SITE INFORMATION AND CORRESPONDENCE

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Doug O'Neal . • February 7, 2002 Page 2 COMMENTS • GSU has several concerns regarding the facility's proposal to install a single well (Figure 1) screened at the same depth interval as well KI-12. Multiple factors, discussed below, indicate that a single monitoring well is inadequate to fulfill the monitoring objectives. These objectives must be addressed in the offsite area south of the facility in order to achieve compliance with California Code of Regulations, title 22, section 66264.100. GSU previously discussed this issue in our comments on the Plan for Additional Remediation (DTSC 2000). It would be more expedient for the facility and more efficient for achieving compliance with 22 CCR, if the Workplan addressed the multiple monitoring objectives for the offsite area. These objectives include: ➢ determine the nature and extent of the VOC plume south of the facility (Health & Saf. Code §25200.14(e)(3)). [Note: This objective is not focused south of well KI-12, but the entire area south of the facility.] ➢ demonstrate hydraulic control of the plume and the effectiveness of the corrective action (Cal. Code Regs., tit. 22, § 66264.100(d)); ➢ demonstrate that contamination is not migrating away from the facility (Cal. Code Regs., tit. 22, § 66264.100(c)); and ➢ demonstrate that groundwater quality is improving (Cal. Code Regs., tit. 22, § 66264.100(c)). Multiple additional piezometric and water quality monitoring points are necessary to fulfill these monitoring objectives in the off-site area. - Water level contour maps of the Upper Intermediate Zone under non-pumping conditions indicate that groundwater is flowing toward the west-southwest (Figure 2; from Dudek 2000). The existing and proposed monitoring network is inadequate to evaluate groundwater plume migration in this direction. The plume was likely migrating in this direction prior to any groundwater extraction at the facility and during the recent two-year cessation of groundwater extraction. Water level contour maps of the Upper Intermediate Zone under pumping conditions (Figure 3; from Dudek 2001 b), including groundwater extraction from well KI-12, indicate that hydraulic control of the VOC plume cannot be demonstrated because there are no water-level monitoring points to assess the radius of influence around well KI-12, and thus hydraulic control of the plume. The width of the TCE plume at the southern boundary of the facility (Figure 1) indicates that a single monitoring point is inadequate to assess the extent of the VOC plume. Figure 3 of Dudek (1997) (included as Figure 4 of this memorandum) depicts a sand unit in the Upper Intermediate Zone that is approximately 550 to 600 feet wide and intersected by wells KI-5, KEW-4U and KI-12. The sand unit geometry in the north- south cross section appears to be that of a fluvial channel. The groundwater plume is likely migrating in a westerly direction within the channel. The Workplan should propose monitoring point(s) to evaluate off-site plume migration in this feature.