2013-11-19_HYDROLOGY - M2005059 - Laserfiche WebLink

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Nelson Mining Resource SWSP November 12, 2013 Page 2 of 6 provided a monthly breakdown of the annual depletions at this site in your attached Table All. 1. The following parameters were used in the model: transmissivity (T) = 157,000 gallons per day per foot; specific yield (SY) = 0.2, the distance (X) from the exposed ground water surface to the creek = 2,013 feet, and the distance (W) from the aquifer boundary through the well to the creek channel = 2,126 feet. The total lagged depletions were determined to be 30.75 acre -feet for this plan period. The distance (X) from the exposed water surface to the creek was measured from the edge of the pit, instead of from the centroid of the exposed water surface area. Since the modeled lagged depletions assume steady -state conditions have been reached, the use of the X value shown above will be accepted for the purposes of this SWSP. However, if depletions at the site change in the future, the Applicant must calculate the lagged depletions with the X value measured from the centroid of the exposed water surface area. Replacements The proposed source of replacement for this pit is the historical consumptive use from the dry-up of 30 acres of land historically irrigated with 0.434 shares of Last Chance Ditch diverted from St. Vrain Creek and previously used for irrigation of the mined property. Irrigation has ceased on the Nelson Pit site due to the mining operation and groundwater lakes. The landowner, Harold Nelson, originally owned 1 and 5 /9th share (1.55 shares) of the outstanding 20 shares of the Last Chance Ditch. In the 1990's Mr. Nelson sold 1/18 1h of a share, leaving 1.5 shares under his ownership. The subject Last Chance Ditch shares were historically used for irrigation on the Nelson Pit mining site. The historical use of the irrigation supply was quantified using the average monthly diversion from 1950 through 2005. The total irrigated crop acreage varied from 101 acres (1950 -1965) to 121 acres (1966 -1990) to 72 acres (1991- 2005). The ditch loss used the consumptive use analysis was 10 percent based on the ditch location, its length and previous experience and the irrigation efficiency was taken as 50 percent. Water in excess of the irrigation requirement was added to the soil moisture bank, which was assumed to have a water holding capacity of 1.95 inch /ft. The consumptive use analysis for the 1.55 shares averaged 160.6 acre -feet per year with a non - irrigation season return flow obligation of 14.6 acre -feet (see Table AI.7). The pro -rata historical consumptive use credit for the 0.434 shares is 48.18 acre -feet with a non - irrigation season return flow obligation of 4.34 acre -feet. The historic consumptive use analysis for the Nelson Pit mining site was based on continuous ownership of the 1.55 shares. This analysis will be accepted for this SWSP plan period; however, any future SWSP request must include a revised historic consumptive use analysis based on ownership of 1.55 shares from 1950 to 1993 and 1.5 shares from 1994 to 2005. After applying the 0.434 shares of Last Chance Ditch water, there will be 16.87 acre -feet of uncompensated depletions. Additional replacements for depletions during the non - irrigation season and during months with insufficient credits will be made by releases of fully consumable wastewater treatment plant effluent or reservoir releases from a lease with the City of Longmont ( "Longmont "). The Applicant provided a copy of the renewed lease agreement with Longmont, dated September 3, 2013, to this office and the lease is attached to this letter. In accordance with the lease, Longmont will provide a total of 17.31 acre -feet of fully consumable water during the period of November 2013 through April 2014, and October 2014. The Longmont wastewater treatment plant is located approximately 10.5 miles upstream of this new mining operation. A 0.25 percent per mile transit loss (2.625 percent overall loss) has been accounted for in the amount of replacement water obtained from Longmont. The monthly depletions and replacement requirements are found on the attached Table All. 1.