2018-10-16_PERMIT FILE - M2018039 (2)

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feet of clay overburden (Empire 1975). In undisturbed areas, as much as a foot of topsoil exists (Empire 1975). However, the historic mining and reclamation activities on the site have significantly influenced soil and overburden structure and depth. Groundwater in the Cache la Poudre alluvium near the site generally has a saturated thickness less than 20 feet, is less than 10 feet below the land surface, and is under unconfined water table conditions (Robson 2000 and Empire 1975). The groundwater in the alluvium adjacent to the river represents a stream-aquifer system in which the groundwater and the surface water are in close hydraulic and hydrologic connection. The static water table fluctuates seasonally and year to year, but is generally no higher than the top of the alluvial sand and gravel deposit. The groundwater is tributary to the Cache la Poudre River and generally flows from west to east through the alluvium, parallel to and slightly towards the river (Robson 2000). The Cache la Poudre River is used extensively for irrigated agriculture in the valley and on the adjacent lands. Water is supplied for irrigation by diversion of surface flow directly from the river and its tributaries through a network of irrigation canals and ditches. Pumping by shallow wells for domestic use and irrigation represents a principal source of alluvial groundwater depletion. The primary source of alluvial aquifer recharge is the river and its tributaries. Recharge to the alluvial aquifer also consists of infiltration of diverted ditch flows, applied irrigation water and deep percolation of precipitation. Water levels in the aquifer in close proximity to the river channel can be influenced by changes in stream flow over a short period of time, whereas water levels in the aquifer distant from the river are likely influenced by changes in stream flow over the long term and an overall increase in availability of surface water for irrigation. 2.2 Site Conditions Information on the subsurface conditions for the project site was derived from site boring logs (Empire 1975) and associated sample analyses performed prior to the original mining activities. Additional subsurface, groundwater, and alluvial aquifer characteristics were obtained from published studies and reports prepared by federal and State water resource agencies. Selected reference material is provided in Appendix A. Records were also obtained from the Colorado Division of Water Resources, Office of the State Engineer (SEO) for site and nearby alluvial well information. Geotechnical information from slurry wall construction at the Treiber Lakes site was also used for alluvial characterization. During the August 1975 geotechnical investigation (Empire 1975) exploration holes were drilled to define sand and gravel reserves for the mining and reclamation operation. The drilling included a total of 6 holes on or near the site. Applicable data from the report are summarized in Table 1 and the report and other selected geotechnical information is provided in Appendix B. Although the 1975 exploration provides estimates of alluvial thickness and groundwater elevations, sieve analyses and other geotechnical evaluations were not available. Generally, however, the report describes the overburden as a damp clay containing varying amounts of silt and sand extending from 2.5 to 4 feet below the ground surface. The alluvial material is generally less than 20 feet thick, consists of well-graded sand and gravel with cobbles and boulders up to 18 inches in diameter (Empire 1975). Overall, the samples indicate that the characteristics of the alluvium are somewhat consistent across the site. The upper one-foot of the underlying claystone material is weathered, but the deeper material is described as firm (Empire 1975). This clay layer is interpreted to be the underlying Pierre Shale bedrock (Robson 2000). (blue earth Page 3 of 12 SOLUPONS