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Geosyntec
<br /> consultants
<br /> • 1,4-Dichlorobenzene
<br /> • 1,3,5-Trimethylbenzene
<br /> • Chlorobenzene
<br /> The VOC concentrations are generally stable to decreasing, and only 21 wells sampled at RRI
<br /> between 2018 and 2021 had increasing concentration trends for one or more VOCs (Appendix C;
<br /> Geosyntec, 2020d; Geosyntec, 2021b). In some of these cases, the increasing concentration trend
<br /> is for the daughter product of a VOC with a decreasing concentration trend,providing evidence
<br /> of degradation and natural attenuation. Cis-1,2-DCE and VC are currently the most commonly
<br /> detected VOCs in groundwater and serve as a proxy for the chlorinated solvent groundwater
<br /> plume. Figure 3 displays isoconcentration contours of cis-1,2-DCE and VC in groundwater with
<br /> concentrations above their respective MCLS based on the most recent sample collected from
<br /> each well. The concentration trends for cis-1,2-DCE and VC at each well are shown in Figures 4
<br /> through 8. During the sampling events conducted at RRI in 2020 and 2021, PCE was only
<br /> detected above the MCL or DTSC Tap Water screening criteria at W-064, W-071, W-172, and
<br /> W-325 in the Landfill Area, W-103 at Site 3, and P-173, W-053, and W-104 at Site 4. TCE was
<br /> only detected above the MCL at W-230 and W-325 in the Landfill Area, W-103 at Site 3, and
<br /> P-173 and W-020 at Site 4 (Geosyntec, 2021b).
<br /> During the sampling events conducted in 2020 and 2021, benzene was detected above the MCL
<br /> and/or DTSC Tap Water screening levels at wells 917-03, 917-04, 917-10, and W-292 in the
<br /> Central Area, W-036, W-064, and W-227 in the Landfill Area, and W-052 and W-103 at Site 3.
<br /> Total petroleum hydrocarbons as gasoline (TPHg), total petroleum hydrocarbons as diesel
<br /> (TPHd), and/or total petroleum hydrocarbons as motor oil (TPHmo)were reported above 100
<br /> micrograms per liter(gg/L) at 12 wells in the Central Area(917,01, 917-03, 917-04, 917-05,
<br /> 917-06, 917-07, 917-09, 917-10, W-341, W-310, W-292, and 816-MW 1), and TPHg was
<br /> reported above 100 µg/L at three wells in the Landfill Area(W-225, W-226, and W-227).
<br /> One or more dioxins and/or furans were detected above the laboratory reporting limit in 2020
<br /> and/or 2021 at five wells at Site 3 (W-076, W-077, W-099, W-210, and W-338) and one well
<br /> (W-031) in the Landfill Area.
<br /> Radionuclides were not detected in groundwater in the Landfill Area at concentrations above the
<br /> screening levels in 2020 (Appendix B; Geosyntec, 2020d).
<br /> Organochlorine pesticides have been below screening levels in all of the wells sampled for
<br /> organochlorine pesticides since 2018 (Appendix B; Geosyntec, 2020d; 2021b).
<br /> Metals have been reported across RRI at concentrations above screening levels that may be
<br /> naturally occurring. Arsenic in groundwater at RRI has been demonstrated to be naturally
<br /> occurring (ERS, 2016; CVRWQCB, 2016). A work plan to evaluate the background
<br /> concentrations for other metals at RRI was prepared in 2021 (Geosyntec, 2021c). Following the
<br /> completion of the proposed background metals concentrations work, an evaluation will be done
<br /> to determine if metals are a groundwater COC at sites at RRI.
<br /> RRI Groundwater Monitoring Work Plan 6 June 2022
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