2022-03-10_REVISION - C1981044 - Laserfiche WebLink

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23. Please provide the missing data for well 9BF as outlined in the permit monitoring requirements. a) Well 9BF has been monitored as outlined in the monitoring requirements.Well 9BF is a newer well and was never added to the AHR layout. MCM will be adding the water year and the historic data for 9BF to the 2021 AHR and future AHRs. 24. Please provide an analysis of the excursions in Well 8 1-01 and the No. 9 Mine Well. a) Well81-01 • Monitoring well 8 1-01 is screened within the Middle Sandstone located approximately 60 ft above F coal seam mined at the facility. The well is located immediately above the Williams Fork Mine shadow area. Review of the 2020 monitoring data indicates that the water level in well 81-01 is consistent with historic water levels,which have been stable since monitoring was reactivated in 2006. Although the iron measured at 8 1-01 exceeded the Domestic Water Supply Drinking Water standard(0.3 mg/L)it remains within the Agricultural Standard of 5 mg/L. This is characteristic of the Middle Sandstone which exhibited an average dissolved iron concentration of 0.41 mg/L(range: <0.02— 1.51 mg/L)that was greater than the Drinking Water Standard prior to mining (Well TR-4). Furthermore the Drinking Water Standards should not be applied to deep bedrock wells where there are no known users and limited potential for future use due to the high economic cost for drilling to the depth of the Middle Sandstone (-533 ft bgs) and location within the mine boundary. Similarly manganese exceeds the Drinking Water Standard but does not exceed the Agriculture standard, as the 0.2 mg/L Agricultural Standard is only applicable to areas where irrigation water is being applied to soils with pH values less than 6 (see Regulation 41). The Middle Sandstone is not being used for irrigation purposes and acidic soils are not present at Williams Fork,therefore this standard should not apply. The pH in the 2020 remained neutral indicating the iron and manganese where not mobilized by acidic conditions. Its possible that the filter may have burst during sample collection and the iron and manganese concentrations were elevated by solids greater than 0.45 micron that would normally be removed. Magnesium does not have a Drinking Water or Agricultural Groundwater Quality Standard and has remained fairly stable over the last few years. An increased emphasis on monitoring techniques and evaluation of the wells condition will occur during the 2022 monitoring event. b) No. 9 Mine Well(9MN) • Monitoring well 9MN is located between the Williams Fork F and E Seam Workings and Empire's P Seam Workings. The well is screened within the Twentymile Sandstone,which is several hundred ft above the F coal (shallower of two seams mined at Williams Fork) and is separated by several units of low permeable siltstone and mudstone which act as an aquitard. The groundwater levels within 9MN have remained stable since the mine ceased using the well for water supply purposes and are above the water levels present within the lower sandstone units further indicating the Twentymile Sandstone remains disconnected from deeper groundwater that could potentially be influenced by the Williams Fork underground mine area. Groundwater within the Twentymile Sandstone generally flows from the southeast to the northwest.Well 9MN is located approximately 1000 ft 4 1 P a g e