INVESTIGATING TRAPPED GAS PROCESSES IN HYDROCARBON-CONTAMINATED GROUNDWATER

Microbial degradation of hydrocarbons in groundwater plumes can lead to the production of dissolved gases, and result in the formation, growth and subsequent vertical mobilization of a trapped gas phase. In this study, temporal and spatial changes in trapped gas saturations were investigated during the biodegradation of ethanol, methyl tert-butyl ether (MTBE) and toluene, representing a generic petroleum and/or gasohol plume composition. The experiment was performed in a highly-instrumented, pilot-scale laboratory tank (80 cm wide x 525 cm long x 175 cm deep), simulating an unconfined sand aquifer with a residual gas phase following a water table fluctuation. Dissolved hydrocarbon contamination was injected into the aquifer over 330 days at a constant rate. In situ Time Domain Reflectometry (TDR) probes directly measured changes in the trapped gas phase. Additionally, various sampling was performed for groundwater geochemistry and dissolved gas composition, while dissolved oxygen and total dissolved gas pressure (P_TDG) were monitored continuously via probes in a series of wells. Denitrification was the dominant gas-producing reaction initially, with methane generation commencing after an approximate 80-day acclimation period. Following this, dramatic increases in trapped gas were observed; however, maximum trapped gas saturations never exceeded 27% of the pore volume even during continued methane generation, suggesting active vertical mobilization of the gas phase. Further details on the nature of gas production and its implications for groundwater flow and contaminant transport will also be discussed.