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Investigative Methods for Controlling Groundwater Flow to Underground Mine Workings
Applying the various methods of controlling groundwater flow into mine workings requires knowledge of the source and pathways of the portion of the aquifer hydrologically connected to the mine. The following discussion of methods is an overview of the investigation. No single method will provide all the information. Compiling information from several sources and integrating the information into a conceptual model of the groundwater flow system near the mine is necessary.
Changes in groundwater chemistry and flow paths induced by mine openings sometimes can be observed throughout the watershed—certainly within the vicinity of the mine. Several methods can help quantify these changes and develop a conceptual model of sources and pathways. Precisely measured, detailed seepage measurements of streams and springs near the mine may show a net loss that should approximate the adit’s discharge rate. In addition to surface flows, precipitation throughout the watershed and measured or estimated evapotranspiration are used to determine the overall water balance.
The age of a given volume of groundwater is the time that has elapsed since the water infiltrated the ground surface to become part of the groundwater flow system. The age is related to how far and how deeply the water has traveled. Determining the age of a given volume of groundwater, along with the hydraulic characteristics of the aquifer, will provide a means of locating the recharge source for that groundwater.
Tritium (3H) is a radioactive isotope of hydrogen that has a half-life of 12.45 years in the atmosphere. Nearly all of the tritium in the atmosphere and water worldwide is a result of aboveground nuclear weapons testing that began in the 1950s. Atmospheric tritium levels in rainwater before the 1950s have been estimated to be 5 to 10 tritium units. The level increased to over 1,000 tritium units in the United States during the peak of testing in 1963 and has since declined to less than 100 tritium units. Tritium is unaffected by chemical reactions, and sources other than nuclear weapons are rare. The decay process essentially stops when the tritiated water leaves the atmosphere and enters the groundwater. These qualities make tritium particularly useful in determining the age of groundwater. The concentration of tritium in a sample of groundwater can be compared to the historic concentrations of tritium in rainwater. Using tritium as a tracer, the age of groundwater can be estimated with a precision of a few years.
One of the decay products of tritium is helium-3 (3He), a stable, inert isotope. In the atmosphere, 3He escapes and does not accumulate. In groundwater, 3He accumulates and is referred to as excess 3He. The ratio of tritium and the excess 3He can be used to determine the time elapsed since the groundwater was in contact with the atmosphere (Poreda, Cerling, and Solomon 1988) with a precision of a few weeks to a few months. This technique is particularly useful in determining the age of groundwater in shallow-flow systems. Helium-tritium age dating has been used in conjunction with other information related to the aquifer when identifying recharge areas.
In addition to helium and tritium, other isotopes can be used for dating, including anthropogenic chlorofluorocarbons, oxy-gen (16O and 18O) and deuterium (2H). Kendall and McDonnell (1998) have compiled investigative methods used in catchment studies.
Tracer studies have been applied at the watershed scale to estimate metals loading from various contaminant sources (for one example, see Kimball and others 1994). In general, a chemical unique to the local environment is introduced to surface water or groundwater. Streams and wells are monitored to determine when and where the tracer travels. Choices for the tracer include bromide (usually sodium bromide or potassium bromide), fluorescent dyes, and isotopes. Tracer tests are used commonly, but the failure rate is quite high. Reasons for failure vary, but often tracer studies are begun without sufficient knowledge of the fracture density, fracture orientation, and so forth. Tracer studies should be applied toward the end of the overall investigation rather than during an initial investigation, as is often the case.
“Natural” tracers associated with unmined host rock and acid rock drainage may be used to define flow paths in and around the mine. This method requires rather specific information on the mineralogy of the ore, gangue minerals (waste rock), alteration and weathering products, and the host rock. If available, detailed sampling and analysis of waters inside and outside the mine’s suspected zone of influence can produce information. In areas where there are many rock types, each rock type often is fingerprinted by its water chemistry. Computer-based geochemical equilibrium modeling programs can be used to estimate concentrations based on mineralogy, and to define water quality resulting from the mixing of two waters (such as host-rock water and acid rock drainage). For example, a model could be based on the chemistry of the adit discharge and the mineralogy of the ore body to help define the “required” chemistry and relative quantities of the different waters flowing into the mine.
Often, the type of drilling required to develop a thorough understanding of the groundwater system near a mine opening is used in the final phase of an investigation when grouting is being considered. Drilling targets should be based on other less intrusive and less expensive methods. A thorough discussion of the many drilling methods, tools, and strategies is beyond the scope of this paper. Mobilization, road building, and the logistics of any type of drilling are expensive and often are reserved for larger, more complex mine sites. Core drilling to determine such parameters as fracture density, orienta-tion, and hydraulic conductivity can add to the expense.
Remote sensing provides a relatively inexpensive, efficient process for mapping hydrologic features of the landscape. Remote sensing often relies on images obtained by a low- to moderately high-elevation aircraft flight or by satellites.
Image Scale—Various scales of imaging are available with fixed-wing flights, ranging from large scale (for example, 1:2,000) to fairly small scale (for example, 1:63,000).
The larger the scale, the more detail that can be extracted from the images, assuming that high-quality equipment and imaging techniques are used. However, costs generally increase as the scale of imaging increases. Because wetlands and riparian areas associated with groundwater sources to mine workings are often quite small, large-scale images will improve mapping accuracy. Generally, an image scale of 1:3,000 to 1:7,000 is adequate for delineating small areas.
Smaller scale images such as 1:16,000 to 1:24,000 are still quite useful for mapping wetlands and riparian areas. However, images at scales greater than about 1:50,000 provide only marginally useful detail for the needs discussed in this paper.
Satellite imaging such as the Landsat and the French SPOT series provide various image scales, along with a variety of spectral capabilities that can be quite useful for mapping wetland and riparian areas at a coarse scale. The ability to discern small wet areas depends on the electronic resolution associated with the image. Applicable Landsat bands for vegetation mapping have 30-meter pixel resolution, while the SPOT black-and-white panchromatic band has 10-meter resolution and the color bands have 20-meter resolution. Landsat image resolution may not be adequate for locating small areas related to the recharge of mine workings.
Other innovative remote-sensing methods include using hand-held or video cameras to record landscape features during overflights in small, fixed-wing aircraft and helicopters. Distortion, relatively poor image quality, and difficulty orienting images make these methods less desirable as a primary mapping resource. However, they can be useful when interpreting other remotely sensed images.
Another similar—but less expensive—method is to photograph or videotape from various fixed points on the landscape, preferably high-elevation, open areas, such as peaks. Even though heavy forest cover and intervening ridges may obstruct the view of some areas, these photos can be used to improve understanding of other images and maps. Tripod-mounted cameras can provide excellent image quality, but georeferencing the images is difficult.
Imagery—A popular type of imaging is conventional color film in a large-format mounted camera, such as the 1:16,000 resource photos used by the Forest Service. These photo sets also allow stereo viewing, which is especially helpful when the image is magnified using a stereo-scope. The three-dimensional (3-d) image shows the relief of the land and allows interpretation of landforms, an important part of identifying wetlands and riparian areas. These resource photos are readily available, are fairly inexpensive for small assessment areas, and are reimaged about every 5 years by the Forest Service. Image quality is excellent.
False-color infrared photography is even better for mapping wetlands and riparian areas. The photos are similar to the resource photos and can be viewed as 3-d images. The colors of the printed image are drastically different from those the eye sees when viewing the landscape. This is difficult for some viewers to get used to, but for an experienced mapper, the information is much improved over conventional color photos. These false-color infrared images are especially useful for locating vegetation associated with wet areas, such as wetlands and riparian areas.
Because of the spectral sensitivity of the film in the very near infrared portion of the electromagnetic spectrum, vegetation that is well supplied with water, such as vegetation growing in wet areas, shows up as brilliant red on the print. This is especially true for the wet-site deciduous trees and shrubs, as well as forbs and grasses.
Vegetation growing in relatively dry soil and unhealthy, diseased vegetation reflects very little of the red wavelengths and shows up as gray to grayish pink. Experience is required to properly interpret these photos, because healthy vegetation on upland, well-drained sites may also appear quite red on the photo. Viewing in 3-d helps interpret these images properly.
Both the color and the false-color photo series contain significant distortion, particularly toward the edges of the photo. Wetlands and riparian areas can be delineated on these photos, but for project planning and implementation, the delineations are best transferred to a georeferenced, distortion-free image. Orthophotos provide such a base-photo series. They are available in the same size, scale, and coverage as the U.S. Geological Survey 1:24,000 topographic maps. Perennial and intermittent streams, as well as topographic contour lines, can (and should) be added to these orthophotos. The combination of a distortion-free, georeferenced photo image and contour lines is a very powerful mapping tool. These images also serve as useful field tool, especially when combined with global positioning system (GPS) capabilities.
The Landsat satellite images are available in a variety of spectral bands, including very near infrared, which is useful for identifying vegetation associated with wet areas. Black-and-white photography has limited application for mapping wetlands and riparian areas. If the only available images are black and white, a well-trained mapper can do fairly well with these images, preferably using good-quality stereo pairs.
Compiling the pertinent spatial information narrows the geographic area that may be contributing water to mine workings. This compilation can include delineating the surface watershed boundary and circumscribing lands at elevations higher than the mine portal, both within and adjacent to the watershed. Local knowledge of groundwater pathways and bedrock permeability, as well as time and budget constraints, are factors that may determine the geographic limits of the area where wetlands and riparian zones are mapped.
If color or false-color infrared stereo photos are available, they should be used to delineate areas that are likely to be wetlands or riparian areas. At this point, it would be helpful to adapt an existing wetland and riparian area classification system (see Soil and Wetlands Interpretation in the References section). These choices include classification based on potential natural vegetation (plant associations), existing vegetation (community types and riparian-dominance types), hydrogeomorphic features and functions (U.S. Army Corps of Engineers), and on various combinations of hydrologic systems, soils, and vegetation types (U.S. Fish and Wildlife Service’s wetlands classification and the U.S. Army Corps of Engineers’ Wetlands Delineation Manual). The U.S. Fish and Wildlife Service’s Classification of Wetlands and Deepwater Habitats of the United States is especially useful. This classification is hierarchical, allowing the user to select the appropriate level or rank that works best for the project. The more detailed levels of classification allow a better understanding of the hydrologic regime. The Corps of Engineers’ manual is not a classification system, but it does present a well-recognized system for identifying jurisdictional wetlands.
Both polygons and line segments may be needed to delineate the wet areas; polygons are most appropriate when the area is large enough to circumscribe at the mapping scale being used. Line segments are appropriate for long, narrow areas, such as along minor streams. These delineations can be transferred to a field map, such as an orthophoto with contours and streams, by referring to common landscape features in the stereo photos and the orthophotos. Map symbols for the various classification units that reflect the classification hierarchy should be developed. Human-caused sources of water, such as irrigation and drainage ditches, stock ponds or other impoundments, and mine portals should also be located and mapped. Both the stereo photos (preferably large scale) and the orthophotos are useful field references for locating wet areas on the ground and for choosing sampling sites.
Some field time should be devoted to correlate remotely sensed image features with on-the-ground data. Remotely sensed data is often quite good, but is incomplete by itself. Depending on the landscape and the particular needs of the project, field study may become a major part of the identification of wetlands and riparian areas. Fieldwork can greatly improve the accuracy of remotely sensed delineations. Fieldwork is also needed to understand the relationships of wet areas to hydrologic features, flow paths, and patterns. Field documentation is required to understand water discharge, recharge, and storage.
Once the mapped delineations have been classified as categories of wetland and riparian types, the field sampling strategy can be developed. The classification and mapping can reduce the amount of field documentation needed to verify the mapped delineations and to characterize their hydrology, soils, and vegetation. Based on the degree of confidence needed in the mapped delineations, as well as the time and money available for fieldwork, delineations representative of the classified map units may be selected for field study. In other situations, the needs of the study may require that every delineation be field checked.
The following is a suggested methodology for field investigations that should produce the data needed to characterize the wetland and riparian area map units for mine workings recharge studies. The specific needs of the map user should determine how much field data to collect.
Traverse representative portions of the mapped polygon or line segment, noting the various landforms and positions on landforms. Streams commonly have a low, active, poorly drained floodplain and a higher, older, better drained stream terrace. Slumps may have a shallow, ponded basin near the base of the head scarp with poorly drained slide debris below. These different topographic features should be characterized individually as follows.
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Topographic Features |
Site Features and Supplementary Information
For the dominant vegetation classes of a given landform or landform position, describe the major features of the upper portion of the soil profile. Initially, to develop correlations between landforms, vegetation, and the soil taxa or soil moisture regime, the upper 30 to 40 inches of soil should be described. Once the correlation of landscape features, soil drainage, and permeability is better understood, the soil may need to be characterized only for the upper 12 to 24 inches. Descriptions of the soil profile should include:
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Geoscientific methods use existing information, which can usually be obtained at low cost. Some historical information, such as underground mine maps of collapsed mines, could not be gathered today.
Before going into the field, review the literature. The amount of geologic information about a particular area may vary greatly. Some mines with a great deal of development were never documented—neither in published materials nor in proprietary, unpublished materials. Other highly speculative properties may have numerous reports written on them, but with embellished information. Mine maps are a great asset when they are available, but the level of detail varies greatly from mine to mine. Some maps include notes detailing where water was encountered. Other geologists only sketched in major structures and rock types.
Many of the techniques applied to soil and wetland delineation also apply to structural and geologic mapping. Aerial photo interpretation before and during field mapping can help analysts interpret geologic features that may control the flow of groundwater. Geologic features that may be differentiated through aerial photos include lithologic changes such as alluvial rather than bedrock substrate, intrusive rather than sedimentary rocks, or welljointed rather than unfractured formations. Recent landforms such as landslides may control, or be controlled by, groundwater and are usually easy to discern on aerial photos. Interpretation includes studying vegetation differences, lineations, locations of seeps, and rock types.
Aerial photos are relatively inexpensive and are available in a variety of scales. Aerial photos have been taken for the past 50 years, allowing changes to be documented over time (Sciacca and Ault 1993). Because most aerial photos date from the 1950s and later, older mine activity may not be documented.
The scale of most publicly available aerial photos ranges from 1:62,500 to 1:15,000. You may have to arrange your own flights to get larger scale photos, but they will show more detail. Orthophotographs (a compilation of aerial photos that have the qualities and position of maps) can be used to plot features seen in stereographic projections.
Factors to consider in photo interpretation include tone, texture, and pattern. Tone is related to the amount of light reflected by an object. It is influenced by the location of the sun, the amount of haze, the latitude (and the time of year), the sensitivity of the film used, the camera filters used, and the type of image processing. Texture is a tonal change within the image produced by an aggregate of unit features too small to be resolved or distinguished individually. Texture may also be a function of the drainage density. Pattern is an orderly spatial arrangement of geologic, topographic, and vegetation features. Patterns may be a result of faults, joints, dikes, or bedding. Faults tend to be single and more striking or intense when they have more offset. Joints are more numerous and less intense.
Lineations caused by fracturing and faults may be evidenced by the locations of springs, angular drainage patterns (especially with abrupt directional changes), changes in the type and health of vegetation, and unexplained changes in soil color and texture. Joint patterns in rocks can be observed on some aerial photos, especially in granitic terranes. Aerial photos are not perfect tools. In heavily vegetated areas, vegetation may mask lineation produced by fracturing of bedrock.
Aerial views may show the presence of highly permeable surface materials or show vegetation growth that indicates the presence of water. To some degree, the amount of plant material may be used in the water-budget calculations of transpiration in an area.
Sometimes the recharge area is so extensive that flow will be unaffected by plugging, grouting, or other local control measures. Large geologic structures sometimes can be discerned from aerial photographs, even when they are not recognizable from the ground. Drainage patterns and, more specifically, their offsets, often reveal fault traces. Other subtle differences in tone, texture, or patterns may reveal geologic differences.
Field reconnaissance and detailed map-ping are also an important part of a regional interpretation. They are used to ground truth the interpretation from aerial photographs and to add details that are not evident from even the largest scale (and most detailed) aerial photographs.
Field mapping is essential to discovering the sources of discharge and recharge in mine areas. Underground mine mapping is one method of gathering data for adit discharge control. Ideally, the sources of water would be mapped while the mine is still open. Although rare, opportunities do exist in active operations. Examples would be the recently inactive TVX Gold, Inc., Mineral Hill Mine in Jardine, MT. A large quantity of water was encountered in the crosscut driven to the Crevasse deposit in 1997. Individual fractures and faults could be recognized easily and mapped as discharging the majority of the water.
Abandoned workings of varying ages are sometimes accessible if properly trained personnel can enter them. Recent U.S. Geological Survey wilderness mineral resource study reports contain maps produced while the study was underway, even though the mines had been abandoned for years (Stotelmeyer and others 1983).
A third possibility would be to rehabilitate mine workings that have caved in or become unsafe. Although costly (often cost prohibitive), the information gleaned from such an exercise would be invaluable in planning mine reclamation and, later, in the actual process of adit plugging, grouting, or implementing another method for controlling adit discharge.
Consider using a geophysical program to locate fractures and the water they may carry. Various electrical methods may help identify voids created by mining and the presence of water in fractures and mine workings. This option would be preferred to amass more detail where fractures are the primary water conduits. Bither and Tolman (1993) found that a combination of aerial photo interpretation and geophysics (specifically, very low-frequency electromagnetics or terrain conductivity) were useful in locating the preferred sites for drilling water wells. Well yields were greater when the two methods were combined than when fracture-trace studies based on aerial photo interpretation alone were used. This combination of methods may be useful in discharge and recharge studies.
Ground-penetrating radar may help detect voids left by mining that may be filled with water, so long as the voids are not more than 100 feet underground. Ground-penetrating radar can provide a cross section of soils and subsurface features by measuring transmitted energy reflected by differing medias and buried objects. Steep and uneven terrain provides a physical challenge for the implementation of surface geophysics.
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