Laserfiche WebLink
r <br /> Review of OU-1 FS Report -7- 7 August 1992 <br /> DDRW, Tracy <br /> wells may be able to be pumped at higher rates. The extraction wells should be <br /> designed to maximize extraction rates from the contaminated zones. This would <br /> generally require screen lengths based on lithologic unit thicknesses rather than <br /> limiting screen lengths to 10 to 20 feet, increasing the diameter of the casing from <br /> six inches and designing the filter pack to minimize well losses. If the wells are <br /> designed to maximize extraction rates and well efficiency, then the cleanup of the <br /> aquifer to the aquifer cleanup level may occur more quickly than the 30 years <br /> currently estimated to be needed. Details on the design will have to be presented <br /> for the Remedial Design/Remedial Action (RD/RA) . <br /> Injection Well Placement <br /> Figures 3.3-4 and 3.3-8 depict the placement of the injection wells in each of the <br /> horizons for Alternatives 3 to 7. The injection wells are tentatively located along <br /> the western and southeastern boundaries of the Tracy site. Further consideration <br /> may need to be given to the placement of some of the injection wells because the <br /> supply wells for the base are in the southern portion of the site. The hydraulic <br /> affects of large supply wells and many injection wells will need to be considered <br /> for the RD/RA. <br /> No Action Alternative <br /> The Draft OU-1 FS Report did not include a detailed evaluation of the no action <br /> alternative with institutional controls. The evaluation of the no action <br /> alternative in the Report included the IRM. The Draft Final FS Report for OU-1 <br /> should include an evaluation of the no action alternative without the IRM for the <br /> purposes of comparison. <br /> Source Areas <br /> The Draft OU-I FS Report briefly discusses the different source areas in Section <br /> 1 .2.2.3. 1 and indicates that Alternative 6, if selected, will significantly reduce <br /> the amount of the contaminant sources through biotreatment of the sorbed <br /> contaminants (page 4-17) . However, if Alternative 6 is selected, not all of the <br /> source area will be remediated through in-situ bioremediation because this remedial <br /> alternative Only addresses th•e saturated zone. The source areas Within the <br /> unsaturated zone may also need remediation. Source abatement work in the <br /> unsaturated zone will help to expedite the ground water remediation. Remediation of <br /> the unsaturated zone for VOCs, such as through vacuum extraction, should be included <br /> in the site-wide RI/FS. <br /> Granular Activated Carbon <br /> The Draft OU-1 FS Report indicates that a GAC will be used in series after air <br /> stripping for the preferred Alternatives 3 and 6. It is unclear why GAC is needed. <br /> The Draft FS Report for OU-1 indicates that the influent concentrations for the IRM <br /> are between approximately 10 to 15 pg/1 (page 1-17) . These concentrations are <br /> relatively low. The Report did not clearly indicate if GAC was a requirement of the <br /> San Joaquin Air Pollution Control District (SJAPCD) . If the influent concentrations <br /> are low enough, GAC may not be required by the SJAPCD. For example, at the DDRW, <br /> Sharpe site, GAC was not required for the North or South Balloon treatment plants, <br /> but is required for the Central Area treatment plant where the TCE concentrations in <br />