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Investigative Methods for Controlling Groundwater Flow to Underground Mine Workings

Introduction


In 1998, an adit discharge water-quality and quantity monitoring project began at the Elkhorn and Charter Oak Mines in Montana. These mines were chosen as representative of hard-rock mines with discharging adits (horizontal entrances) throughout the United States. Three reports have been published detailing the findings of this project:

This report, the fourth in the series, discusses continued investigations at both sites showing how additional data from conventional resources available to USDA Forest Service staff can be used to help determine a specific recharge control method for a particular mine discharge.

An image of an adit at the Charter Oak Mine.
The acid mine discharge at one of the adits at the Charter Oak Mine southeast of Elliston, MT, in the Helena National Forest.

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 following discussion is meant to provide an overview of the investigation. Information from several sources must be compiled to compose a conceptual model of the groundwater flow system near the mine. No one method will provide all the necessary information. The case-study portion of this report is divided into two sections: one for the Elkhorn Mine and the other for the Charter Oak Mine. Each section provides details of the methods that were used to investigate environmental problems at the mine.

Controlling groundwater recharge to the workings requires identifying, characterizing, and controlling surface water and groundwater that may infiltrate mine workings. Good-quality water is diverted from the workings to prevent it from interacting with pyrite and metal-bearing minerals. Infiltration controls may include grouting from outside the workings, soil or streambed treatment to reduce infiltration capacity, storm water and runoff management, and recontouring or regrading natural recharge areas. This discussion explores methods for controlling groundwater flow into mine workings. Many of the methods should be applied to investigations of all three treatment categories: capture, treatment, and disposal; adit plugging or grouting; and controlling groundwater recharge to the mine workings.

Numerous investigations of mining sites throughout the Western United States have identified thousands of discharging mine adits. Many of these adits discharge poor-quality water that is created when groundwater circulates through the mine workings and interacts with mineral-rich rock exposed by mining. Many mines also contain abundant iron pyrite. Interaction of pyrite, oxygen, and water can produce acidic waters quickly as the water circulates through the rock and mine workings. A pyrite content of only a few percent by volume can produce acid for hundreds or thousands of years. Other metal-bearing minerals dissolve in the acidic water, producing high concentrations of metals in the discharging waters.

Hargrave and others (2000) identified about 336 discharging adits in Montana based on work conducted by the Montana Bureau of Mines and Geology, the USDA Forest Service’s Northern Region, and the Montana office of the U.S. Department of the Interior, Bureau of Land Management. Although the quality of the discharging water varied widely, nearly all discharges indicated elevated concentrations of metals. The rate of discharge from the adits ranged from less than 1 gallon per minute to more than 100 gallons per minute. However, the vast majority of the adits discharged water at a rate of 5 to 10 gallons per minute. This report will review methods of investigation for controlling groundwater flow to underground mine workings.

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