2015-01-12_REVISION - M1985112 (9)

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Mr. J.C. York July 13, 2015 Page 3 structures with basements. JT has indicated that the installation of the drain around the slurry wall at other similar gravel pit locations has been successful in mitigating the mounding and shadow effects. The depth, location, and size of a drain will depend on the timing and location of rising water and hydrologic properties of the aquifer. Recommendations We recommend: 1. Canvasing the area within the area of influence to confirm well locations and determine whether any basements or structures could potentially be impacted; 2. Evaluating whether the existing monitoring well network is adequate to monitor recovery after the slurry wall is installed, and installing additional wells if necessary; 3. If actual recovery appears to be excessive, utilize the model to evaluate drain locations and designs (depths and size) to mitigate the situation. Intercepted groundwater could be piped down gradient and recharged to prevent impacts to senior water rights. Hydrologic Setting Up to approximately 100 feet of saturated sand and gravel make up the alluvial aquifer located within 2 miles of the Poudre River. The stratigraphy of the valley fill and particle size distributions beneath the Poudre River observed in gravel pits was examined in detail by Langer and Lindsey, 1999. The aquifer consists primarily of sand and gravel with minor fine-grained interbeds. The aquifer was evaluated and modeled in detail by CDM -Smith, 2013 to support the Colorado Water Conservation Board's (CWCB) South Platte Decision Support System (SPDSS). Model Construction MWE constructed a groundwater model of the alluvial aquifer north centered on the Loloff pit, extending approximately one mile on either side. The model domain is 10,000 feet square on each side, consisting of 100 rows and 100 columns with model 100 foot square model cells. The model includes all of Sections 4 and 5 in Township 5 North, 65 West (6th PM), and the northern half s of Sections 8 and 9. The model also includes the southern half's of Sections 31 and 32 in Township 6 North, 65 West (6th PM). Approximately 15,000 feet of the Poudre River crosses the model from west to east. The river is hydraulically connected to the alluvial aquifer at all times, and the groundwater gradient is from west to east. This is consistent with Figures 4-3 and 4-4 of the South Platte Alluvial groundwater model (CDM -Smith, 2013). Well Data Appendix A includes well data from 104 wells located within the model area. The database consists of only active well permits with the exception of a few abandoned wells with usable well data, none of which are included in Table 1. We removed shallow monitoring wells that did not penetrate the alluvium, wells in excess of 119 feet deep thought to be completed in bedrock, and all duplicate well permits. The depth of wells range from 21 to 119 feet and averages 45 feet. The depth to water ranges from 2 to 46 feet and averages 16 feet. The reported