Comparisons of Geochemical Signatures of Biotransformation of Fuel Hydrocarbons in Groundwater

Abstract

Biotransformation processes play an active role in reducing the environmental impact of fuel hydrocarbon releases to groundwater. Because monitoring data at release locations are typically sparse, spatial variations in geochemical indicator parameters are often called upon as indirect evidence of biotransformation. These parameters include concentrations of electron acceptors (O2, NO3-, SO> 4 2- , reduced redox reaction by-products (Fe2+, Mn2+, CH4), as well as bicarbonate alkalinity, pH and Eh. However, background variability in a number of these parameters complicates the task of data interpretation, particularly in the case of small data sets. In this study, correlation analyses are applied to geochemical indicator data at six hydrocarbon groundwater contamination sites in California to identify which parameters are the most reliable indicators. The results of the analyses suggest that the most direct indicators of the local redox environment – Fe2+, Mn2+, CH4, Eh – yield the most consistent evidence of hydrocarbon biotransformation. Indicators which rely largely on mass balance – O2, NO 3 - , SO 4 2- , alkalinity – appear to be less reliable. These findings may provide guidance in both the collection and interpretation of groundwater monitoring data at hydrocarbon contamination sites.