2013-04-11_HYDROLOGY - M1980149 - Laserfiche WebLink

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Ms. Michelle L. Hatcher Hokestra SWSP April 10, 2013 Page 5 of 9 preventing the refill of the pit. However, dewatering of the alluvial aquifer will result in depletions to the stream system. The initial dewatering will be made up of both lake water and alluvial aquifer water. The maintenance dewatering will be made up of only alluvial aquifer water. For the purposes of this SWSP, it is assumed that the initial dewatering will deplete the alluvial aquifer at the maintenance pumping rate. Therefore, the post - pumping depletions was modeled to show 50 gpm of the initial rate of 1, 500 gpm as causing a depletions and the remainder (1,450 gpm) will not be included in the depletion analysis as long as the liner has successfully been installed preventing the pit from refilling with ground water. Since Cell 2 has been continuously dewatered for some time, and because Cells 3 and 4 do not have exposed ground water, only maintenance dewatering will be required for these Cells. The maintenance dewatering activity of 50 gpm for all of the cells (Cell 2 through 6) will create lagged depletions to the stream as this water is coming from the surrounding alluvial aquifer outside the perimeter of the proposed slurry wall. The County will continue dewatering the cells until liner approval is obtained. Lagged dewatering depletions from dewatering that occurred during the liner construction will be replaced once the dewatering has ceased due to the presence of the liner. All dewatering operations must be metered with a totalizing flow meter. The County has projected an initial and maintenance dewatering rates of 1,500 and 50 gpm, respectively. In order to receive accretion credit, and in order to properly determine post - pumping depletions, submitted accounting must use the actual dewatering volumes as shown through a meter. The Applicant's consultant has confirmed that Hall Irwin will be the slurry wall contractor and that they will be installing their dewatering equipment complete with meters. After dewatering ceases, the river is expected to experience a post - pumping depletion in the amount of 50.3 acre -feet during this approval period. Post - pumping depletions will continue to impact the river after this approval period and must therefore continue to be included in future SWSPs. Replacements The sources of replacement water are 3.75 shares in the Rural Ditch Company owned by the County, storage of excess credits of these shares in Varra Companies' 112 Reservoir, and net accretions from dewatering activities. Dewatering operations will create accretions to the stream system sufficient to replace all depletions from pit operations from April 2013 through August 2013. September 2013 will be covered by Rural Ditch water that is immediately returned to the river. October 2013 through March 2014 will rely on a release of the Rural Ditch water that was stored in priority during the previous May through September 2013. In order to claim credit for the delivery of the 3.75 shares of the Rural Ditch to the river, the shares must be measured to the satisfaction of the water commissioner. You indicated that the County's Rural Ditch farm headgate has been improved and that construction plans are being developed to line the return ditch and install appropriate measuring devices. The County anticipated that construction on the return ditch would commence by summer 2012. This did not occur as scheduled. The County and Varra Companies now have a completed agreement for this work and construction is set to occur spring 2013, with the installation of the measuring devices by summer or fall 2013. The water commissioner must approve the measurement structure prior to use. Until the measuring device is installed, the County is unable to measure water downstream of their Rural Ditch farm headgate. Until the pipeline and measuring devices are installed, the applicant proposes to run the 3.75 shares of the Rural Ditch through the lined Varra Pit No. 112, which lies directly to the