Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and
Convergent Flow Systems, Southern California, 2000
By Barbara J. Milby Dawson, Kenneth Belitz,
Michael Land, and Wesley R. Danskin
U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 03-4059
Sacramento, California 2003
Prepared in cooperation with the
California State Water Resources Control Board
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Abstract
Ground water is a major source of drinking water in southern
California. In an effort to understand factors influencing the susceptibility of ground water tapped by
public supply wells, the U.S. Geological Survey has undertaken studies in cooperation with the California
State Water Resources Control Board. The vertical and lateral distribution of stable isotopes (deuterium
and oxygen-18) and volatile organic compounds (VOC) were examined along seven ground-water flow paths in
three urban ground-water basins in southern California: Central Basin in Los Angeles County, Main Basin in
Orange County, and Bunker Hill Basin in San Bernardino County. Forty-seven monitoring wells and 100 public
supply wells were sampled.
The results of this study suggest that the direction of flow and perhaps the degree
of confinement in an aquifer system are important controls on the distribution of VOCs. Ground-water flow in
the Central and Main Basins in the southern California coastal plain is characterized as radially divergent,
with ground-water flow directions moving outward from focused areas of recharge in the unconfined part of the
aquifer system toward dispersed areas of discharge in the more confined part. In these basins, there is a volume
of water containing VOCs that extends out into a volume of water containing no VOCs. This pattern suggests that
radially divergent flow systems disperse VOCs in distal areas. The overall pattern also suggests that ground water
in the pressure area is generally insulated from compounds introduced at land surface. These two factors—dispersion
of VOCs due to divergence of flow and insulation from land-surface inputs—suggest that the susceptibility of public
supply wells to surface contamination decreases with distance in radially divergent, well confined ground-water flow
system.
In the inland Bunker Hill Basin, ground-water flow is characterized as radially convergent;
ground-water flow directions move inward from dispersed recharge areas in the unconfined part of the aquifer system,
toward an area of focused discharge in the more confined part. The number of VOCs increased and the concentrations of
individual VOCs increased, or remained the same, with increasing travel distance. Methyl tert-butyl ether was detected
only in wells in the confined part of the aquifer system, suggesting that the confining units present in the distal part
of the Bunker Hill Basin do not prevent VOCs from reaching ground water. These results suggest that VOCs in the Bunker
Hill Basin are collected and concentrated as ground water moves downgradient because of radial convergenence of flow. They
also suggest that ground water in the Bunker Hill Basin has an increasing opportunity to pick up VOCs introduced at land
surface as it moves along a flow path. Some of the downgradient increase in VOC occurrence and concentration may be due to
pumping that selectively removes cleaner ground water, thus leaving ground water containing more VOCs in the aquifer. These
two factors—collection of VOCs due to convergence of flow and increasing opportunity to collect surficial contaminants perhaps
due to a relative absence of confinement—suggest that the susceptibility of public supply wells to surface contamination
increases with distance in radially convergent ground-water flow systems, particularly those that are unconfined.
CONTENTS
Abstract
Introduction
Study Area
Study Design
Field Methods
Laboratory Methods
Quality Control
Data Analysis Methods
Accessing Data
Acknowledgements
Central Basin, Los Angeles County
Hydrogeology
Stable Isotopes
Volatile Organic Compounds
VOC Score
Chloroform
MTBE
TCE
PCE
CFC-11
Main Basin, Orange County
Hydrogeology
Stable Isotopes
Volatile organic compounds
VOC Score
Chloroform
MTBE
TCE
PCE
CFC-11
Bunker Hill Basin, San Bernardino County
Hydrogeology
Stable Isotopes
Volatile organic compounds
VOC Score
Chloroform
MTBE
TCE
PCE
CFC-11
Summary and Conclusions
References Cited
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Water Resources of California