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Water-Resources Investigations Report 01-4188

U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 01-4188

Low-Level Volatile Organic Compounds in Active Public Supply Wells as Ground-Water Tracers in the Los Angeles Physiographic Basin, California, 2000

National Water-Quality Assessment Program
By Jennifer L. Shelton, Karen R. Burow, Kenneth Belitz, Neil M. Dubrovsky, Michael Land, and JoAnn Gronberg

Abstract

Data were collected to evaluate the use of low-level volatile organic compounds (VOC) to assess the vulnerability of public supply wells in the Los Angeles physiographic basin. Samples of untreated ground water from 178 active public supply wells in the Los Angeles physiographic basin show that VOCs were detected in 61 percent of the ground-water samples; most of these detections were low, with only 29 percent above 1 µg/L (microgram per liter). Thirty-nine of the 86 VOCs analyzed were detected in at least one sample, and 11 VOCs were detected in 7 percent or more of the samples. The six most frequently detected VOCs were trichloromethane (chloroform) (46 percent); trichloroethene (TCE) (28 percent); tetrachloroethene (PCE) (19 percent); methyl tert-butyl ether (MTBE) (14 percent); 1,1-dichloroethane (11 percent); and 1,1,1-trichloroethane (TCA) (11 percent). These VOCs were also the most frequently detected VOCs in ground water representative of a wide range of hydrologically conditions in urban areas nationwide. Only two VOCs (TCE and PCE) exceeded state and federal primary maximum contaminant levels (MCL) for drinking water in a total of seven samples. Because samples were collected prior to water treatment, sample concentrations do not represent the concentrations entering the drinking-water system.

Ground water containing VOCs may be considered to be a tracer of postindustrial-aged water—water that was recharged after the onset of intense urban development. The overall distribution of VOC detections is related to the hydrological and the engineered recharge facilities in the Coastal Los Angeles Basin and the Coastal Santa Ana Basin that comprise the Los Angeles physiographic basin. Most of the ground-water recharge occurs at engineered recharge facilities in the generally coarse-grained northeastern parts of the study area (forebay areas). Ground-water recharge from the land surface is minimal in the southwestern part of the basins, distal from the recharge facilities, where clay layers impede the vertical migration of ground water (pressure areas).

VOCs are not uniformly distributed over the study area. Most of the wells with multiple VOC detections, which also have the highest concentrations, are in the forebay areas and are clustered proximal to the recharge facilities. In addition, the number of VOC detections and VOC concentrations decrease beyond about 10–15 kilometers from the recharge facilities. The distribution of individual VOCs is also related to their history of use. MTBE traces ground water recharged during about the last decade and is detected almost exclusively in the forebay areas. Chloroform, which has been used since the 1920s, is more widely distributed and is detected at the greatest distances from the recharge facilities.

Downward migration of VOCs from the land surface may be a viable process for VOCs to reach aquifers in parts of the forebay areas, but there is little indication that the same process is active in the pressure area. The lack of contrast in the number of VOC detections between wells of different depths over most of the study area suggests that the downward migration from the land surface is not a dominant pathway for VOCs to travel to the capture zones of public supply wells. Isolated occurrences of multiple VOC detections and high concentrations of VOCs in individual wells may indicate rapid vertical transport from a localized source.

Stable isotope data indicate that ground water containing VOCs is a mixture of local precipitation and runoff with water that is isotopically lighter (more negative) than the local sources. The isotopically lighter water could either be Colorado River water or State Water Project water, both of which are imported to the basin and used as a source of recharge to the ground-water flow system. The stable isotope data support the interpretation that VOCs in ground water are associated with the engineered recharge facilities.

Two of the most frequently detected VOCs in ground water, MTBE and chloroform, were detected more frequently and at higher concentrations in the Santa Ana River than in ground-water samples from active public supply wells, suggesting that the Santa Ana River may be a source of these compounds. In contrast, the maximum concentrations and frequency of detection of TCE and PCE are much higher in the ground-water samples than in the Santa Ana River. The VOCs detected in surface water sampled in the present time are only an approximation of past surface-water quality, which likely varied as a complex function of urbanization, water-quality regulations, and climate; and current surface-water quality alone can not explain VOC occurrence in ground water.

Correlations between VOC occurrence and sources of VOCs on the land surface do not explain VOC occurrence in this study. The overall lack of correlation between these surficial features and the spatial distribution of VOCs in active public supply wells in this study indicates that vertical migration of contaminants is not a dominant process outside the susceptible forebay areas. Hydrogeology and the acceleration of the lateral rate of ground-water flow produced by engineered recharge and groundwater pumping in the Coastal Los Angeles Basin and the Coastal Santa Ana Basin appear to be the dominant factors in controlling the distribution of VOCs in active public supply wells in these coastal basins. Overall, the data show that active public supply wells in the forebay areas are more vulnerable to contamination than those in the pressure area in the Los Angeles physiographic basin study area.

Contents

Abstract
Introduction
Hydrogeologic Setting
Study Design and Methods
Active Public Supply Wells Frequently Contain Low Concentrations of VOCs
VOC Distribution Indicates Enhanced Recharge and Ground-Water Pumping are Primary Controls on Ground-Water Flow System
Stable Isotope Data Relate Source of Recharge Water to VOC Occurrence
Assessment of Relation Between VOC Distribution and Potential Sources of VOC
Summary and Conclusions
References Cited

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Send questions or comments about this report to the author, Jennifer L. Shelton,, (916) 278-3068.

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