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SAFEWAY MEAT PACKING FACILITY, STOCKTON, CALIFORNIA TIER II RISK BASED CORRECTIVE ACTION FEBRUARY 26, 1996 PAGE 4 - 3 Estimated Area Characteristics Average wand speed for Stockton was used to represent the wand speed above the ground surface in the ambient mixing zone for the average case Based on the concrete paving of the area of concern, the particulate emission rate was set to 0 grams per square centimeter for the average case The infiltration rate of water through the vadose zone was estimated from the annual rainfall in the Stockton area From a caval engineering perspective, concrete paving is considered impervious to water from most rainfall events The RME case infiltration rate was assumed to be 15% of the annual rainfall based on engineering considerations from the perspective infiltration, a worst-case runoff coefficient (factor of rainfall forming runoff) is 0 85 (Lmdsley, et al , 1992) That is, the possible infiltration of water during a storm is 15%, however actual infiltration will be less as the rate slows to a steady state as pores cracks, and construction points in the concrete become completely filled with water In addition, effective infiltration will be less because the evapotranspiration potential in Stockton is high By consideration of all these factors, a value of 2 percent of the rainfall per year infiltrates to the groundwater has been estimated for the average case Default Values RBCA or USEPA risk assessment default values were used to define the lower depth of the surface soil zone, the ambient air mixing zone, and the averaging time for the vapor flux Information on the volumetric fraction of water in the capillary fringe was not available For the Stockton site it was assumed that 90% of the pore space in the capillary fringe was filled with water to be consistent with the assumption in the RBCA standard The fraction of organic carbon in the soil at the site has not been measured Based on the RBCA standard, it was assumed that the soil contains I% organic carbon (fraction of organic carbon is 0 01) 0131CDJI