This article describes 1) methods of locating abandoned boreholes within a side hill pit, and 2) the design and operation of in-line pH adjustment facilities for low-pH pit water.
Historic exploration boreholes in the base of Pit 1 at the Tonkin Springs Mine in central Nevada intercepted confined groundwater at depth. These boreholes were originally abandoned without sealing, which resulted in artesian groundwater flow into the pit base, contact with in-pit sulfide rock, and formation of a perennial “pit lake” in the pit base containing low-pH mine drainage.
SRK Reno developed a plan for locating and sealing known boreholes, including the construction of a system directing flows to the tailings impoundment to render the pit base “free-draining”. During drying out of the pit base, SRK used survey coordinates and tracked “permanent” wet spots to locate open boreholes. The mine continued the process, closing more than 100 open boreholes over a three-year period, and reducing post-closure water management flow by up to 15gpm.
The pit sump is gravity drained via a pre-constructed HDPE pipeline that exits the low point of the pit and flows through an in-line pH-adjustment system and then to the tailings impoundment. The pipeline is buried below ground surface for protection against freezing and is laid within a secondary containment pipeline.
The in-line pH-adjustment system sits in a buried precast concrete vault. High-frequency measurements from flow and pH meters continuously adjust dosing pump speed and stroke length, adding caustic for a range of incoming seepage flow rates (0 to 52gpm). Caustic is injected into the pipe flow upstream from the meters, and water then flows through an in-line static mixer before reaching the flow and pH meters for continuous pH adjustment. A data logger is used to record and transfer pH and flow data to a laptop computer.
The pH adjustment system includes a sump-pump to keep the vault dry, a strobe to alert mine staff of power outages, a wall heater to prevent freezing/condensation, air vents with motion-activated fans to circulate fresh air, and sensors to detect pump diaphragm or caustic feed problems and automatic shut-off flow valves.
