ARCHIVED REPORTS_THIRD QUARTER 2014 GROUNDWATER MONITORING REPORT

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1URS ' Mr. Durin Linderholm Page 6 of 10 Regional Water Quality Control Board—Central Valley Region ' October 30, 2014 ' MW-2-BP through 3Q13 (to a high of 100 gg/L), then declined from 4Q13 through 2Q14 (when the PCE concentration was 0.7 µg/L) but rebounded slightly to 2.7 µg/L in 3Q14. There was also a slight increase in TCE (0.9 µg/L to 2.2 µg/L) and an increase in DCE (1.7 lig/L to 6.7 µg/L)in this well from 2Q14 to 3Q14. However, with the exception of these low-level TCE and DCE concentrations, PCE daughter products never increased to levels that would indicate all the PCE in the well had been reductively dechlorinated. Therefore, a reduced mass flux from the upgradient treatment area and ' migration of CVOC mass from the well may be at least partially responsible for the PCE concentration reductions in this well. By contrast, Figure C-5 indicates a decline in PCE concentrations in MW-24-BP in 3Q 12, concurrent with a large increase in DCE concentrations through 3Q]3. This increase may have been due to migration from the upgradient treatment area, reductive dechlorination, and desorption of CVOCs bound to soil. As of 4Q13, however, DCE concentrations in the well decreased from a high of 130 gg/L to 16 ltg/L in ]Q14, with no apparent VC formation. There has since been a continuing ' rebound trend in DCE concentrations to 24 lig/L in 2Q14 and to 33 pg/L in 3Q14 but VC remains undetected. Molar concentration trends (i.e., micromoles per liter, [µmoUL] versus time) also can be useful for ' evaluating destruction of CVOCs. Figure C-8 depicts these trends for MW-6-BP, showing that as of 3Q14, total CVOCs decreased from a baseline concentration of approximately 3.9 µmoUL to 0.26 µmoUL. However, DCE and VC now make up the bulk of the molar composition, compared to the ' baseline event, when PCE, TCE, and DCE comprised 22 percent, 8 percent, and 70 percent, respectively, of the molar composition. This observation supports the hypothesis that the reductions, especially in PCE and TCE concentrations, in MW-6-BP are primarily due to destructive microbial processes. With regards to DCE concentration reductions, however, partial migration to other site wells, specifically MW-24-BP, cannot be ruled out. Based on hydraulic conductivity of site-specific soils (assumed to be between 10e-06 and 10e-02 centimeters per second), the historic groundwater flow directions, typical hydraulic gradient (historically about 0.001 foot/feet), an assumed effective porosity of 0.25 based on site soils, and a retardation factor of DCE compared to groundwater seepage velocity of approximately 2 (assuming an average soil fraction of organic carbon of 0.005 and an organic carbon-water partition coefficient for DCE of approximately 40 grams per cubic centimeter), it is ' possible that DCE could have moved not only to MW-24-BP but downgradient from there up to 8 feet (in the general southeast direction), up to 35 feet (in east-southeast and south directions), and up to 180 feet (in the northeast direction) over the 3 years since the FOS injection. Note that there is inherent ' uncertainty in these travel distance estimates because of the assumptions made regarding the soil properties. However, using these estimates, URS has proposed an area of shallow soil gas investigation, followed by installation of soil borings and monitoring well(s)to finther characterize CVOC presence in the subsurface downgradient of the treatment area (URS, 2014). Figure C-9 indicates that the total CVOC molar concentration in MW-24-BP started increasing roughly at the same time when concentrations in MW-6-BP decreased to trace levels. Additionally, the molar composition in ' MW-24-BP consists primarily of DCE, indicating a large portion of the DCE may have originated from MW-6-BP. 1\SACDATA0J0miplme6WpmCms%CDCR0eud VOMOUS msfimdou(Bum PuYM-BP-3QM 14 Repos doe