2017-03-23_HYDROLOGY - M1980149 - Laserfiche WebLink

Home Browse Search

Hokestra SWSP Page 3 of 11 March 20, 2017 and 6 will be mined in the future. No mining will occur outside of the lined areas during the period of this SWSP. According to the information provided by the applicant, the County has lined Cells 2 through 6. Liner tests were approved on October 7, 2014. For the proposed mining operations in Cell 3 through 6, water within the liner boundary is now classified as trapped native ground water. The trapped native ground water may be removed from within the lined area and returned to the stream system through surface flow or ground water recharge without the need for replacement so long as the operator does not put the water to beneficial use. All native trapped ground water remaining that is put to beneficial use (except for the water removed in product) must be replaced by the operator. All inflow of water into the liner boundary from any source, including precipitation and ground water inflows shall be removed by the pit operator so as not to cause out-of-priority storage. Currently 46.7 acres of ground water surface area is exposed at the Hokestra Pit site. The Applicant is claiming 14.1 acres of pre-1981 exposed area under this SWSP. Previous SWSPs (prior to 2009) recognized a pre-81 area of 14.1 acres; the Applicant was not required to replace evaporative depletions for 14.1 acres of exposed surface area regardless of where within the permit boundary water was exposed. Since the Applicant was previously allowed to reallocate 14.1 acres of pre-81 area, the State Engineer’s Office will allow the 14.1 acres of pre-81 area to be set at a location other than the location of the original pre-81 exposure. The area to receive the pre-81 credit is shown on the attached “Figure 2”. The exemption from augmentation requirements for the pre-1981 area is tied to the physical location identified on this figure and may not be applied to other areas of ground water exposure within the gravel pit permit boundaries. Net evaporative depletions were calculated using a gross annual evaporation of 40 inches from the exposed water surface, with a credit of 9.38 inches for effective precipitation. No phreatophyte credit has been applied to this plan. The net depletion of ground water due to evaporation from the 32.6 acres of ground water exposed at the site after December 31, 1980 was calculated to be 83.2 acre-feet. This assumes that Cells 2 through 6 do not expose any ground water due to their liners or dewatering activities. See Table 3 for a monthly breakdown of evaporative depletions at the site. Weld County uses ground water from the southwestern reclaimed cell (Cell 1) to irrigate landscape at the Weld County Southwest Service Center (SWSC). Weld County conducted a field survey on February 7, 2008 and reported that the landscaped area consists of 0.15 acres of blue grass and 5.5 acres of native grass. This area will be irrigated using water pumped from Cell 1 during the growing season of April through October. According to a Modified Blaney-Criddle analysis and the data from the Longmont 2ESE weather station, the net irrigation requirement for the 0.15 acres of blue grass is 0.3 acre-feet, and the net irrigation requirement for the 5.5 acres of native grass is 9.4 acre-feet per year (Table 4). Since the irrigation occurs directly around the reclaimed cell, the plan assumes that irrigation return flows will return to the river in the same timing as the depletions. The total consumptive use of ground water associated with the Hokestra Pit is estimated to be 92.8 acre-feet per year, consisting of evaporation loss from 32.6 acres of post-80 exposed surface area and water used for irrigation at the SWSC. The timing of depletions to the Saint Vrain River due to consumptive use of ground water at Cell 1 and 7-12 will be calculated using the Alluvial Water Accounting System (“AWAS”) model developed by the IDS Group at Colorado State University. The aquifer parameters used in the model are shown in Table 2: