Comparison of three field screening techniques for delineating petroleum hydrocarbon plumes in groundwater at a site in the southern Carson Desert, Nevada

Abstract

Three types of field screening techniques used in the characterization of potentially contaminated sites at Naval Air Station Fallon, Nevada are compared. The methods and results for each technique are presented. The three techniques include soil-gas surveys, electromagnetic geophysical surveys, and groundwater test hole screening. Initial screening at the first study site included two soil-gas surveys and electromagnetic geophysical studies. These screening methods identified localized areas of contamination, however, results were inconclusive. Monitoring well placement was postponed and groundwater test hole screening was performed. The groundwater screening consisted of auger drilling down to the shallow alluvial aquifer. Groundwater samples were collected from the open drill hole with a bailer. Onsite head-space analyses for volatile organic compounds (VOCs) was performed using a field portable gas chromatograph (GC). Five areas of floating product and the overall dissolved contaminant plume boundaries were identified. Well placement was re-evaluated and well sites were relocated based on the screening information. The monitoring wells were placed at the perimeter of the plume in locations expected to yield groundwater samples with no detectable voes. The program was successful as demonstrated by monitoring well sample results. The most effective technique for identification of petroleum hydrocarbon contaminant plumes was groundwater test hole screening. Groundwater screening was subsequently performed at 19 other sites. A total of 450 test holes were analyzed resulting in the delineation of six plumes. Comparisons of contaminated versus uncontaminated designations as determined by open hole PID measurements and field GC sample analysis revealed a 90% agreement between the PIO readings and GC results . Field GC screening results were confirmed by sending 10 duplicate samples to an independent laboratory for overnight analysis of voes. Laboratory results were consistent with the field analyses on all 10 samples. Of the 66 monitoring wells installed based on the groundwater screening results, only 2 did not fit the predicted status of either ''clean" or "contaminated" . Thus, the technique provided 97% confidence that a well could be located either within or outside of contaminant plume boundaries as desired. The technique optimized the placement of and minimized the number of monitoring wells. Cost savings were realized because fewer wells were required to define a plume. In addition, a high degree of certainty about plume boundaries and overall data quality was maintained.