Environ. Sci. Technol. 2005, 39, 3563-3570
Tracking Sources of Unsaturated Zone and Groundwater Nitrate Contamination Using Nitrogen and Oxygen Stable Isotopes at the Hanford Site, Washington M I C H A E L J . S I N G L E T O N , * ,†,‡ KATHARINE N. WOODS,† MARK E. CONRAD,† D O N A L D J . D E P A O L O , †,§ A N D P. EVAN DRESEL| Earth Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Building 70A4418, Berkeley, California 94720, Department of Earth and Planetary Science, 307 McCone Hall, University of California, Berkeley, California 94720, and Pacific Northwest National Laboratory, Richland, Washington 99352
The nitrogen and oxygen isotopic compositions of nitrate in pore water extracts from unsaturated zone (UZ) core samples and groundwater samples indicate at least four potential sources of nitrate in groundwaters at the U.S. DOE Hanford Site in south-central Washington. Natural sources of nitrate identified include microbially produced nitrate from the soil column (δ15N of 4-8‰, δ18O of -9 to 2‰) and nitrate in buried caliche layers (δ15N of 0-8‰, δ18O of -6 to 42‰). Isotopically distinct industrial sources of nitrate include nitric acid in low-level disposal waters (δ15N ≈ 0‰, δ18O ≈ 23‰) and co-contaminant nitrate in high-level radioactive waste from plutonium processing (δ15N of 8-33‰, δ18O of -9 to 7‰). The isotopic compositions of nitrate from 97 groundwater wells with concentrations up to 1290 mg/L NO3- have been analyzed. Stable isotope analyses from this study site, which has natural and industrial nitrate sources, provide a tool to distinguish nitrate sources in an unconfined aquifer where concentrations alone do not. These data indicate that the most common sources of high nitrate concentrations in groundwater at Hanford are nitric acid and natural nitrate flushed out of the UZ during disposal of low-level wastewater. Nitrate associated with high-level radioactive UZ contamination does not appear to be a major source of groundwater nitrate at this time.
Introduction Nitrate is the most abundant anion reported in contaminated groundwater, soils, and sediment at U.S. DOE facilities (1). A considerable mass of nitrate may accumulate by natural processes in the unsaturated zone (UZ) in arid and semi* Corresponding author phone: (925)424-2022; fax: (925)422-3570; e-mail:
[email protected]. † Lawrence Berkeley National Laboratory. ‡ Present address: Lawrence Livermore National Laboratory, 7000 East Ave., L-231, Livermore, CA 94551-0808. § University of California. | Pacific Northwest National Laboratory. 10.1021/es0481070 CCC: $30.25 Published on Web 04/15/2005
2005 American Chemical Society
arid climates, where infiltration of water at the surface is low (2). Groundwater quality may be significantly impacted when this sink of nitrate is mobilized by activities that enhance natural infiltration to the UZ such as irrigation, storage of water in aquifers, or wastewater disposal in infiltration ponds. The dual isotopic composition of nitrate (δ15N and δ18O) can be used as a tracer for industrial and natural sources and can give valuable insight into transport through unsaturated zone and groundwater systems. However, few studies have examined the processes that affect the isotopic composition of nitrate in contaminated industrial settings (e.g., refs 3 and 4). The isotopic composition of natural nitrate has been documented in a diverse range of ecosystems (5-10). The cycling of nitrate in soils is complex, with inputs from precipitation and surface water combined with numerous co-occurring microbial processes. Soil microbes tend to favor uptake of the lighter isotopes of N and O, leaving the residuum enriched in the heavier isotopes. For example, the process of denitrification (microbially mediated reduction of nitrate to N2) results in a trend of higher values (both for δ15N and δ18O) and has been of particular interest to groundwater and surface water studies (3-6, 11). A recent study of bioremediation of uranium has indicated that the high concentrations of nitrate commonly associated with uranium in industrial contamination can strongly affect uranium mobility by inhibiting the ability of metal-reducing bacteria to convert the soluble U(VI) to the insoluble U(IV) (12). Bioremediation efforts therefore require a detailed understanding of local sources of nitrate as well as abiotic and biological processes affecting nitrate in industrial settings. Our investigation of nitrate at the Hanford Site uses borehole sediment samples to determine sources of nitrate in the UZ. Potential industrial sources of nitrate in the UZ are evaluated based on sediment samples from two boreholes through radionuclide-contaminated sediments at high-level waste tank farms. Natural sources of nitrate are determined from two boreholes in relatively undisturbed areas. The isotopic compositions of UZ nitrate sources are then compared with nitrate from the local unconfined aquifer to determine the sources of groundwater contamination. Measurements of the nitrogen and oxygen isotopic compositions of nitrate provide a promising method to identify sources and track the transport of co-contaminant nitrate and associated radioactive contaminants from the UZ into groundwater. Site Description. The Hanford Site is located in the Pasco Basin in south-central Washington, USA (Figure 1). The Pasco Basin consists of mostly unconsolidated sedimentary deposits overlying gently folded and faulted Miocene tholeiitic basalt flows of the Columbia River Basalt Group. The UZ is generally 50-90 m thick and overlies a 40-180 m thick unconfined aquifer that flows from west to east across the site. Both the UZ and unconfined aquifer are hosted in the gravel silts and clay of the fluvial Pliocene Ringold Formation and the Pleistocene flood deposits known locally as the Hanford formation. A “caliche” zone of pedogenic carbonate, part of the Cold Creek Unit (CCU), was developed on top of the eroded surface of the Ringold Formation and in overlying gravel deposits during arid climate conditions (13, 14). Nitrate concentrations >2000 mg/L (all nitrate concentrations reported here as NO3-) have been reported for Hanford groundwaters (15), where nitrate commonly accompanies radionuclide contamination. High levels of nitrate in drinking waters can lead to heath problems such as “blue VOL. 39, NO. 10, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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along with high-level waste, and a small fraction was released with atmospheric emissions.
Materials and Methods
FIGURE 1. (A) Location of the Hanford Site and 200 areas where cores were collected in contaminated areas near two tank farms and in two areas with no known contamination. Contours are shown for nitrate concentrations in the Hanford unconfined aquifer (simplified after ref 15). Groundwater samples from 97 wells have been analyzed for nitrate isotopic composition (dots). Nitrate concentrations in these wells range from 1.5 to 1290 mg/L (15). (B) Schematic cross section of the Pasco Basin from east to west across the 200W and 200E chemical processing areas. baby” syndrome, and the U.S. EPA has established a maximum contaminant limit (MCL) of 44 mg/L in drinking waters (16). Many wells at the Hanford Site exceed the drinking water standard for nitrate, although currently no remediation of the nitrate contamination is occurring or anticipated. It is, however, critical to understand the source and transport of nitrate at Hanford because nitrate is intimately linked with the sources and processes affecting radioactive contaminants. There are a variety of natural and industrial sources that may contribute to high nitrate concentrations in the groundwater at Hanford. Natural sources include atmospheric deposition, microbial fixation of N2, and buried paleosol or evaporite deposits such as the CCU. There are currently no major agricultural operations at the site, although prior to Hanford Site operations there was a limited amount of agricultural development along the Columbia River near the old Hanford townsite. Starting in the mid-1940s, nitrogenbearing chemicals were used extensively in plutonium extraction and purification processes carried out at the Hanford Site. Most of the nitrate was discharged to UZ in low-level waste streams, some is now stored in buried tanks 3564
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Core Samples. Shorthand designations are used here to refer to the four cores sampled for this study. Samples from core collected during the installation of two monitoring wells are referred to as BG1 (now the location of well 299-W22-48) drilled in October 1999 and BG2 drilled in April 2002 (now the site of well 299-E17-22). Boreholes drilled to characterize UZ contamination near tank farms include TF1 (core C3832) near tank TX-104, drilled during May 2002, and TF2 (core C4105) near tank T-106 drilled from January to March 2004. Subsamples consisting of 80-150 g of sediment were collected from each core and sealed in plastic bottles. Pore water was vacuum distilled from the sediment samples prior to dissolution of the nitrate in the pore water extraction method described below. Analytical Procedure. For the pore water extracts, 30 g of dry sediment was combined with 30 mL of 18 MΩ deionized water, shaken for 90 min at room temperature, and allowed to stand for 24 h. The nitrate contained in the rinse water is assumed to be that which was originally dissolved in the pore fluid plus any readily dissolvable sources of nitrate in the soil. After 24 h, the leach solutions were vacuum filtered through a 0.45 µm polyethylene filter into sealed glass vials and kept in the dark and refrigerated at 4 °C until analysis. Until recently, methods for analyzing δ15N and δ18O values in dissolved nitrate were unwieldy for routine analyses of dilute groundwaters and (small volume) pore water extracts from UZ sediments. A recently developed method uses denitrifying bacteria to generate N2O from NO3- and NO2in dilute and saline samples (17, 18). This method allows for simultaneous δ18O and δ15N analyses on low concentration samples (down to 0.5 mg/L NO3-) and low sample volume (
