Laserfiche WebLink
Mr. Maurice Benson <br /> December 30, 2009 <br /> Page 3 <br /> 4.) Analytical groundwater results for vinyl chloride (VC) showed increases at <br /> MW321 B1 at the 12-month event and at MW322B2 during the 6-month sampling <br /> event. The corresponding VC concentrations increased from <0.600 pg/L to 1.50 <br /> pg/L at MW321 81 and from <0.200 pg/L to <1.30 pg/L. Ethene, the reductive <br /> dechlorination by-product of VC did not appear to increase during the pilot test <br /> indicating that a "stall" likely occurred in the breakdown of cis-1, 2-DCE. <br /> Conclusion <br /> Emplacement of EHC in the saturated soil created enhanced anaerobic reductive <br /> dechlorination of TCE and its by-products in groundwater. TCE concentrations <br /> throughout the EHC distribution area showed noticeable reductions and in one location <br /> levels decreased from 760 pg/L during baseline, to 37pg/L at the 12-month sampling <br /> event. Although the EHC reduced the contaminant concentrations throughout the pilot <br /> study area up to 95% of the initial values at select locations, the EHC did not remediate <br /> TCE or its breakdown products to the ACL of 5 pg/L. Conversely, the EHC reaction with <br /> TCE did not fully breakdown the contaminant and seemed to "stall", creating elevated <br /> concentrations of cis-1, 2-DCE up to 37 times greater than the baseline values. <br /> Monitoring at the performance monitoring wells MW321 B1 and MW322B2 is planned for <br /> first quarter 2010 (1 Q10) and 3Q10 as part of the DDJC-Sharpe groundwater <br /> monitoring program. These planned sampling efforts should determine whether the <br /> increases in VC concentrations are still occurring or remain at the 12-month levels. <br /> Dissolved metal levels increased throughout the pilot study leaving much higher <br /> concentrations of manganese, barium, boron, iron, and strontium compared to initial <br /> values. DTSC agrees with the report's recommendation that additional sampling is <br /> needed at MW321 B1 and MW322B2 to assess whether the cis-1,2-DCE stall is still <br /> occurring, and whether the dissolved metals concentrations and TDS are transitory or <br /> permanent. Additionally, the report should further discuss the "stalling" of TCE <br /> breakdown products and whether this is attributed to an insufficient amount of EHC <br /> injected in the source area, or if it is an artifact of the different biodegradation rates for <br /> various chlorinated compounds. Also, the report should discuss how if any, the <br /> dehalococcoides bacteria counts played a role in the pilot study results. <br />