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approaches 6 feet, the drain will be installed. A conceptual design of the groundwater <br />underdrain is included as Figure E-5. <br />CONCLUSIONS <br />This groundwater impact analysis was performed to evaluate the mounding and shadowing effect <br />the reclaimed condition has on the local groundwater table. It also was used to evaluate a <br />potential mitigation alternative for draining the groundwater mound up-gradient of the property <br />line. The model accurately replicated the conditions of the Arkansas River alluvial aquifer based <br />on data available from recent geotechnical investigations and from well data available from the <br />SEO. Model construction was facilitated by using an extensive GIS to inventory, analyze and <br />present the data. The boundary conditions consisted of constant head boundaries for the western <br />and eastern ends of the domain as well as for the Arkansas River and the unlined pond west of <br />the site. The model was calibrated by adjusting constant heads at the river to minimize the <br />difference between pre-mining groundwater levels at the site and simulated groundwater levels. <br />The mass balance had a-0.0018 percent error. <br />The model simulated the hydrologic changes caused by emplacement of the slurry wall and the <br />pit backfilled with wash fines. The predictive simulation showed that the magnitude of the <br />maximum groundwater mounding upgradient of the backfilled pit is about 1.65 feet, or a depth to <br />groundwater of about 6.5 feet. The simulation also indicates that the maximum shadowing effect <br />caused by the reclamation activities is about 1.5 feet downgradient of the proposed slurry wall. <br />The model also predicted the effect of a groundwater underdrain installed up-gradient of the <br />wash fines pit to lower the groundwater mound. This simulation results in a maximum <br />groundwater mound up-gradient of the property line of approximately one foot. The height of <br />the mound is dependent on the beginning elevation of the invert of the drain and the groundwater <br />head at the below-grade discharge point. Although the mound is lowered by about half a foot, <br />the drain would result in a groundwater table that roughly approximates the pre-mining heads. <br />The results of the groundwater modeling and conclusions drawn from them represent an <br />approximation and are based on the best available data. Conservative assumptions were made <br />during the calibration process so that groundwater mounding was not under-predicted. Given the <br />unknown heterogeneity of the aquifer in the field, groundwater mounding and/or drainage <br />mitigation may deviate from the model simulation. There is a possibility that mounding may be <br />higher than predicted, although the conservative assumptions of this work make the deviation <br />toward a lower mound in the field a more likely possibility. <br />Q:\0212 Stonewall Springs\0212.010 Southwest Reservoir Slurry Wall\DRMS Comment And Response\September 2017 Responses\Appendix E.Docx <br />E-7