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Mr. Dave Bates • 736.01 <br /> Maria Cooling Tower Company Page 2 <br /> Concentrations of total dissolved chromium in both the MSD and MSI wells have steadily <br /> decreased since 1989 (Figures 2 and 3). Total dissolved chromium has been non- <br /> detectable in the MSD and MSI wells since 1993 and 1995, respectively. Total dissolved <br /> chromium in nearby reclamation wells I-4 and I-5 have also steadily decreased since 1989 <br /> (Figures 4 and 5). Total dissolved chromium has been non-detectable in reclamation well <br /> I-4 since April 1995. <br /> The MODFLOW model has been used previously at the Marley site to design the remedial <br /> wellfield and adjust pumping rates (Hargis + Associates, Inc. 1989a and b, 1990, 1991, <br /> and 1993). Using the MODFLOW groundwater flow model and SURFACT contaminant <br /> transport model, Dudek simulated the potential changes in groundwater flow and <br /> contaminant migration due to future usage of MSD and MSI as water supply wells. <br /> The MSD well has not been used for residential water supply since 1989. However, it <br /> appears that the MSI well continues to be used as an irrigation well for the approximately <br /> 1.55 acres of mature walnuts on the property. <br /> MODEL PARAMETERS <br /> Most of the MODFLOW model parameters used previously were retained for these <br /> simulations. Only groundwater, gradient, chromium concentration, and pumping rate data <br /> were updated. Starting chromium concentration data were obtained from the January <br /> 1996 annual monitoring round (Dudek, 1996a). Reclamation wellfield pumping rate data <br /> were obtained from the July 1996 quarterly monitoring round (Dudek, 1996b). <br /> In order to simulate a potential worst case condition, drought conditions for example, the <br /> model water levels used were the 1993 water levels, which are approximately 14 feet <br /> lower than 1996 water levels. However, groundwater flow directions are the same for <br /> 1996 as 1992, to the south and southeast, in the intermediate zone. Model simulations <br /> (Figures 6 through 8) will have drought condition water levels with predominant <br /> groundwater flow directions. <br /> The pumpage estimates used in the model for the MSD well were compiled using two <br /> years of actual usage by the two households on East Main Street (Appendix A). These <br /> data indicated that approximately 0.5 acre-feet per year of water was used from 1994 to <br /> early 1996. This amount is an average annual household usage. If the MSD well is <br /> required to supply the same amount of groundwater over a year, the average annual <br /> pumping rate to achieve 0.5 acre-feet per year is 0.34 gallons per minute (gpm). Because <br /> water usage is seasonal, the monthly usage changes were also reviewed. These data <br /> indicated that the highest monthly usage occurred in July or August at approximately <br /> 8,100 cubic feet per month. If the MSD well is required to supply the same amount of <br /> groundwater for one month, the highest monthly pumping rate to achieve 8,100 cubic feet <br /> per month is 1.36 gpm. <br /> Potential Impacts from Main Street Wells December 1996 <br /> C:\gfv\admin\marley\batl2ms.doc <br />