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Review Comments for Revised Site Investigation Work Plan — February 27,2018 Former Helena Chemical Facility Page 2 surrounding sites have documented groundwater flow directions to the southeast, which is consistent with groundwater flow beneath the Site in September 2017 and December 2017. 3. Central Valley Water Board staff also recommend that at least three additional groundwater monitoring wells be installed at the Site to fully define the extent of the nitrate plume and monitor potential migration of the plume as discussed below: The extent of the nitrate (as nitrogen) plume that exceeds the maximum contaminant level (MCL) of 10 milligrams per liter (mg/L) is not defined to the south of boring B32/GW10. Nitrate (as nitrogen) was detected at a concentration of 47 mg/L in B32/GW10. However, there are no sampling points to the south of B32/GW10 to show the extent of the nitrate plume that exceeds the MCL concentration. In addition, based on the September 2017 groundwater monitoring data, groundwater flow direction was to the south. Therefore, a monitoring well is needed to the south of B32/GW10 to define the boundary of the nitrate plume that exceeds the MCL concentration and to monitor potential migration of the plume to the south. Response: Condor proposes a monitoring well south of well MW-3/southeast of boring B32/GW10, to evaluate groundwater quality in the assumed downgradient direction (south-southeast as discussed in the previous comment). The additional well would be placed very near the existing building in order to provide the desired information while minimizing potential disruption to future tenants. Similarly, the extent of the nitrate plume that exceeds the MCL concentration is not defined to the east/southeast of boring B39/GW12. Nitrate was detected at a concentration of 150 mg/L in a groundwater sample from B39/GW12. Central Valley Water Board staff recommend installing a monitoring well to the southeast of B39/GW12 to better define the extent of the nitrate plume in this portion of the Site. Response: Condor proposes a monitoring well southeast of boring B39/GW12, in the presumed downgradient direction. The additional well would be placed very near the existing building in order to provide the desired information while minimizing potential disruption to future tenants. Nitrate has been detected at concentrations ranging from 140 to 240 mg/L in MW-1 which is located in the northwestern portion of the Site and groundwater monitoring data has shown groundwater flow direction towards the northwest during May 2016 and June 2017. Therefore, a monitoring well is needed downgradient of MW-1 to monitor potential plume migration towards the northwest. Response: As discussed in the response to Comment #2 above, the predominant groundwater flow beneath the Site appears to be toward the south-southeast. As such, no additional monitoring well is proposed northwest of well MW-1. However, nitrate concentrations in historical "grab" groundwater samples collected from B28/GW6 (5.4 mg/L) and B29/GW7 (1.2 mg/L), both of which are located northwest of well MW-1, were below the drinking water MCL of 10 mg/L. 4. The Work Plan lacks quality assurance/quality control (QA/QC) procedures needed to ensure that data obtained by the proposed sampling and analytical techniques is reliable, repeatable, and representative of conditions at the Site. The Work Plan should be revised to indicate the number and types of field QC samples (e.g. duplicate samples, rinsate samples, or trip blank samples) that are planned for this investigation. Response: QA/QC procedures have been added to the revised Work Plan. C, CONDOR