2002-02-20_REVISION - M1999021 - Laserfiche WebLink

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southeast corner of the Stegner Fann slurry wall and is expected to be least affected by groundwater level changes caused by the Stegner Fann slurry wall. 5.1 Pre-Slurry Wall Groundwater Levels Pre-slurry wall groundwater conditions were evaluated using monthly groundwater monitoring data collected at the Stegner Farm wells between ApriUAugust 1999 and April 2000 and at the LaFarge wells between February 1998 and April 2000. Groundwater hydrographs were prepared using these data to show fluctuations in groundwater elevation prior to the construction of the slurry wall. Groundwater levels at the Stegner Farm wells aze generally measured during the middle of each month; whereas, groundwater levels at the LaFarge wells are generally measured during the first week of each month. Figure 5 shows the pre-slurry wall groundwater levels for both the Stegner Farm and LaFarge monitoring wells. Figure 5 shows pre-slurry wall groundwater elevations at the eight (S-1 through S-8) Stegner Farm monitoring wells for the period April 1999 through April 2000. Groundwater levels at wells S-7 and S-8 were measured between April 1999 and April 2000; whereas groundwater levels at wells S-1 through S-6 were measured between August 1999 and April 2000. The groundwater levels for the four LaFarge monitoring wells (L-1, L-2, L-3 and L-5) are included for comparison. Groundwater levels were not obtained at the LaFarge wells in June 1999. Examination of Figure 5 indicates that a generally decreasing trend in groundwater levels occurs between ApriUAugust 1999 and April 2000. The average groundwater level decrease during this period is about 4.6 feet at wells S-1 through S-6 and 6.6 feet at wells S-7 and S-8. The decreasing trend depicted in Figure 5 is typical of the decreasing groundwater levels following the peak groundwater elevation that typically occur during the summer irrigation season. The LaFarge well hydrographs indicate that groundwater levels increased between May and August 1999, the summer irrigation period. Peak groundwater levels at the Stegner Farm wells during the pre- slurry wall monitoring period occurred in July 1999, whereas peak groundwater levels in the LaFarge wells occurred in Augusi 1999. Figure 6 is a potentiometric map that shows the elevation of the water table in the Stegner Farm area in April 2000, the month that slurry wall installation was initiated. This map was prepared using groundwater elevation data from both the Stegner Farm and LaFarge monitoring wells, a total of 12 wells. Each contour line shown on the map represents a line of equal groundwater elevation. For example, the groundwater elevation anywhere along the 5020 contour is equal to 5020 feet above mean sea level (ft amsl). The potentiometric map indicates that the water table decreases in elevation from the northwest (well S-3) to the southeast (well L-1). Groundwater 8