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

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Recharge flux within the radius of influence (W) was estimated based on annual precipitation (WRCC 2018) and inputs from irrigated agriculture (CSU 2009). Annual precipitation in the region averages 15.1 inches/year (WRCC 2018). With the majority of precipitation occurring during sporadic, high intensity thunderstorms, it was assumed that 20 percent contributes to alluvial groundwater recharge. The recommended average annual water need for grass pasture crops in the Longmont/Greeley area is about 26 inches/year (CSU 2009). Irrigation, therefore, must contribute 10.9 inches/year to make up for the precipitation deficit. Considering the soaking nature of irrigated agriculture, it was assumed that 50 percent of the irrigation water contributes to the aquifer recharge. To estimate the potential dewatering impacts, the radius of influence was assumed to extend to a point where analysis results showed an alluvial groundwater drawdown of approximately 20 percent of the saturated thickness or 3 feet. This assumption is also reasonable considering groundwater elevation and saturated thickness can vary up to several feet seasonally. Within the estimated radius of influence, obstructions will increase and decrease the affects of dewatering. To the northeast, the Cache la Poudre River borders the site boundary. It is assumed that the river is in direct connection with the alluvial groundwater and, therefore, continuous recharge from the river will significantly influence the affects of dewatering. Impacts from dewatering will not extend north or west of the river, and the radius of influence will be reduced immediately south and east of the river due to the increased recharge flux. Alluvial aquifer saturated thickness and recharge are very important parameters in the analytical solution. In addition to assumptions made estimating the saturated thickness and input from recharge, these two parameters also vary seasonally and from year to year. Values for the parameters, however, are used in the analytical solution based on annual averages. Therefore, variability in these parameters will make observed impacts from the analytical solution differ seasonally and year to year. Additionally, affects from obstructions such as seasonal recharge from the river and irrigation canals such as the Fort Collins Irrigation Canal Ditch are not considered in the analytical solution. Considering all assumptions and parameter variability, it needs to be noted that results of the analytical solution should only be taken as a rough approximation of potential dewatering impacts and alluvial groundwater elevation monitoring is recommended to evaluate actual groundwater impacts. For the Lamb Lakes dewatering analysis, it is assumed that only one lake will be dewatered at a time with each lake creating a circular pit with a radius of 600 feet. Based on available information and parameter assumptions, the unobstructed radius of influence was estimated to extend over 1300 feet beyond the outer-most cell wall with dewatering flows from the cells of almost 130 gallons per minute. However, if three feet of drawdaown from the existing groundwater surface is considered to be the significant affect, the significant radius of influence was estimated to be approximately 650 feet from the cell walls (Figure 3). Drawdown of the alluvial groundwater elevation is illustrated in Figure 4 and the spreadsheet analysis results are included in Appendix C. As discussed above, the drawdown affects will be reduced by the Cache la Poudre River recharge and drawdown affects will not extend north or west of the river. The result of these affects on the radius of influence has been estimated in Figure 4. Although Figure 4 presents an estimate of potential dewatering impacts, alluvial groundwater elevation monitoring is recommended to evaluate actual impacts. (blue ear'fh Page 8 of 12 SOLUliONS