2025-08-08_PERMIT FILE - C1982057

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TR-83 15-3a-22 Revised 3/25 Technical Revision 82 (2016) Monitoring Reductions Additional reductions of the monitoring plans were requested. Table 1, Current Hydrologic Monitoring Program (starting on page 15-3a-4), has been revised to reflect these changes. The details of the changes are provided below. 1 - Remove continuous flow recording requirements at all six NPDES outfalls and surface water site WSHF1 (lower Hubberson Gulch). Continuous flow recording is not required in the NPDES permit (SCC was performing this function voluntarily). Continuous flow recording at Site WSHF1 was initially performed to gather baseline data for the mine permit application, but was never dropped after the application. Again, continuous recording at this site was voluntary, it was not required by any other permit. This recording is time-consuming and expensive to maintain. Continuous flow data is not required for final bond release. 2 - Remove all monitoring requirements for alluvial Well WHAL6-2 located on Hubberson Gulch. This well was plugged with a wooden fence post (by the landowner) in late 2014 or early 2015. Data from Hubberson Gulch alluvial Well WHAL7-2, located about a mile downstream of Well WHAL6-2, will be used for the final bond release application. 3 - Remove all monitoring requirements for Trout Creek Sandstone Well WTC201. This well was the shop water supply well. It is currently not in use, and cannot be sampled when the power is off. Power is no longer available at the shop facilities, and the transformer has been removed. This well is 760 feet deep, and should be unaffected by mining as confining layers of shale separate it from the coal seams mined. The table presented on the following page gives a statistical summary of the water quality of this well for the last ten years (2006 thru 2015). Water quality data at this well for the last ten years was compared to CDPHE drinking water standards found in their Regulation 41. No Table 1 (human health) standards were exceeded. Three Table 2 (secondary) standards were exceeded. The iron standard, 0.3 mg/l, was exceeded in ten out of twelve samples, with values of 0.35 to 7.86 mg/l. The manganese standard, 0.05 mg/l, was exceeded in eleven out of twelve samples, with values of 0.051 to 0.181 mg/l. The sulfate standard, 250 mg/l, was exceeded once, with a value of 260 mg/l. However, a 1988 sample had an iron value of 1.31 mg/l and a manganese value of 0.22 mg/l. A 1991 sample had an iron value of 9.58 mg/l, a manganese value of 0.22 mg/l and a sulfate value of 241 mg/l. Mining began at Seneca IIW in 1990. Technical Revision 83 (2025) Monitoring Reductions Discontinue all ground water monitoring in preparation for abandonment procedures to take place prior to final release of SL8 mid 2025. Past Annual Hydrology Reports show ground water is with in compliance and stable at the site. Wells will be abandoned in spring 2025.