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Mr. Carl Mount <br />September 14, 2005 <br />Page 3 <br />percent in the modeling versus 0.22 percent from the piezometric surface contours), and the <br />groundwater elevations for the non-growing season surface. The elevations from the monitoring <br />data during the growing season are around ]-foot lower than those in the modeling. This <br />comparison suggests that the steady-state modeling was reasonable in terms of predicting existing <br />goundwater levels and piezometric surface shape. <br />Possible Impacts. (I+tigure 2 of the March 2003 report). You indicated in your conversation with <br />Ms. Duncan that you would like to understand what could happen with installation of the slurry wall <br />(e.g., basements flooded, cottonwood trees killed, etc.). The modeling showed that the magnitude <br />of effects due to the Heit Pit will be relatively small (2-feet of mounding and 3 feet of shadow <br />effect). The eighteen months of monitoring data indicate that the predicted changes in groundwater <br />levels are close to the range of natural variation in groundwater levels. In addition, the county's <br />Koenig pit, which is lined, has been in place immediately south of the proposed Heit Pit without any <br />known negative impacts on groundwater from this pit. These factors indicate that any negative <br />impacts will likely be small, and possibly difficult to detect. <br />Additional information on mitigation measures. The mitigation measures included in our July <br />2003 report included: <br />• Pipeline to equalize groundwater. <br />• Deepening of existing wells. <br />• Drilling new wells. <br />• Delivery of water from the Chambers Island Ditch. <br />• Relocation of wetland nursery stock (for Aquatic and Wetland Company nursery to the <br />north). <br />• Excavation of area for nursery stock (for Aquatic and Wetland Company nursery to the <br />north). <br />• Emergency pumping. <br />A reassessment of mitigation measures that could be used reaffirms that these are appropriate <br />mitigation measures. The specific measure used (if needed) will depend on the nature of any impact <br />on groundwater levels. It is not possible to pre-determine the appropriate mitigation measure. <br />The mitigation measures have the potential to adjust groundwater. Because groundwater flow <br />through the permit area is dynamic, it is difficult to calculate an exact rate and volume of water <br />required to attain the required adjustment. The amount will depend on the specific location and <br />nature of the impact. <br />