ARCHIVED REPORTS_20100427 ATC 1Q2010 GWMR COMPLETE

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Ms.Amy Terrell April 27,2010 Page 5 of 10 Risk Information System (IRIS) reference dose for drinking water (14 µg/L). The highest diuron concentration was reportedly 50 µg/L in the groundwater sample collected from well MW-2 in March 1997. • Well MW-6R is the intergrid monitoring well for the Expanded Pilot Test. Well MW-6R had the highest concentrations of dissolved phase pesticide and volatile organic compound concentrations detected during the March 2010 groundwater sampling event. Historical results for these constituents of concern (COC) are tabulated in Table 5 and graphically presented on Figure 8. o TON—MW-6R The TON concentration was observed to be 116 mg/L. This concentration exceeded the MCL of 10 mg/L. TON concentrations have generally been increasing in groundwater samples collected from well MW-6R since 2007 o Ammonia MW-6R—MW-6R The ammonia concentration was observed to be 347 mg/L. Ammonia concentrations have been relatively consistent since 2007. There is currently no established MCL for ammonia. 0 1,2,3-TCP and 1,2-DCP—MW-6R Dissolved phase 1,2,3-TCP and 1,2-DCP were detected at concentrations of 3.3 pg/L and 38 pg/L, respectively. These concentrations exceed the California Notification level of 0.005 µg/L and MCL of 5 µg/L, respectively. There has been no significant change in 1,2,3-TCP or 1,2-DCP concentrations since the 2007 injection event. o EDB—MW-6R Dissolved phase EDB was detected at a concentration of 0.51 µg/L. This concentration exceeds the MCL of 0.05 µg/L and the local background level of 0.26 µg/L (Geosyntec, 1998). EDB concentrations have been declining in well MW-6R since 2008. 0 1-Chloropropane—MW-6R Dissolved phase 1-chloropropane was detected at a concentration of 1.1 µg/L. There is currently no established MCL for 1-chloropropane. • Well MW-7, located immediately downgradient of MW-6R (Figure 2), exhibited lower dissolved phase COC concentrations than MW-6R. Historical results for these COC are tabulated in Table 5 and graphically presented on Figure 9. o TON—MW-7 Dissolved phase TON was observed to be 21 mg/L. This concentration exceeded the MCL of 10 mg/L. TON concentrations have fluctuated between 2.73 mg/L and 49 mg/L since 2007.