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<br /> Ms.Kathy Amaru Page 5 of 9
<br /> Regional Water Quality Control Board—Central Valley Region
<br /> April 30, 2014
<br /> ' Possible reducing conditions were also observed in MW-25-BP in December 2013, when ORP levels
<br /> decreased to -34 millivolts; however, ORP increased back up to 38 millivolts in 1Q14. The ORP
<br /> measurements in the remaining two wells, (i.e., MW-5-BP and MW-7-BP) have remained positive
<br /> throughout the pilot test. Since baseline sampling, pH levels decreased consistently in MW-6-BP and
<br /> MW-24-BP but remained practically unchanged in other site wells,although as of 1Q14,pH levels have
<br /> decreased slightly in most wells. Fe" has been increasing in MW-6-BP, MW-2-BP and MW-24-BP
<br /> compared to baseline.
<br /> Therefore, field parameter trends point to the establishment and continuation of reducing conditions in
<br /> MW-6-BP, MW-2-13P, and MW-24-13P. MW-25-BP also has shown some evidence of reducing
<br /> conditions. The presence of reducing conditions for nearly 3 years, following EOS injections, points to
<br /> the longevity and effectiveness of these injections under site conditions.
<br /> Figures C-1 through C-6 (Appendix C) provides time series plots showing concentrations of PCE and
<br /> its daughter products (i.e., TCE, cis-l,2-dichloroethylene [DCE], and vinyl chloride [VC]).
<br /> Concentrations of PCE, TCE, and DCE in MW-6-BP (Figure C-3) had decreased to historically low
<br /> ' levels by December 2012, approximately 20 months after vegetable oil injection. Since that time, there
<br /> has been a slight rebound in concentrations (fable 3),but the residual concentrations in 1Q14 represent
<br /> only 6 percent of the original mass (i.e., from a baseline CVOC concentration of 442 micrograms per
<br /> ' liter [gg/L] to 26.6 gg/L). VC, the dechlorination daughter product of DCE, was first detected in MW-
<br /> 6-BP in December 2012 and has increased to 8.6 gg/L in 1Q14, indicating reductive dechlorination of
<br /> DCE was occurring. However, the slight increase in VC concentrations does not account for the large
<br /> reduction in DCE concentrations. This reduction could be due to two possible scenarios: (1) VC was
<br /> formed but was destroyed at a faster rate than could be captured during quarterly sampling; and (2)
<br /> some of the DCE concentration decrease is due to migration to downgradient areas (DCE has a higher
<br /> solubility in water than either PCE or TCE).
<br /> As mentioned above, in addition to MW-6-BP, reducing conditions have been well-established in
<br /> MW-2-BP and MW-24-BP. Figure C-1 indicates that PCE concentrations actually increased in
<br /> ' MW-2-BP through 3Q13, but there was a sharp decline in PCE concentrations in 4Q13, which has
<br /> continued into 1Q14, with PCE at 5 gg/L. However, with the exception of low-level TCE and trace
<br /> levels of DCE, daughter products are not present in the well, indicating a reduced mass flux from
<br /> ' upgradient locations and mass migration from the well may be at least partially responsible for the
<br /> concentration reductions. By contrast, Figure C-5 indicates a decline in PCE concentrations in
<br /> MW-24-BP in 3Q12, concurrent with a large increase in DCE concentrations through 3Q13. This
<br /> ' increase may have been due to migration from upgradient areas, reductive dechlorination, and
<br /> desorption of CVOCs bound to soil. As of 4Q13, however, DCE concentrations in the well decreased
<br /> from a high of 130 gg/L to 16 gg/L in 1Q14, with no apparent VC formation. Therefore, the recent
<br /> decrease in concentrations partially could be due to mass migration away from the well.
<br /> ' Molar concentration trends (i.e., micromoles per liter, [gmol/L] versus time) also can be useful for
<br /> evaluating destruction of CVOCs. Figure C-7 depicts these trends for MW-6-BP, showing that total
<br /> ' CVOCs decreased from a baseline concentration of approximately 3.9 gmol/L, to 0.31 gmol/L.
<br /> However, DCE and VC now make up the bulk of the molar composition, compared to the baseline
<br /> event, when PCE, TCE, and DCE comprised 22 percent, 8 percent, and 70 percent, respectively, of the
<br /> ' molar composition. This observation supports the hypothesis that the reductions, especially in PCE and
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