2012-02-03_HYDROLOGY - M1998013 - Laserfiche WebLink

Home Browse Search

Mr. Andy Rodriguez, P.E Page 2 of 5 Shores SWSP January 26, 2012 stream depletions that result from mining related exposure of ground water. The DRMS letter identifies four approaches to satisfy this requirement. In accordance with approach nos. 1 and 3, you have obtained a bond for $1,045,840 through DRMS for lining or backfilling of the exposed ground water. Depletions Currently 11 acres of ground water surface is exposed at the Shores Pit within the silt pond. The net depletion of ground water due to evaporation from these 11 acres is calculated to be 29.45 acre -feet. Net evaporative depletions were calculated using a gross annual evaporation of 42 inches from the exposed water surface, with a credit of 10.56 inches for effective precipitation. Computation of evaporation under this plan was also reduced during the ice covered period. You have assumed the ice covered period to occur during the months of January and December, based on the average temperatures of 26.99 °F for January and 28.62 °F for December; taken from the Longmont 2ESE weather station. However, for the purpose of this SWSP, the Applicant shall replace the net evaporation depletions from the exposed ground water surface area that may occur during the assumed ice covered period (the months of January and December) for any time that the water surface is not completely covered by ice. Computation of the net evaporation during any time that the water surface is not completely covered by ice shall be determined as the pro -rata amount of the monthly gross evaporation rate distribution amount identified in the State Engineer's General Guidelines for Substitute Supply Plans for Sand and Gravel Pits, subtracting the pro -rata amount of the effective precipitation for that period. Ground water from the 11 acre silt pond is used for washing aggregate, dust control and concrete batching. The Applicant anticipates that 387,502 tons of aggregate will be mined during this plan period. The material is mined below the ground water table and is washed; therefore the water retained in the mined product is considered to be 4.0% of the mined material by weight, equal to 11.40 acre -feet. The Applicant estimates that 1.66 acre -feet of water per year will be used for dust control. Approximately 85,000 cubic yards of concrete will be batched onsite at a depletion of 40 gallons per cubic yard, resulting in an annual depletion of 10.43 acre - feet. The net evaporation and operation depletions associated with the Shores pit are estimated to be 52.93 acre -feet, as shown in the attached Table 4 & 5. Dewatering of the site commenced in February 1999 at a rate of 300 gallons per minute and had remained constant during mining of cells A, B, and C. For modeling purposes dewatering was stopped on December 2007 which is the year the slurry walls for the three cells were completed. As long as dewatering operations in cell D remain constant, it is assumed that the accretion credits are sufficient to offset any lagged depletions as this water is not used and immediately returns to the stream. The SWSP accounts for the net depletions at the river that occur due to the past dewatering operations (February 1999 through December 2007) The depletions from past dewatering at the site during the period of this plan total 35.17 acre -feet. The depletions from evaporation, water removed in the mined product, dust suppression, concrete batching and dewatering were lagged to Saint Vrain Creek using a Glover analysis. The following parameters were used in the model to lag operational and evaporative losses: transmissivity (T) = 72,300 gallons per day per foot, specific yield (SY) = 0.2, the distance from the exposed ground water to the creek (X) = 5,409 feet, and the location from the parallel impermeable boundary to the creek (W) = 10,000 feet. The attached Table 5 provides a monthly schedule of lagged depletions to the river system due to operational losses, shown to be 57.22 acre -feet for this plan period. Including depletions due to past dewatering activities, the total lagged depletion to the river system in 2012 is 92.39 acre -feet, as shown on Table 7.