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Virus Attenuation at a Research Basin Receiving Reclaimed Water for Artificial Recharge, Los Angeles County, California

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Problem: The use of treated municipal waste- water effluent for artificial recharge of ground water is increasing in the western United States. In California, additional regulations are being proposed to ensure that adequate soil retention time requirements are met for pathogen removal. As a result, there is a need for predicting the degree of virus attenuation in the subsurface to protect ground-water resources. The current knowledge about virus attenuation as reclaimed water moves through the subsurface has largely been derived from mathematical models and laboratory studies. Extension of virus attenuation studies to the field scale is now needed.

Objectives: Determine under actual recharge conditions (1) the degree to which a surrogate for human enteric viruses is attenuated as reclaimed water percolates through the unsaturated zone to the water table; (2) the degree to which the surrogate is attenuated at the water table and as reclaimed water moves downgradient to extraction wells; and (3) the degree to which changes in hydraulic conditions and water quality affect virus attenuation.

Relevance and Benefits: The results of these experiments will provide a better understanding of virus attenuation under actual recharge conditions. These results may be useful in developing well-setback requirements for ground-water recharge with recycled water.

Approach: The study will be done at the U. S. Geological Survey (USGS)/Water Replenishment District of Southern California (WRDSC) research basin located at the San Gabriel River Spreading Grounds in Pico Rivera, in Los Angeles County, California. The USGS/WRDSC research basin is currently being used to study the fate of nitrogen and attenuation of organic carbon during artificial recharge with reclaimed water. The site is instrumented to collect water samples from the unsaturated and saturated zones during recharge experiments, and additional sampling points have been installed downgradient to collect water samples as the reclaimed water mixes with the ground water and moves away from the research basin. USGS personnel will operate the research basin during two proposed experiments involving bacteriophage surrogates and bromide. Two experiments are planned; one in summer 1997, and a second in summer 1998. Water samples will be collected for field measurements of temperature, pH, specific conductance, dissolved oxygen, and redox-sensitive species such as nitrogen, iron, and manganese. Bromide will be added to the reclaimed water during each recharge experiment at a concentration of about 10 mg/L. County Sanitation Districts of Los Angeles County (CSDLAC) personnel will perform the bacteriophage assays and coliform testing. The male-specific RNA coliphage, MS2, or the Salmonella typhimurium phage, PRD1, will be the surrogate virus used for the two proposed experiments. The concentration of the selected bacteriophage will be augmented by several orders of magnitude in the research basin to obtain attenuation estimates and increase the downgradient distance at which the phage can be detected.

Progress and Significant Results in FY 1999: A third tracer experiment that was scheduled to be done in fiscal year 1999 was postponed until the summer of fiscal year 2000.

Plans for FY 2000: The third tracer experiment will be conducted in the summer of fiscal year 2000. Techniques will be developed to quantify bacteriophage concentrations to a depth of 25 feet. Also, instruments will be installed to measure changes in soil moisture.

Progress and Significant Results in FY 2001: A third field-scale virus infiltration experiment was conducted in the summer of fiscal year 2000. Data collected during this third infiltration experiment provided additional information about bacteriophage attenuation to a depth of 25 feet. In addition to the data collected during the field-scale infiltration experiment, instrumentation installed at the research basin are providing continuous soil moisture measurements at the research basin.

Plans for FY 2002: Laboratory-scale virus transport experiments will be designed to measure the effects of soil moisture fluctuations on virus sorption and inactivation. The results from these laboratory-scale tracer experiments will be used to simulate the data collected during the three field-scale infiltration experiments.

Number: CA523
Cooperating Agencies: County Sanitation Districts of Los Angeles County
Project Chief: Robert Anders
Period of Project: October 1996 through September 2000
Team: Robert Anders, Roy Schroeder

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