An Approach to Differentiate Gasoline and Non-Gasoline Sources of TBA inGroundwater

摘要

Detection of TBA in groundwater at fairly high concentrations (tens to hundreds of mg/L) at a number of sitesrnhas raised concerns about the potential sources of TBA. One obvious possibility is that the TBA in the groundrnwater was originally present in reformulated gasoline as an oxygenate or as a trace impurity in the MTBErnblended with gasoline. Another possibility is biological transformation of MTBE to TBA. This paper presentsrna data analysis approach to convincingly differentiate TBA originally present in a release of gasoline from TBArnproduced in ground water by MTBE transformation. Using equilibrium gasoline-water partitioningrncalculations, it is possible to predict the aqueous equilibrium concentrations of MTBE and TBA from therncontent of MTBE and TBA originally present in the gasoline and from the ratio of gasoline to water in contact.rnBy analyzing recently published literature data on a number of gasoline-water partitioning experiments, it wasrnshown that trace levels of TBA (< 0.1% v/v) in gasoline could result in very high TBA concentrations inrngroundwater. A ratio of gasoline to water was assumed that is a reasonable upper boundary on the content ofrngasoline at residual saturation in a sand aquifer. The partitioning calculations were used to generate a cross-plotrnof expected concentrations of TBA and of MTBE on a log-log scale. The calculations bound a region in therncross plot where TBA can reasonably be attributed to TBA emanating from gasoline dissolution (assumingrngasoline/water ratio = 1 and TBA/MTBE ratio in gasoline between 0.02 and 0.2). When the TBA-MTBE datarnlie above this region, at least some portion of the TBA in groundwater can reasonably be attributed to MTBErntransformation in groundwater. This approach was validated using data from two sites where there was directrnevidence of MTBE biotransformation to TBA. This cross-plot could also provide a qualitative indication of thernextent of in-situ transformation of MTBE to TBA, if the range of concentrations of TBA and MTBE originallyrnpresent in the gasoline used at the site is known.rnThis is an abstract of a proposed presentation and does not necessarily reflect EPA policy.