USING PUSH-PULL TESTS TO EVALUATE FATE AND TRANSPORT OF MIXED HYDROCARBON PLUMES (BTEX AND ETHANOL) IN AQUIFER AND WETLAND SYSTEMS

With careful interpretation, push-pull tests are a powerful in-situ tool to investigate the physical and biogeochemical controls on the fate and transport of mixed hydrocarbon plumes in subsurface systems. Using new numerical interpretation techniques (recently released code PPTEST), we can quantify processes such as sorption, enhanced mobility of contaminants, and complex rate data analysis. Understanding these and other compound-specific biodegradation rates is critical to support effective use of natural attenuation of subsurface systems where, commonly, mixed contaminants are present. This study shows the results of multiple single-well push-pull tests to quantifying the physical (ex. sorption and enhanced solubility) and biogeochemical processes (ex. microbial degradation of hydrocarbons) that occur when there exists a mix of monoaromatic hydrocarbons, principally benzene, toluene, ethylbenzene and xylenes (BTEX) and ethanol. We evaluated effects under iron-reducing and methanogenic conditions in an aquifer and wetland contaminated with crude oil in 1979 near Bemidji, MN, USA. In June 2011, four push-pull tests were conducted over 6 days in the aquifer and 13 days in the wetland environments in the presence and absence of ethanol. Push-pull tests consist of a controlled rapid injection of a test solution (conservative tracer and various combinations of mixed hydrocarbons) into a single well followed by the slow recovery of the test solution from the same well to evaluate rates of physical, microbiological, and geochemical processes. Given the increased use of ethanol as fuel, it is prudent to quantitatively understand the physical and biogeochemical effects of a combined BTEX-ethanol plume. We hypothesize that when ethanol is released to sediments coated in crude oil, the ethanol will act initially as a solvent, mobilizing oil from sediment grains. Increased concentration of BTEX compounds in the aqueous phase has implications for transport, including total plume length and maximum concentrations, as well as biodegradation. Few studies have looked at the impact of multiple competing organic compounds, such as occurs when BTEX and ethanol are both present, on overall contaminant fate. Push-pull tests are an effective tool to conduct such a controlled experiment within the natural environment.