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Quarterly Monitoring • 2 23 Marc. <br /> Marley Cooling Tower Company <br /> Chromium concentrations in well D-2 at a depth of 180 - 200 feet (Hargis & Assoc. Feb. 1989) <br /> have been at 90 ppb for last two quarters which means that we do not have adequate definition of <br /> the chromium plume in the deep zone. Based on review of this data, well clusters are needed at <br /> greater depth in the area of D-2 and along Mormon Slough. We conclude that additional extraction <br /> is necessary and monitoring wells are needed to define the eastern boundary of the plume and the <br /> vertical extent at this time. <br /> Continued pumping of the irrigation wells appears a major impediment to the clean up from the <br /> Marley site. It is therefore critical that pumping from those wells cease. An alternative water <br /> supply should be provided to the farmers so that the irrigation wells can be taken out of use. We <br /> request Marley set up a meeting with the land owners between Main St. and Mormon Slough to <br /> discuss access agreements, and alternative water supply options. In addition, we suggest Marley run <br /> it's ground water model for a scenario with and without agricultural pumping, as a visual tool for <br /> presentation in this meeting. We will be available to participate in this meeting to represent the <br /> Board's concerns. <br /> Dudek recommends sampling all monitoring wells, reclamation wells, and soil flushing extraction <br /> and monitor wells annually. Dudek notes that changes in chromium concentrations in most wells <br /> have occurred over a six to twelve month time period rather than a three month time period. <br /> Currently the performance monitoring program includes sampling all monitoring wells in January <br /> and July, and key monitoring wells (located along the longitudinal axis and the down gradient edges <br /> of the plume) in April and October. Water level measurements in all wells are collected quarterly. <br /> The sampling schedule should be re-evaluated based on the systematic sample decision process <br /> provided in the attached document, entitled Long-Term Ground Water Monitoring Program <br /> Guidance. This decision process should be tailored to Marley's specific site situation where <br /> necessary. In concurrence with the recommendations by Dudek, the monitoring program could be <br /> reduced to semi-annual water level measurements to evaluate variation in flow direction. The <br /> frequency of monitoring well sampling should follow a basic plan of 1) new wells should be <br /> sampled quarterly, 2) horizontally or vertically downgradient wells need to be sampled quarterly to <br /> monitor leading edges of contaminant plumes for each water bearing zone, impacted or threatened 3) <br /> quarterly sampling of private and municipal wells is necessary 4) semi-annual sampling of interior <br /> portions of plumes and 5) annual sampling to monitor background or upgradient water quality would <br /> be acceptable. Annual and quarterly reporting should follow the outline in the attached guidance. <br /> A memo on the site inspection I performed in November 1993 is also attached for your review. If <br /> you have any questions or comments please call me at (916) 255-3047. <br /> 240NDO'N)14� <br /> Associate Engineering Geologist <br /> cc: Duncan Austin, Department of Toxics, Sacramento <br /> Gary Vargas, Dudek & Associates, Encinitas <br /> Harlin Knoll, Environmental Health, San Joaquin Co. <br /> Howard Renne, Marley Cooling Tower Company, Stockton <br />