighter water that originated in the Montebello Forebay, local recharge, and water containing a mixture of seawater and imported water. Tritium data were used to identify recent water (less than 50 years old) present in the Upper and Lower aquifer systems inland from the seawater barrier projects, and present locally near the Dominguez Gap. Carbon-14 data indicate that water with uncorrected ages ranging from about 4,000 years to more than 20,000 years before present occurs in the Lower aquifer systems and in the Pico unit.

     Borehole electromagnetic conductivity logs, combined with gamma-ray logs, were used to identify potential saline zones throughout the entire well depth; this provides an indicator of intrusion in zones that are not being monitored by a piezometer. Temperature logging was used to track the thermal effects of the cooler water used for injection. Specific-conductance logs provided a screening-level indicator of poor-quality water in monitoring wells.

CONTENTS

 

Abstract

Introduction

Purpose and Scope

Previous Investigations

Acknowledgments

Description of Study Area

Physiographic Setting

Geohydrologic Setting

Hydrostratigraphy

Recharge and Discharge

Ground-Water Movement and Levels

Methods of Investigation

Well Network

Water Quality

Core Pore-Water Extraction

Geophysical Logging

Electromagnetic Induction

Gamma-Ray Emission

Temperature

Specific Conductance

Quality Assurance

Ground-Water Chemistry

Major Ion Composition

Dissolved Chloride

Dissolved Bromide

Dissolved Iodide

Dissolved Boron

Ground-Water Isotopes

Oxygen and Deuterium

Tritium

Carbon

Boron

Strontium

Summary of Ground-Water Chemistry and Isotope Data

Geophysical-Log Analysis

Evaluation of Log Data near the Dominguez Gap Barrier Project

Evaluation of Log Data near the Southern Part of West Coast Basin Barrier Project

Summary Analysis of Geophysical Log Data

Ground-Water-Quality Conditions along a Geohydrologic Section

Summary

References Cited

Appendixes


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