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Investigative Methods for Controlling Groundwater Flow to Underground Mine Workings
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| Figure 4—Discharge from the main adit of the lower Elkhorn Mine ranged from 80 to 135 gallons per minute over the period of record. |
The discharge from the lower adit at the Elkhorn Mine was monitored over a period of about 1 year. The discharge ranged from a minimum of about 80 gallons per minute to a maximum of about 135 gallons per minute during the period of record (figure 4). The greatest discharge occurred in May and June during snowmelt and spring storms. Discharge generally declined throughout the rest of the year. Field parameters such as temperature, pH, and specific conductivity (SC) show some seasonal trends, particularly in the spring months. Water temperature is lowest in late winter. The annual range in temperature is less than 2 degrees Celsius, suggesting a deep groundwater flow source. Overall, water quality ap-peared to be best just before the spring snow melt and poorest during higher flows. The range of values throughout the year is small.The discharge from the lower adit at the Elkhorn Mine was monitored over a period of about 1 year. The discharge ranged from a minimum of about 80 gallons per minute to a maximum of about 135 gallons per minute during the period of record (figure 4). The greatest discharge occurred in May and June during snowmelt and spring storms. Discharge generally declined throughout the rest of the year. Field parameters such as temperature, pH, and specific conductivity (SC) show some seasonal trends, particularly in the spring months. Water temperature is lowest in late winter. The annual range in temperature is less than 2 degrees Celsius, suggesting a deep groundwater flow source. Overall, water quality appeared to be best just before the spring snow melt and poorest during higher flows. The range of values throughout the year is small.
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| Figure 5—The upper Elkhorn shaft/adit (center of photo with lumber over it) discharges a small amount of water throughout the year. |
The upper Elkhorn Mine adit discharge (figure 5) ranges from less than 1 gallon per minute to about 5 gallons per minute. As with the smaller adit at the lower Elkhorn Mine, this site was measured four times over a period of 2 years. Water from the area drains through the wasterock dumps and down a small tributary. A second adit in the area of the upper workings does not show any evidence of discharge at any time of the year.
The workings of the Park Mine are near the head of a small tributary of Elkhorn Creek. A flooded shaft and several cuts and prospect pits are flooded throughout the spring and during most of the summer. A spring originates near the surface workings and flows throughout the workings most of the year (figure 6).
Springs that were apparently unrelated to mining activities were found throughout the area around the upper and lower Elkhorn Mines and the Park Mine (figure 7). The largest spring originates on the scree slope between the upper and lower Elkhorn Mines and flows onto the wasterock dump of the lower adit. During the spring months, this spring flowed more than 20 gallons per minute, but it was dry by late summer during the 3 years of observations. The other springs had much lower flows. By late summer, many springs were dry and none was found to flow more than about 1 gallon per minute. Field chemistry (pH, specific conductance, and temperature) was unremarkable. All of the springs had nearly neutral pH and low specific conductivity (less than 50 micromhos per centimeter). A visit in late 2000 found springs and wet areas paralleled the west side of the Elkhorn Creek drainage at a consistent elevation. They were apparent at the toes of the scree slopes that commonly formed on the west side of the valley.
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| Figure 6—The workings at the Park Mine include a flooded shaft and several flooded cuts and prospects. The area is especially wet during snowmelt, but the workings are flooded throughout the year. |
Two sets of aerial photos were interpreted for the Elkhorn area: a black-and-white set taken from high altitude (at 1:24,000 scale) and a set of color photos at a larger scale (about 1:48,000). The smaller scale photographs were more useful for mapping the regional structures in the area. A large north-south structure (figure 8) was inferred to pass between the lower and upper Elkhorn Mines. That structure would correspond with the Comet Fault as shown on the unpublished map in the Montana Bureau of Mines and Geology mineral property files. A lineament was drawn east-west through the lower Elkhorn workings that may reflect the vein system. Areas showing a sudden change from upland vegetation to vegetation indicative of wet environments were also mapped on the aerial photos. The regional structures at the Elkhorn area extended thousands of feet. Aerial photos at commonly available scales were of limited use in mapping the smaller structures in the area. When known geology was transferred to the aerial photos, the large structures could be discerned, but it was difficult to pick them out when the geology was unknown.
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