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Investigative Methods for Controlling Groundwater Flow to Underground Mine Workings
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| Figure 7—Adits at the upper and lower Elkhorn Mine—as well as shafts and cuts at the Park Mine—discharge water. Symbols indicate the approximate location and direction of flow for the larger springs. Sustained surface-water flow (measured in gallons per minute, gpm) near the adits is also indicated. |
Wetland and riparian sources of groundwater entering mine workings can be identified using several levels of mapping intensity. These include remote sensing, field verification and delineation, and physical and chemical characterization of the hydrological features of the study area. Depending on time and funding available for identifying the source areas, various levels of these methods may be preferred. The discussion that follows summarizes a full study (McBride 2002).
The assessment area for the riparian and wetland characterization was designed to follow roughly the 7,600-foot-elevation contour; 7,600 feet is the approximate elevation of the lower workings of the Elkhorn Mine. Lands higher than 7,600 feet were presumed to be potentially contributing groundwater to the underground mine workings through bedrock fractures.
Some subjective judgment was used to adjust the assessment area boundary, based on broad landscape drainage patterns. The assessment area encompasses about 12,000 acres, extending about 2 miles north of the Elkhorn Mine area, 4.5 miles to the west, and 4 miles to the south. This large area ensured that all potential sources of water were included. It is unlikely that locations farthest from the mine are contributing groundwater to the mine’s underground workings.
The characterization and mapping of riparian and wetland areas included stereoscopic review of standard 1:16,000-scale color aerial photography, commonly called resource photography in the National Forest System, as well as field documentation of soil profiles, soil moisture status, plant species, plant associations, slope hydrology, and landforms.
The initial stereoscopic review of aerial photographs was used to select sampling sites that would be representative of various combinations of vegetation, soil parent material, landforms, and landform positions observed on the photos. In the photos, nearly all of the sites that were selected appeared to be moderately wet or wet near the surface. About 10 to 15 sampling sites were located to represent each unique combination of vegetation type, landform, soil parent material, and landform position to characterize the variability of the soils, vegetation, and hydrology.
Field investigations documented these sites as well as others that would provide useful information. After field operations, these landscape and ecological parameters were formulated into map units that represent a unique combination of features that recur in patterns across the assessment area. The map units were designed to keep variability low and to be different enough to allow meaningful map unit interpretations.
The assessment area was satisfactorily characterized and mapped using a total of seven map units. Once these map units had been described, a stereoscope was used to produce a preliminary map of the assessment area on clear acetate overlain on the color aerial photos. Based on field data and a review of these preliminary riparian/wetland delineations, adjustments were made to the characterizations of the map units. A followup step in this iterative process was to correct the delineations on the aerial photos. These delineations were transferred to a clear acetate film registered to a paper black-and-white orthophoto that included all of the assessment area. This orthophoto was retrieved in digital format from the Natural Resource Information System database at the Montana State Library in Helena, MT. The digital file was printed at 1:24,000. This paper map became the base map for the transferred polygon delineations. The orthophoto allowed the riparian and wetland polygon lines to be located precisely on a base corrected for distortions. The acetate map of these polygons was scanned. Map unit symbols were added to provide digital files for the project’s final report. Locations of field sampling stops and discharge or recharge areas also were mapped and scanned.
A total of 102 field sites were documented. Nearly all of the sites were in areas with moderately high to high water tables at some time during the growing season. The soil profile and soil moisture conditions were characterized on 60 of these sites. Soil was excavated with a shovel, or where feasible, samples were retrieved using a hand-operated soil auger. Depth of sampling depended on site characteristics and site-specific data needs. Typically, soil was sampled from 12 to 40 inches deep. Soil-related data included horizon designations, textures, clay content, color, redoximorphic features (Vepraskas 1992, USDA NRCS 1998), soil moisture status, and the depth to saturated soil. Not all parameters were determined at each site.
Data collected on vegetation included dominant and characteristic plant species and the potential natural plant community (referred to as plant associations). Other field data collected included slope, aspect, pertinent landform or landscape features, and whenever feasible, latitude and longitude determined with a GPS receiver. The data collector also made observations of slope hydrology, including determinations of water discharge out of the soil (to become surface water) or recharge into the soil (to become groundwater). Rough visual estimates of flow rates of small perennial and intermittent streams also were noted. About 40 sites were assessed for hydrologic conditions. In addition, 15 determinations of the pH of stream and pond water were made using a field meter.
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