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Mr. Dave Bates • 736.01 <br /> Marley Cooling Tower Company Page 3 <br /> The pumping estimates used in the model for the MSI well were estimated using <br /> University of California Cooperative Extension, consumptive use of selected crops in the <br /> Stanislaus - San Joaquin area for walnuts (Appendix B). The annual consumptive use of <br /> walnuts for this area is approximately 39.19 inches. Assuming an irrigation efficiency of <br /> 0.67, no carryover moisture, no growing season precipitation, or other water demand, the <br /> total irrigation usage for the approximately 1.55 acres of walnuts is 7.6 acre-feet per year. <br /> If the MSI well is required to supply this amount of groundwater over a year, the average <br /> annual pumping rate to achieve 7.6 acre-feet per year is 4.69 gpm. Because water usage is <br /> seasonal, the monthly consumptive use changes were also reviewed. These data indicated <br /> that the highest monthly consumptive use occurred in July at 7.9 inches. If the MSI well is <br /> required to supply groundwater during the highest consumptive use month, the highest <br /> monthly pumping rate is 111.6 gpm. <br /> MODEL SIMULATIONS <br /> Two sets of model simulations were prepared. The first set of simulations involved <br /> pumping the reclamation wellfield and most irrigation wells at average annual pumping <br /> rates, except for MSI or MSD (Figure 6). Then, the reclamation wellfield, most irrigation <br /> wells, and MSI and MSD wells were run at average annual rates to compare the impacts <br /> of the two additional wells (Figure 7). <br /> The second simulation involved pumping the reclamation wellfield, most irrigation wells <br /> for one month, and the MSI and MSD wells were tun at their highest monthly rate (Figure <br /> 8). This was performed to compare the average annual impacts to the highest possible <br /> monthly impact of MSI and MSD well pumping simulations. <br /> The first set of simulations, involving average annual pumping rates, indicated that the <br /> remedial wellfield maintained capture regardless of MSD or MSI pumpage (Figures 6 and <br /> 7). This is because the average annual pumpage from these two wells is small in <br /> comparison with the remedial wellfield. <br /> The second simulation, involving the highest monthly pumping rates, indicated that the <br /> remedial wellfield maintained capture, even with the higher rates. <br /> Comparison of the two sets of simulations indicates that there is minimal adverse impact <br /> to the MSD or MSI wells from pumpage. Review of the model estimated chromium <br /> concentration changes in the MSD and MSI wells (Figures 9 and 10) indicates that even is <br /> the MSD/MSI wells pump at their highest rate for a year, chromium concentrations should <br /> not exceed MCLs (Figure 10). Chromium capture is still maintained by the reclamation <br /> wellfield and is not altered by pumping of the MSI or MSD wells. <br /> SUNWARY <br /> These model simulations indicate that the MSI and MSD wells could return to operation <br /> as discussed above and not adversely impact remedial wellfield capture. These model <br /> Potential Impacts from Main Street Wells December 1996 <br /> C:\gfv\admin\marley\batl2ms.doc <br />