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I1 <br />1 Figure 10 shows the change in groundwater level between the low groundwater levels in winter <br />and the high groundwater levels during the spring/summer irrigation season at the LaFarge wells. <br />' Depending on the particular year, the peak groundwater elevation at these wells generally occurs <br />in June, July, or August. The seasonal groundwater level fluctuation at the LaFarge wells is <br />1 typically about 4 feet. However, in 1998, the seasonal groundwater level fluctuation was as much <br />as 9 feet (L-2), about twice the typical seasonal groundwater level fluctuation. <br />1 5.4 Comparison of Pre- and Post-Slurry Wall Groundwater Levels <br />I 1 The most reasonable approach to determine pre- and post-slurry wall groundwater level changes <br />is to compaze groundwater levels for the same time period pre- and post-sluny wall. A <br />1 comparison of time periods representing the potentially highest and lowest seasonal groundwater <br />levels in the area was made by comparing potentiometric maps for August 1999 and 2001 and <br />December 1999 and 2001. <br />1 <br />To determine the maximum groundwater level increases caused by spring precipitation and <br />' summer irrigation, the potentiometric maps for August 1999 (Figure 11) and August 2001 <br />(Figure 12) were compared by subtracting the groundwater elevations depicted on the August <br />1 1999 map from those shown on the August 2001 map. Positive differences indicate that the water <br />table was higher in 2001 than 1999; whereas negative differences indicate that the water table <br />was lower in 2001 than 1999. Figure 13 is the resulting map of groundwater level changes <br />between August 1999 and 2001, the seasonally high groundwater period. <br />Figure 13 shows the complex groundwater level change pattern created by irrigation seepage <br />during the summer. In general, this map shows that groundwater levels are up to 3 feet higher in <br />the area of the Roe and Martin properties since the slurry wall was installed and as much as 4 <br />feet lower in the immediate vicinity of the slurry wall. This figure (Figure 13) also shows that the <br />groundwater levels south of the Roe and Martin properties aze 1 to 2 feet lower since the slurry <br />wall was installed, suggesting that Local groundwater withdrawal may occur in this area. <br />To minimize the effect of spring precipitation and summer imgation on groundwater levels, a <br />comparison of the potentiometric maps for December 1999 (Figure 14) and December 2001 <br />(Figure 15) was also made as described above. December was chosen because it represents the <br />period of seasonally low groundwater levels. Figure 16 is the resulting map of groundwater level <br />changes between December 1999 and 2001, the seasonally low groundwater period. The <br />groundwater level changes depicted on Figure 16 are discussed in the sections below. <br />11 <br />