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EPA Comments on the <br /> Memorandum, Recommendation to <br /> Shut Down Extraction Wells EW014A,EW016A,and EW017A, <br /> Defense Distribution Depot San Joaquin (DDJC) Tracy Site, Tracy, California <br /> June 2003 <br /> GENERAL COMMENTS <br /> 1. We concur with the intent of the Memorandum, Recommendation to Shut Down <br /> Extraction Wells EW014A, EW016A, and EW017A (the Memorandum), and with the <br /> second, third, and fourth of the four recommendations. The objective of optimizing <br /> performance by shutting down wells that dilute flow to the treatment plant and increasing <br /> the pumping rate in wells with concentrations above aquifer cleanup levels (ACLS) is <br /> worth pursuing. The comments that follow request further discussion of the rationale <br /> supporting the selection of wells and the first recommendation. <br /> 2. The Memorandum states that EW015A and EW018A are not recommended for shut down <br /> because of the "slight increasing visual trend" of trichloroethene (TCE). However, based <br /> on Figure 2 of the Memorandum, EW014A also appears to have an increasing trend. The <br /> rationale used to determine why EW014A should be shut down while EW015A and <br /> EW018A remain on-line is unclear and requires further discussion. Please explain why <br /> EW014A should be shut down while showing an increasing trend that appears similar to <br /> the increase in EW015A. Alternatively, recommend that EW014A remain on-line and be <br /> re-evaluated with EW015A and EW018A for potential future shut down. <br /> 3. The first recommendation in the Memorandum states that the shut-down extraction wells <br /> (EW014A, EW 16A, and EW017A) will not be returned to operation until concentrations <br /> exceed ACLS for three successive quarters, but the rationale for waiting three quarters is <br /> not presented. It is also unclear how long the extraction well will operate once it is <br /> restarted, but the recommendation implies that once the concentration decreases to less <br /> than the ACL for one quarter, the well will be shut down again immediately. This strategy <br /> may present a risk to maintaining plume capture and minimizing plume expansion. Given <br /> that the wells have been extracting groundwater at concentrations below ACLS for several <br /> years, a sudden increase to concentrations exceeding ACLS would be a concern that would <br /> appear to warrant quicker action, especially at EW014A, which is the northernmost well in <br /> the line of extraction wells along Banta Road. It is recommended that the Regional <br /> 1 <br />