SOIL VENTILATION - EFFECTS ON MICROBIAL-POPULATIONS IN GASOLINE-CONTAMINATED SUBSURFACE SOILS

Short- and long-term effects of vapor extraction (VE) in an unsaturate d subsurface soil and in situ biodegradation of gasoline were evaluate d in a field study, Subsurface temperature, moisture, solid- and gas-p hase contaminant levels, atmospheric gases, nutrient levels, and micro bial population densities were measured during and after soil VE for 4 62 d. Microbial activity, based on in situ O-2 consumption rates, meas ured 7 d after VE started averaged 3.8% O-2 d(-1); by Day 62 these Fat es dropped to 0.2% O-2 d(-1). Soil VE was stopped on Day 180 and about 70 d elapsed before renewed, low-level (0.05% O-2 d(-1)) activity was detectable. Following a second round of VE, average O-2 consumption r ates increased to 0.11% Oz d(-1). Microbial population densities did n ot consistently reflect activity changes measured by O-2 consumption. Activity increases in the latter part of the study were not adequately accounted for by changes in subsurface moisture levels, temperature, or contaminant vapor concentrations. At the study's completion, 400 kg of gasoline was volatilized from the soil and another 139 kg estimate d to be biodegraded in situ. A two-phase process is proposed to accoun t for the effects of VE on microbial activity. The initial phase is ch aracterized by declining microbial activity levels in response to subs trate reduction. Microbial activity slowly increases as a result of in teractions between gasoline vapor concentrations and possibly changes in degradative activities of the microbial population. More work is ne eded to identify the gasoline constituents serving as substrates for m icrobial populations before and after ventilation.