--> Abstract: Geochemical and Hydrologic Characterization to Identify the Source of Low Level Benzene in the Chicot Aquifer Groundwater, by Henry B. Kerfoot, Warren Brady, Bill Schramm, and Mark A. Allendorf; #90069 (2007)

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Geochemical and Hydrologic Characterization to Identify the Source of Low Level Benzene in the Chicot Aquifer Groundwater

Henry B. Kerfoot1, Warren Brady2, Bill Schramm3 and Mark A. Allendorf4
1 Geosyntec Consultants, 2100 Main St., Ste. 150, Huntington Beach, California 92648
2 Geosyntec Consultants, 5420 Corporate Blvd., Ste. 303, Baton Rouge, Louisiana 70808
3 Louisiana Department of Environmental Quality, P.O. Box 4314, Baton Rouge, Louisiana 79821
4 Allied Waste Industries, Inc., 1001 S. Dairy Ashford Rd., Ste. 400, Houston, Texas 77077

A case study describing how multiple lines of geochemical evidence were used to identify the source of benzene detected in groundwater samples at concentrations above the federal maximum contaminant level (MCL) of 5 μg/L is presented. These wells are located in the vicinity of a municipal solid waste landfill, situated within a larger region that has been widely explored and developed for oil and gas resources. The affected monitoring wells were limited to two located over 1000 ft hydraulically upgradient of the waste placement area, with no benzene detections in samples from wells located downgradient of the waste. Although benzene concentrations were not correlated with increases in chloride concentrations, the site leachate had relatively low chloride concentrations (approximately 30 mg/L) and dissolved methane was present in the samples with benzene. The approach used for the evaluation was to evaluate groundwater data for the geochemical fingerprint of the three potential fluids that would transport the benzene from one of three potential sources of (a) landfill leachate, (b) landfill gas, and (c) field gas (unprocessed natural gas). Multiple forms of geochemical data from site leachate and landfill gas along with literature information on natural gas were used in combination with geochemical and hydrogeologic data for the affected groundwater to identify the most likely source of the detected benzene. The parameters considered included 14C (Carbon-14), 13C (Carbon-13) / Deuterium, Tritium, C2 and C3 alkanes, dissolved CO2, and the direction of disequilibrium between groundwater and the gas phase. The results of all of the multiple evaluations were consistent with the natural gas as the source of the benzene, with no evidence of landfill gas or landfill leachate effects on the groundwater samples. The groundwater tritium levels were less than 2 Tritium Units, inconsistent with the landfill leachate tritium level of over 1200 Tritium Units, the methane 14C content was 2 percent Modern Carbon (pMC), inconsistent with the landfill gas methane 14C content of 125 pMC. The 13C and deuterium of dissolved methane was consistent with thermogenic (natural gas) methane, the presence of dissolved C2 and C3 alkanes was consistent with natural gas and not landfill gas. The lack of a correlation between dissolved CO2 and benzene was inconsistent with landfill gas but consistent with natural gas, and the direction of methane partitioning (water-to-gas) showed that the methane source was below the water table, consistent with natural gas.

 

AAPG Search and Discover Article #90069©2007 GCAGS 57th Annual Convention, Corpus Christi, Texas