2020-06-25_REVISION - C1981019 (6)

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RULE 2 PERMITS This is more flow than originates from the potentially-impacted springs, which have an average annual flow of 77 gpm. The alluvial aquifer associated with Good Spring Creek has a high transmissivity and is unconfined. Possible impacts to this aquifer would be associated with the infiltration of water from the pit and water quality deviations caused by infiltration of runoff water. The preferential flowpath of bedrock groundwater from the reclaimed pits would tend to be down-dip through and between the different strata of the Williams Fork Formation. The discharge would be to springs and, thus, some groundwater could eventually recharge the alluvial material of Good Spring Creek. Transmissivity of the Williams Fork Formation is presented in Section 2.04.7. Measured and published transmissivities of the upper Williams Fork Formation average about 50 square feet per day (ft2/d). The average hydraulic conductivity of the formation is about 1 foot per day (ft/d). The values utilized to calculate these averages are presented in Table 2.04.7-26 and are from published data (Robson and Stewart, 1990;tables 5 and 6; upper member Williams Fork Formation). A rectangular infiltration area in the undisturbed pit highwall of 133 feet long by 133 feet high could transmit all of the estimated 92 gpm (approximately 150 acre-feet) of annual recharge from the reclaimed pit. This is calculated as follows: Annual seepage from the pit= (133 ft high)(133 ft long)(I ft/d)= 17,710 ft3/d= 150 ac-ft/yr. With approximately 400,000 square feet of buried highwall, all of the meteoric water infiltrating into the reclaimed pit that contacts the pit wall is expected to enter the strata of the Williams Fork Formation. Most of this water is expected to eventually contribute to seeps and springs tributary to Good Spring Creek. This suggests that it is possible that a reclaimed pit aquifer (if it develops) will flow entirely into the undisturbed strata, and that there will be no or limited discharge into the surficial alluvium/colluvium from the reclaimed pit. Whether the pit aquifer discharges into the bedrock of the Williams Fork Formation or into surface colluvium, it will eventually contribute to the alluvial aquifer and springs tributary to Good Spring Creek. To evaluate the possible effects of infiltration from the pit areas, a velocity calculation for average groundwater flow can be performed. The calculation is based upon the parameters determined for the Williams Fork Formation as discussed above. Seepage velocity (vs), the true velocity representing the rate the groundwater flows through the pore spaces can be calculated utilizing the following formula(Fetter 2001): vs=Kdh/nedl where: • K is the hydraulic conductivity, • dh is the vertical difference in groundwater elevations between two points, and • ne is the effective porosity, and dl is the distance between the two points. Although the strata between the pit and the creek are discontinuous, the elevation difference between the pit aquifer and Good Spring Creek (500 feet) and the horizontal distance between the edge of the pit and South Taylor/Lower Wilson—Rule 2,Page 81 Revision Date: 6/22/20 Revision No.: MR-220