LONG-TERM EVALUATION OF VOLATILE ORGANIC COMPOUND DEGRADATION DUE TO BIOAUGMENTATION IN A FRACTURED-ROCK AQUIFER

An in situ bioaugmentation experiment to stimulate the degradation of volatile organic compounds (VOCs) was initiated at the Naval Air Warfare Center fractured-rock research site in West Trenton, New Jersey in October 2008 by injecting emulsified soybean oil as an electron donor and a consortium of bacteria. Groundwater flow from the injection well was predominantly along fractures within mudstone beds toward a pump-and-treat withdrawal well located 45 meters downgradient. Baseline geochemical conditions were established during the six months prior to injection by analyzing samples for VOCs, bacteria, inorganic constituents, and dissolved gases from the injection and withdrawal wells and from two multi-level observation wells located along a flow path. Following injection, the same constituents were monitored in these wells for six years to assess the effects of the bioaugmentation.

From 2008-2014, reductions of two orders of magnitude were found in trichloroethene (the original contaminant) concentrations in groundwater from the injection well and in a monitoring well 18 meters downgradient. In water from these same wells, 1-2 orders of magnitude increases were found in concentrations of dechlorinating bacteria, cis-1,2-dichloroethene, vinyl chloride, ethene, and chloride, indicating that reductive dechlorination had been stimulated.

Because production of chloride occurs on a 1:1 molar basis with the degradation of each chlorinated VOC, increased chloride concentrations after injection were used as a measure of increased VOC degradation caused by the bioaugmentation. Chloride increases measured in the injection well indicated that bioaugmentation had increased VOC degradation by a maximum factor of 4.9 times that of natural biodegradation after 7 months. Chloride increases measured in the monitoring well 18 meters downgradient indicated that bioaugmentation had increased VOC degradation by a maximum factor of 6.3 times that of natural biodegradation after 3.8 years. The increased degradation due to bioaugmentation was still measureable in these wells after 5.8 years.