2024-01-31_REVISION - M1977344 (33)

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Mr. Michael Toelle Holcim US, Inc. January 31, 2024 Page 6 Holcim RCQ Letter Report 20240131 Table 2. Infiltration Results Area Runoff Volume (acre-feet) Infiltration Area (acres) Infiltration Rate (feet/day) Infiltration Time for 97% of Volume (hours) Backfilled pit 46.937 370.8 0.5 5.9 Pit 1 1.925 113.2 0.011 36.0 Pit 2 5.315 226.4 0.011 49.7 Pit 3a 18.376 99.0 0.011 392.3 Pit 3b 4.485 30.7 0.011 308.8 Summary of Findings Precipitation runoff resulting from a 5-year, 24-hour storm event was calculated to deter- mine the volume of precipitation that would infiltrate within 72 hours of the storm event. Results of the limited hydrologic analysis indicate accumulated water from a 5-year storm event in the eastern portion of the reclaimed pit will likely not infiltrate within the 72-hour requirement. Areas of the reclaimed pit that meet the 72-hour requirement are the backfilled pit and pits 1 and 2 where the predicted water pool is shallow (less than 0.5 inches). There is uncertainty of the final pit reclamation design meeting the 72-hour infiltration requirement due to the assumption of the western portions of the reclaimed pit being large, level areas. Therefore, based on results of the limited hydrologic analy- sis, the reclaimed pit may not meet the 72-hour infiltration requirement without any ad- ditional mitigation measures to enhance infiltration. BC appreciates that Holcim has requested our services in assisting with this project. Should you have any questions, please do not hesitate to call me at 575-418-7069. Very truly yours, Brown and Caldwell Kelly Donahue, P.G., Project Manager Golden, Colorado cc: Mary Jay Martens, P.E., Brown and Caldwell Attachments (3) • References • Attachment A: NRCS Soils Information • Attachment B: HydroCAD model output files Limitations: This document was prepared solely for Holcim in accordance with professional standards at the time the services were performed and in accordance with the task order between Holcim and BC dated November 30, 2023. This document is governed by the specific scope of work authorized by Holcim; it is not intended to be relied upon by any other party except for regulatory authorities contemplated by the scope of work. We have relied on information or instructions provided by Holcim and other parties and, unless otherwise expressly indicated, have made no independent investigation as to the validity, completeness, or accuracy of such information.