By Robert A. Anders and Roy A. Schroeder
U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 03-4279--ONLINE ONLY
Sacramento, California 2003
Prepared in cooperation with the
Water Replenishment District of Southern California
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Tertiary-treated municipal wastewater (recycled water) has been used to replenish the Central Basin in Los Angeles County for over 40 years. Therefore, this area provides an excellent location to investigate (1) the fate and transport of wastewater constituents as they travel from the point of recharge to points of withdrawal, and (2) the long-term effects that artificial recharge using recycled water has on the quality of the ground-water basin. The U.S. Geological Survey has been conducting such investigations in this area for about 10 years, beginning in 1992. For this investigation, a variety of inorganic, organic, and isotopic constituents were analyzed in samples from 23 production wells within 500 feet of the San Gabriel and Rio Hondo Coastal Basin Spreading Grounds, and tritium/helium-3, chlorofluorocarbons, dissolved gases, and nitrogen isotopes were analyzed in five multiple-well monitoring sites along a 10-mile flow path extending from just upgradient of the spreading grounds southward through the Central Basin.
Spearman rank-order correlation coefficients and level of significance calculated for about 40 water-quality indicators and several physical features show significant correlations between numerous inorganic and organic constituents that indicate the presence of wastewater. On the basis of a simple two-member mixing model, chloride, boron, ultraviolet absorbance at 254 nanometers, and excitation-emission fluorescence yielded the most reasonable estimates of wastewater percentages in the production wells. Tritium/helium-3 age determinations indicated that samples of ground water tested range in age from less than 2 to more than 50 years. Chloride and boron concentrations, along with tritium/helium-3 age determinations, indicate more rapid recharge and (or) displacement of pre-existing ground water at the San Gabriel Coastal Basin Spreading Grounds than at the Rio Hondo Coastal Basin Spreading Grounds. Nitrogen-15 enrichment of the ground-water nitrate and dissolved nitrogen indicates that denitrification, an important process for the removal of nitrate at the shallower depths beneath the spreading grounds, continues to occur at distances of several miles from the spreading grounds and over a period of many years. Analysis of dissolved gases shows that areas that contain recycled water have no detectable methane, whereas methane is present in the native ground water older than 50 years. The absence of methane in the younger ground water suggests that artificial recharge using recycled water has the desirable effect of increasing slightly the redox potential of the ground-water basin. Finally, measured chlorofluorocarbon concentrations and tritium/helium-3 age determinations indicate that chlorofluorocarbon concentrations are markedly elevated above atmosphere-water equilibrium in ground water older than about 20 years but still young enough to contain recycled water.
Abstract
Introduction
Artificial Recharge Using Wastewater
Previous Studies
Purpose and Scope
Description of Study Area
Acknowledgements
Field and Analytical Methods
Production Wells Adjacent to Spreading Grounds
Downgradient Monitoring Wells
Water-Quality Indicators and Environmental Tracers
Production-Well Results
Statistical Relations
Nonparametric Test
Graphical Representation
Two-Member Mixing Models for Selected Constituents
"Excess" Chloride and Boron
Boron Isotopes
Nitrogen Isotopes
Organic Constituents
Tritium and Well-Water Ages
Multiple-Well Monitoring-Site Results
Tritium-Helium-3 Age Determinations
Tracers of Recycled Water
Chloride and Boron
Nitrogen
Dissolved Gases
Chlorofluorocarbons
Summary and Conclusions
References Cited
Appendixes
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