NEWS and TRENDS A newsletter about soil, sediment, and groundwater characterization and remediation technologies Issue 46 February 2010 This issue of Technology News and Trends highlights the use of compound specific isotope analysis Contents (CSIA), an environmental forensics technique used to characterize contaminated sites and the progress of bioremediation and natural attenuation. CSIA measures and compares the ratios of stable isotopes found in compounds of suspected contaminant sources or plumes as well as the CSIA at the Bandera feedstock or manufacturing process of materials historically used in a site’s vicinity. Isotopic Road Site Reveals a analysis can help discern the potential for multiple spills of the same compound based on their Second Contaminant different isotopic “signatures.” An isotopic signature can be used to associate a contaminant Source page 1 plume with a particular spill or potentially responsible party. It also can be used to evaluate the Combined Dual extent of contaminant degradation caused by microbes during natural attenuation. Isotope and Dissolved Gas Analyses Used to CSIA at the Bandera Road Site Reveals a Second Contaminant Source Evaluate Nitrate Contamination at The U.S. Environmental Protection Agency characterize, and quantify biotic and abiotic LLNL Site 300 page 3 (EPA) Region 6 office recently conducted a transformation reactions. CSIA Discounts PCE CSIA investigation at the Bandera Road Biodegradation as Groundwater Plume Superfund site in Leon The study area encompassed approximately 162 acres in a commercial area with nearby Source of TCE in Valley, TX, just outside San Antonio. Municipal Wellfield page 4 Previous groundwater investigations residences. Potential contaminant sources included several dry cleaners, automotive identified chlorinated solvents exceeding CLU-IN Resources groundwater standards. Determining the businesses, and a former municipal airfield. The primary contaminants of concern are carbon isotope ratios (13C/12C) for tetrachloroethene (PCE), trichloroethene PCE, TCE, and cDCE due to their detection EPA’s CLU-IN Web host (TCE), and cis-1,2-dichloroethene (cDCE) in frequency and concentrations in local provides information on methods groundwater samples helped to distinguish groundwater. Of the 10 wells with PCE or TCE and standard operating procedures for site character- release sources and assess biodegradation. concentrations exceeding the 5 ppb federal drinking water standard, four had been ization, such as A Guide for Gas chromatography-isotope ratio mass Assessing Biodegradation and spectrometry (GC-IRMS) and sample plugged and abandoned prior to CSIA work because they were open to the Edwards Source Identification of preparation techniques that could detect Organic Groundwater Contami- chlorinated solvents at concentrations as Aquifer, the sole-source drinking water aquifer for the San Antonio area. nants Using Compound low as 20 ppb were used. Specific Isotope Analysis (CSIA) (EPA 600/R-08/148), at: Many processes affecting contaminants in The other six impacted wells extend to water- bearing formations (Austin Chalk and Buda www.cluin.org/characterization/. groundwater, such as dilution, sorption, CLU-IN information on the and volatilization, have little or no effect Limestone) above the Edwards Aquifer. The highest PCE concentration of 11,600 ppb was related issue of natural on isotopic ratios. Processes such as attenuation is available in detected in an Austin Chalk monitoring well biodegradation and abiotic degradation, guidance such as Monitored however, are associated with significant (USGS-42) adjacent to a former dry cleaning Natural Attenuation of Inorganic facility. Another Austin Chalk well (DW-404) isotopic fractionation. In addition, the Contaminants in Ground isotopic ratios of PCE and TCE vary approximately 150 feet from an operating dry Water (Volumes 1 and 2, EPA cleaning facility exhibited PCE concentrations depending on the manufacturer. As a 600-R-07-139 and EPA 600-R- result, CSIA can be used at a contaminated as high as 1,100 ppb. 07-140), at: www.cluin.org/ remediation/. site to identify sources of chlorinated CSIA was conducted in June 2009, in solvent; help determine the sequence of accordance with EPA’s Guide for Assessing multiple releases; and identify, [continued on page 2] 1 Table 1. Results of CSIA for PCE and its degradation products in groundwater samples helped identify a second source of contamination at the Bandera Road site. [continued from page 1] more negative δ13C value (Table 1). This contaminant ratios indicate that Biodegradation and Source Identification indicates that either a more recent release contamination in other wells (DW-407 of Organic Groundwater Contaminants has taken place near DW-404 and/or less and DW-36) may originate from a source Using CSIA. The guidance allows for a degradation is occurring near this well. other than than the former dry cleaner. typical analytical uncertainty of ±0.5‰ for Subsequent passive soil gas sampling at Using the CSIA results, Region 6 plans to carbon isotopes. Results of the analysis are the operating dry cleaner near DW-404 conduct a tracer study to help identify reported as δ13C, which represents a identified PCE, TCE, cDCE, and vinyl contaminant migration pathways. Analysis comparison between the ratio of 13C to 12C chloride contamination in soil gas. of additional CSIA techniques involving in a sample and the ratio in an international The spatial distribution of δ13C for PCE chlorine and hydrogen istopes (39Cl/37Cl standard, expressed in parts per thousand and its degradation products (TCE and and 2H/1H) may be used to further (‰). A δ13C of -30‰, for example, indicates cDCE) and standard sampling results were distinguish and delineate the plumes. that the ratio of 13C to 12C for the sample is examined to determine potential trends in 3% lower than the standard. Biodegradation Contributed by Chris Villarreal, EPA biodegradation (Figure 1). Biodegradation induces a shift of the residual compound to impacts also were observed in well DW- Region 6 ([email protected] or less negative values of δ13C; the more 214-665-6758) and Yi Wang, Ph.D., DPRA- 408, which is located approximately 400 negative the CSIA values are, the closer to feet from the former dry cleaner and ZymaX Forensics ([email protected] the source and/or the more recent the or 760-781-3338) monitoring well USGS-42. CSIA data and contaminant release is likely to be. Techniques for collecting and preserving groundwater samples for CSIA are identical to those used in collecting samples for volatile organic analysis using EPA Method 8260. All samples are packed in the same manner and then shipped to an offsite laboratory. For Bandera Road CSIA, laboratory costs averaged $600 per sample. Comparison of carbon isotope ratios for PCE indicates that monitoring well USGS-42 contains the highest PCE concentration, but well DW-404 has a Figure 1. Spatial distribution of CSIA results at Bandera Road suggests the presence of two or more contaminant sources. 2 Combined Dual Isotope and Dissolved Gas Analyses Used to Evaluate Nitrate Contamination at LLNL Site 300 Lawrence Livermore National Laboratory isotopes, systematically shifting the field. Costs for combined isotope and (LLNL) developed an integrated approach isotopic composition of both nitrogen and dissolved gas analysis averaged using groundwater nitrate (NO3) isotopic oxygen in residual nitrate along a approximately $600 per sample. composition and dissolved gas analyses characteristic trend line. Dissolved nitrate isotopic compositions to help identify nitrate sources and demonstrate natural attenuation at the The approach used to determine nitrate in the samples were consistent with a LLNL Site 300 Superfund site, east of San isotopic composition relied on a specific natural source (non-impacted soil and Francisco Bay, CA. The U.S. Department strain of denitrifying bacteria. The bedrock) or a septic effluent source. of Energy has conducted high explosives denitrifier approach can be two to three In addition, groundwater isotopic orders of magnitude more sensitive, less signatures differed markedly from (HE) formulation and testing at Site 300 since 1955. In the 1990s, site subject to contaminated groundwater isotopic signatures of materials used in matrix interferences such as sulfate, and onsite HE operations, including nitric acid, investigations identified nitrate in groundwater at concentrations reaching less labor-intensive than other methods for barium nitrate, and degraded RDX. isotope composition analysis. The Although the HE materials dataset is too 90 mg/L (as NO3), significantly higher than the average 30 mg/L background technique also allowed collection of sample small to be definitive, it supports volumes as small as 40 mL and determination of a non-HE source for site level at Site 300. As a result, the HE process area was studied to determine determination of nitrate isotopic nitrate contamination. composition in low-nitrate groundwater, whether elevated groundwater nitrate where denitrification effects on isotopic Groundwater sampled in the upgradient was due to degradation of past releases unconfined aquifer had high concentrations composition are most pronounced. of HE to unlined rinse-water lagoons and of nitrate and oxygen and no excess landfills, effluent from site septic Dissolved gases were measured in the nitrogen. In the downgradient confined discharge systems, or natural nitrate same set of samples to more conclusively