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22 October 1996 <br /> AGE-NC Project No. 95-0100 <br /> ' Page 7 of 9 <br /> With all information is entered, SESOIL provides a predicted mass loading to ground water for each <br /> Year of the test. The AT123D modeling program used the yearly predicted mass load to ground water <br /> from SESOIL and hydraulic properties of the aquifer to estimate the contaminant concentrations in <br /> ' ground water at a receptor well. The maximum permitted running time of 100 years was used for this <br /> site. <br /> The volume of hydrocarbons was calculated for impacted soil from the surface to a depth of 55 feet <br /> bsg. The total volumes of benzene, toluene, ethyl benzene 'and xylene were defined as an <br /> instantaneous release to ground water over the area of impacted soil. A zero degradation rate for <br /> ' breakdown of hydrocarbons was used for the model. The concentrations of contaminants in ground <br /> water at the down-gradient edge of the release were predicted using the AT12313. <br /> ' The AT123D model was also processed to calculate the effects of an increase in the ground water <br /> elevation to a depth of approximately 35 feet bsg, currently ground water is at an estimated depth <br /> of 55 feet bsg. The SESOIL data was utilized for the AT123D ground water model. <br /> 3.3. SOURCE OF MODEL VARIABLES <br /> Intrinsic Permeability: Two reference sources were used to obtain the intrinsic permeability (IP) <br /> ' values for the site. Appendix F lists various IP values and the sources of each Table. Each reference <br /> provided similar ranges for IP. The IP used to generate the model were for silty sand, sand and <br /> gravel, all textural compositions likely to be more permeable than the actual soils encountered. <br /> ' Porosity: Average porosity of the standard model 30% was obtained from <br /> ( ) Hydrology For <br /> Engineers (1982), and also summarized in Appendix F. The PHS EHD has typically requested that <br /> an effective porosity of 55 percent be used when modeling soils. For this model the PHS-EHD allow <br /> a value of 30 percent to be utilized. <br />' Tranmissivity (hydraulic conductivity): A transmissivity of 20,000 m/yr was used in the standard <br /> model. This value was obtained from an example problem, generated in the SESOIL program with <br /> soil types similar to the study area, but including silt; 20,000 m/yr is within the predicted range for <br /> silty sand, but is above the range for clayey silt. <br /> Precipitation: An average precipitation rate of 14 in/yr(35.6 cmlyr) was confirmed,by the Stockton; _ <br /> - "'-Reeord. Evapoiatiori: An average evaporation rate of 6 in/month (0.5 em/day)'was obtained from <br /> the Stockton Wastewater Treatment Plant by pan evaporation tests performed at the facility. <br /> The risk model was developed with the.ground water depth input at 55 feet below surface grade. <br />. . p � <br />