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with similar soil types to the site; 3,000 m/yr is within the predicted range for silty sands, but <br /> ! above the range for clayey silts. <br /> Precipitation: An average precipitation rate of 14 in/yr (35.6 cm/yr) was confirmed by the <br /> weather department of a local television station. <br /> Evaporation:_ An average evaporation rate of 6 in/month (0.50 cm/day) was obtained from <br /> the Stockton Wastewater Treatment Plant by pan evaporation tests performed at the facility. <br /> NEW RISK ASSESSMENT MODELS <br /> Model No. I <br /> A new risk assessment model was generated using several different variables (explained <br /> above) and a groundwater depth of 38 feet. The bottom of the impacted soil was defined <br /> as the interface between the vadose zone and groundwater. The laboratory detection limits <br /> 'for toluene, ethylbenzene and xylene were used as the minimum contaminant concentrations <br /> for the leaching potential models (no benzene was detected at the site and, therefore, not <br /> included in the model). No target constituents were predicted to impact groundwater with <br /> impacted soil immediately above the groundwater surface (Attachment A). <br /> . Model No. 2 <br /> If groundwater were to return to a depth of 35 feet, approximately 3 feet of impacted soil <br /> would be in contact with groundwater. The masses of toluene, ethylbenzene and xylene <br /> located in the soil from 35 feet to 3S feet were estimated using the laboratory results from <br /> borings MW-4, B-17 and B-18. <br /> The mass of contaminants was then defined in the model as an instantaneous release to <br /> groundwater. This would represent 100% of the contaminants immediately dissolving into <br /> groundwater over the area of the impacted soil. It should be noted that degradation rates <br /> for the contaminants were not used in generation of the risk assessment model. <br /> The predicted impact to groundwater at monitoring well MW-7 was calculated (Attachment <br /> B). The predicted maximum concentrations at MW-7 are 0.0558 ppb, 0.0604 ppb and 0.210 <br /> ppb, respectively. All predicted concentrations are well below laboratory method detection <br /> limits (MDLs) and State of California Maximum Contaminant Levels (MCLS) or Action <br /> Levels (ALs) for drinking water. <br /> Model No. 3 <br /> A third model was generated using identical parameters as Model No. 2, except a fictitious <br /> well was placed in Harding Way half way between the contaminant source and MW-7 to <br />