2023-12-26_GENERAL DOCUMENTS - C1981044

• Permit No: C1981044
• IBM Index Class Name: General Documents
• Doc Date: 12/26/2023
• Doc Name: Proposed Decision and Findings of Compliance
• Type & Sequence: RN8
• Media Type: D
• Archive: No

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expected impact.
12. The No. 9 Mine will not refill to the surface. It will refill to an equilibrium level
between the Twentymile Sandstone and the overlying White Sandstone.
13. The No. 9 Mine portal backfill area is too small to generate enough leachate to have a
measurable effect on nearby aquifers.
The monitoring plan contained in the permit application has been designed to verify the permittee's
projected hydrologic impacts of mining. Section 2.05 of the permit application includes a description of
the observed hydrologic impacts caused by mining at the Williams Fork Mines. Each year, MCM
assesses the on-going impacts to the hydrologic system in its annual hydrologic report. Observed impacts
are summarized below.
Observed Ground Water Impacts
Mine Inflows and Discharges
Annual hydrology reports show the total discharge for the 5 and 6 Mines was a constant 600 gpm during
active mining in the early 1990s. After mining ceased in 1995, sumped water was pumped down. The
pumped mine water was discharged to the Williams Fork River at NPDES outfalls 003 (5 Mine well)and
024 (7 North Angle well). The No. 5 Mine pump was then turned off in August of 2013;no further
pumping at this site is anticipated. Monitoring data through 2013 indicate the mine water was
consistently alkaline,with total dissolved solids less than 1800 mg/l, and low concentrations of iron and
manganese. As the pump is turned off data is no longer collected.
Trout Creek Sandstone
The water level in the Trout Creek Sandstone monitoring well (No. 5 Mine well) fluctuated over the life
of the mine. When monitoring resumed in 2006 to July of 2013,the water level remained relatively
stable. The No. 5 Mine pump was turned off in July of 2013. Historical field conductivity data for the
Trout Creek Sandstone wells indicate no adverse water quality impacts related to mining. The
conductivity levels appear elevated to the levels observed in the 80's. As the pump is turned off data is
no longer collected.
Middle Sandstone
The most significant hydrologic impact caused by Mines 45 and 46 may be an approximate 60-foot
drawdown of ground water in the Middle Sandstone as observed in wells TR-4, TR-7a, 81-01 and 83-03.
Historically these wells have exhibited water level fluctuations. As of this writing,water levels have
stabilized.
Section 2.04.7 of the permit describes the pre-mine characteristics of the bedrock aquifers and indicate
that the Middle Sandstone aquifer often exceeds the primary and secondary EPA drinking water
standards. Monitoring is ongoing. Conductivity in Well TR-7a exhibits consistency over time.
Conductivity in wells 8 1-01 and TR-4 range between 1000 to 1600 umhos/cm and 2000 to 2300
Williams Fork Mines Prepared by: R ReiRey M.S. GISP
C1981044 December 2023