SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

t Mr. Maurice Benson - 3 - 40 19 March 2008 l Defense Logistics Agency `7f there are multiple wells that may provide similar information, a "preferred" well will be selected for sampling and the others will not be sampled." Regional Water Board staff agrees that, where nearby wells are screened in the same water-bearing zone, some of them may be sampled at a reduced frequency if it is demonstrated that they are providing redundant data. However, the Draft Report does not provide such demonstrations. The Final Report must present the sets of "multiple wells"from which each `preferred well"was selected, define the method used to select each preferred well, and demonstrate that the wells not selected provide redundant data only. For consistency and Remedial Program Manager consensus, a valid scientific approach needs to be selected, defined, and presented. The results should be presented for each of the site's Well Monitoring Program wells listed on Table 6.3-1. GROUNDWATER TREATMENT-PLANT PERFORMANCE AND COMPLIANCE 4. Please correct the following typographical error in the first line of page 4-4, Section 4.2.3.2 of the Draft Report: change mg/L to pg/L. 5. DESJC should consider monitoring extraction well EW030C for four consecutive quarters. The Decision Logic for Operation of Extraction Wells for Application to Each Well Annually establishes the rules for monitoring frequency of an extraction well, such as this one, that has been shutdown and is being monitored for rebound. As described in the decision logic, if the concentration of the last sample is greater than 1.2 times the ACL, the well must be sampled during the next three consecutive quarters. The concentration of tetrachloroethene (PCE) in EW030C was 5.8 micrograms per liter (Ng/L) during third quarter 2007. Although 1.2 times the ACL for PCE (5.0 pg/L) is 6.0 pg/L, the observed concentration of 5.8 pg/L is the highest concentration observed in EW030C to date. The well was constructed in 1997. From 1997 to second quarter 2007, the concentration had fluctuated from an estimated low of 0.33 Ng/L to a high of 1.10 pg/L and averaged 0.98 pg/L. Furthermore, the reported concentrations of PCE during four of the last five sampling events were estimated between the laboratory reporting limit (0.50 pg/L) and the method detection limit (0.20 to 0.25 pg/L). The fifth sample PCE concentration was slightly above the laboratory reporting limit at 0.52 pg/L. Regional Water Board staff believes it would be prudent to sample the well quarterly, given the fact that the concentration increased more than ten fold and the fact that the sample was collected from the Lower Hydrologic Zone. 6. Section 4.3.2.1 on page 4-5 indicates that the capture zone depicted on Figure 4.3-3a was interpreted using second quarter 2007 data. The map key on the figure should be amended to indicate this. 7. Section 4.3.2.2 on page 4-5 references the Sharpe groundwater model rather than the Tracy model.