2021-07-21_PERMIT FILE - C1981019 (5)

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Point of Compliance Well Investigation AECOM 10 Creek at some point down hydraulic gradient from the mining impact areas. [Comment 109] AECOM estimates that on average, a permanent well near POC-2 has an 85 to 95 percent chance of meeting groundwater standards, depending on which of the three threshold values defined in this report is used for determining compliance. Similar to the Division’s recommendation, AECOM also suggests installing a second permanent compliance well in the Good Spring Creek drainage near the present location of POC-10 (Figure 13). This recommendation is based on the relatively low average concentrations of dissolved manganese (0.29 mg/L), sulfate (610 mg/L), and TDS (1,331 mg/L) compared to the other temporary monitoring well locations, including some that are farther downgradient in the Good Spring Creek valley. We estimate that on average, a permanent well near POC-10 has an 85 to 99 percent chance of meeting groundwater standards, depending on which groundwater threshold value is used for comparison. If the Division accepts these proposed point of compliance well locations, we recommend installing the permanent wells within 10 to 20 feet of the existing temporary wells. The permanent wells should also be drilled to a similar depth as the temporary wells to ensure an adequate water column for sampling. This will likely require drilling the permanent well near POC-2 about five feet deeper than the temporary boring (30 feet total) since POC-2 only contained five feet of water at the time of installation (Table 1). The permanent well near POC-10 can be drilled to the same depth as the temporary well (25 feet) because POC-10 contained over ten feet of water when it was first installed. AECOM recommends installing 2-inch wells for the permanent point of compliance locations. The permanent wells should be completed using 2-inch inner diameter, flush-threaded, Schedule 40 PVC well casing and 0.010-inch factory-slotted PVC screen. We recommend installing the well screens across the lower 10 feet of each boring. Consistent with the Rules and Regulations for Water Well Construction (2 CCR 402-2), the point of compliance wells should be completed with a silica sand filter pack that extends from the bottom of the boring to 2 feet above the top of the well screen. A 3-foot thick bentonite seal should be placed on top of the filter pack and hydrated prior to adding grout. After the bentonite seal has had sufficient time to hydrate, the remaining annular space in each wellbore should be grouted from the top of the bentonite seal to ground surface using cement-bentonite grout. AECOM recommends constructing each well as an above-grade completion with approximately two feet of casing stickup secured by a lockable steel well cover. The inner PVC casing should be sealed with an expandable J-plug well cap, or similar. AECOM also recommends constructing a concrete well pad around the steel cover that is at least 4 inches thick and slopes away from the wellhead. The concrete pad should extend at least two feet laterally in all directions from the outside of the steel cover. Following construction, the new point of compliance wells should be developed in accordance with Rule 10.6 of the Rules and Regulations for Water Well Construction (2 CCR 402-2) to remove drill cuttings and fines from the well screen and filter pack. 3.4 Proposed Compliance Standards The analysis presented in this report identified three different comparison standards that were used to evaluate concentration data from the temporary monitoring wells. However, once permanent point of compliance wells are installed at the Mine, a single comparison standard needs to be established to determine if groundwater quality in the permanent compliance wells