Trichloroethene (TCE) concentrations of up to 930 mg/L (85% saturation) were detected in the source area groundwater of a manufacturing facility in Virginia as a result of historic site operations. Natural attenuation sampling at the site indicated the presence of TCE daughter products, volatile fatty acids and dissolved hydrogen, indicating that enhanced reductive dechlorination (ERD) can be promoted further. Published information on ERD application at near saturation conditions is rather limited. Therefore, a two-phase laboratory treatability study has been undertaken to: (1) select an appropriate electron donor; (2) determine the need for bioaugmentation; and (3) determine ERD effectiveness at high TCE concentrations.
Groundwater samples were collected from two monitoring wells one in the source area (MW-2S) and one just outside of the source area (MW-18S). TCE concentrations in these two wells were 140 and 930 mg/L, respectively. Seven microcosms were prepared using the groundwater from MW-18S (one killed control, three bioaugmented and three un-augmented). Bioaugmentation was achieved using Bioremediation Consulting, Inc.(BCI) mixed culture consisting of TCE degraders, sulfate reducers, fermenters, and methanogens. Each of the three augmented and un-augmented pairs received one of the three electron donors tested (sodium lactate, a refined soy oil product, and corn syrup). In addition, all microcosms except the killed control were also amended with sulfate, ammonia-nitrate, phosphorus, yeast extract, B12 and trace minerals. The microcosms were monitored for TCE daughter products, dissolved hydrogen, and volatile fatty acids at regular intervals over a 90-day period. Significant reduction of TCE or sulfate was not observed in the un-augmented microcosms. Complete conversion of TCE to daughter products was achieved within 60 days by all three donors in the bioaugmented microcosms. By day 111, 140 ppm TCE had been converted to 75% ethene, 15% VC and 10 % cis-DCE. The data also indicated that soy oil supported faster dechlorination, while sodium lactate supported faster sulfate reduction. Soy oil did not produce significant propionate unlike the other two electron donors. Based on these results, soy oil with lactate accelerator was selected as the electron donor for the second phase of the investigation.
In the second phase, groundwater from MW-18S and MW-2S were mixed in certain ratios to set up two additional microcosms with 400 and 800 PPM of TCE. The second phase is primarily designed to investigate the effectiveness of ERD at higher TCE concentrations. The microcosms completed 60 days and will be maintained for a total period of 120 days. Both microcosms were initially inoculated with material from the first phase MW-18S microcosm; however, additional bioaugmentation with the BCI culture was necessary. Day 42 analyses indicated 100% and 0% sulfate reduction in the 400 and 800 PPM microcosms, while TCE reduction was 0% and 12%, respectively. At Day 61, the 800 PPM microcosm exhibited 50% sulfate reduction, but TCE reduction did not improve. On the other hand, the 400 PPM microcosm exhibited comparable TCE reduction (11%) at Day 61. Based on the data available to date, it appears that TCE reduction followed sulfate reduction at lower concentrations. Such clear trend is not apparent at higher concentrations. In general, TCE reduction appears to be slower at higher concentrations.