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Review of the Draft Final Extraction Well Optimization Work Plan DDJC Sharpe Site,Lathrop, California September 2006 GENERAL COMMENTS 1. Regulatory Agencies commented on the Extraction Well Optimization Work Plan ()A ork Plan) that was issued in a Draft version on September 20, 2005, but no response to comments was included with this Draft Final version. Please explain why no respon a to comments was prepared for the Draft Final Work Plan. 2. A general statement could be added to the text of the Work Plan to explain how the modeling layers (1, 2, 3, and 4) as seen on the figures for Section 2 correlate to the groundwater zone depth intervals(A, B, C, and D) discussed in the text. This information would make the Work Plan more user friendly. Please consider adding t xt to help the reader correlate the layers as seen in the figures for Section 2 with the groundwater zone depth intervals discussed in the text. SPECIFIC COMMENTS 1. Section 1.2, Introduction, Page 2: The Work Plan refers to the aquifer beneath the DDJC-Sharpe facility as "arsenic-contaminated" in discussing the "unreasonable cost'of aquifer remediation, but arsenic is one of the contaminants of concern (COCs) identif ed in the DDJC-Sharpe Record of Decision(ROD) and is also subject to remediation un er the ROD. Please delete the characterization of the aquifer as arsenic-contaminated in this section of the Work Plan. 2. Section 2.3, Introduction, Page 4: It appears that the D Zone depth interval should be included in the statement regarding trichloroethene (TCE) concentrations exceeding 100 micrograms per liter (ug/L). The first sentence of this section states B and C Zone depth intervals have TCE concentrations exceeding 100 ug/L, however, as seen in Figure 2. it appears layer 4 (D Zone depth interval) in the Central Area also contains TCE concentrations exceeding 100 ug/L. Please discuss the D Zone depth interval with TCE concentrations exceeding 100 ug/L, including any plans to remove mass from the D Zone, or explain why the elevated concentrations in the D Zone were not discussed in the text. 3. Figure 2-10,TCE Plume in Layer 4, Scenario 28, 15 Extraction Wells Operating, Page 23: It appears from the modeling figures that the TCE Plume after 30 years in Layer 4 is larger with higher concentrations for Scenario 28 (in situ technologies, as seen in Figure 2-10) than for Scenario 17 (no in situ technologies, as seen in Figure 2-6). It is unclear why the modeling results using in situ technologies (Scenario 28) show a larger TCE Plume than Scenario 17, which only includes pumping and treatment of the groundwater. These figures also contradict the statements in Section 2.2.2 that the 1