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5 Mining: Methods and Impacts OVERVIEW OF SURFACE MINING, SPOIL HANDLING, AND RECLAMATION There are three major types of surface mining procedures, which in the context of the whole mining and reclamation process have common features as well as significant differences. The three mining types contour mountaintop removal and , . . . area mining, differ in their size of operation, equipment use, spoil handling, and final landscaping. The mechanics of the mining process involve several steps, including: 1. planning and permit approval (for the complete operation); 2. site preparation (topsoil storage, sediment pond construction); 3. mining (blasting, spoil dumping, coal extraction); 4. reclamation (landscape stabilization and revegetation); and 5. bond release (operator no longer responsible for the site). Two reports from the National Research Council (1981a,b) provide comprehensive summaries of surface mining methods, impacts, and land -60-
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-61- restoration. One of these reports, prepared by the Committee on Ground-Water Resources in Relation to Coal Mining, provides a full description of each of the main surface mining methods (NRC, 1981a). A brief summary is provided here as a framework for addressing the recharge focus of this report. Contour Mining The overburden above the approximately horizontal coal beds in the mountainous areas (Figure 5.1) of the Appalachian Coal Basin is removed in a stepwise procedure that includes topsoil removal and overburden blasting and removal to expose the coal seam. The spoil from the first cut in a contour operation may be stored for replacement in the reclamation of the last cut (generally not favored due to double handling), reclamation of an adjacent abandoned surface mine (encouraged), or by head-of-hollow fill (most frequent). Contour mining is often accompanied by lateral angering to extract the unexposed coal seam. Horizontal holes up to 60 m long at spacings from 15 to 60 cm can become subsurface reservoirs influencing the occurrence and movement of ground water in the reclaimed site. In general the landscape is returned to approximate original contour unless a variance has been granted in the mine plan. Previously stored topsoil or a substitute material is placed over the spoil and is stabilized by mechanical compaction on sloping terrain, and the area is revegetated with herbaceous, shrub, and/or tree species. The main features of this mining method that influence recharge include initial vegetation removal, compaction of the reclaimed soil profile and the mine floor (e.g., clay material underlying the coal seam), spoil generation with an increase in porous material volume, and change in vegetation type.
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-63- Mountaintop Removal In this procedure complete extraction of upper coal seams in a mountain (Figure 5.2) is achieved by sequential removal of all overburden above the seams. Spoil material is placed and graded to favor surface runoff toward the center of the mountain in the restructuring of the landscape. The volume increase in spoil due to the porosity being greater than that in the original overburden rock is usually handled by deposition in a head-of-hollow fill. The main features of this mining method that influence recharge are similar to those described for the contour method. However, the greater size of the operation and the longer duration of the mining process may result in greater impacts on recharge in the vicinity of the mine site. Area Mining This method uses the largest equipment. It well established in the western United States (e.g., the Northern Great Plains Region), southern Illinois, Indiana, and in western Kentucky, and is also being practiced in mountain areas of Appalachia where several mountaintop removal operations are combined. These operations have common features with the other mining methods in terms of vegetation removal, topsoil storage, blasting, overburden removal, coal extraction, landscape restructuring, and revegetation. Sequential mining of the area is usually practiced whereby current overburden is used to fill the previous extraction pit (Figure 5.31. Sometimes the last pit is landscaped as a water body, thus eliminating the cost of hauling large amounts of stored overburden. Recharge and ground water systems can be subject to the greatest disturbance when the area mining method is used, due to the large mine size, heavy equipment use, and long duration of the mining operations. .
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-64- , FIGURE 5.2 Surface mining mountaintop removal and valley fill method
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- 6 5 - ~~ 14~:'. . ~ W~ ~i. GRADED AR:| HDISTURBED LAND ~.~ R ~ ~ ~ ~—~ Of —~ ~:——_ ~ '_·_~ FIGURE 5.3 Typical area mining method with s tr ipp ing shove 1 .
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-66- Common Features of Three Mining Methods The common features of these three mining methods that can influence recharge of ground water in the reclaimed landscapes are 1. initial vegetation removal (eliminates transpiration); 2. blasting (increases volume of overburden, fracturing of adjacent and underlying bed rock); 3. mine floor compaction (reduces recharge to lower aquifers); 4. disruption of aquifers) (dewatering, destruction of storage zone); 5. water storage in spoil (greater porosity, extra fill areas); 6. surface compaction (greater surface runoff); 7. unfavorable reclaimed soil (poor water storage in root zone); and 8. change in vegetation type (change in rooting depth and growing season). MINESOIL PROPERTIES Surface coal mining and other land disturbances often significantly change soil properties. Minesoils, which are the materials on the restored land after mining, have properties that reflect the character of the coal overburden that becomes the parent material of the soil. Proper placement of soils and overburden after mining produces minesoils suitable for plant growth, but haphazard placement of these earth materials may result in minesoils that are difficult to vegetate even with lime and fertilizer amendments. Haphazard placement of overburden materials generally results in extreme variability of minesoil properties. Some minesoil properties change rapidly over time, especially for the first few years after revegetation. These changes are caused by the weathering of fresh, unweathered, or partially weathered overburden materials, a process that may
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-67- be accelerated or decelerated by applying amendments such as lime, fertilizer, or sewage sludge to the minesoil. Most minesoil textures are loamy, but some are clayey or sandy. Rock-fragment content varies, but most minesoils in areas without an original loess cover of 60 cm or more have greater than 35 percent by volume rock fragments in subsoil horizons (some as high as 80 to 90 percent) and fewer than 35 percent fragments in the surface horizons (Ciolkosz et al., 1985; Thurman et al., 1985~. Minesoils, in general, have more rock fragments than do contiguous native soils (Bussler et al., 1984; Pedersen et al., 1978; Thurman and Sencindiver, 1986). Soil structure develops very quickly in some minesoils, but it is generally more strongly developed in older minesoils. New minesoils constructed with scrapers tend to have massive, compacted layers, but minesoils constructed with a mining wheel excavator in combination with belt transportation tend to have a fritted structure, which is a porous structure with rounded aggregates loosely compressed together (McSweeney and Jansen, 1984). Surface horizons of minesoils generally have higher bulk density, lower porosity, and lower water-holding capacity than do those of contiguous native soils (Bussler et al., 1984; Potter et al., 1988; Smith et al., 1971; Thomas, 1987; Thurman and Sencindiver, 1986; Younos and Shanholtz, 1980~. As minesoils age, however, these properties become more like those of the native soils. The subsoil horizons of minesoils also may have properties that differ from those of the subsoil horizons of native soils. Infiltration rates and saturated hydraulic conductivity of minesoils are highly variable and may be lower or higher than are those of contiguous native soils (Hnottavange, 1987; Pedersen et al., 1978~. Large macropores in some minesoils cause water to move rapidly through the profile, but compaction of clayey-textured material may cause water to move very slowly through the soil.
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-68- Other factors a~ffecting physical and hydraulic properties of minesoils are the presence or absence of topsoil and land use. Topsoil layers on minesoils typically have higher water-holding capacities than do the surface layers of non-topsoiled minesoils, because topsoil generally has fewer rock fragments (Thurman et al., 19859. However, water may infiltrate more rapidly and move through the profile more quickly in non-topsoiled . . ~ . ~ . · . ~ ._ r_ __ Layers (Rogowski and Jacoby, 1979~. Grazing of minesoils by livestock may significantly lower the infiltration rate (Hnottavange, 1987~. m~nesoils than in minesoils with topsoil ACTIVE SURFACE MINING EFFECTS ON RECHARGE In the site-preparation phase of mining, vegetation is removed, topsoil is scraped off and stored, and overburden is blasted and excavated to expose the coal. Significant changes in the landscape's water-budget components (evapotranspiration, drainage, storage) result from these activities; however, the effect on recharge depends on the season of the year and the coal field's location. Differences in mining effects on water-budget components for an eastern and a western coal region are given as examples. Appalachian Coal Basin Summer Operations Site preparation that is initiated in May, followed by mining, site reconstruction, and completion of revegetation by September may have a small impact on long-term recharge. Recharge is v Removal of A —-r - usually negligible cur: no the .~'mm~r vegetation eliminates ~ . ~ transpiration at the site 2 and summer ralntall can result in increased recharge if infiltration is not restricted. However, surface compaction usually limits
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-69- infiltration and causes surface runoff to dominate the hydrologic processes during mining. In undisturbed forest, surface runoff is essentially nonexistent due to the high infiltration rates of forest soils. Ephemeral stream flow (apparent surface runoff) is generated by storm events from subsurface water exfiltrating to the surface by convergent flow induced by topography. Surface water diversion into sedimentation ponds and pond discharge into stream waters are the dominant impacts of summertime mining operations , replacing the normal evapotranspiration water-loss component of the water budget Winter Mining . Most recharge in eastern coal basins occurs during the winter and the spring into a natural fracture system within the outer rock zones of mountains. These recharge zones have formed as a result of stress-relief fracturing during landscape erosion (Wyrick and Borchers, 19819. Mining operations destroy some of the natural fracture system, and during the winter period mining can significantly reduce recharge through surface compaction effects caused by mining equipment. Reduced infiltration leads to enhanced surface runoff, which is routed to sedimentation ponds if the mine operation is conducted according to approved procedures. Overflow from the ponds is channeled into local stream waters. The net effect is a bypassing of the natural ground water recharge-discharge process by overland routing to streamflow. Summer and winter mining represent two extremes of direct mining effects on recharge, with patterns of impact expected to be intermediate for operations conducted during the spring or autumn.
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-70- Western Coal Basins The large mining operations of western coal basins have a long period of coal extraction often lasting for a decade or more, and they occupy large areas relative to most eastern mines. ~ ~ The aquifer systems in western coal basins can be extensive, and several different aquifers, with differing recharge areas, may exist at a mine site. Mine operations often require dewatering of the intended mining area since the coal seams are usually a significant component of the aquifer system. Ground water pumping causes a drawdown of the aquifer, forming a cone of depression in the water table (Figure 5.4~. The extent of drawdown may be small in relation to the whole aquifer, but the local effect is necessarily large (Woessner et al., 19791. Van Voast and Reiten (1988) estimated that the aquifer drawdown at the Decker mine in southeastern Montana extended over several kilometers (Figure 5.51. The surficial and deep aspects of recharge are noted for the western coal basins. v Surficial Recharge Processes There is a definite seasonality to recharge in the western coal basins. The occurrence of frozen soil and the dynamics of snowmelt result in runoff to nearby alluvial depressions and valleys during spring. Snowmelt can be a very dramatic pulse event resulting in water accumulation in the lower landscape positions. Recharge to the upper aquifers in the landscape takes Place largely during the snowmelt period. ~ ~ , Rainfall during winter and early spring can also ce effective in recharging the upper aquifers in the landscape. The operations at a mine site disrupt recharge within the mine area. However, this local effect may be offset to some extent by the collection and discharge of mine site precipitation through
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-71- COMBINED , . MINE ONLY C_ - ~1 D~ ~ W~ ~ NON—MINE __ _I it_ '! ~ '~1 ''~1 '2~ my. _ C-A W~ SCENARIO D YEAR 2032 .,: - /~\~N' /' ~'~" / ~ ma\ ~ Ky3&r_ -~ ~ / \~ K~ ~ ~ R r :~.~7< t FIGURE 5.4 Three-dimensional representation of drawdown and percent of that drawdown resulting from mining for Scenario D in 2032. SOURCE: Office of Surface Mining Reclamation and Enforcement, 1988.
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-72- ~ / (a) ~ ~ ~ j —~ w r O ~ Drawdown (~) c `,, ,,~ land surface ~ _ i_ 2 ~ Km 2 1 Mi '< Fault C1 Min. pit :: ~ WHIP =' \D-1 and D-2 coal bedim— ~ _ _ potent~om~ric profile FIGURE 5.5 Aquifer drawdown at the Decker mine in southeastern Montana. (a) An area of potentiometric decline more than 15 miles long and 5 miles wide has developed for the D-2 coal bed; (b) lowered potentiometric levels pass unaffected beneath valley bottoms and perennial streams. SOURCE: Van Voast and Reiten, 1988. (b)
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-73- sedimentation ponds to surface waters, which may contribute to recharge of surficial aquifers outside the area of active mining. Deep Recharge Processes Mine site operations may have little effect on the recharge of deep aquifers since in many cases the recharge occurs in permeable upland areas remote from the mine site. These permeable areas are formed in the approximate location of an outcrop of a former coal seam. Coal at or near the surface usually had been ignited by natural causes in earlier geological times, and the heat from the combustion had fractured the surrounding rock, forming the highly permeable material called clinker or scoria. Part of the precipitation received on clinker percolates through this material into adjoining coal seams, recharging deep aquifer systems. Recharge to such deep aquifers proceeds unimpaired during mining operations. RECLAMATION EFFECTS ON RECHARGE AND ON WATER QUALITY Water Quantity Restoration of the mine site to approximately the pre-mining landscape by spoil placement, surface application of topsoil or other approved material, and revegetation can lead to an increase or decrease of recharge to aquifer systems relative to that of the original landscape. The factors contributing to an increase in post-mining recharge are Poor vegetation establishment. Reduced evapotranspiration increases soil water drainage. · Revegetation with species having more shallow root systems. Smaller available soil water storage favors reduced transpiration.
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-74- · Reduced soil water storage in root zone--leads to increased drainage below the root zone and to lower transpiration. ~ Increased porosity of the vadose zone--provides greater water storage for potential recharge to aquifers. · Increased permeability of the vadose zone--allows more rapid movement of water to the aquifer. · Increased fracture porosity in the aquifer (from blasting--results in greater aquifer water storage to receive and transmit recharge. o Water impoundments. ~ nine plans allowing lakes and ponds may increase recharge through reduced runoff and increased seepage from the impoundments into aquifers. · Enhanced zones of stress-relief fracturing in the rocks above buried head walls, especially in sites that received extensive angering. Post-mining recharge may be decreased by: · Reduced infiltration of the reclaimed surface. Greater surface runoff bypasses aquifer recharge. · Slope instability and erosion--expected to increase surface runoff in channel flow bypassing recharge to the aquifer. o Enhanced evapotranspiration. Prolific vegetation may have greater canopy interception and transpiration than did the original vegetation. ED Reduced effective corositv in the acuifer (from blasting--less storage to receive and transmit recharge. · Reduced permeability in vadose zone due to compaction during spoil placement and reshaping. · Mine floor compaction--limits recharge to deeper aquifers. o Elimination of surficial scoria or other high-recharge areas from within the mine site boundaries.
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-75- One of the dominant effects of the mining operations on recharge is the conversion of rock to rubble. In the pre-mining landscape, deep percolation may often be restricted by layers with low hydraulic conductivity, leading to perched water table development and lateral outflow as seeps or springs. In the post-mining landscape, deep percolation is often enhanced by the high hydraulic conductivity and high porosity of the spoil material. One critical zone in the restoration of recharge occurs at the surface. Restoration of the infiltration rate of the reclaimed site is a prerequisite for restoration of recharge. Reduction in infiltration rates below pre-mining rates can be caused by compaction by earth-moving equipment during the landscape reconstruction. Compaction is deliberately used on sloping terrain to stabilize the spoil material and minimize erosion during revegetation. Some spoil materials from eastern Kentucky can withstand compaction without lowering infiltration to unfavorable levels (Wells et al., 19829. Not all spoil materials exhibit favorable infiltration properties. In particular, spoils from a western Kentucky surface mine were shown to have extremely low infiltration rates that were associated with high bulk density and a well-graded particle size (Wells et al., 1982~. Deep ripping of the restored profile prior to revegetation can have a helpful influence on infiltration in some cases. Materials that form surface seals during wetting and crusts during drying will also reduce recharge by enhancing surface runoff rather than infiltration. Vegetative cover is one of the better means of overcoming surface sealing and crusting problems. Several non-mining changes in land use due to natural and human-induced causes can alter landscape water budgets, and these provide some comparisons with the effects of surface mining on recharge. A discussion of these other land use effects on recharge given in Appendix C.
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-76- In low-rainfall areas, recharge may not be limited by recharge capability, and restoration of recharge capability could be viewed as an excessive requirement. In high-rainfall areas such as the coal provinces east of 100° W longitude, recharge capability can limit recharge rates, as evidenced by lateral subsurface flow and wet weather seeps. Such seeps have been observed, for example, in vertical faces of rock formations with vertical fractures that terminate on horizontal strata of low hydraulic conductivity. Seeps then occur above the outcrops of the less-permeable strata. Restoration of recharge capability to pre-mining conditions at a surface mine permit area requires two main component steps: (1) the infiltration characteristics of the reclaimed site need to favor/produce recharge rates that equal or exceed the pre-mining rates, or exceed the highest rainfall intensity of the area, whichever is smaller, and (2) the hydraulic-conductivity values of the root zone and the vadose zone need to equal or exceed the equivalent pre-mining values. Also, revegetation should be carried out in such a way that the evapotranspiration rates after mining are not greater than those that occurred before mining. Due to soil and subsurface heterogeneity, it is not practical to undertake major field hydraulic-conductivity measurements (Klute, 1986), and indirect methods of evaluation should be used. In most cases, the hydraulic-conductivity characteristics of the restored vadose zone are much higher than the corresponding pre-mining values due to the greater porosity of the reclaimed spoil materials relative to the original rocks and other geologic deposits and due to the breakup of layers of low hydraulic conductivity. Restoration of infiltration rates is needed to initiate the recharge process. Compaction of spoils is the major factor that may inhibit the recharge capability at a mine site, and such effects can usually be identified by the occurrence of surface erosion and poor establishment of vegetation. The reclaimed permit area should not
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-77- have any visible signs of particle erosion rates exceeding pre-mining values. This requirement provides evidence that the surface hydrologic regime has been restored and that surface runoff is not exceeding pre-mining levels. Additionally, the requirements for revegetation specified in SMCRA favor the restoration of recharge capability by maintaining the integrity of the soil surface for infiltration as well as the permeability of the root zone for soil water drainage. Special attention needs to be given to evaluating compaction in reclaimed spoils as part of the post-mining assessment of recharge capability, particularly in areas with loess deposits, such as in western Kentucky. Water Quality From the standpoint of water quality, coal surface mining has its greatest impact on the shallower aquifers. The most notable effect is increased total dissolved solids and increased sulfate, calcium, and magnesium concentrations. In some cases there have been increases in the concentration of trace metals, including lead, manganese, nickel, chromium, cadmium, zinc, arsenic, and selenium (NRC, 1981a; Appendix D of this resort). - ~ , Increased selenium content in ground water is a particularly onerous problem. The selenium content of some coals is 10 to 200 times the crustal abundance of this element Waken, 1973~. Selenium is mobilized as selenate (SeO4 ~) in alkaline coal spoils and is readily transported by ground water. Selenate is the form most available for accumulation by plants (Presser and Barnes, 1984~. Some species of Astragalus accumulate up to several thousand ppm selenium (Walter et al., 1972), and selenium is concentrated by animals. Fish in the selenium-contaminated waters of the Kesterson National Wildlife Refuge were found to have selenium concentrations 100 times that of fish
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-78- found in selenium-free waters (Presser and Barnes, 1984~. Above levels required for good health (0.04 to O.1 mg/liter) selenium is toxic, causing mutations in waterfowl hatchlings (Presser and Barnes, 1984) and acute selenosis in other animals (Fishbein, 1977~. Ground water quality data that have been collected to date are obtained from samples collected on or very near coal surface mine sites. Data are collected from probable hydrologic consequence (PHC) reports and compiled by the regulatory authority into cumulative hydrologic impact assessments (CHIAs) to evaluate cumulative effects of multiple coal surface mines in a given area. Although no long-term consistent trend (i.e., several decades to centuries) has been confirmed through evaluation of analytical data, (Groenewold et al., 1983; Van Voast and Reiten, 1988), it is anticipated that there may be long-term water quality impacts. CONTROL OF ADVERSE EFFECTS OF COAL SURFACE MINING ON GROUND WATER QUALITY All three phases of coal surface mining--pre-mining exploration, active mining, and post-mining reclamation--can potentially have negative impacts on ground water quality. Exploration boreholes, drilled to determine the extent and quality of the coal seams, are, after data collection, now plugged as mandated by the Federal Surface Mining Control and Reclamation Act of 1977. This action is an important step in preventing contamination of deeper aquifers that often have better water quality than do shallower aquifers. Many coal companies now have a routine program of sealing all exploration drillholes that they find even though they may have existed from exploration periods predating the 1977 Surface Mining Control and Reclamation Act (W. A. Van Voast, Montana Bureau of Mines and Geology, personal communication, 1989~.
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-79- During mining, blasting can create vertical fractures or widen existing fractures, allowing percolation of contaminated water into deeper aquifers of higher quality. Little can be done during the actual mining phase to prevent this from occurring other than continual dewatering to minimize the amount of water flowing to or from the deeper aquifer. During post-mining reclamation, considerable effort is devoted to minimizing the short- and long-term impacts of coal surface mining on ground water quality. Blending of spoils and selective placement of spoils are techniques now being developed rurcner, and in some cases practiced, to minimize the deterioration of water percolating through the backfill of reclaimed coal surface mines (Phelps and Saperstein, 1982; Groenewold et al., 1983; Caruccio and Geidel, 19891. One example of spoil blending is the mixing of acid-producing spoils with alkaline spoils to "neutralize" acid produced by iron sulfide oxidation. Selective placement of weathered overburden may play an important role in controlling ground water quality in the western United States. The uppermost zone of the alkaline overburden of the West is highly weathered, and soluble salts have migrated only a few meters into the underlying zone over geologic time, because of low precipitation. It is this zone of very soluble salts that has often been placed on the mine floor as a result of overburden handling. Lateral and vertical ground water recharge into the spoils forms a new aquifer with the mine floor as the aquitard. Material high in water-soluble salts is readily leached by the new mine floor aquifer, resulting in serious _ deterioration of the new aquifer (Pagenkopf et al., -- ~ 1979 W A N7an Voast 1977; Woessner et al., _ . , ~ a_ , Montana Bureau of Mines and Geology, personal communication, 19899. To avoid increasing the salinity of the new aquifer, it has been proposed (Pagenkopf et al., 1977) that the zone containing the water-soluble salts be perched below the root zone and above the mine floor aquifer by selective spoils handling.
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-80- Toxic and acid-producing spoils, identified during the pre-mining evaluation of the overburden, are often isolated with clay barrier materials and placed well below the root zone and above the mine floor aquifer. This technique establishes differential permeabilities, reducing percolation through the undesirable spoils, which could lead to ground water deterioration. However, this technique may only provide a temporary solution, since clay barriers may eventually leak. The reduction of recharge by decreasing permeability may at first blush appear inconsistent with the mandate of the SMCRA to restore ground water recharge capacity. However, if local reduction of recharge preserves water quality, then isolation of toxic and acid-producing spoils should be implemented. Because such isolation practices are localized and percolation is diverted, not prevented, the overall recharge may be preserved. Should recharge, however, be substantially reduced over a given area because of isolation practices, then artificial recharge through zones, engineered to assure good water quality, should be implemented. If isolation practices are required to protect water quality and the isolation technique reduces recharge and this reduction in recharge cannot be overcome by engineered recharge zones, then the area should be evaluated under the "unsuitable-for-mining" provision of SMCRA (Sect. 522).
Representative terms from entire chapter: