|
6.3 Miti_gation
<br /> This section describes potential mitigation actions to decrease the mounding effects of slurry wall construction on the
<br /> surrounding properties and the Town. The shadow effects to the north will likely be minimized by the proximity to the
<br /> unnamed slough and the infiltration ponds to the northeast of the Site. During the mining phases at the Site, an unlined
<br /> siltation pond will be present in the northwest corner of the Site which will also mitigate some of the shadow effects. At
<br /> this time no mitigation is recommended to address shadowing to the north unless groundwater monitoring demonstrates
<br /> that the drop in groundwater head elevations is larger than anticipated.
<br /> The groundwater table to the south of the Site and underlying the Town of Gilcrest is anticipated to rise as a result of the
<br /> full build out of the slurry walls at the Site, mitigation for mounding is warranted at the southeastern end of the Site. The
<br /> mounding is expected to be greatest in the southeast corner of the Site and decreases to the west. Mounding at the
<br /> southern end of the Site boundary and in Town is not expected to rise more than 2 feet before the construction of the
<br /> Cell 5S slurry wall so mitigation is not warranted until the installation of the Cell 5S slurry wall is constructed or Site
<br /> monitoring observes a higher than anticipated groundwater elevation at the southern monitoring wells(MW-5 and MW-6).
<br /> If the Cell 5S slurry wall is not constructed (see discussion below),then the underdrain would not be warranted until the
<br /> construction of the Cell 6 slurry wall. See Table 4 and 5 below.
<br /> Table 4
<br /> Base Model and Modeled Site Groundwater Level After Cell 3 4 and 5N Slurry Walls
<br /> Well Base Modeled Cell 3,4, and 5N Difference Cell IJK
<br /> MW-1 4,727.3 4,727.6 0.3 146,111,1
<br /> MW-2 4,718.2 4,717.1 -1.1 136,119,1
<br /> MW-3 4,725.7 4,726.5 0.8 142,136,1
<br /> MW-4 4,733.0 4,734.9 1.9 153,136,1
<br /> MW-5 4,736.8 4,737.5 0.7 162,133,1
<br /> MW-6 4,737.8 4,738.0 0.2 162,115,1
<br /> MW-7 4,729.2 4,729.0 -0.2 148,122,1
<br /> BBA-MW4 4,740.8 4,741.1 0.3 148,122,2
<br /> BBA-MW11 4,737.9 4,738.2 0.3 148,122,3
<br /> BBA-MW18 4,738.4 4,738.8 1 0.4 1 148,122,4
<br /> BBA-EC 4,734.0 4,734.3 0.3 1 148,122,5
<br /> Groundwater Model Report—Section 20 Gravel Mine Site -9- August 2022
<br />
|