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Mr. Michael T. Goldstein October 18 , 1985 Page 3 The transmissivity, storage coefficient , and hydraulic conductivity were determined from aquifer tests conducted by Brown and Caldwell . The effective porosity was estimated and confirmed during calibration. The regional flow used was 1.7 ft/day north northwest and was calculated from pre—pumping hydraulic gradients measured across the site. The dispersivities were determined by calibration. The retardation was set equal to 1 in lieu of the site specific tests ( according to Prickett in Bulletin. 65 of the Illinois State Water Survey) in order to product distributions of concentrations as a result of pure mixing . And finally, a convenient partical mass was selected that causes a 100 ppb increase in chloroform concentration in an element for each particle introduced. Assumptions The following assumptions were utilized to develop the simulations : 1 . A uniform aquifer thickness of 40 feet, confirmed from well lithology logs. 2. Uniform hydraulic properties in the shallow aquifer, derived from the recovery well (RW1 ) aquifer test. 3 . The storm water basin was the only source of chloroform. 4. The initial concentration of chloroform in the basin was 200 ,000 ppb, derived from analyses of basin water samples taken in August 1984 . 5 . The production from the recovery well was 110 ,000 gallons per day ( 76 gallons per minute) after June 30 , 1985. 6 . The infiltration into the basin averaged 33 , 200 gpd after June 28 , 1985 , based on analysis of records of discharge to the basin and did not significantly effect the groundwater velocity. 7 . RW1 will cease operation in mid November 1985 . 8 . Reactions which retard the movement of chloroform such as adsorption and organic fixation are negligible. Calibration The calibration process involved developing a scenario that approximated the measured concentrations in wells and reproduced known events. Simulations were run with varying transport coefficients, amounts of chloroform and times of introduction. The final calibrated model simulated chloroform concentrations that were reasonable when compared with measured values. The calibration results were compared to measured concentrations in Table 1 . BROWN AND CALDWELL 1 0 14, (416193/1311111 II If r 13 f4111,•UI I ClIAI}490 If116K I FIK AVI NL)I III I ASANI 11111 1W,'A