Using chemical and isotopic tracers to assess hydrogeologic processes and properties in aquifers intended for injection and recovery of imported water

By John Izbicki
U.S. Geological Survey, 5735 Kearny Villa Road, Suite O, San Diego, California 92123

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Abstract

During the late 1800's and early 1900's, aquifers underlying the East Bay Plain on the densely populated eastern shore of San Francisco Bay, California (fig. 1), were pumped for water supply. Since that time, pumping has decreased as a result of the availability of imported water from the Sierra Nevada. Injection and recovery of imported water has been proposed by the East Bay Municipal Utilities District (MUD) for aquifers 500 to 650 feet beneath the East Bay Plain (CH2M-Hill, Inc., 2000; Fugro West, 1998, 1999). However, successful injection and recovery of imported water requires a thorough understanding of hydrologic processes within these aquifers including understanding recharge processes, sources of high-chloride water to wells, and isolation of deep aquifers from surface sources of contamination. Further, concern regarding flow of water from shallow to deeper aquifers has increased because of the large number of abandoned wells that may serve as conduits for contamination from the upper aquifer to the lower aquifer (San Francisco Bay Regional Water Quality Control Board, 1999). During this study,chemical and isotopic data, in conjunction with test-drilling and hydrogeologic data collected by East Bay MUD, were used to evaluate hydrologic processes and confirm inputs for ground-water flow models developed to estimate the effects of injection. Depth-dependent water-quality and velocity-log data were collected from wells to measure the flow of water between aquifers for pumped and unpumped conditions.

Nobel gas data were used to determine the timing of natural recharge and to evaluate contributions from focused recharge sources, such as infiltration of winter streamflows, and from diffuse sources, such as direct infiltration of precipitation (Stute and Schlosser, 2000). Nobel gas data, corrected for excess air concentrations, indicate recharge temperatures ranging from 11.0 to 18.3°C. The average annual air temperature in the study area is 13.2°C and the average winter air temperature is 9.0°C. Depending on the thickness of the unsaturated zone, recharge temperatures at the water table are typically between 0.4 and 1.4°C greater than the average air temperatures (Stute and Schlosser, 2000). Cooler recharge temperatures are consistent with water recharged fromwinter stormflows directly to the water table. Warmer recharge temperatures are consistent with water recharged by direct infiltration of precipitation through a thick unsaturated zone. Nobel gas data suggest that both recharge processes occur at this site.

Oxygen-18 (δ¹⁸O) and deuterium (δD) data were used to determine if leakagefrom water supply and sewer pipes is a large source of ground-water recharge. Previous researchers believed this was the largest source of ground-water recharge to aquifers underlying the East Bay Plain (Muir, 1996). The median δ¹⁸O and δD composition of fresh water from wells was -6.6 and -44 per mil, respectively. These values are only slightly lighter than the median isotopic composition of precipitation (-6.1 and -39 per mil, respectively) in coastal California (measured at Santa Maria, California about 250 miles south of the study area) (International Atomic Energy Agency, 1981). The δ¹⁸O and δD composition of imported water from the Sierra Nevada is about -12 and -82 per mil, respectively, and is more negative than water from wells in the study area suggesting that recharge from leaking water-supply and sewer pipes may not be as large a source of recharge as previously believed.

Depth-dependent water samples collected from wells and trace-element data were used to determine the sources of high chloride concentrations in water from wells. The high chloride concentrations measured in water from wells completed in deep aquifers were believed the result of seawater that intruded from San Francisco Bay and entered these aquifers through gaps in confining clay layers or through the casings of wells (San Francisco Regional Water Quality Control Board, 1999). The downward flow of high-chloride water from the overlying aquifers intruded by seawater into the deep aquifers was measured at one well which demonstrated that this process can occur. However, the quantity of water moving through this well was small, about 0.1 acre-feet per year. High-chloride water from surrounding and underlying partly consolidated deposits also may be a source of high-chloride water to wells. For example, the depth-dependent samples of water collected from selected wells at depths of 1,000 feet below land surface have chloride concentrations as high as 640 mg/L.  Iodide and other trace-element concentrations in water from these depths are similar to concentrations in water from the partly consolidated deposits that underlie coastal aquifers in other parts of California (Piper, Garrett, and others, 1953; Izbicki, 1991) and this water may be a source of contamination to deep aquifers as injected water is withdrawn.

Carbon-14 data were used to evaluate the age of water from wells and the isolation of deep aquifers from surface sources of recharge and contamination. Water from the deep aquifer at the proposed injection/recovery site had a carbon-14 activity of 18.9 percent modern carbon (pmc) and an interpreted age of 9,400 years before present. These data suggest that the deep aquifer is isolated from surface sources of recharge and, presumably, from surface sources of contamination. A regional ground-water flow model, with a particle-tracking package, was used to simulate the movement of a particle of ground-water as it flowed from recharge areas to the injection site and to calibrate aquifer porosity values (Dan Wendel, CH2M-Hill, Inc., oral commun., 2000).

References Cited

CH2M-Hill, Inc., 2000, Regional hydrogeologic investigation of the south East Bay Plain, Oakland, Calif.: variously paged.

Fugro West, Inc., 1998, East Bay injection/extraction groundwater pilot project well construction and performance testing Oro Loma Phase III injection/extraction well--summary of operations report: Ventura, Calif. Fugro West, Inc., variously paged.

Fugro West, Inc., 1999, Oakport groundwater storage pilot project: Volume I--Technical Memorandum Number 3, Phase 2 field investigation, Ventura, Calif., Fugro West, Inc., variously paged.

International Atomic Energy Agency, 1981, Statistical treatment of environmental isotope data in precipitation, Technical Report Series, No. 206, 255 p.

Izbicki, J.A., 1991, Chloride sources in a California coastal aquifer. in: Peters, Helen, ed., Ground water in the Pacific Rim countries: American Society of Civil Engineers, IR Div/ ASCE Proceedings, p. 71-77.

Muir, K.S., 1996, Classification of ground water recharge potential in the East Bay Plain area, Alameda County, California: Hayward, Calif., Alameda County Flood Control and Water Conservation District, C-92-320, 10 p.

Piper, A.M., Garrett, A.A., and others, 1953, Native and contaminated ground waters in the Long Beach-Santa Ana areas, California: U.S. Geological Survey Water-Supply Paper 1136, 320 p.

San Francisco Regional Water Quality Control Board, 1999, East Bay Plain groundwater basin beneficial use evaluation report, Oakland, Calif., variously paged.

Stute, Martin, and Schlosser, Peter, 2000, Atmospheric noble gasses, chap. 11, in: Cook, Peter, and Herezeg, A.L., eds., Environmental tracers in subsurface hydrology: Boston, Kluwer Academic Publishers, pp. 349-377.

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

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