Condensate liquids have been found to contaminate soil and groundwater at two gas production sites in the Denver Basin operated by Amoco Production Co. These sites have been closely monitored since July 1993 to determine whether intrinsic aerobic or anaerobic bioremediation of hydrocarbons occurs at a sufficient rate and to an adequate end point to support a no-intervention decision. Groundwater monitoring and analysis of soil cores suggest that intrinsic bioremediation is occurring at these sites by multiple pathways, including aerobic oxidation, Fe(III) reduction, and sulfate reduction.
Accurate delineation of the extent of subsurface hydrocarbon contamination in soils and ground water is important when initiating a monitoring plan or considering remediation options at E&P sites. Traditional site-assessment techniques used to delineate subsurface hydrocarbon contaminants (e.g., soil boreholes, excavation, monitor well installation, etc.) can be expensive, time-consuming, and disruptive to local land use or production operations. Soil gas surveys can provide a rapid, cost-effective, nonobtrusive alternative to traditional site-assessment techniques. Introduction Accurate delineation of the extent of subsurface hydrocarbon contamination in soils and ground water is important when initiating a monitoring plan or installing a remediation system. Traditional site-assessment techniques utilized (soil boreholes, excavation with backhoe, monitor well installation, etc.) can be time-consuming, expensive, and/or disruptive to local land use. Soil gas surveys can provide a rapid, cost-effective, unobtrusive alternative to traditional site-assessment techniques. Soil gas surveys are based on the following principles:Most subsurface hydrocarbon contaminants have a volatile component (VOCs) which is present in soil gases; andAerobic (oxygen-consuming) bacteria associated with subsurface hydrocarbon contaminants consume oxygen and produce carbon dioxide more rapidly than bacteria present in uncontaminated, background soils. Consequently, soil gases associated with subsurface hydrocarbon contamination will typically have elevated VOCs carbon dioxide (CO2) levels and decreased oxygen (O2) levels when compared to background, uncontaminated soil. These soil gas constituents can be inexpensively and rapidly sampled and measured in the field. If the hydrocarbon contaminants have a significant volatile constituent, measurement of soil gas O2 and CO2 levels may not be necessary. However, soil gas O2 and CO2 measurements provide independent data which increase the confidence level of the soil gas survey. This paper describes how soil gas surveys can be used as a site-assessment technique and the application of this technique at two production sites. Site characteristics which make soil gas surveys either practical or impractical as a site-assessment technique are also discussed. Site Descriptions Soil gas surveys were used to delineate the extent of subsurface hydrocarbon contamination at two production sites in the Ft. Lupton, Colorado, area. The contamination at both sites was the result of the chronic leakage of gas condensate from a concrete sump over a period of years. The concrete sumps at both sites have been removed and replaced with fiberglass tanks. Contaminated soil immediately adjacent to the concrete sumps was removed to a depth of approximately 8 to 10 ft. The Brewer Pooling Unit No. 2 site is characterized by a sandy clay loam top soil which is approximately 1 to 1.5 ft thick. Beneath the top soil is a homogenous gravelly sand alluvium to a depth of at least 16 ft. Depth to ground water at the site varies from approximately 4 to 5 ft. At the Kuipers Pooling Unit No. 2 site, topsoil is a sandy loam, although a sandy gravel road fill has been spread over a significant area of the site. Beneath the topsoil/road fill sediment type varies from a gravelly sand to silt-rich sand horizons. Depth to ground water varies from approximately 2 to 4 ft in the area where the soil gas survey was performed. Soil Gas Samples At the Brewer Pooling site, soil gases were sampled and measured from a subsurface depth of 2.5 ft at 15 locations on the site. At the Kuipers Pooling site, soil gases were sampled and measured from subsurface depths of 1 to 2.5 ft at 22 locations. Soil vapors were sampled with the AMS Soil Gas Vapor Probe (SGVP; Forestry Suppliers, Jackson Mississippi). P. 749
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