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Greg Vaughn 5 24 July 1995 The closure plan suggests that after a year of detection monitoring, the number of wells included in the monitoring array may be reduced from 9 to 4. Before a modification in the number of monitoring wells for the site can be approved, we must have a clearer understanding of the present ground water hydrology, including seasonal patterns in depth to ground water and in flow directions. The influence, if any, of Duck Slough should be assessed. The Closure Plan states (p. 19) that access to the property north of the cut wall of the former clay pit has been denied, so that a geotechnical rig cannot get in to do investigatory work on the top of the slope at the property line. Some of the monitoring wells appear to be near or even on the north side of the same property line. Access to the wells for sampling on a regular basis will have to be confirmed in order for these wells to be used in a water quality monitoring program. Review of the Closure Plan shows that several areas of concern remain, even though the plan's two principal proposals, the use of a geomembrane instead of a compacted clay cover, and institution of detection monitoring instead of evaluation monitoring, are acceptable in concept. The plan as submitted needs significant clarification of several issues. Therefore, I recommend that Cove Contractors submit the following: 1. An engineering analysis showing that the proposed geomembrane cap is effective and presents an economic benefit. An analysis showing that filling the former clay pit, now proposed to be left as a topographic deep area in the final configuration, is uneconomic. 2. Detailed design drawings showing layout and anchoring of the geomembrane, and designs for the fitting of any surface accoutrements, such as inlet/outlet piping, that may penetrate the cover. 3. Slope stability analyses on both the waste materials and the cover material overlying the geosynthetic membrane. 4. Detailed Construction Quality Assurance (CQA) Plan for subgrade preparation, geomembrane installation, and vegetative layer construction.. 5. A revised drainage plan maintaining stormwater separation from wastes, and/or a rationale and engineering/econonuic analysis justifying the use of the deep area in the north for impounding water and showing that the impoundment will not pose a threat to infiltration through the cover or to stability of the cover. The final closure plan, design drawings, CQA plan, and reports must be certified by a civil engineer or a certified engineering geologist licensed in California. MAM