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Investigative Methods for Controlling Groundwater Flow to Underground Mine Workings
Application of the various methods of controlling groundwater flow into mine workings requires knowledge of the source and pathways of the portion of the aquifer that is hydrologically connected to the mine. The previous discussion of methods provides an overview of the investigation. No single method will provide all the pertinent information. It will be necessary to compile information from several sources and compose a conceptual model of the groundwater flow system near the mine.
The discharge of 80 to 130 gallons per minute from the main adit of the lower Elkhorn Mine far exceeds that of most mines in Montana. Discharge from most adits is in the 5- to 10-gallon-per-minute range (Hargrave and others 2000). The Elkhorn Mine is also one of the largest abandoned mines in Montana with acid mine drainage problems. The mine has a reported 24,000 feet of workings. This investigation was limited to a survey of surface features, surface water, soils, vegetation, and existing information on geology and workings.
Although information is incomplete and sometimes contradictory, the mine probably intersects a number of faults and welljointed granitic rock capable of transmitting groundwater. The volume of rock available for groundwater recharge to the mine is quite large. This is evidenced, in part, by the high sustained discharge from the adit. The workings of the Elkhorn Mine extend toward the Park Mine. Flooded shafts and prospects prevent us from knowing whether there is a direct connection between the two mines, but there could be some contribution of groundwater from the Park Mine to the Elkhorn Mine workings.
Any attempt to control or eliminate adit discharge from the lower Elkhorn Mine would have to include an investigation of the underground workings. The true extent of the workings, the effects of the faults on groundwater flow, and fracture/joint orientation and density are unknown. Extensive tree cover severely limited the use of aerial photos or field mapping to identify fractures and joints on the surface.
The unnamed tributary flowing through the disturbed area, along with several closed depressions throughout the area, probably contribute a significant portion of the groundwater recharge to the adjacent underground workings. Infiltration controls on the streambed and surface disturbances may reduce discharge from the adits. Further investigations are warranted, particularly investigations to identify the recharge area. For example, age-dating the adit discharges would provide a means to estimate flow paths and residence times. Similarly, tracer tests and detailed flow measurements of the stream may identify segments losing water.
A portion of the discharges at the Charter Oak Mine site may be the result of fault-related flow; faults were described in the underground workings on the unpublished maps of the area. No mention of water in the workings was made in the sparse information available for the mine. However, the amount of water contributed from these fault conduits is probably small. The seasonal response in adit discharge and the shallow workings suggest a similarly shallow groundwater flow system that may be a good candidate for source control.
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