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,,_,~, ~ ,..~,~_~ z zz .,~f i~• ~^ m~i z~a~ aa~+ rrnrtm .T or a <br />Stream depletion was simulated over 5 years to represent hydrologic <br />conditions during the first 5 years of mine development. The <br />computer analysis shows that there should be no impact to the St. <br />Vrain if the range of hydraulic conductivity of near surface <br />deposits is approximately lo' cm/s. However, if hydraulic <br />conductivity is approximately lo•~~cm/s, the analysis indicates that <br />the stream could first be impacted by dewatering activities during <br />the last several months of the first year. The attached computer <br />simulation (assuming 30'' cm/s) indicates a total stream depletion <br />volume of about 40 acre-feet after 5 years. Considering that an <br />estimated volume of 45,000 acre-feet/month of water is available at <br />any point along the St. Vrain River, this volume of depletion may <br />be insignificant. <br />DETERMINATION OF POST RECLAMATION WATER TABLE SURFACE ELEVATION <br />The steady state water table surface elevation for the reclaimed <br />mine pit was estimated through a two-phased modeling analysis. The <br />first phase of modeling was conceptual while the second entailed <br />preliminary 3-dimensional modeling to lend general support to the <br />conceptual analysis. The two-phased analysis is described below. <br />Modeling Approach <br />Since very little site specific data is available, the first aspect <br />of modeling involved simplification of the post reclamation <br />groundwater regime. In order to simplify the system and still <br />approximate the post reclamation groundwater regime, it was <br />necessary to make several assumptions. The primary assumptions are <br />as follows: <br />• The post reclamation groundwater system and the natural <br />groundwater system which immediately surrounds the <br />reclaimed pit area are each essentially homogenous and <br />isotropic. <br />• The reclamation fill material will have a K of <br />approximately 100 feet per day (.04 cm/s) and the <br />surrounding native material will have a K of approximately <br />.03 feet per day (10-5 cm(s). <br />• The size of the post reclamation pond will be approximately <br />20 acres and will be situated in the north central portion <br />of the reclaimed pit area. <br />• Annual lake evaporation is approximately 40 inches per <br />year. Average precipitation is approximately 15 inches per <br />year and recharge to the reclaimed pit ranges from a rate <br />4 <br />