Transport and Fate of Water-Quality Indicators after 40 Years of Artifical Recharge with Treated Municipl Wastewater to the Central Ground-Water Basin in Los Angeles County

By Roy A. Schroeder and Robert Anders
U.S. Geological Survey, 5735 Kearny Villa Road, Suite O, San Diego, California 92123

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Introduction

The use of treated municipal wastewater effluent (recycled water) to replenish potable ground-water supplies through artificial recharge is increasing, especially in the arid southwestern United States. Artificially recharging ground water basins is accomplished by delivering large amounts of water, including recycled water, into large holding ponds, or spreading grounds, where the recycled water infiltrates through the unsaturated zone to the water table and moves downgradient to drinking-water aquifers (National Research Council, 1994). In Los Angeles County, the Los Angeles Department of Public Works, using water supplied from the Water Replenishment District of Southern California, has been augmenting the potable ground-water supply in the Central Ground-Water Basin with recycled water for the past four decades. Therefore, this area provides an excellent opportunity to investigate the transport and fate of wastewater constituents as they travel long distances and over long time periods from the point of recharge to points of withdrawal. The U.S. Geological Survey (USGS) has been engaged in such investigations in this area for the past 10 years. Manuscripts and abstracts based on these investigations are listed in chronological order of their publication in the last section of this abstract.

The California Department of Health Services (DOHS) regulates artificial recharge using recycled water under Title 22 of the California Administrative Code. In addition to ensuring compliance with the common drinking-water standards for all constituents, the DOHS also proposed additional regulations for organic carbon, nitrogen, and pathogens. These proposed additional regulations require that no more than 1 mg/L organic carbon and nitrogen in the ground water be of wastewater origin. As a result, the proportion of recycled water in the total recharge to the area described in this study is limited to 50 percent annually and to 35 percent within three contiguous years. Likewise, protection from exposure to pathogens is achieved by requiring separation between points of recharge and withdrawal that results in a ground-water travel time of at least 6 months (Hultquist and others, 1991).

Figure 1. Location map.

Artificial recharge at the Montebello Forebay in the Los Angeles area takes place at two spreading grounds (constructed infiltration basins) using an annual delivery of about 150,000 acre-feet of water. One spreading basin is adjacent to the San Gabriel River and includes a short unlined part of the down-stream river channel (see location map). The other is adjacent to the Rio Hondo. Total area of recharge is almost 1,000 acres, and the spreading grounds typically are operated in "wet/dry" cycles wherein each part of the cycle lasts about a week. The recharged water moves from the spreading grounds southward in the direction of regional ground-water flow. The recharge is composed of imported water (from northern California and the Colorado River), recycled water (secondary- and tertiary-treated wastewater from treatment facilities located north of the Montebello Forebay and outside the Central Ground-Water Basin), storm water (local runoff), and a small amount of underflow rom the San Gabriel Valley to the north. Tertiary treatment includes chlorination-dechlorination and dual-media (charcoal and sand) filtration. Since recycled water was first introduced in 1961, its proportion has gradually increased with time as the proportion of imported water has decreased and now constitutes about 35 percent of annual recharge.

Studies during the last 10 years were conducted in stages, beginning at the spreading grounds and moving farther away from the area of recharge. Water-quality changes occurring over intervals of several days and vertical and horizontal distances of about 25 ft were investigated at a small research basin constructed on the north side of the San Gabriel River spreading grounds in Pico Rivera (see location map). These studies were followed by comprehensive inorganic, isotopic, and organic analyses at production wells near the spreading grounds, and finally by analysis of samples from multilevel monitoring wells as deep as 1,000 ft and located as much as 10 miles from the research basin. Recently, virus tracer studies have begun at the research basin along with laboratory column experiments. Principal findings from these investigations are summarized here.

Research Basin

Study results at the research basin indicate that dissolved organic carbon (DOC) and dissolved nitrogen (present almost entirely as ammonia and nitrate) in percolating recycled water decreased by about one-third and by about 20 to 60 percent, respectively. The decrease in DOC is independent of operating conditions, such as duration of recharge and length of the subsequent desiccation cycle; however, the decrease in dissolved nitrogen concentration does depend on operating conditions to the extent that they influence the establishment of local anoxic conditions in the subsurface. In the subsurface, reduced nitrogen in the form of ammonium and organic nitrogen in recycled water initially is rapidly oxidized to nitrate as oxygen is incorporated with recharge, but the nitrate is partly removed through denitrification that ensues as the subsurface becomes increasingly reducing after several days. Denitrification is confirmed by enrichment in 15N in the residual nitrate.

Production Wells

A wide variety of inorganic, organic, and isotopic constituents were analyzed in 23 production wells located within 500 ft of the spreading grounds to ascertain which constituents of wastewater origin could be identified and used as tracers to determine percent of recycled water. The production wells range in depth from 250 to 900 ft, are screened over intervals ranging from 100 to 400 ft below land surface, and were yielding 500 to 3,000 gallons/min at the time of sampling. Spearman Rank Order correlation coefficients and level of significance were calculated for about 40 water-quality indicators and several physical features such as well depth and distance from spreading grounds to the production wells. About half of the correlations are significant at α<0.01, and several are even significant at α<0.001. The DOHS would like to base regulations on the percentage of wastewater present in a well by using internal tracers; therefore, an attempt was made to calculate values based on a two-member mixing model. One end member is the recycled water itself and the other is either a background well known to be from outside the area containing recycled water or the production well with the lowest measured concentration of each constituent. No completely satisfactory tracer was identified although chloride, boron, ultraviolet absorbance at 254 nanometers, and excitation-emission fluorescence seemed to yield the most reasonable estimates. The large number of statistically significant nonparametric correlations results from the fact that the wastewater signal is preserved in the production wells. However, the failure of parametric calculations to yield precise estimates of the wastewater percentage is caused by the fact that the wells draw water from various depths and ages (with variable extent of degradation) that are not accurately represented by a simple two-member mixture.

Multilevel Monitoring Wells

Five multilevel monitoring well sites along a section line A-A' (see location map) extending from upgradient from the spreading grounds to about 10 miles downgradient were analyzed for wastewater (and other) constituents and to ascertain the long-term effects that artificial recharge has on the ground-water basin. The southernmost site along A-A' serves as a background insofar as absence of tritium confirms that the ground water at this site is older than 1950s recharge and, therefore, is still ahead of the advancing wastewater front. Individual organic constituents present in recycled water and in the 23 nearby production wells described earlier, such as ethylenediaminetetraacetic acid (EDTA), are not in ground water that was more than 20 years old (on the basis of tritium/helium-3 dating). The partly degraded sulfonates present in detergents were the only organic chemicals of recycled-water origin that were detected, and they appear to comprise a substantial portion of the DOC present in the ground-water samples containing recycled water. They also contribute to the finding that THM (trihalomethane) formation potentials are less than for ground-water samples containing only natural organic matter. An additional desirable effect of recharging using recycled water here seems to be a slight increase in the redox state of this generally suboxic (less than 1 mg/L dissolved oxygen) basin insofar as methane is absent where age dating and chemical analyses indicate recycled water is present, whereas methane is present at distances and depths where recycled water has not yet penetrated. Chlorofluorocarbon (CFC) data suggest that older recycled water may have contained more volatile organic contaminants than today insofar as CFC concentrations in ground-water samples older than about 20 years are enriched by as much as 100-fold over equilibrium with the historical atmospheric level-in contrast to current samples of recycled water, which are only slightly enriched in CFCs.

Virus Transport and Attenuation

Recent experiments in which recycled water delivered to the research basin has been amended (seeded) with high concentrations of the bacteriophages PRD-1 and MS2 have been completed to determine the potential for virus transport and attenuation in the subsurface. Extrapolation from results obtained over short distances indicates that 7-log removal, an amount required by the DOHS for human enteric viruses, occurs for these surrogates over a distance of about 100 ft in the Montebello Forebay.

USGS publications on SAT studies in Los Angeles County

Schroeder, R.A., and Anders, Robert, 1996, Transport and attenuation of wastewater constituents during recharge at a research basin in Los Angeles County: American Geophysical Union Fall Meeting, San Francisco, December 15-19, 1996, EOS Transactions supplement, v. 77, no. 46, Abstract no. H42F-2, p. F281.

Barber, L.B., II., Brown, G.K., Kennedy, K.R., Leenheer, J.A., Noyes, T.I., Rostad, C.E., and Thorn, K.A., 1997, Organic constituents that persist during aquifer storage and recovery of reclaimed water in Los Angeles County, California, in Kendall, D.W., ed., Proceedings of the AWRA Symposium, Conjunctive Use of Water Resources: Aquifer Storage and Recovery, American Water Resources Association 33rd National Meeting, Long Beach, California, October 19-23, 1997: American Water Resources Association, Herndon, Virginia, TPS-97-2, p. 261-272.

Schroeder, R.A., Anders, Robert, Böhlke, J.K., Michel, R.L., and Metge, D.W., 1997, Water quality at production wells near artificial-recharge basins in Montebello Forebay, Los Angeles County, in Kendall, D.W., ed., Proceedings of the AWRA Symposium, Conjunctive Use of Water Resources: Aquifer Storage and Recovery, American Water Resources Association 33rd National Meeting, Long Beach, California, October 19-23, 1997: American Water Resources Association, Herndon, Virginia, TPS-97-2, p. 273-284.

Anders, Robert, and Schroeder, R.A., 1997, Water-quality changes during recharge with recycled water at a research basin in the Montebello Forebay, Los Angeles County, California, in Kendall, D.W., ed., Proceedings of the AWRA Symposium, Conjunctive Use of Water Resources: Aquifer Storage and Recovery, American Water Resources Association 33rd National Meeting, Long Beach, California, October 19-23, 1997: American Water Resources Association, Herndon, Virginia, TPS-97-2, p. 285-296.

Anders, Robert, and Schroeder, R.A., 1998, Correlations between various water-quality indicators of recharged recycled water in production wells in Los Angeles County: American Geophysical Union Spring Meeting, Boston, May 26-29, 1998, EOS Transactions supplement, v. 79, no. 17, Abstract no. H51B-23, p. S141.

Anders, Robert, Yanko, W.A., Schroeder, R.A., and Jackson, J.L., 1999, Attenuation of PRD-1 and MS-2 during recharge at a constructed research basin in Los Angeles County: 4th International Symposium on Subsurface Microbiology, Vail, Colorado, August 22-27, 1999, Programs and abstracts, p. 28.

Anders, Robert, Yanko, W.A, Schroeder, R.A., and Jackson, J.L., 2000, Attenuation rates for PRD-1 and MS2 during recharge with recycled water at a research basin in Los Angeles County: American Geophysical Union Spring Meeting, Washington, D.C., May 30-June 3, 2000, EOS Transactions supplement, v. 81, no. 19, Abstract no. H41C-04, p. S226-S227.

Leenheer, J.A., Rostad, C.E., Barber, L.B., Schroeder, R.A., Anders, Robert, and Davisson, M.L., 2001, Nature and chlorine reactivity of organic constituents from reclaimed water in groundwater, Los Angeles, County, California: Environmental Science & Technology, v. 35, p. 3869-3876.

Anders, Robert, and Schroeder, R.A., 2001, Using environmental tracers to determine long-term effects of recharging recycled water in the Central Ground Water Basin, Los Angeles County, California: American Geophysical Union Fall Meeting, San Francisco, December 10-14, 2001, EOS Transactions supplement, v. 82, no. 47, Abstract no. H51C-0338, p. F492.

Acknowledgments

This investigation was funded under the USGS cooperative program with the Water Replenishment District of Southern California and the County Sanitation Districts of Los Angeles County.  We thank numerous individuals from the USGS National Research Program for providing analyses of various chemical constituents and isotopes.

References cited

Hultquist, R.H., Sakaji, R.H., and Asano, T., 1991, Proposed California regulations for ground water recharge with reclaimed municipal wastewater, in Proceedings of the 1991 Specialty Conference, Environmental Engineering, Reno, Nevada: American Society of Civil Engineers, p. 759-764.

National Research Council, 1994, Ground Water Recharge using Waters of Impaired Quality: Washington, D.C., National Academy Press, 182 p.

In George R. Aiken and Eve L. Kuniansky, editors, 2002, U.S. Geological Survey Artificial Recharge Workshop Proceedings, Sacramento, California, April 2-4, 2002: USGS Open-File Report 02-89

The use of firm, trade, and brand names in this report is for identification purposes only and does not consitute endorsement by the U.S. Government.