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Greg Vaughn 4 • 24 July 1995 recommend that one bench a minimum of five feet wide be cut at a vertical elevation of 15 feet. The bench(es) should be designed with positive drainage away from the wall to reduce infiltration These comments are based on the assumption that the text description (p. 19) of the slope as 30 feet high is accurate; however, the rendering in Drawing I appears to show a pit that is only approximately 15 feet deep and a slope that is steep, but slightly less than vertical. Stability of a 15 -foot wall is a lesser safety concern and can probably more easily be enhanced with a bench at 6 feet. On the other hand, the lesser height of 15 feet leads to a lesser volume in the former clay mine excavation, and suggests that partial or complete infilling of the area might be both economically reasonable as well as preferable from a drainage management viewpoint. While the principal concern with stability of a 30 -foot high cut slope is safety, the failure of any of the steep slopes may impair the integrity of the final cover of the landfill. Failure of any of the slopes may damage the barrier layer, permit water to infiltrate into the wastes, and promote erosion of the vegetative cover. For this reason, the design drawings, engineering analyses and CQA plan should address method of anchoring or keying the barrier layer into the slopes, steps to be taken to ensure that the vegetative layer, if placed on a geomembrane, will be stable and not subject to sloughing -off, and an engineering economic analysis justifying retention of any slopes at greater than 3:1 H:V. 6. Water Quality Sa=ling ands' sure Monitoring: The plan notes that water quality sampling was last conducted in April 1991. Sampling also was conducted in 1986, 1988, and 1989. Results suggest that water quality is variable in the area and may respond in part to seasonal variations. Ground water depth and flow directions also may be variable, and regionally, ground water depth and flow are influenced by a pumping depression around Stockton. The plan suggests also that some "dilution" of the shallow upgradient ground water may be occurring from a slough south of the site. The available information shows that benzene, ethylbenzene, toluene, and elevated levels of some inorganic parameters (notably TDS, iron, manganese, and sulfate) have been detected in one or more samples and sampling rounds. It is not clear that all of these detections can be related to effects of the landfill. The plan proposes that postclosure water quality monitoring be, undertaken in a detection, rather than evaluation, mode, given that (1) 13 years have passed since wastes were last deposited at the site, (2) there is relatively little water quality data for the site, and no data more recent than four years ago, and (3) that the existing data suggest, but do not conclusively demonstrate, a water quality impact from the landfill. The proposal to do detection monitoring is reasonable, but note that several of the 13 years since waste disposal ceased have been drought years. The water chemistry may show changes after the past wet season. That this is an unlined landfill and that some or all of the wastes may be in contact with ground water raises additional concern. Therefore, a regular water quality monitoring program should be undertaken as soon as practicable. To this end, after a detailed review of the Sampling and Analysis Plan proposed in Appendix B of the CPC MP is completed, I recommend that we prepare a Monitoring and Reporting Program for submittal to the Board.