HYDRAULIC RESPONSE TO EMULSIFIED VEGETABLE OIL BIOSTIMULATION: IN-SITU TEST IN A HIGHLY HETEROGENOUS URANIUM CONTAMINATED AQUIFER

The purpose of this study was to determine if the injection of emulsified vegetable oil (EVO) to remediate a uranium-contaminated aquifer can result in a reduction in hydraulic conductivity. The secondary purpose was to determine if there was evidence of a “memory effect,” a phenomenon where the second time an electron donor is injected, the environment responds to it faster. This has been observed at many remediation sites. A previously treated (2009) uranium contaminated aquifer at Y-12 National Security Complex located in Oak Ridge, Tennessee, was injected with EVO to determine whether hydraulic conductivity changes and to assess the effectiveness of EVO treatment for reducing dissolved uranium. Acetate was monitored in downgradient wells as an indicator of biodegradation. On December 13, 2017, a 20% EVO and groundwater mixture was injected within the Y-12 FRC Area 2 site. Periodic measurements of hydraulic conductivity and dissolved uranium concentration were taken from a control wells, three injection wells, and four down-gradient wells for 134 days. During the experiment, hydraulic conductivity in the injection wells decreased by up to two orders of magnitude but only up to one order of magnitude in half of the down-gradient monitoring wells located 2.5 to 11 m away. Dissolved uranium concentrations significantly decreased in the injection wells, but not in the monitoring well directly down-gradient of injection because dissolved uranium concentrations increased by day 78 and surpassed pre-injection concentrations due to oxidation of reduced uranium in those wells. Acetate concentrations indicated an accelerated response to EVO compared to the 2009 study results. However, this was the only evidence of “memory response.” The results of this study show that injecting EVO can have unintended consequences related to hydraulic conductivity, which can reduce EVO effectiveness or even cause bioremediation using EVO to fail. The effects of EVO interacting with aquifer media and injection well spacing should be carefully considered to minimize changes in preferential flow, limit oxidation of reduced uranium, and maximize the effectiveness of the treatment.