2023-02-16_HYDROLOGY - M2000158 - Laserfiche WebLink

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David M. Heintz S &t H Mine SWSP February 16, 2023 Page 2 of 11 Combined Plan from a lease of effluent from the City of Aurora; and excess Whitney Ditch consumptive use credits owned by MMM. Depletions The Platte SFIG Mine includes three unlined ponds that currently expose a total of 13.39 acres of ground water of which 9.74 acres are exposed within a bigger pond known as the Sharkey's Pond and the remaining 3.65 acres exposed within two small ponds. The evaporation depletions from the 9.74 acres have historically been replaced under SWSPs approvals for the S&tH Mine. MMM started mining within the two small ponds in October 2018. Dewatering at the site started to take place in two mine cells during the previous SWSP, as shown in Figure 1. According to the information submitted, no water surface was exposed within the permit boundary prior to January 1, 1981. The Applicant proposed to replace evaporation from exposed ground water at the site based upon evaporation atlases in NOAA Technical Report NWS 33 and the SEO monthly distribution factors for sites below 6,500 feet, as shown in attached Table 1. Gross annual evaporation at the gravel pit location is estimated to be 43.00 inches per year. Net evaporation is defined as gross evaporation less the consumptive use of water by vegetation that naturally occurred at the site prior to construction of the pit. The historical consumptive use was assumed to be equal to the effective precipitation, which was estimated based on the data from the Greeley UNC climate station for the time period of 1967-2017. The net evaporation from the exposed water surface is 36.60 acre-feet during the period of this SWSP, as shown in Table 1, column 7. In addition to the evaporation, water is lost with the mined product removed from the mine site. The Applicant projected that they will produce 680,800 tons of gravel during the period of this SWSP. All mined material from the site will be washed and transported offsite. Mining operation at the site occurs within the approved liner of a slurry wall. The slurry wall for the Milliken C-2 Reservoir (WDID 0203388) was provisionally approved on December 9, 2020. The mined material will be washed therefore the ground water lost with the mined product during this period is estimated at 2 percent by weight. The water lost with the mined product is projected to total 10.02 acre-feet during the period of this SWSP, as shown in Table 1, column 9. Water for gravel washing and dust control will be provided by pumping water from the 9.74 acre unlined pond. The estimated water used for dust suppression during the period of this SWSP total 9.99 acre-feet, as shown in Table 1, column 10. Dewatering will continue within the slurry wall prior to the mining activities. Any dewatering water discharged to the South Platte River must be measured by totalizing flow meters that can accurately show the monthly volume of dewatered water that is pumped and returned to the stream, and such volumes must be reported on the submitted accounting as part of this SWSP. MMM is not seeking to use any dewatering accretion credits for the replacement of depletions at the SFIG Mine or at any other sites under this SWSP. The dewatering water needs to be returned to the stream without consumption.