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0 0 TECHNICAL MEMORANDUM Evaluation of Bed Ash Disposal Forward Landfill San Joaquin, California November 1, 2016 Page 10 • Closest Monitoring Well and Thickness of Saturated Zone. The distance to the closest monitoring well to each of the disposal areas was measured from Figure 1. The thickness of the saturated zone was based on the Forward Landfill Revised Engineering Feasibility study and Annual Corrective Action Report (Andrew's Engineering). The well screen depth fraction for each well was based on ratio of screen length to the aquifer thickness. • Hydraulic Conductivity, Hydraulic Gradient, and Seepage Velocity. The hydraulic gradient and conductivity of the subsurface materials were based on site monitoring report data. The seepage velocity below the disposal site was calculated by the program using these values and the effective porosity of the materials. • Aquifer Temperature, pH, and Organic Content. The groundwater temperature and pH were estimated based on recent groundwater monitoring data. The organic content of the saturated zone was assumed to be zero. The retardation coefficient for the saturated zone was conservatively set at 1, which means the contaminant will move at the same rate at the groundwater and there is no retardation in the subsurface. The parameters that were used in the model are summarized in Table 5A and 5B for the alternative disposal area/source duration scenarios. The model was run in a transient mode to calculate the predicted maximum concentration in the closest downgradient monitoring well and the time required to reach this concentration. The results of these analyses are included in Tables 5A and 5B and are summarized below. As shown by these results, if copper were to leach from the bed ash, the worst-case condition indicates the maximum concentration in the closest downgradient monitoring well would be more than 1,900 times lower than the typical copper detection limit of 0.0011 mg/L and would require more than 10,000 years to migrate from the source to the monitoring wel1.6 These long transit times and low concentrations are primarily a 6 For comparative purposes,the most critical of the different disposal scenarios that were analyzed (Alternative 1,Area 2)was evaluated by running the MULTIMED model in steady-state mode. This is extremely conservative because under steady-state,the source never decays(i.e.,it leaches forever),leakage from the landfill continues forever,the thickness of the vadose zone is irrelevant,attenuation is not allowed,and partially- penetrating monitoring wells are not considered. The steady-state DAF under these conditions is calculated to be on the order of 1,932. This means that even if the source of copper never decayed and the landfill leaked forever, the maximum concentration in groundwater for the worst-case scenario would be about 0.017 mg/L. For comparison,this value is about 10 to 100 times lower than the copper Primary Maximum Contaminant Level(MCL) of 1.3 mg/L,Secondary MCL of 1.0 mg/L,California Public Health Goal(PHG)of 0.30 mg/L,and Agricultural Water Quality Goal of 0.20 mg/L.