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RE: L. G. Everist, Inc., Fort Lupton Sand and Gravel Mine, DMG #M-1999-120; <br />Second Adequacy Response Letter <br />10. Little Dry Creek...capacity... <br />The groundwater mounding that is projected by the model to occur on the west side of the <br />expansion will occur along the gentle valley bottom of Little Dry Creek. The topography of this <br />area is such that shallow groundwater and any standing surface water will naturally flow towards <br />the creek, since is it located in the valley bottom. The creek is incised 3 to almost 6 feet. <br />Groundwater higher than the creek bottom will drain into the creek. That is, the creek will act <br />similar to a common agricultural drain. The water in the creek will be conveyed to the north <br />under County Road 18. A 5' by 7' culvert exists under the road. <br />Field measurements were taken of Little Dry Creek to assess its capacity. The average cross- <br />sectional area of the creek along the west side of the expansion is approximately 30 square- <br />feet. The average channel width is 7.4 feet and the average slope is 0.13 percent. At an <br />average velocity of 1 foot per second, the channel capacity would be approximately 30 cfs. <br />Base-flow is typically 1-2 cfs, at a water depth of around 0.6 feet. <br />The projected rate of flow of groundwater into the mound is approximately 0.6 cfs, based on the <br />computer modeling completed by WWE. Groundwater higher than the creek bottom would flow <br />into the creek. This projected flow will occur as diffuse seepage over a relatively long reach of <br />the creek (e.g., the creek is 2500 feet long on the west side of the expansion). The 0.6 cfs of <br />groundwater flow out of the mound can easily be accommodated by the Little Dry Creek <br />channel and existing culvert under County Road 18. Furthermore, this diffused flow will occur <br />over a long reach of the creek. Therefore, there will not be any significant effects on the <br />channel or hydrologic balance from the mounding. <br />There are two monitoring wells along Little Dry Creek, which will continue to be monitored on a <br />monthly basis. <br />11. Wetland adjacent to Little Dry Creek <br />The wetland along Little Dry Creek is sustained by shallow groundwater and periodic overbank <br />flooding from the creek. As previously described, shallow groundwater and any standing water <br />will flow towards the creek. Significant surface ponding will not occur along the creek on the <br />west side of the expansion because there are no large depressions or areas where water can <br />pond, and water will naturally flow to the north. <br />The mounding that is projected to occur will increase the frequency and extent of saturated <br />soils. This increased water supply will be favorable to the wetlands, which currently have some <br />facultative species and upland weeds. The only potentially negative impact from increased <br />water on wetlands is with significant inundation (i.e. greater than 3 feet of permanently standing <br />water), and this will not occur at the subject wetlands. The increased water supply from the <br />mounding will favor a shift to more obligate wetland species such as bulrushes, spikerush, <br />cattails, sand bar willow, etc. <br />There are two monitoring wells along Little Dry Creek, which will continue to be monitored on a <br />monthly basis. If the groundwater levels change enough to hit the trigger, then LGE will start an <br />assessment, and if responsible, will implement necessary mitigation measures. <br />LM/FL-Resp2ndAdeq Rev-dmgKP-012105.doc 5 <br />