GAS GENERATION IN SAND COLUMNS CONTAMINATED WITH SMALL VOLUME RELEASES OF ETHANOL

Ethanol is increasingly being used as biofuel and fuel oxygenate, and thus, its potential release into the subsurface has generated interest for investigating biodegradation and remediation of ethanol in aquifers and the vadose zone. One aspect of potential significance is that ethanol, which is highly soluble and easily biodegradable, may lead to substantial gas production, including methane (CH₄). This may have detrimental effects, as CH₄ may accumulate below low permeability layers or under sealed ground surfaces. Moreover, the formation and exsolution of CH₄ in the saturated zone may enhance the upwards transport of gas bubbles (i.e., by ebullition) into the overlying vadose zone, possibly increasing the CH₄ content of the soil atmosphere. Our study aims to investigate the potential for gas generation and biodegradation of ethanol under natural attenuation conditions. A series of experiments was conducted in acrylic columns that contained a stagnant volume of water and Ottawa sand, varying amounts of ethanol (0.00%, 0.10%, and 0.15% by volume), and small amounts of methanogenic organic rich sediment for inoculation. The columns were monitored for approximately 2.5 months under anaerobic conditions, and the gas composition (e.g., CH₄, CO₂, O₂, N₂ and Ar) in the headspace was quantified using a mass spectrometer. Following ethanol injection, gas analyses showed a steady increase in CH₄ concentration in ethanol-amended columns, indicating that microbial organisms have a significant potential for natural attenuation of ethanol in contaminated aquifers, and that gas exsolution and gas transfer to the vadose zone occur. The composition of gases in the headspace as a function of time and the rate of CH₄ production will be addressed.