Source Zone Delineation Demonstration Report - Triad Resource ...

20Porosity = 30%NAPL Dilution Factor161284Moisture = 5 to 20 vol%foc = 0.002 mg/mgMoisture = 10 vol%foc = 0.001 mg/mg00 100 200 300 400 500TCE Sample Concentration (mg/kg)Figure 7-3. NAPL dilution factor as a function of TCE sample concentration.7.3 RecommendationsDuring the OU 12 soil investigation, a separate LNAPL phase was identified in the upper portions of thevadose zone located in the suspected source area adjacent to vapor extraction wells U12-VP1 and U12-VP2. Soil samples that exhibited LNAPL also were shown to have elevated concentrations of TCE. Themodeling of TCE contamination in the soil illustrated that the highest levels of soil contamination arelocated near the top of the screened intervals of U12-VP1 and U12-VP2. In addition, the upper nine feetof the screened interval in both wells are located in an extensive silty-sand region, similar to the highestsoil contamination. These observations are manifest in the high TCE concentrations observed in theextracted vapor from U12-VP1 and U12-VP2. Because the measured concentration of TCE in theextracted soil vapor corresponds to an average vapor concentration for the entire length of the screenedinterval, it is impossible to delineate the relative contribution to the total measured contaminationsupplied by the various soil intervals spanned by the screen. However, due to the close proximity of U12-VP1 and U12-VP2 to high soil contamination and the local soil stratigraphy of their screened intervals, itis inferred that the upper few feet of the intervals (typically 30 to 40 feet bgs) for these wells yielded thebulk of the contaminated vapor.Thus, it is recommended that vapor extraction operations be conducted using the existing extraction wellsU12-VP1 and U12-VP2 to evaluate contaminant mass (including both petroleum hydrocarbon andchlorinated ethenes) removal over a 30-day period. Results obtained from the 30-day investigationshould be evaluated to determine whether continued vapor extraction operations are beneficial forreduction of source zone contamination. In addition, it is recommended that any additional investigationsshould focus on determining the presence or absence of DNAPL in the saturated zone, and evaluating thepotential presence of contaminant mass in the vicinity of the two geophysical anomalies previouslyidentified to the northeast and southwest of the excavated drum area.March 2003 7-7 OU 12 Demonstration ReportFinal