2024-01-31_REVISION - M1977344 (33)

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Mr. Michael Toelle Holcim US, Inc. January 31, 2024 Page 5 Holcim RCQ Letter Report 20240131 Infiltration Calculations The RCQ will consist of an open-pit limestone quarry and two sandstone pits. The site is comprised of Cretaceous age marine sediments of inter-bedded limestones, sand- stones, and shale (Scott and Cobban 1964). During mining, four open pits will be exca- vated to the Codell Sandstone layer, which slopes toward the north. At reclamation, one of the pits will be partially backfilled with shale and the other three pits will have ex- posed pit sidewalls and Codell Sandstone bottoms. The backfilled pit surface and the pit bottoms are shown on the reclamation plan as being level surfaces that cover a rela- tively large area (approximately 840 acres total). As part of a multi-year baseline investigation to document and characterize the site hy- drologic system, five monitor wells were installed at the RCQ in November 2021 and are screened in the Codell Sandstone (BC 2023). The wells were spaced to ensure the data collected from each well was spatially distributed to characterize the hydrologic proper- ties of the water bearing units. Four of the wells were completed in the Codell Sandstone and one well was completed in the Fort Hays Limestone. The hydrologic properties of the Codell Sandstone were evaluated to assess the ground- water system at the Site. Slug tests were conducted to estimate hydraulic conductivities of the sandstone and limestone layers. The hydraulic conductivity for the Codell Sand- stone was determined from these tests to be approximately 0.011 feet per day (ft/day; BC 2023). The hydraulic properties for the Fort Hays Limestone were estimated based on previous groundwater modeling efforts in the area to be 0.0054 ft/day (Resource Ge- oscience, Inc. 1999). No hydraulic information currently exists for the Smokey Hill Shale pit backfill material; therefore, the RCQ groundwater model used an estimated hydraulic conductivity of 0.5 ft/day, which is two orders of magnitude larger than the Fort Hayes Limestone. For the infiltration calculations, the backfilled pit surface and the exposed pit bottoms were assumed to be completely level (as shown by the lowest closed contour elevation). The backfilled pit was assumed to have an infiltration rate (i.e., hydraulic conductivity) of 0.5 ft/day and the exposed pit bottoms were assumed to have an infiltration rate of 0.011 ft/day. The time required to infiltrate the runoff from a 5-year, 24-hour storm event was calculated using the following equation: 𝑇𝑖𝑚𝑒=𝐷𝑠𝑤 𝐼 Where: Time = time to infiltrate (hours) Dsw = depth of stormwater pool (feet) I = infiltration rate (ft/hour = ft/day ÷ 24 hours/day) Table 2 summarizes the stormwater runoff to each area and time required to infiltrate 97 percent of the stormwater volume. Because the results show that some areas of the pit (i.e., Pits 3a and 3b) may fail to infiltrate 97 percent of the stormwater within 72 hours, no further analysis was performed to assess the 120-hour, 99-percent require- ment.