By John A. Izbicki, Gregory O. Mendez, and Carmen A. Burton
U.S. GEOLOGICAL SURVEY
Water Resources Investigation Report 00-4127
Sacramento, California 2001
Prepared in cooperation with the
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The Santa Ana River drains about 2,670 square miles of the densely populated coastal area of southern California, near Los Angeles. Almost all the flow in the river, more than 200,000 acre-feet annually, is diverted into ponds where it infiltrates and recharges underlying aquifers. About 2 million people are dependent on these aquifers for water supply. Stormflow in the Santa Ana River is considered a source of "high-quality" water suitable for use as a source of ground-water recharge. To test this assumption, stormflow samples were collected at two locations--below Prado Dam and at the diversion point downstream from Imperial Highway--for 12 winter storms between 1995 and 1998.
Nitrate concentrations decreased during stormflow from a median concentration of 7.8 milligrams per liter in base flow to concentrations less than 1 milligram per liter in some large storms. Concentrations of chemically reduced forms of nitrogen (nitrite, ammonia, and organic nitrogen) increased during stormflow and are the predominant forms of nitrogen in large stormflows. Dissolved organic carbon (DOC) concentrations increased from a median concentration of 4.6 milligrams per liter in base flow to more than 20 milligrams per liter in some stormflows. Concentrations of DOC were especially high during the first storm of the rainy season, and large increases in DOC concentrations were measured even as a result of small early season storms that did not cause large increases in streamflow. DOC present during early season stormflow had less ultraviolet absorbance at 254 nanometers (UV254 ) per unit of carbon than did DOC from late season stormflows. DOC in water held in storage behind Prado Dam had the highest UV254 absorbance per unit of carbon. Maximum pesticide concentrations in stormflow did not exceed U.S. Environmental Protection Agency Maximum Contaminant Levels. Most pesticide concentrations were less than 1 microgram per liter and less than the detection limits obtained using standard drinking water analyses. Increases in concentrations of pesticides such as diazinon, malathion, and chlorpyrifos in stormflow result from runoff from urban areas downstream from Prado Dam. In general, large late season stormflows have the most pesticide detections of all stormflows sampled. Concentrations of methyl tert-butyl ether (MTBE), a gasoline additive, during base flow were as high as 0.9 microgram per liter and concentrations decreased during stormflow. Like pesticides, the concentrations did not exceed the U.S. Environmental Protection Agency Maximum Contaminant Levels for MTBE.
Abstract
Introduction
Purpose and Scope
Acknowledgments
Description of the Santa Ana River Basin
Population, Land Use, and Water Use
Streamflow Characteristics
Approach
Sampled Stormflows
Timing and Distribution of Precipitation and Runoff
Regulation of Streamflow at Prado Dam
Stormflow Chemistry
Changes in Nutrient Concentrations During Stormflow
Nitrate
Reduced Forms of Nitrogen
Phosphate
Changes in Concentration and Composition of Dissolved Organic Carbon During Stormflow
Dissolved Organic Carbon Concentrations
Dissolved Organic Carbon Composition
Ultraviolet Absorbance
Nitrogen-to-Carbon Ratios
Comparison with Dissolved Organic Carbon Concentrations and UV Absorbance in Water from Other Sources
Changes in Selected Organic Compound Concentrations During Stormflow
Pesticides
Occurrence of Pesticides During Stormflow
Comparison with Reported-Use Data
Principal-Component Analysis
Volatile Organic Compounds
Changes in Selected Trace-Element Concentrations During Stormflow
Diversion of Stormflows and the Effects on the Quality of Recharge Water
Summary
References Cited
Appendix A--Quality-Assurance Procedures
Appendix B--Sample Collection, Preservation, and Analytical Methods
Appendix C--Data for Field Parameters, Major Ions, Nutrients, and Selected Trace Elements
Appendix D--Data for Organic Carbon and Ultraviolet Absorption
Appendix E--Data for Pesticides
Appendix F--Data for Volatile Organic Compounds
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