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Subsurface Hydrologic Controls <br />Subsurface hydrologic controls aze in-mine measures that inhibit or prevent the <br />process of acid formation andlor heavy metal dissolution into the ground or surface water <br />system. If it is possible to prevent water from entering a mine, or from coming into <br />contact with sulfide ores or wastes, or mixing with contaminated water plumes in the <br />workings, this can be the best, most cost effective remediation approach, because it helps <br />prevent the problem, rather than treating its symptoms in perpetuity. The success of most <br />hydrologic controls depends on understanding the sources and hydrologic pathways of <br />waters that enter the mine workings and discharge from the mine workings through <br />groundwater and surface pathways to determine how best to segregate or seal off <br />particular water sources in the workings. Here is more discussion from Herron et al <br />{1999, p. 30}. <br />In-mine diversions aze effective where clean groundwater inflows aze degraded by <br />flowing through drifts {on veins) and slopes in the mine workings. The concept is <br />to intercept the inflows before they come in contact with metals loading source <br />areas in the mine, thus circumventing metals contaminant production in the mine <br />workings/ore body. The "clean" inflows aze then diverted to the surface stream <br />through a collection and piping system. Though in many cases it may not be <br />possible to intercept all inflows before they become contaminated through contact <br />with the ore body, it is often possible to segregate and divert much of the <br />groundwater inflow before it mixes with the contaminated plume. This can <br />greatly reduce the overall quantity of polluted outflow. By significantly reducing <br />mine discharge, it may then become cost-effective and feasible to treat the <br />segregated contaminate plume through passive or semi-passive techniques; the <br />ei~luent flow is minimized, and concentration may be adjusted for optimum <br />system performance through dilution with part of the diverted clean flows. <br />Grout sealing a fracture inflow zone at a discrete location can prevent <br />groundwater from entering the workings, using proven, existing "ring-grouting" <br />methods and technology. The concept for this technique is to seal water inflows <br />through a grouting program, similaz to those used to seal dam foundations, and <br />control water inflows to active underground mining operations. Chemical or <br />cement grout is pumped under pressure into an array of holes drilled radially out <br />from the drift in and along the plane of the water bearing fracture or fracture <br />zones. The grout enters and seals the fracture pathways that communicate with <br />the mine opening. If engineered and executed correctly, the vrrater is prevented <br />from entering the excavation, and is forced far enough back into the rock away <br />from the mine workings so that it resumes its pre-mining course, flowing azound <br />the grout "curtain''. Depending on conditions and the layout of the workings, care <br />must be taken to ensure the inflows are not simply diverted to a point where they <br />enter another part of the ore body. Ideally, the grout curtain would be in position <br />where no other lower or upper levels aze nearby, and where numerous small <br />fractures or one discrete structure is draining groundwater into the workings along <br />a relatively short section of drift. <br />7 <br />