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I BROWN AND CALDWELL CONSULTING ENGINEERS �lV! OCT 22 1985 V ENVIROMENTAL HEALTH October 18 , 1985 PERMIT/SERVICES Mr. Michael T. Goldstein Georgia-Pacific Corporation 760 South Vail Avenue Montebello, California 90640 11-39-2092-05/1 Subject: Summary of Computer Modeling of Groundwater in the Vicinity of the Tracy Plant Dear Mr. Goldstein: This letter summarizes the results of our computer modelling of the chloroform plume in the shallow groundwater in the vicinity of the Tracy Chemical Packaging Plant. The purpose of this modeling is to estimate the present position of the 50 parts per billion (ppb) concentration in the plume and to better define the shape and extent of the plume. The model can also be used to predict' the future movement of the plume. The remainder of this letter describes the model , assumptions , calibration, and results. The estimated extent of the plume as of the beginning of September 1985 , and the predicted extent in January 1986, are illustrated. Emphasis was placed on determining quantities of chloroform introduced into the shallow aquifer and the distribution of this chloroform with time. The simulations indicate that 125 pounds of chloroform entered the shallow groundwater and RW1 removed more than 95 pounds of this chloroform, as of October 15 , 1985 . This simulation determined this quantity closely matched the total concentration of chloroform discharged from RW1 , calculated using operating records and laboratory analyses from the recovery well. Further, the model indicated concentrations at BC-12 will not exceed 350 ppb, shown on Figure 1 , and will be below 250 ppb at the beginning of 1986 , shown on Figure 2. Random-Walk The National Water Well Association interactive version of the Random-Walk solute transport model was utilized to evaluate chloroform transport in the vicinity of the Georgia-Pacific Tracy plant. Random-Walk is a two-dimensional, finite difference computer 1'0 IIUi tl(L WAI Nl q 11111 Y. CA Q V4h I:91•141W tiP 9000 1111 X.43 I491)• Of 11,'.1 At 341106tZ61I1k AV NUI PI I AtiANI 11111 94�?a