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Investigative Methods for Controlling Groundwater Flow to Underground Mine Workings

Case Studies

Figure 8—Several faults have been mapped in the Elkhorn Mine area. Solid lines indicate faults mapped previously and documented in published and unpublished reports. Dashed lines indicate lineations (fractures).

Mapping Results

Three digital map layers were produced from the field data and aerial photo interpretations:

• A layer with polygons of seven map units that partition the area into distinct, relatively homogeneous ecosystems based mainly on soil wetness and natural vegetation (figure 9).
• A layer showing locations of groundwater discharge, recharge, and of complex patterns of discharge and recharge. As used here, discharge refers to groundwater becoming surface water as typically occurs along streams, seeps, and springs. Recharge occurs where surface water becomes groundwater. Because of the large size of the assessment area, some discharge or recharge areas were not inventoried in the field.
• A layer showing the locations of all field sampling points.

These three layers can be overlain on the geologic structure layer to help locate potential connections of groundwater or surface water with the mine’s underground workings. Appendix A has detailed descriptions of the map units.

The map of riparian and wetland ecological units shows that much of the assessment area has soils that are saturated to the surface or to within a few inches of the surface at some time during the year. These wet areas are generally associated with valley bottoms, alluvial basins, stream headlands, and pitted glacial deposits (see appendix A).

Discharge and Recharge Areas

Most of the riparian and wetland areas are associated with groundwater discharge in which the groundwater becomes surface water associated with streams, seeps, and springs. During the latter part of the growing season, some areas have complex patterns of alternating discharge and recharge. Although these patterns are not documented, these areas are likely to be dominated by discharge during the wetter, early part of the growing season and during snowmelt. Indications of 10 sites of discharge, 12 sites of recharge, and 8 sites of complex discharge and recharge were observed in the study area.

Figure 9—A portion of the riparian and wetland map unit delineations for the Elkhorn Mine study area (see appendix B for the detailed map unit descriptions).

This study focuses on the recharge areas as potential contributors of groundwater to the mine’s underground workings. Of particular interest are a few areas near the mine workings. One site is the partially collapsed portal of the Park Mine in the southwest corner of section 14. Standing water occurs at a depth of about 20 to 25 feet in the vertical opening. Another excavated depression has a couple of feet of standing water. The small grassy meadow to the east of these sites has several ditches that apparently were constructed to divert water away from the downslope shafts and workings, possibly for domestic use at the cabin sites in the meadow (stop 29 on the maps and in the notes of the project file at the Montana Bureau of Mines and Geology). One of these ditches runs roughly along the contour at the top of the meadow heading north-northeast. Near the north edge of the meadow, this ditch enters a drainageway that flows into the Elkhorn valley below. The increased flows from this ditch have destabilized the old channel, leading to considerable erosion and sediment deposition. It appears that at least some of this drainage water becomes groundwater about 300 to 400 feet from the edge of the park before it heads down the very steep glacial trough wall of Elkhorn valley. At this point, the Elkhorn Mine adit portals are only about ¹¦₃ mile away.

Along the upper reaches of St. Louis Gulch, the workings of a remote, small, collapsed mine was observed. At the time, a trickle of water was draining from the collapsed opening. After flowing about 30 feet, this trickle disappeared into the ground. The pH of the surface water was 4.2. The water was depositing bright red iron coatings on the soil and rocks of the channel.

Water Samples

The pH of surface water of the lower adit and various locations in Elkhorn Creek was collected on Aug. 30, 2002 (table 3). These data indicate that the lower adit water is 10 times more acidic than Elkhorn Creek water that is unaffected by the adit discharge. The adit water lowers the pH of Elkhorn Creek from 7.1 to 6.8, and 6.5 downstream of the junction with the adit discharge. The pH values of the adit discharge water are higher than those reported in a previous Montana Bureau of Mines and Geology report in which the discharge water had a pH of 4.8, compared to a pH of 6.1 in 2002. The use of a relatively inexpensive field pH meter for these field samples may account for some—but probably not all—of this difference.

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