WORK PLANS_2006-2007 - Laserfiche WebLink

Home Browse Search

LFR Inc. vicinity of the proposed ERD treatment zone. These results indicate that conditions are generally anaerobic within the core plume in the B zone and more aerobic in the A zone. 3.2.4 Summary of Geochemical Results The geochemical sampling conducted during May 2007 provides evidence that the natural geochemical conditions in the B zone favor reductive dechlorination of the contaminants of concern; specifically, the following lines of evidence support biotic reductive dechlorination: low DO, negative ORP values, relatively elevated ferrous iron, the presence of ethane and ethene, and the presence of significant concentrations of PCE daughter products (TCE and cis-1,2-DCE). In the A zone, the geochemical conditions indicate that the geochemistry is not at present as favorable for reductive dechlorination by anaerobic processes as the B zone. This conclusion is based on the following lines of evidence indicating the A zone is aerobic or weakly anaerobic: DO (>1.0 mg/1), ferrous iron (<0.10 mg/1), ethane (<0.025 ttg/1), and the relative low ratio of degradation products compared to the concentration of PCE in A-zone groundwater. Based on these geochemical results, the proposed ERD injection in the A zone will likely yield results favorable for reductive dechlorination in the A zone. 3.3 Evaluation of cis-1,2-DCE Stall Several different approaches can be used to determine if the microbial community at a site is capable of complete dechlorination of PCE, TCE, and cis-1,2-DCE to ethene. Newly developed laboratory techniques are available to determine if Dehalococcoides- like organisms are present at a site and if they have the required genes to reduce vinyl chloride to ethene. In LFR's experience at similar sites, Dehalococcoides numbers may initially be very low or patchy due to the absence of a suitable electron donor. Also, once a biodegradable organic substrate is added (e.g., emulsified food-grade oil), the number of dechlorinators may increase rapidly, resulting in rapid and complete dechlorination of PCE and TCE to ethene. For this reason, it is preferable to rely on the presence of vinyl chloride, ethane, and ethene, even at low levels, as an indication that the indigenous microbial community is capable of complete dechlorination. The groundwater results for the 25 monitoring wells sampled in the A-zone aquifer during the December 2006 event indicated that cis-1,2-DCE was detected in 14 of the 25 groundwater samples. The cis-1,2-DCE concentrations ranged from 0.27 J µg/I in MW-116A to 51 µg/1 in MW-012A, with the exception of MW-113A, which had a cis- 1,2-DCE concentration of 260 µg/l. The cis-1,2-DCE concentrations in MW-113A and MW-114A have detected PCE and TCE concentrations that are the same order of magnitude. In MW-012A, the cis-1,2-DCE concentration is higher than the PCE and TCE concentrations; however, vinyl chloride was detected at a concentration of 0.42 lAg/1, indicating dechlorination past cis-1,2-DCE. In addition, the ethane and ethene detections from the May 31, 2007 Geochemical sampling in B-zone well MW-020B, wp-PhV_IRASep07-Fhu1-06750.dm:1fr Page 13