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Mr. Michael T. Goldstein
<br /> October 28 , 1985
<br /> Page 3
<br /> The transmissivity, storage coefficient, and hydraulic conductivity
<br /> were determined from an aquifer test of RW1 conducted by Brown and
<br /> Caldwell . The effective porosity was estimated and confirmed during
<br /> calibration. The regional flow used was 1 .7 ft/day north-northwest
<br /> and was calculated from pre-pumping hydraulic gradients measured
<br /> across the site. The dispersivities were determined by calibration
<br /> and are typical values for this type of aquifer. The retardation was
<br /> set equal to 1 in lieu of the site specific tests ( according to
<br /> Prickett in Bulletin 65 of the Illinois State Water Survey) in order
<br /> to produce distributions of concentrations as a result of pure
<br /> mixing. And finally, a convenient partical mass was selected that
<br /> causes a 100 ppb increase in chloroform concentration in an element,
<br /> 100 feet by 100 feet by 40 feet , for each particle introduced.
<br /> Calibration
<br /> The calibration process involved developing a scenario that
<br /> approximated the measured concentrations in wells and reproduced
<br /> known events. Simulations were run with varying transport coeffi-
<br /> cients, amounts of chloroform, and times of introduction. The final
<br /> calibrated model simulated chloroform concentrations that were
<br /> reasonable when compared with measured values. The calibration
<br /> results are compared to measured concentrations in Table 1 .
<br /> The calibrated model indicates that initially 14 pounds of chloroform
<br /> leaked from the storm water basin into the shallow groundwater zones .
<br /> The leak occurred between July 28 and July 29 , 1984, during the
<br /> hyperchlorination of the basin. Chloroform concentrations in moni-
<br /> toring wells BC2, BC3, BC4 , and BC5 measured on October 9 , 1984 ;
<br /> BC10 on April 28 , 1985 ; and concentrations in BC12 and BC14
<br /> September 9 , 1985, were used to fix the amount of the initial leak.
<br /> The low concentration in BC2 , BC3, BC4 , and BC5 on December 7, 1984 ,
<br /> and the low concentration in BC6 and BC7 on December 27 , 1984, indi-
<br /> cate there was no significant leakage in the period from the initial
<br /> leak to the start of sludge removal.
<br /> Excavation of sludge from the basin, which began in early 1985 ,
<br /> caused intermittant infiltration of water through chloroform
<br /> containing soils. This allowed another 15 pounds of chloroform to
<br /> enter the shallow aquifer before April 20 , 1985; 13 pounds of this
<br /> chloroform entered on January 12 , 1985 . Chloroform concentrations in
<br /> BC2, BC3 , BC4, BC5, and BC7 on January 31 , 1985, in conjunction with
<br /> field observations , fix the time. Concentration in BC-10 on February
<br /> 28 , 1985, and March 11 , 1985; and BC-12 on September 9 , 1985,
<br /> September 16 , 1985 , September 23 , 1985 , and October 7 , 1985 fix the
<br /> amount. Another pound of chloroform was introduced into the aquifer
<br /> on February 15 , 1985 , and the other pound infiltrated between January
<br /> and April. Concentrations in BC7 and BC11 on February 28 , 1985, RW1
<br /> on May 2 , 1985 , and RW1 and BC7 on June 24 , 1985 , fix the time and
<br /> the amount.
<br /> BROWN AND CALDWELL
<br /> P.O.BOX 8045 WALNUT CREEK,CA 94595-1220• (415)937-9010 TELEX 33-6490-OFFICE AT 3480 BUSKIRK AVENUE PLEASANT HILL 94523
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