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C TECHNICAL MEMORANDUM Evaluation of Bed Ash Disposal Forward Landfill San Joaquin, California November 1, 2016 Page 8 SUMMARY OF BED ASH ALTERNATIVE DISPOSAL LEAKAGE RATE AND SOURCE DURATION EVALUATION Area/Thickness Area 1 1 Area 2 Area 3 Area 4 Disposal Alternative 1 Disposal Area (ftZ) 429,540 828,793 458,778 225,038 Ash Thickness (inches) 0.73 0.38 0.18 0.07 Leakage Rate (m/yr) 3.13E-5 6.03E-5 3.34E-5 1.65E-5 Time to Source Depletion (yr) 0.52 0.52 0.59 0.52 Disposal Alternative 2 Disposal Area (ftZ) 13,000 13,000 3,395 617 Ash Thickness (inches) 24 24 24 24 Leakage Rate (m/yr) 9.47E-7 9.47E-7 2.47E-7 4.49E-8 Time to Source Depletion (yr) 17 17 20 17 Multimedia Exposure Assessment Model Potential impacts to water quality associated with the bed ash disposal were evaluated using the U.S. Environmental Protection Agency (USEPA) Multimedia Exposure Assessment Model (MULTIMED; Sharp -Hansen et al. [1995]) to calculate maximum copper concentrations in groundwater at the closest downgradient monitoring well to each of the disposal areas shown in Figure 1. Both disposal alternatives were evaluated to calculate the maximum concentration and the time required for the maximum concentration to be reached at the closest downgradient monitoring well to each of the disposal areas. Several conservative assumptions were incorporated into the model, including: • Size of Disposal Area and Recharge Rate. The sizes of the disposal areas were measured based on the information shown in Figure 1. The recharge rate outside the landfill footprint was assumed to be zero (this is conservative because it prevents dilution). • Source Decay Constant and Duration of Leakage. The source duration for the transient analyses was based on mass balance calculations (Tables 4A and 413) indicate that the time required to deplete the bed ash of soluble copper ranges from less than one year assuming uniform distribution of bed ash through the disposal area to about 20 years or less assuming a 2 -feet -thick layer of ash in each disposal area. The source decay constant was assumed to be zero and the duration of leakage was assumed to be infinite for steady-state analyses. • Initial Concentration. An initial contaminant concentration equal to the median WBP STLC concentration for each disposal area (summarized in Table 2) was assumed. It