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TECHNICAL MEMORANDUM Evaluation of Bed Ash Disposal Forward Landfill San Joaquin, California November 1, 2016 Page 6 over time. Therefore, as described in more detail below, leakage through this system (if it were to occur) would be very low; and • The Forward Landfill is underlain by lenticular deposits of clay, silt and sand with minor gravel of the Pleistocene Victor Formation and the first encountered groundwater occurs about 60 to 80 feet below the native ground surface. The generally fine-grained nature of the subsurface soils suggests that if leakage were to occur, the time required for the leachate to reach groundwater would be very long and any copper remaining in solution would likely adsorb to the subsurface silt and clay. The qualitative conclusion that bed ash disposal is unlikely to affect groundwater was further evaluated by calculating containment system leakage rates and then using a transient/steady- state model to calculate maximum copper concentrations in groundwater at the closest downgradient monitoring well to each of the disposal areas shown in Figure 1. The results of these evaluations are summarized below. Containment System and Leakage Rate All the bed ash disposal locations shown in Figure 1 are underlain by a Subtitle D composite liner system that includes (from top to bottom): (i) a LCRS; (ii) a 60-mil high density polyethylene [HDPE] liner; and (iii) a 2-foot-thick compacted clay liner [CCL] with a maximum hydraulic conductivity of 1E-7 centimeters per second (cm/sec). Because an intact geomembrane has extremely low permeability, if leakage occurs, most of the liquid migration occurs through geomembrane defects. In general, if there is a defect in the geomembrane, liquid passes first through the defect, then it flows laterally some distance between the geomembrane and CCL before ultimately infiltrating into and through the CCL. Other factors being equal,higher leakage will occur if there is poor contact between the geomembrane and the underlying clay. Giroud (1997) proposed a series of equations to calculate the rate of liquid migration through composite liners that accounts for these migration mechanisms and is based on the head of liquid on the top of the liner, the number, size, and geometry of defects, the quality of the contact between the geomembrane and underlying CCL, and the thickness and hydraulic conductivity of the CCL. For the purposes of this analysis, circular defects were assumed and the leakage rate