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Investigative Methods for Controlling Groundwater Flow to Underground Mine Workings

Case Studies

Figure 11—Two adits discharge water throughout the year at the Charter Oak Mine and mill just south of the Little Blackfoot River. A stream originates above the site and flows into the disturbed area. Springs near the stream flow only during spring and early summer.

Environmental Conditions

The mine and mill were identified in 1998 by the Montana Bureau of Mines and Geology and the Forest Service as having several potential environmental impacts (Hargrave and others 1998). Additional flow and chemistry data were collected from the lower adit in 1998 and 1999. The concentration of several dissolved constituents in the lower (west) adit discharge exceeded secondary and primary drinkingwater standards as well as acute and chronic aquatic life criteria (table 4). The main adit discharge and the small stream flowing through the site also exceeded several standards but generally had much better water quality.

Underground Workings

The area immediately around the mine is steep and covered with trees. Above the mine, bedrock crops out and talus slopes are common. The mine has at least six adits, but only the lower two adits discharge water (figure 11). A third adit, the Kineo vein and drift, may contribute water to the unnamed tributary that has been partially reclaimed. Seeps emerge from the northeast side of the unnamed tributary but it was not evident whether they were the result of a buried adit discharge.

Unpublished maps in the Montana Bureau of Mines and Geology files provide some indication of the extent of the workings and their geology. The plan map (figure 12) was drawn at a reported scale of 1 inch equals 30 feet. It conservatively estimates 2,350 feet of workings on at least three levels in 1943. Similarly, the accompanying cross section (figure 13) shows three or four levels with several stopes and raises. Four adits on the 1943 map include:

• The lower, discharging adit, trending south 62 degrees east, southeast of the mill that was designated as “0.0” elevation
• One adit at 146 feet relative elevation
• Two adits at 206 feet relative elevation

The map also shows a shaft at 275 feet relative elevation. The Kineo workings or adit (north 55 degrees east-trending) has about 1,000 feet of drifts and crosscuts. A fault was noted in the Kineo crosscut. The same fault was the terminus for the 146-foot-elevation adit and the two drifts off of the shaft. Raises and stopes helped to connect the various levels. Between 1904 and 1908, most of the Kineo drift that followed the vein was driven at a relative elevation of 130 feet. This adit correlates with the partially reclaimed portal to the northeast of the unnamed tributary that flows to the northeast into the Little Blackfoot River, south of the main mine area.

If the notes on the map are accurate, the workings are shallow—most are less than 200 feet below the surface. As noted, the mine operated for about 20 years after the date of the maps, so they probably represent the minimum extent of workings. The lower discharging adit south of the unnamed tributary is not shown on the mine map. The small size of the waste-rock dump indicates that the tunnel is fairly short, probably less than 50 feet

Figure 12—An unpublished sketch map from the Montana Bureau of Mines and Geology files provides some detail of the Charter Oak Mine workings. Although the map does not follow convention, the orientation of the map was preserved (note north arrow).

Figure 13—The unpublished work in the Montana Bureau of Mines and Geology files included a cross section of the Charter Oak Mine workings. The scale of the maps and cross section had to be estimated because they did not include a bar scale.

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