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0 Greg Vaughn 3 24 July 1995 and potentially into wastes, and to prevent erosion of the cover. The CPCMP describes (p. 19) a proposal to divert rainfall to the deepest portion of the landfill (a former clay mine area that has been partially filled with wastes). The plan states that although the northern sidewall of this deep area contains no waste, the southern, western, and eastern sidewalls do. The plan also proposes to line the bottom and sidewalls with the geomembrane. We have already expressed our concern over the ability to place the geomembrane on steep slopes. Given that concern, adding questions about stability of the very steep north sidewall (as discussed on pp 19-20 of the plan and in Item 5 below), and considering Chapter 15's prescription against allowing water to be impounded over waste -bearing areas, we cannot approve the proposal to divert and impound storm waters in this area., as presented. As noted below (Item 4), construction of this impoundment in the former mine pit area is a key portion of the storm water management proposal. If Cove intends to pursue this approach as an engineered alternative under Chapter 15, economic and engineering analyses are required to justify it. Double -lining of the pond area may be required. 4. Storm Water Management Proposal: The plan proposes drainage diversion berms, corrugated pipe downdrains, and hydroseedng to control erosion and drainage, but little detail is given. Chapter 15 requires that precipitation and drainage controls be designed to accommodate flows from a 100 -year, 24-hour storm. Flows should be diverted away from the waste management units. Section H.1.5 of the plan (p. 19) proposes a berm and two corrugated pipe downdrains for diversion of surface waters into the low area in the north part of the site. An estimate of the volume of runoff that can be handled by the proposed system should be given. The plan proposes that stormwater accumulated to a depth of six feet in the north side impoundment be pumped into the drainage ditch west of the landfill site, adjacent to El Dorado Avenue. Under present conditions (without final cover) water impounded in the unfilled pit potentially has come into contact with waste. Discharge of this water to the drainage ditch, or via the ditch to Duck Slough, may require a National Pollution Discharge Elimination System (NPDES) storm water permit. Closure of the landfill may eliminate the need for this permit, provided there is no other potential for discharge of wastewaters to surface water systems. A letter ("Notice to Comply") explaining the stormwater permitting process has been sent separately. 5. Stability of the Mine Pit Sidewalls: According to the CPCMP, the north sidewall of this pit is a 30 -foot high, vertical, cut slope that abuts against the property line between Cove and Martin Metals Finishing. Concerns about the stability of the slope are based on its steepness and on the possibility that soils which become saturated from water ponded on the top of the slope or in the pit adjacent to the slope, might fail. Raveling and destabilization due to erosion are not considered. The Plan proposes to place geomembrane to a height of 6 feet above the slope base. I question whether this will do much to aid stability of a 30 -foot vertical slope. I assume that placement of a geomembrane will require construction of a bench in the slope wall in order to cut an anchor trench. If equipment is able to access the wall for this purpose, then the same equipment should be able to cut one or more benches at intervals on the slope. If a bench is cut at 6 feet height to accommodate anchoring of the geomembrane, I recommend that an additional bench be cut ten feet above the anchor trench; or, if for some reason, a bench is not cut for the geomembrane, then I