USGS


Occurrence and Distribution of Volatile Organic Compounds and Pesticides in Ground Water in Relation to Hydrogeologic Characteristics and Land Use in the Santa Ana Basin, Southern California

By Scott N. Hamlin , Kenneth Belitz , and Tyler Johnson

 

U.S. GEOLOGICAL SURVEY

National Water-Quality Assessment Program

Scientific Investigations Report 2005-5032

Sacramento, California 2005


In cooperation with the California State Water Resources Control Board



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Abstract

     This report presents an evaluation of the occurrence and distribution of VOCs and pesticides in the Santa Ana ground-water basins in relation to two types of explanatory factors: hydrogeologic characteristics and land use. The Santa Ana Basin is subdivided into the San Jacinto, the Inland, and the Coastal ground-water basins. Most wells sampled were deep and used for public supply. Data from regional studies were used to evaluate the occurrence and distribution of pesticides and volatile organic compounds (VOCs) in relation to hydrogeologic characteristics and land uses that could potentially explain variations between basins. Additional data from special studies (flow path and aquifer susceptibility) were used to evaluate potential factors affecting water quality for individual basins. The hydrogeologic characteristics evaluated in this report were hydrogeologic setting, ground-water age, depth to the top of the well screen (top of well perforations), and proximity to engineered recharge facilities. Urban land use, agricultural land use, and population density were characterized within a 500-meter radius of sampled wells and at the basin scale.

    Aquifers in the San Jacinto Basin are generally unconfined, and major land-use categories are urban (33 percent), agricultural (37 percent), and undeveloped (25 percent). Recharge is primarily from the overlying landscape, but engineered recharge is locally important in the Hemet area. VOCs and pesticides were detected more frequently in younger ground water (less than 50 years old) than in older ground water, and more frequently in shallower wells than deeper wells; the numbers of VOCs and pesticides detected also were significantly higher in the younger ground water and in the shallower wells. In the Hemet area of the San Jacinto Basin, VOCs and pesticides were detected more frequently in wells proximal to engineered recharge facilities than in distal wells. These patterns illustrate the importance of proximity to sources of recharge in relation to the occurrence and distribution of VOCs and pesticides in ground water.

    Aquifers in the Inland Basin also are generally unconfined, and the major land-use category is urban (58 percent), with lesser amounts of agricultural (13 percent) and undeveloped (28 percent) land. Recharge is from engineered facilities that utilize local runoff and imported water and from vertical infiltration. VOCs and pesticides were detected more frequently in younger ground water than in older ground water, and more frequently in shallower wells than deeper wells. The number of VOCs detected per well also was significantly higher in the younger ground water and in the shallower wells. Several solvent plumes extending between 5 and 10 kilometers illustrate the large distances that contaminants travel in basins with intensive use of ground water.

    Aquifers in the Coastal Basin, in contrast to the other basins, are generally confined. Land use in the basin is largely urban (80 percent), with lesser amounts of agricultural (7 percent) and undeveloped (12 percent) land. Recharge is primarily from engineered facilities that utilize water diverted from the Santa Ana River and imported water. Consequently, VOCs and pesticides were detected more frequently in wells proximal to engineered recharge facilities than in distal wells. These compounds were also detected more frequently in the unconfined area than in the confined area of the basin. In the confined area, the numbers of VOCs and pesticides detected per well were not significantly different in wells with shallower and deeper screens. This distribution reflects the dominance of lateral flow and insulation from overlying land use in the confined aquifers of the Coastal Basin.

    In the unconfined area of the Coastal Basin, the numbers of VOCs and pesticides detected per well were significantly higher in shallower wells than in deeper wells. VOC and pesticide detections were not statistically correlated to urban land use, agricultural land use, or population density near the sampled wells, in contrast to national findings.

    Nationally, positive correlations have been observed between the percentage of urban land use near sampled wells (500 meters) and pesticide and VOC detection frequencies in shallow ground water. Similarly, national studies have shown correlation between the percentage of agricultural land use near sampled wells and pesticide detection frequency. In contrast, pesticide and VOC detections in the Santa Ana ground-water basins were not positively correlated with urban land use, agricultural land use, or population density near the sampled wells. These contrasts with national findings could be due to differences in the types of wells sampled in the Santa Ana ground-water basins: deep, production wells compared with shallow, domestic wells in the national evaluation. The capture zones for these deep wells are complex (asymmetrical) and commonly extend beyond 500 meters, more than 10 kilometers in some cases as illustrated by solvent plumes. Another explanation for the contrasts with national findings, particularly in the San Jacinto Basin, is that current land use may not reflect land use at the time of recharge. An additional explanation for the lack of agreement with national findings in the Coastal Basin is the predominance of confined aquifers that are insulated from overlying land use.


CONTENTS

Abstract

Introduction

Description of Study Area

Purpose and Scope

Acknowledgments

Approach

Description and Comparison of Explanatory Factors in the Ground-Water Basins

Occurrence and Distribution of VOCs and Pesticides in the San Jacinto Basin

Detection Frequencies of Tritium, VOCs, and Pesticides

Relation to Ground-Water Age and Depth to the Top of the Well Screen

Relation to Engineered Recharge in the Hemet Area

Relation to Land Use and Population Density

Occurrence and Distribution of VOCs and Pesticides in the Inland Basin

Detection Frequencies of Tritium, VOCs, and Pesticides

Relation to Ground-Water Age and Depth to the Top of the Well Screen

Relation to Land Use and Population Density

Occurrence and Distribution of VOCs and Pesticides in the Coastal Basin

Detection Frequencies of Tritium, VOCs, and Pesticides

Relation to Ground-Water Age and Depth to the Top of the Well Screen

Relation to Hydrogeologic Setting

Relation to Proximity to Engineered Recharge Facilities Along the Santa Ana River

Relation to Land Use and Population Density

Summary and Conclusions

References

Appendix : Classification of Wells in the San Jacinto, Inland, and Coastal Basins


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