  Privacy
| Legal |
Table of Contents Back | Next | Home | Cover Page |
 Technology & Development Center |
Investigative Methods for Controlling Groundwater Flow to Underground Mine Workings
General information about the discharge and recharge areas, including St. Louis Gulch (figure 8), is included in the Discharge and Recharge Areas section (pages 18 and 19). Another area with several sites of apparent groundwater recharge is around the Upper Camp of Elkhorn Mine. A 100- by 300-foot wet sedge meadow (figure 9) is at the toe of the flat, large fill immediately east of the collapsed upper adit portal.
 |
| Figure 8—Looking upslope into collapsed hillside mine workings about 3¦4 mile south of the Elkhorn Mine upper adit. A small, very acidic flow (pH 4.2) emerges from the workings, then becomes groundwater 30 feet downstream. The water deposits thick iron coatings on soil and rocks in the shallow channel. |
This meadow consists of about 5 inches of saturated organic soil material over 8 inches of extremely acidic, fine sandy loam and silt loam alluvium which is saturated and gleyed. These recent deposits of alluvium buried the native wetland soil. The buried soil profile is still intact and consists of buried organic horizons over dark, gleyed silty clay loam, which overlies gleyed very cobbly sandy loam, likely from glacial till. The recent alluvium is probably associated with the mine operations. Part of the alluvium collected behind a constructed dam at the base of the wet meadow (a complete soil profile description of this site is in the project file). Immediately above this dam is a deposit of saturated silty clay loam that appears to be bentonite or drilling mud with 30- to 35-percent clay. It varies from red to buff grey or black. This is not native material, but its purpose in the mine operation is unknown—perhaps it is drilling mud that was used for exploratory drilling. This clayey alluvium continues in the wet meadow below the dam (stop 67). The native wetland was buried by the clayey alluvium in this lower meadow. The pH of the recent organic deposits and the recent alluvium ranged from 4.3 to 5.5 at a ratio of 1:1 as soil: distilled water. Surface water adjacent to the meadow is pH 4.3.
 |
| Figure 9—Buried wetland soil at the Elkhorn Mine’s upper adit. The upper 13 inches of soil is saturated and covers the original saturated wetland soil. The upper 5 inches has dark brown to reddish fibric organic horizons overlying gleyed, mucky silt loam. The original soil surface (below 13 inches) is fibric to hemic organic material. The tape measure shows inches. |
At the upper end of this wet meadow is a small pond about 7 by 10 feet with several inches of standing, stagnant water (stop 65). This spot appears to have been previously excavated. The pH of the pond water is 2.9 and the pH of the exposed sediment adjacent to the pond ranges from 2.5 to 4.4 (stops 65 and 101). Sediment in the bottom of the pond is less acidic (pH ranged from 4.5 to 6.0) and more reduced (redox potential ranged from +39 to +186 millivolts) than the exposed sediment next to the pond (redox potential ranging from +249 to +341 millivolts). It appears that the surface of the pond represents the level of the water table under the wet meadow discussed above. The source of the acidity of the pond and adjacent wet meadow is unknown. The pH of the fillslope soil, as well as the soil in the large cutslope above the buried upper adit portal, is about 6.3 to 7.4 (stops 65 and 71).
To the east of the large, flat fill area in the Upper Camp is a large deposit of sandy loam to gravelly sandy clay loam wasterock material that has very steep sideslopes (stop 100). This material apparently came from the upper workings underground system and was dumped from a rail system. The deposit is about 200 feet wide and 400 feet long. This material was excavated to 52 inches for study. Seven layers were described regarding pH, color, texture, redox potential, percent clay and gravel, temperature, and soil moisture status (stop 100). The pH of this material ranged from 2.9 for red soil to 3.5 for light yellow soil. The redox potential ranged from +433 (light yellow soil) to +543 millivolts (red soil). Colors to 52 inches are red (surface), grayish-yellow, and light yellow. The observed red soil only occurred in the first 16 inches, where it is about 70 percent of the soil volume. Water leaching through this extremely acidic deposit could become quite acidic. If such water reached the underground workings, it would contribute to the problem of acid drainage from the lower adit.
This acidic waste dump is bare of vegetation and shows signs of serious erosion in the form of numerous rills and several gullies. The eroded material was deposited on the forested slope (15-percent gradient) immediately below the waste dump (stop 92). Much of the sediment is deposited within 200 to 250 feet of the toe of the dump. The thickness of the overburden south of the tramway is about 5 to 8 inches. Some sediment was carried another 200 feet through the tramway corridor before spilling over the steep slope below. Some of the surface water probably becomes groundwater in these deposition zones as the flow rates decrease and surface water flows across the native volcanic ashcap soils of the area. The pH of the overburden and of the buried silt loam volcanic ash soil is 4.2. This contrasts with a pH of 5.2 for native silt loam volcanic ash surface soil in an adjacent area that was not affected by the acidic sediment. A 10-fold increase in the acidity of the native buried soil has occurred since its burial. Acidification and burial of this area south of the tramway has resulted in the death or severe defoliation of most lodgepole pines. North of the tramway, the overburden occurs in patches and is less than 2 inches thick. In this area, the tree canopies appear healthy. Acidification of the buried native soil by water leaching through the acidic overburden indicates that this area has the potential to supply acidified water to the groundwater.
Near the bottom of the tramway corridor is a recent, long narrow slump scar (stop 92). In the forested site nearby, there is a second slump in extremely bouldery glacial till. The slumps are probably due to a seasonally high water table as evidenced by several large Engelmann spruce trees and a few alder shrubs.
Soil pH of the lower workings below the lower adit were alkaline, ranging from a pH of 7.8 at a depth of 4 to 6 inches, to a pH of 8.5 at 12 to 24 inches (stop 93). The upper 6 inches is gravelly, sandy clay loam that overlies very gravelly sandy loam. Evidence of seasonally saturated soil (fine reddish iron masses) begins at about 12 inches. The upper 10 inches of the profile appear to be topsoil that was added to the site as part of previous reclamation.
The pH of surface water of the lower adit and various locations in Elkhorn Creek was measured on August 30, 2002. The results follow:
These data indicate that the lower adit water (stops 97 and 98b) is 10 times more acidic than Elkhorn Creek water that is unaffected by the adit discharge water (stop 98a). The addition of the adit water lowers the pH of Elkhorn Creek from 7.1 (stop 98a) to 6.8 and 6.5 (stops 98c and 98d) downstream of the junction. The pH values of the adit discharge water are higher than those reported in a previous Montana Bureau of Mines and Geology report. The use of a relatively inexpensive field pH meter on the field samples mentioned above, may account for some—but probably not all—of this difference.
The adit discharge water is more reduced (redox +129 millivolts at stop 97) than that of Elkhorn Creek above the junction with the adit discharge (redox +220 millivolts at stop 98a). The redox potential of the adit water increased from +129 millivolts (stop 97) to +176 millivolts by the time it reached the junction with Elkhorn Creek (stop 98b). This discharge water runs across several hundred feet of wetland before reaching this junction. The wetland appears to influence the redox potential as well as water temperature, which rises from 45 °F at the adit portal to 68 °F by the time it reaches Elkhorn Creek. The adit discharge water does not appear to affect the redox potential of Elkhorn Creek 100 feet downstream from this junction (+228 millivolts at stop 98c), but within another couple hundred feet downstream, the redox drops again to +176 millivolts at stop 98d. The pH drops from 6.8 to 6.5 and the water temperature increases from 62 to 65 °F between these two points of Elkhorn Creek. Perhaps unidentified underground sources between the adit and the mill sites are responsible for these changes in water characteristics.
 |
E-mail: