Geochemical Reactions During Biodegradation/Vapor‐Extraction Remediation of Petroleum Contamination in the Vadose Zone

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Abstract
Hydrocarbon concentrations in soil water and vapor are generally used to evaluate the progress of biodegradation and vapor extraction of petroleum contamination in the unsaturated zone. This study shows that changes in the inorganic composition of vadose zone water samples can be used to evaluate the reactions that occur in the unsaturated zone during such a remediation effort. Chemical analyses were completed on water samples collected from alluvial sediments contaminated with diesel fuel and gasoline at the Gallatin Farmers Cenex, Belgrade, Montana. The samples were collected from 7 suction lysimeters for 3 months after fertilization, but before vapor extraction, and then for 6 months following the start of vapor extraction. The geochemical reaction progress code SOLMINEQ.88 is used to calculate the aqueous equilibria in the samples and to simulate possible reaction pathways.Reduction in TPH and BTEX concentrations indicated that biodegradation of the petroleum began after fertilization, prior to vapor extraction, and continued after the start of vapor extraction. SO4=, HCO3‐, pH, and PCO2 show large systematic variations with both time and depth. These variations are independent of evaporation, mixing, sample extraction time, and soil moisture content and thus appear to be a direct consequence of bioremediation and vapor extraction. PCO2 is found to be a measure of the effectiveness of vapor extraction. The chemical mass transfer calculations also show that if vapor extraction occurs alone removing CO2 from solution without coupling of a process to buffer the solution pH, large amounts of carbonate minerals could precipitate, significantly reducing sediment permeability. These data suggest that analyses of inorganic compounds in lysimeter samples can be used to evaluate geochemical changes during vadose zone remediation and can be used to improve remediation design.