TPH Criteria Working Group Demonstration Field Sampling Report: Marine Corps Air Station, El Toro, CA

A demonstration of the Total Petroleum Hydrocarbon Working Group approach for risk assessment was conducted on two former underground storage tank sites (Sites 380A and 529) at the Marine Corps Air Station in El Toro, CA. Site 380A is impacted by diesel fuel and Site 529 is primarily impacted by heating oil, as well as some lighter fuels. Samples were collected in side-by-side brass sleeves and submitted to different laboratories for fractionation (Direct Method) analysis and conventional total petroleum hydrocarbon (TPH) analysis, specifically EPA Modified 8015. The results of the conventional method were consistently two to three times higher than the results from the fractionation analysis. In addition, the Direct Method yielded volatile aromatic fractions concentrations that differed from the benzene, toluene, ethylbenzene and xylene (BTEX) analyses (EPA Method 8020). The reason for the discrepancies are not clear but may be due to intralaboratory variability. The RBSLs developed for both sites were higher than current state criteria with the exception of the RBSLs for the indoor air pathway. It was found that detection limits, particularly for the lightest aromatic fractions (EC5-8), dramatically affect the RBSLs pathways requiring partitioning. In effect, by using half the detection limit, nondetects in the EC5-8 aromatics contributed 20% of the risk for the indoor air pathway. The effect on the leaching pathway was similar. The sensitivity of the RBSLs to EC5-8 detection limits warrants additional BTEX analysis as part of the protocol for assessing risks.