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Soil Ventilation: Effects on Microbial Populations in Gasoline-Contaminated Subsurface Soils

Department of Soil Science, Univ. of Wisconsin, Madison, WI 53719.

^* Corresponding author (wjhickey{at}vms2.macc.wisc.edu).

ABSTRACT

Short- and long-term effects of vapor extraction (VE) in an unsaturated subsurface soil and in situ biodegradation of gasoline were evaluated in a field study. Subsurface temperature, moisture, solid- and gas-phase contaminant levels, atmospheric gases, nutrient levels, and microbial population densities were measured during and after soil VE for 462 d. Microbial activity, based on in situ O₂ consumption rates, measured 7 d after VE started averaged 3.8% O₂ d^–1; by Day 62 these rates dropped to 0.2% O₂ d^–1. Soil VE was stopped on Day 180 and about 70 d elapsed before renewed, low-level (0.05% O₂ d^–1) activity was detectable. Following a second round of VE, average O₂ consumption rates increased to 0.11% O₂ d^–1. Microbial population densities did not consistently reflect activity changes measured by O₂ 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 estimated to be biodegraded in situ. A two-phase process is proposed to account for the effects of VE on microbial activity. The initial phase is characterized by declining microbial activity levels in response to substrate reduction. Microbial activity slowly increases as a result of interactions between gasoline vapor concentrations and possibly changes in degradative activities of the microbial population. More work is needed to identify the gasoline constituents serving as substrates for microbial populations before and after ventilation.