Laserfiche WebLink
• <br /> Draft OU-1 ROD Memorandum -3- 4 June 1993 <br /> DDRW, Tracy <br /> background to require treatment for pesticides. With the ground water remedial <br /> action for OU-1, the pesticide concentrations may decrease due to dilution from <br /> the large volumes of water requiring treatment for VOCs. However, in the event <br /> that the pesticide concentrations do not decrease with full OU-1 extraction in <br /> place, DDRW, Tracy must determine that discharge of the treated ground water <br /> through reinjection or by discharging to on- or off-site ponds will not be a <br /> degradation of the ground water for pesticides or other potential pollutants. <br /> The WDRs issued by the Board will require that background concentrations of <br /> pesticides be established in the receiving aquifers and that pesticides will be <br /> monitored for in the groundwater treatment plant effluent. <br /> Our primary concern with the disposal of the treated ground water would be with <br /> reinjection where pollutants found and extracted from the shallow water bearing <br /> zone could be directly discharged into lower water bearing zone of better water <br /> quality. We believe that with the proposed placement of the injection well <br /> field up gradient from the plume and the location of the existing on-site ponds . <br /> within the area of contamination and the proposed placement of an off-site <br /> pond(s) cross gradient from the contamination, are the most optimal locations <br /> for disposal to prevent degradation from other pollutants due to the location of <br /> the extraction well field. In addition, with more than one disposal option, if <br /> the pesticide concentrations become of concern, the treated ground water could <br /> be directed to existing ponds in order to minimize potential affects on the <br /> ground water. <br /> 3. Extraction and Disposal Systems. In our review of the Draft Final FS Report, we <br /> expressed concern regarding the conceptual plans for the extraction and disposal <br /> systems for OU-1 . Our concern regarding the extraction system was that all of <br /> the extraction wells would be placed at the leading edge of the plume rather <br /> than placing some of the wells in the "hot spot" areas within the plume. We <br /> believe that the placement of wells at the hot spots in addition to the leading <br /> edge of the plume is a more efficient method of cleanup of the contaminant <br /> plumes and complete hydraulic control . Sections 7.3.1.1 .1 (page 7-10) and <br /> 9.1 . 1.1.1 (page 9-2) discuss the conceptual plans for the OU-1 extraction system <br /> and reference Figures 7.3-1 to 7.3-3 in support of the conceptual design. These <br /> sections and figures clearly portray the concept of extraction wells being <br /> placed only at the leading edges of the plumes. These sections and figures must <br /> be revised to reflect a conceptual plan that supports placement of extraction <br /> wells at hot spots and near the leading edges of the plume. <br /> With respect to the disposal system, we expressed concern that disposal through <br /> reinjection may be limiting and stressed the importance of a flexible system <br /> with multiple disposal alternatives for OU-1. It appears that some language has <br /> been incorporated into the Draft ROD regarding the discharge to surface <br /> impoundments. However, clarification regarding the disposal to ponds is needed. <br /> Sections 7.3.1. 1.3 (page 7-10) and 9.1. 1.3. 1 (page 9-2) should include some <br /> additional language regarding discharge to existing ponds and ponds that may be <br /> constructed off-site. A map, which identifies the general areas on- and off- <br /> site that pond disposal may occur would be helpful . <br /> 4. Monitoring Program Frequencies. Ground water monitoring of the cleanup of the <br /> OU-1 contamination is presented as occurring on a quarterly frequency for the <br /> first two years after implementation of the remedial action and then being <br />