2024-01-31_REVISION - M1977344 (33)

Home Browse Search

Mr. Michael Toelle Holcim US, Inc. January 31, 2024 Page 1 Holcim RCQ Letter Report 20240131 January 31, 2024 Letter Report Mr. Michael Toelle Holcim (US), Inc. 3500 CO Highway 120 Florence, CO 81226 160041.025.001 Subject: Red Creek Quarry Dear Mr. Toelle, At the request of Holcim US, Inc. (Holcim), Brown and Caldwell (BC) performed a limited hydrological analysis of the Red Creek Quarry (RCQ or the Site) proposed reclamation configuration in response to comments from the Colorado Division of Reclamation, Min- ing, and Safety (DRMS) on the reclamation permit application. The DRMS expressed concern regarding the potential ponding of stormwater in the lowest portions of the pits that will not infiltrate within 72 hours as required by the Colorado Division of Water Re- sources (DWR). Senate Bill 15-212, codified in Colorado Revised Statutes Section 37- 92-602(8) requires that stormwater detention facilities have the ability to: (1) continu- ously release or infiltrate at least 97 percent of all of the water from a rainfall event that is equal to or less than a 5-year storm within 72 hours of the end of the rainfall event, and (2) continuously release or infiltrate at least 99 percent of all of the water from a rainfall event that is greater than a 5-year storm within 120 hours of the end of the rain- fall event BC conducted a limited hydrologic analysis to estimate stormwater flows reporting to the RCQ reclaimed pit bottoms from a 5-year, 24-hour storm and used site-specific infiltra- tion rates from the RCQ Groundwater Modeling Analysis Technical Memorandum (TM; BC 2023) to calculate the time required to infiltrate the modeled stormwater volumes. This document summarizes the hydrologic analysis, assumed parameters, estimated stormwater volumes, and infiltration results. Scope of Work and Limitations BC was retained by Holcim to perform a limited hydrologic analysis of the RCQ in re- sponse to comments received from the DWR on the reclamation permit application. The scope of services, as outlined in the task order dated November 30, 2032, included the following: • Conduct hydrologic modeling to estimate the volume of stormwater that would report to accumulate in the pit bottoms • Calculate the time required to infiltrate the modeled stormwater volumes • Summarize the findings in a letter report In the course of this assessment, BC has relied on information provided by Holcim and other sources, such as the National Oceanic and Atmospheric Administration (NOAA), United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS), and United State Geologic Survey (USGS). BC has made no independent