2025-03-03_PERMIT FILE - M2024056 (2)

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Table G-1. Surface Water Discharge Monitoring Requirements from Existing NPDES Discharge Permit Parameter Monitoring Frequency Sample Type Flow Instantaneous, Monthly In-situ pH 2x/month Grab Total Suspended 2x/month Grab Solids Oil and Grease Visual 2x/month Visual Oil and Grease Contingent on visibility Grab of oil and grease Total Flow Instantaneous, Monthly Calculated Electrical Conductivity Quarterly Grab Arsenic,Total 2x/month Grab Recoverable 3. Floodplain The majority of the site is within the 100-year floodplain and floodway as reported by the Federal Emergency Management Agency. These boundaries are shown in the Exhibit C and F maps. The minimum distance maintained from the South Platte River to excavation activities is 150 feet where river and pit-side armoring will occur. This can be seen on Map C-2 and F-1 in the easternmost pit. All other mining activities will maintain a minimum 400 foot setback from the river which is the minimum setback for no armoring. Additionally, no stockpiling or filling above the natural grade will occur in the floodway. Overall, the downstream flood impacts should remain the same or be reduced from activity at the site as the removal of material results in more storage space for flood water below the existing grade. A no-rise certification will be provided to Weld County as part of its floodplain development permit. In accordance with the Mile High Flood Control District technical guidelines, an inflow/outflow structure will be installed along the riverbank where mining reaches its closest point to the South Platte River to prevent erosion of the bank. Details of these designs can be seen on Map F-2. The inflow/outflow structure design included is from the Mile High Flood Control District's Technical Review Guidelines For Gravel Mining& Water Storage Activities Within Or Adjacent To 100-Year Floodplains dated January 2013, specifically Section 2.4.5 Side Channel Spillway. Using the design guidelines in this section, the design shown on Map F-2 Flow Structures was developed. It should be noted that, in accordance with advice and insight from Division engineers regarding these types of structures, the river side was designed for installation into ground between the pit slope and the riverbank for cases where disturbing the riparian habitats along the river bank was inadvisable. The flow structures will provide an armored channel for flood waters to enter the pit during mining or the lakes following reclamation, preventing erosion that might lead to pit capture. Evans Mining Resource February 2025 G-2 Lewicki&Associates