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As stated previously it was determined that the potential crop consumptive use associated with <br /> the Hill & Brush Ditch shares was 1.63 acre-ft/acre. After analyzing the available irrigation <br /> water provided by the Hill& Brush Ditch, it was determined that the net irrigation requirement <br /> for the historic agricultural practices yielded 1.36 acre-ftlacre. Dry up credit for 15 acres will <br /> therefore provide 20.36 acre-ft annually. Replacement water will be discussed later in this <br /> report. <br /> The historic subsurface return flows were modeled under the following assumptions: <br /> • All water, which was diverted and not used by the crops, was considered to be deep <br /> percolation and returned to the stream system. <br /> • The recharge well was placed at a distance of 511 I0 <br /> r ce was etermined as the centroid of the historically irrigated area that has been <br /> removed from production. <br /> • A transmissivity of 50,000 gaUday/ft was assumed for the site. There was not any <br /> transmissivity data available for this area. A value of 50,000 gal/day/ft was assumed <br /> based on average transmissivity data for South Platte River tributaries similar to the <br /> Big Thompson River. (St. Vrain, Boulder Creek) <br /> • The specific yield was assumed as 0.2. <br /> • The location of the parallel impermeable boundary was assumed at a distance of <br /> 1700' from the live stream. Source: U.S.G.S. Johnstown and Windsor 7.5' <br /> Quadrangles. <br /> The stream depletion model is included in Appendix C for your review. <br /> Projected Stream Depletions <br /> A stream depletion model for the mining operation was completed for the Challenger Pit. Values <br /> presented in Table No. 2 were used in the analysis as the total depletion to the stream system. <br /> These values include water consumptively used for processing,dust control, and evaporation. <br /> The stream depletion model for the mining operation was developed under the following <br /> assumptions: <br /> • The pumping well was placed at a distance of 200' from the active stream. This <br /> location represents the down-gradient extent of the exposed surfaces. <br /> • The transmissivity was assumed as 50,000 gallft/day. This is the same transmissivity <br /> that was derived for the historic subsurface return flow model as presented <br /> previously. <br /> • A specific yield of 0.2 was assumed. <br /> • The location of the parallel impermeable boundary was assumed at a distance of <br /> 1700' from the live stream. Source: U.S.G.S. Johnstown and Windsor 7.5' <br /> Quadrangles. <br /> Table No. 4 represents a summation of the historic depletions versus the current depletions <br /> associated with the mining operation. <br /> 12 <br />