ARCHIVED REPORTS_THIRD QUARTER 2014 GROUNDWATER MONITORING REPORT

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URS ' Mr. Durin Linderholm Regional Water Quality Control Board—Central Valley Region Page 5 of 10 ' October 30, 2014 ' 2.0 mg/L in the other site wells. This decrease in DO levels follows a comparable decrease previously observed in 2Q14 in most site wells (except MW-2-BP). Based on the quarter-to-quarter consecutive decreases in DO concentrations, it is unlikely that field measurement errors were the cause of the low DO readings. ORP measurements have remained negative during the last several sampling events in MW-2-BP, MW-6-BP, and MW-24-BP, indicating reducing conditions were established and persist in these wells. As of the 3Q14 measurements, it appears that ORP levels are now in the negative territory in all site wells, except MW-5-BP, where ORP was measured at a historical minimum of 20 millivolts (mV). with the DO levels, these ORP readings represent historically low/nAs in levels across the treatment area. The performance well MW-6-BP registered the lowest ORP reading at -223 mV, which represents the minimum ORP reading in this well since EOS was injected. Since baseline sampling, pH levels decreased fairly consistently in the performance well MW-6-BP and the transition zone well MW-24-BP but remained practically unchanged in other site wells. As of 3Q 14, PH levels have increased slightly in most wells (except MW-2-BP) when compared to 2Q14 and are ' essentially back to baseline levels. FeZ' has continued to remain somewhat elevated in most site wells, although it decreased slightly in MW-2-BP and MW-6-BP. FeZ' concentrations in these two wells and in MW-24-BP are in a range indicative of strongly reducing conditions. ' Therefore, field parameter trends point to the establishment and continuation of reducing conditions in MW-6-BP, MW-2-BP, and MW-24-BP. Based on 3Q14 field parameter measurements, MW-25-BP, and to a lesser degree the remaining site wells, also are indicating evidence of reducing conditions. The ' presence of reducing conditions for 3 years following EOS injections, points to the longevity and effectiveness of these injections under site conditions; however, unexpected. ' Figures C-1 through C-6 (Appendix C)provide time series plots showing concentrations of PCE and its daughter products (i.e., TCE, cis-1,2-dichloroethylene [DCE], and vinyl chloride [VC]). Concentrations historic low, by December 2012, approximately 20 months after EOS injectioof PCE, TCE, and DCE in the performance well MW-6-BP (Figure C-3) decreased to non-detect, the ns. Since that time, there ' has been a slight rebound in concentrations but the residual concentrations in 3Q14 represent less than 5 percent of the original mass (i.e., from a baseline CVOC concentration of 442 micrograms per liter ' [gg/L] to 21,1 µg/L). VC, the dechlorination daughter product of DCE, was first detected in MW-6-BP in December 2012 and increased to 8.6 µg/L in 1Q14, indicating reductive dechlorination of DCE was occurring. VC decreased to 3.5 gg/L in 2Q14 but increased to 7.8 µg/L in 3Q14. However, there was an overall decrease of 4.1 gg/L in the sum of concentrations of PCE, TCE, and cis-1,2-DCE during the ' same interval, indicating the increase in VC concentrations likely was due to reductive dechlorination of the parent CVOCs. Yet the VC concentrations have remained low in this well and do not account for the large reduction in DCE concentrations observed in MW-6-BP, which had a baseline concentration of 260 gg/L. This reduction could be due to two possible scenarios: (1) VC was formed but was destroyed at a faster rate than could be captured during quarterly sampling; and (2) some of the DCE concentration decrease is due to migration to downgradient areas (DCE has a higher solubility in water ' than either PCE or TCE). As mentioned above, reducing conditions also have been well-established in MW-2-BP and MW-24-BP. Figure C-1 indicates that PCE concentrations initially increased in \\SACDATAOI D l-iP]w6\Wp..s\CDCR\Dud Vownovs Iv dmtiov(Bum PW DVI-BP-3Qh 14 Report do<