Special handling at surface mines includes the selection, handling, and controlled placement of potentially acid and calcareous rock, combined with water management, compaction or other practices to prevent acid drainage. The primary purpose of special handling is to locate acidic or alkaline strata in such a way as to minimize acid production and transport, and to maximize the generation of alkalinity within the mine soil water. Special handling is often used in conjunction with other acid mine drainage prevention techniques such as alkaline addition, water management (e.g., pit floor drains), and surface reclamation (e.g., slope grading to enhance runoff). For example, special handling, in the absence of calcareous material, cannot by itself produce alkaline drainage. Thus, where calcareous strata are absent, offsite calcareous material can be imported to offset natural deficiencies in acid-neutralizing rocks. Pit floor drains can be used, among other purposes, to engineer where the postmining water table will reestablish within the spoil, thus assuring that special handled material will be above the water table.

Typically in Pennsylvania, acidic material is placed above the postmining water table to minimize water and air contact. Calcareous materials, on the other hand, are placed such that their dissolution will be maximized. Use of special handling, alkaline addition, water management, and surface reclamation can allow the mine operator some control over acid- and alkaline-generating processes.

Incorporating special handling into the mining plan may be a difficult and costly task. However, this extra effort is invaluable if it results in the prevention of a costly postmining pollutional discharge.

Probably the first special handling concept involved the recognition of black or very dark colored rocks as potential acid formers. Initially, it was proposed that the material be buried on the pit floor. Deep burial was thought to prevent contact with oxygen, and hence shut off acid production. This approach was discussed as early as 1952 by the Pennsylvania Sanitary Water Board and is shown in Figure 14.1. The Sanitary Water Board also recommended highwall diversion ditches, pit floor drains, contemporaneous backfilling, and grading topography to limit infiltration.

Practical experience with deep burial of the potential acid forming materials showed that water quality problems were not always eliminated and sometimes were more severe. In Pennsylvania and other Appalachian states, special handling strategies began to evolve towards isolation of material above the postmining water table and away from preferred flowpaths. This has become known as "high and dry" placement. This remains the most common special handling technique used in Pennsylvania and is illustrated conceptually in Figure 14.2.

A general set of planning and handling guidelines was presented in 1979 (West Virginia Surface Mine Drainage Task Force, 1979) and later revised (Skousen et al., 1987). These publications identified some of the basic geologic, hydrologic, and mining information needed for the development of special handling plans. The Task Force guidelines emphasized placement above the water table.

Special handling plans are site specific, and should include consideration of the following factors:

• Geologic and Geochemical Conditions - identifying rock types present in the overburden and the determining the distribution, location, and amount of acid and alkaline rocks in the overburden.
• Hydrogeologic Conditions - identifying the geologic structure in relationship to the area to be mined; the occurrence, quantity, and quality of surface and

• Operational Considerations - mining method, sequence and total area to be mined, equipment to be used, and placement and amount of alkaline materials.

Geologic and Geochemical Conditions: Acid and Alkaline Materials

The stratigraphic position, aerial extent and total volume of potentially acid-forming and alkaline materials must be known to formulate a special handling plan. A review and interpretation of the overburden analyses is used for this determination. This topic is discussed in more detail in Chapter 11.

Special handling plans must be clear, simple, and easily implemented by field personnel. Maps and cross-sections should be prepared which show the positions of the materials to be special handled, and locations where these materials are to be placed or are needed. The materials must be readily identifiable in the field, either by color or rock type. Stratigraphic intervals above or below marker beds can also be used but may be less exact. Field identification is necessary to assure implementation as a routine part of the operation, and reduces potential misunderstandings by operators or regulatory personnel.

Segregation of the acid-forming or alkaline-producing strata must also be logistically feasible. Occurrence of acid-forming material in a single or a few discrete zone(s) that can be easily identified is preferable. If blasting is used to break the overburden, then it is preferable that the material to be special handled to lie immediately above or below the coal seam.

Stratagraphic position of the material to be segregated may be a controlling factor depending upon the type of operation. When the material to be special handled lies immediately above or below the coal seam to be mined, segregation is usually not a problem. The feasibility of segregating strata located at an intermediate distance between coal seams is dependent upon the type of operation and whether or not blasting is required. In a dozer-loader operation, the overburden is ripped with the dozer and removed in layers. As long as the material to be special handled is easily recognizable in the field, the operator would have little trouble segregating this material regardless of its stratigraphic position. If blasting is necessary to break the overburden, then segregation becomes problematic if the material to be special handled does not lie directly above or below the coal seam.

Blasting usually involves the total overburden column between coal seams. However, if the strata to be segregated lies at some distance between the coal seams, blasting must be done in lifts. The first lift would incorporate blasting and removing the overburden above the unit to be special handled. The unit to be special handled would be removed separately. The remaining overburden above the coal would then comprise the second blasting lift. However, this process can easily increase blasting costs by more than 50% and may result in poor rock breakage at the top of the lift because of stemming requirements (Mike Getto, personal communication). In this case, the increased blasting costs may rule out segregation and other alternatives may have to be considered.

If the material to be special handled is dispersed throughout the highwall section in thin beds, segregation of the individual rock units may be impossible. Instead of attempting to special handle individual beds, the entire section containing the acid- or alkaline-forming strata may need to be treated as a thick unit and the entire section special handled. Special handling such a large quantity of material will affect profitability of the operation and may be logistically difficult to achieve. In a situation such as this, other alternatives may need to be considered.

A similar problem occurs if the unit to be special handled is not of a distinct color or rock type. If the overburden analysis indicates this material to be laterally continuous across the site and located a uniform distance above a distinct marker bed, then segregation can occur with difficulty. Although fizz tests can be done in the field to identify alkaline strata, there is no comparable test for acid-forming strata. Once again, if segregation of the material is necessary, a thicker, easily identifiable sequence, which would include strata above and below the target strata, may have to be considered as the special handled unit in this situation.

When the overburden contains significant alkaline material and potentially acid-forming material, the location of these two materials relative to each other may determine the special handling plan. For example, if the alkaline section lies adjacent to the potentially acid-forming section, these two materials may become mixed without additional effort during the overburden removal operation. Separation of the potentially acid-forming strata may not be needed. Alternatively, if the alkaline- and acid-forming strata are not adjacent, but both can be segregated, a special handling plan may be implemented to place the alkaline material on top of the acidic material in the backfill as described later in this chapter.

Two conceptual overburden removal plans are shown in Figures 14.3a and 14.3b. In Figure 14.3a, acidic material is located in the upper part of the rock column and requires separate removal. In figure 14.3b, acid material is located directly above the coal. The entire overlying rock column can be blasted and removed in one lift, resulting in a blending of the alkaline- and acid-forming material.

The primary purpose of special handling is to control the location of acidic and alkaline strata in relationship to where groundwater enters and flows through the mine and the location of the postmining groundwater table in the spoil. The postmining water table elevation will influence placement of the potentially acid-forming material. "Rules of thumb" have evolved which recommend placement of acid-forming material at least

(a)

(b)

Figure 14.3a and b The positions of acid-and alkaline-forming material evolved from placement on the pit floor to "high and dry" placement.

10 to 20 ft (3.0 to 6.1 m) off the pit floor and 25 ft (7.6 m) or more away from highwalls. This is to ensure placement above the postmining water table and away from major recharge zones. This placement can be accomplished by a variety of means once the hydrologic conditions are determined. The information needed to predict the postmining water table is obtained from an analysis of the hydrogeologic setting of the mine and coal bed structure. Typically this includes a determination of the type of groundwater system, premining groundwater levels, relationship to adjacent streams, geologic structure, and mine plan.

In the situation where the operator is attempting to special handle acid-forming material by submergence, the length of time required for the postmining water table to re-establish is important. Where the operator wishes to place this material above the postmining water table, then the timing of water table reestablishment is not important. However, estimates of drainage quantities can be used to design dewatering structures such as highwall or pit floor drains, or inundation structures. Additional information and discussion on spoil hydrology, postmining water tables, and anticipated groundwater volumes are contained in Chapter 3 (Spoil Hydrology) and Chapter 16 (Water Management).

Sources of groundwater recharge which the operator must consider include infiltrating precipitation, groundwater recharge through the final highwall or adjacent mined area, and upflow through the pit floor. The ability to quantify these sources allows the operator to estimate when a postmining water table will be established. The development and final configuration of the mine, along with geologic structure and pit floor characteristics, will be significant factors in determining the geometry of the postmining water table. The operator can exert some control over groundwater recharge and the configuration of the postmining water table. Once the hydrologic factors are defined, the operator can then determine where and how to place the alkaline- and acid-forming strata in the backfill to maximize alkaline water and inhibit the formation of acidic groundwater.

The contribution to the postmining water table from infiltrating precipitation during the first few years following reclamation will be less than for an unmined site. Jorgensen and Gardner (1987), Guebert and Gardner (1992), and Ritter and Gardner (1993) investigated infiltration and runoff on newly reclaimed surface mines in central Pennsylvania. They found that infiltration rates on newly reclaimed minesoils are an order of magnitude lower than adjacent, undisturbed soil. However, within four years after reclamation, infiltration rates on some mine surfaces approach premined rates (8 cm/hr). During the topsoiling operation, the soil is compacted by the equipment spreading the soil. This promotes runoff. During freeze/thaw and wet/dry cycles, macropores develop in the surface soils which promote infiltration. Warner (1987) conducting studies on reconstructed soils for landfills under a variety of cover crops, found that runoff accounted for about 5% of the precipitation, infiltration from 42 to 53% and evaporation and stored moisture 41 to 56%.

Drawdown and recharge tests of the aquifers encountered may provide the operator with information, which will allow for an estimate of groundwater recharge rates. However, since fracture flow dominates in the coalfields, wells not located in fractures may underestimate recharge rates. Perhaps a simpler and more useful technique would be to monitor the flows from cropline springs.

Reestablishment of a postmining water table will probably be most rapid for those mines where the lowest seam mined lies beneath the regional water table. In this situation, constant pumping is required to temporarily dewater the pit during mining. Once the pumps are shut off, the regional water will reestablish itself in a very short period of time. It becomes somewhat more difficult to predict the configuration of the postmining water table for those mines associated with aquifers perched above the regional water table.

The hydraulic characteristics of the pit floor will determine whether a postmining water table will be intermittent or permanent if the mine is situated above the regional groundwater table. In some cases, the pit floor might be a thick underclay which tends to serve as an aquitard. In other cases, the floor might be massive, fractured sandstone which will not inhibit the downward percolation of groundwater. The presence and character of cropline springs can provide an indication as to the nature of the pit floor. However, the pit floor should also be investigated during the drilling of exploratory holes.

Experience with postmining water tables in surface mines in similar geologic, hydrologic, and mining situations as the proposed mining operation is useful in predicting the water table location once the permit is mined. The prediction should also be based on the hydrologic properties of the aquifers to be disturbed by the proposed operation. Hydraulic properties and water levels of the aquifers that will likely recharge the site should be measured to determine the volume of groundwater to be produced by the mine.

An understanding of geologic structure is also important in the selection of special handling techniques. Structure of the lowest mined coal seam in conjunction with the area to be mined and the final highwall configuration determines the final pit floor configuration. If a down dip highwall remains after mining and the pit floor retards vertical percolation, groundwater may become impounded on the pit floor against the highwall, resulting in a higher postmining water table than in the case of an up dip highwall. A cropline barrier left in place also has the potential to impound water. In the case where a downdip highwall remains after mining and conditions are present which promote mounding of the groundwater against the highwall, then the "rule of thumb" placement of 10 to 20 ft (3.0 to 6.1 m) above the pit floor may be inadequate. It may be desirable to change the orientation and/or location of the final highwall to avoid impounding water, or to incorporate underdrains to minimize groundwater buildup in the backfilled spoil (Chapter 16).

Premining hydrologic conditions must be carefully considered in light of the experience of the permit preparer and reviewer with the development of water tables in other existing surface mines of like topographic, geologic, hydrologic, and mining scenarios.

The ability to design and effectively implement a special handling plan is influenced to a large degree by several operational considerations. These include the total area to be mined and sequence of mining, time needed to complete mining, the need for blasting, the mining method to be used, and equipment to be used.

The mining plan is often based on the area available for mining (i.e., leased) and the coal quality (need for blending), rather than the optimum configuration for overburden and coal removal. As these constraints are real, they must be considered in preparing a special handling plan. The stratigraphic and areal distribution of the acid- and alkaline-forming materials as they relate to the mining plan will determine to a large extent how these strata realistically can and will be special handled. However, several pit orientations are often possible, and the choice of a particular configuration may have different implications for special handling, especially in a multiple seam operation.

The time needed to complete mining also requires consideration in the special handling plan. When potentially acid-forming strata are exposed, rapidly covering this material helps prevent the onset of acid forming reactions (Skousen et al., 1987). Perry et al. (1997) examined seven sites with special handling and found timeliness of reclamation to have some influence on water quality. Sites where mining ceased for a period of time and then resumed generally produced poorer quality drainage than sites where mining proceeded rapidly to completion. Extended exposure of unreclaimed spoil to infiltration and circulation of water and oxygen apparently allowed more acid production. Some mine permits in Pennsylvania now include a special condition prohibiting temporary cessation of mining to limit exposure of potentially acid-forming materials.

If overburden can be removed without blasting, it is not difficult to isolate thin beds of potentially acid- or alkaline-forming materials. Where necessary for overburden removal, blasting determines the fragmenting and sizing of rocks, their position in a spoil pile, and may affect the operator's ability to selectively handle target strata. Where the target strata does not lie immediately above or below the coal seam being mined, blasting may require three steps: 1) blasting and removal of rocks above the target strata, 2) removal of the acid-forming material itself with bulldozers and highlifts, and 3) removal of the remaining overburden down to the coal. However, this approach may not be realistic considering the increase in blasting costs.

The modified block cut method of mining, also referred to as the haulback method, is used on many Pennsylvania surface mines. This entails hauling the initial cuts of overburden to temporary storage. Subsequent overburden cuts are then placed in the previously created open cut. This cycle is repeated until mining is completed. The final cut is backfilled with the overburden stored from the initial cut. Block cut mining automatically lends itself to special handling material from the operating cut into the cut being reclaimed. Usually the cuts are open for only short periods of time, and are mined with a combination of loaders, bulldozers and trucks.

Area mining, also used on some Pennsylvania mines, generally results in larger pit sizes, and a longer period of time before backfilling and reclamation begins. These often involve multiple seam extraction where it is common for the operator to use a dragline for some seams and bulldozers and loaders for other seams.

The equipment to be used for overburden removal is another important component of a special handling plan. Truck and shovel operations are able to remove distinct portions of the section and to move overburden from one pit to another. Dozers with large rippers attached can often aid in overburden removal without blasting. This can allow for separation of the potentially acid-forming material from other non acid-forming material. In general, segregation of spoil material is more difficult using a dragline. In many cases, the dragline operator does not have visual contact with the spoil he is loading. Cravotta et al. (1994a) compared the ability of dragline and loaders on two areas of the same mine to special handle acid-forming strata. Both handling methods tended to invert the original rock column. Where loaders were used, pyritic shale was placed in pods near the final surface, and only low sulfur material was near the pit floor. On the area mined with a dragline, the overburden with the highest sulfur content was placed near the surface, but the sulfur contents for the material at the bottom of the pit were higher than they were for the area mined with loaders. A special handling study in Montana with dragline mining also reported that the overburden profile was inverted (Dollhopf et al., 1982). Both studies compared chemical and lithological properties of drillholes in minespoil to premining conditions.

The Cravotta et al. (1994a) study compared the distribution of sulfur and neutralization potential in undisturbed overburden strata (Figure 14.4) with the postmining redistribution of these parameters in the disaggregated mine spoil (Figures 14.5 and 14.6) for two mining methods. The mine site they studied was a reclaimed surface mine on two adjoining hilltops in southern Clarion County, PA. The southern area was mined with a 45 yd³ (34.4 m³) dragline. The northern area was mined with bulldozers and front end loaders which selectively handled the high-sulfur strata near the coal.

The original plan for the 16 acre (6.5 ha) northern area called for placing the high sulfur rock in pods 10 ft (3 m) above the pit floor, with low sulfur material placed between the pods and the pit floor. Drill holes N2-0 and N2-2, located 5 ft (1.5 m) apart, encountered one of the specially handled pods. The other drill logs show that mining, in general, inverted the high sulfur (>0.5%) material and located it near the spoil surface. Most logs show low sulfur (< 0.15%) material near the pit floor. Maximum saturated thickness of spoil in the northern area was 18 ft (5.5 m) and in the area of N2-0 the saturated thickness was 10 ft (3 m). The spoil sulfur data and spoil water level data suggests that the high-sulfur spoil was successfully placed above the water table within the northern area. The permit specification for placement 10 ft (3 m) above the pit floor would have been inadequate to keep the high-sulfur material above the spoil water table.

Spoil in the 34 acre (13.8 ha) southern area was also inverted, with the highest sulfur rock predominantly in the upper part of the spoil. The sulfur in the lower part of the spoil is typically between 0.25 and 0.4%. The sulfur concentrations in the lower part of the spoil are higher than what is typical on the northern area where the spoil was selectively handled. The highest saturated thickness in the spoil was about 20 ft (6 m). Thus the highest sulfur material in the southern area was also placed above the water table.

Some inferences about relative compaction of the spoil for the two mining methods can be made. Spoil handled by bulldozers and loaders would be expected to have a more uniform particle-size distribution, exhibit similar or greater compaction, and exhibit lesser hydraulic conductivity than that handled by the dragline (Chapter 3; Phelps and Saperstein, 1982; and Phelps, 1983). Air circulation commonly was lost in shallow spoil during air rotary drilling in the southern area; however, no air losses occurred in the northern area, suggesting greater compaction and more uniform particle size distribution from bulldozers and loaders than from a dragline. Nonetheless, hydraulic conductivities for saturated mine spoil were similar among the two areas. For saturated spoil, median hydraulic conductivities were 10^-3.8 to 10^-3.6 m/s in each area. The similarity in hydraulic conductivities could result from similar lithologies, and piping and settling processes (Chapter 3, and Pionke and Rogowski, 1982) by which fines are transported downward and large voids fill or collapse. Mine spoil in the southern area is several years older than that in the northern area, so a longer time has elapsed for these processes to occur.

Alkalinity, sulfate, iron, and manganese in the spoil groundwater produced by the selective-handling method was similar to that in spoil produced by the dragline method. Median values for alkalinity of groundwater in the saturated zone were between 100 and 400 mg/L. Sulfate ranged from 600 to over 1000 mg/L (Cravotta et al., 1994b).

Design and successful implementation of a special handling plan includes understanding the limitations of mining equipment. The mine operator should have equipment appropriate to the proposed plan and site conditions. Success of a special handling technique will depend on the ability of the equipment operator to identify the acid and alkaline materials during mining, and place those rocks in the locations designated by the handling plan.

Several "special handling" and management techniques are used alone or in combination to dispose of acid materials on mine sites. These include:

• Relocation of potentially acid forming strata above the anticipated postmining groundwater table,
• Constructing "pods" of acid-forming materials
• Submergence or flooding;
• Blending including alkaline redistribution or alkaline addition; and
• Surface and groundwater management.

Discussion of special handling techniques includes explanation of the concept, general level of success, favorable site conditions, considerations in using the techniques, and possible field validation methods. Use of one or a combination of methods may be warranted. Overburden analysis data, hydrologic conditions and mine plan direct the best methods to accomplish this goal.

Placement of acid materials above the water table using segregation, isolation, and encapsulation techniques minimizes contact between acid-forming material and groundwater. Special placement usually occurs in "pods" or discrete piles that are located above the expected postmining water table in the backfill; hence the reference to the "high and dry" method. A few mines have constructed liners and caps to prevent groundwater contact with the acid-forming materials, a method called encapsulation.

Segregation and isolation from the groundwater system does not totally prevent pyrite oxidation. Oxygen, microbes and some water are still present in the pods. Segregation and isolation is directed at preventing downward leaching, or upward migration of oxidation products. The technique is illustrated and described conceptually in Figure 14.7.

Construction of acid-forming material pods is one of the oldest techniques used to isolate potentially acid strata. The purpose is to reduce percolation or recharge of groundwater through the potentially acid-forming strata. Pods are constructed in compacted layers, sometimes with potentially acid-forming material alternated with alkaline strata. Pods are placed above the anticipated final groundwater elevation in the backfill, and usually at least 25 ft (7.6 m) away from the final highwalls and lowwalls and 10 ft (3.1 m) from the surface. Pods are designed with a compacted sloping cap (best constructed of clay or other low permeability material), and are usually covered with and underlain by alkaline material. Potentially acid-forming material needs to be rapidly excavated and covered to prevent prolonged exposure of the materials to oxygen and water.

Improper construction of pods, especially the failure to construct an impervious cap over the top of the pod can result in conditions favorable to the formation of severe AMD. The "high and dry" burial places the pyritic material closer to the surface of the mine where oxygen is more abundant (Chapter 1). This, in conjunction with percolating precipitation and the high concentrations of pyrite, create an environment which would allow T. ferrooxidans to thrive. Schueck (personal communication) found severe AMD formation associated with segregated, but improperly isolated pyritic material on over a dozen mines, which were mapped geophysically. Magnetic mapping was used to locate the pods and electromagnetic terrain conductivity was used to map the pollution plumes through the site. In every case, high conductivity values were associated with and located immediately downgradient of the pods. Conductivity values decreased with increasing distance from the pod, indicating that the AMD was being diluted. Subsequent drilling and groundwater sampling confirmed that the AMD associated with these improperly constructed pods correlated with the higher conductivity values and is more severe than AMD generated elsewhere on the site. In many cases, the operator confirmed that the pods were segregated acid-forming materials, often pit cleanings, but that impervious caps were not constructed on top of the pod.

Placement of acid material into a contour surface mine backfill must fall within a projected target zone (See Figure 14.8). The bounds of this zone are established by the distance from the highwall, height above the pit floor, postmining water table, the depth below the root zone, the distance from the outcrop, and the distance from reestablished drainageways and various barrier areas. In the example provided in Figure 14.8, a simplistic approach is demonstrated to indicate the maximum amount of acid material that can be placed in the target zone. A further reduction to the target zone is then made based on the likely limitations of the mining equipment.

The values used for the Total Mined Area Triangle (TMAT) include:

Maximum Highwall Height 60 ft/18.3 m

Coal Thickness 4 ft/1.2 m

Stripping Ratio 15:1

Landslope 30%

Calculated Maximum Pit Floor Width 200 ft/61 m

The values for the Acid Material Target

(Area Triangle AMTAT) include:

Distance from the highwall 20 ft/6.2 m

Distance above the pit floor 10 ft/3.0 m

Depth below the root zone 10 ft/3.0m

Distance above the postmining water table Variable

Away from re-established surface drains Variable

The TMAT square footage value is 6000 ft² (557 m²) using the maximum pit floor width and highwall height. The maximum height of the AMTAT to which the acid material could be placed (and still meet the segregation and isolation disposal conditions) is 34 ft (10.4 m) on the side nearest the highwall. The maximum width of the AMTAT is 112 ft (34.2 m). At most, only 32% (roughly one third) of the total mined area can be used for acid material placement. This value will change depending on highwall height, land slope and placement constraints. As a general rule, as land slope increases, the size of the target area for acid material will decrease.

Meek (1994) monitored acid production on surface mined areas with segregation and several different alkaline amendments. Acid load, on an area with segregation, was reduced about 50 percent compared to a control area with no segregation or alkaline addition. The Kauffman site in Clearfield County, Pennsylvania, includes placement projected to be above the water table and large alkaline addition rates. Monitoring of Phase 1 of this site showed that a thicker than expected water table has developed in part of the backfill and is in contact with some of the acid material. This site is discussed in Chapters 2 (Groundwater) and 13 (Alkaline Addition).

Phelps and Saperstein (1982) suggested that pods should have a bulk density of 1.1 to 1.5 times the surrounding spoil to minimize infiltration. These investigators also observed that the highest spoil bulk densities occurred at 50 to 80 % depth of spoil for most mining methods. They suggested that the high density spoil zones should be favorable locations for pods if hydrologic requirements are satisfied.

Figure 14.8 Projected target zone determination for placement of acid forming material within the backfill.

Schueck et al. (1996) reported on attempts to grout buried refuse with fluidized bed combustion ash as a form of isolating pods after the fact. This was done on a site where the lower Kittanning seam was mined and most of the overburden is apparently acid-forming. Grout was injected directly into the buried pods to plug void spaces and coat the refuse. Grout caps were also constructed over several of the pods. Combined grouting affected only 5% of the site but resulted in a 50 to 60% decrease in acid loading.

St-Arnaud et al. (1994) observed a "porous envelope" effect at a metal tailings pond in Canada. At this site, low permeability tailings have been placed within permeable soils. A large contrast in permeability promotes the flow of groundwater flow around rather than through the tailings mass, minimizing leaching of the acid material.

Short exposure time before burial and reclamation can reduce weathering and acid generation. As the acid-forming material remains exposed, weathering proceeds to produce acid products and the subsequent buildup of acid salts. If bacteria can be controlled by maintaining alkaline conditions, or through actual destruction of the bacterial population, the oxidation rates for pyrite are slower. In practice, potentially acid-forming materials are often stockpiled until enough material to start pod construction is accumulated. To reduce exposure, some mines in Pennsylvania construct temporary stockpiles covered with soil and vegetation, or cover the material with lime. The addition of lime can also inhibit the initial onset of AMD formation.

When potentially acid-forming material is removed from every cut, the best practice is to advance the construction of pods along with the mining. This ensures that acid-forming material is being rapidly buried. The Kauffman mine includes an area of test cells or pods (Rose et al., 1995). The investigators report that high-sulfur material was stockpiled for several months before construction of the pods. Some pods unexpectedly produced very acidic drainage even though they had been amended with alkaline materials. Delay in construction of the pods may have allowed acid generation to start even before the acid material was placed in pods. Rose et al. (1995) concluded that pod construction required careful implementation including prompt mixing of acid material with the alkaline amendment.

A mine in Greene County, Pennsylvania produced both alkaline and acid water on two phases (Perry et al., 1997). The two segments had similar geology and hydrology, and were mined by the same company. Alkaline drainage was produced on the segment where mining was completed without stoppage and a handling plan was followed. Acidic drainage was produced from the Phase 2 segment where mining ceased for an extended period before the site was completely reclaimed. The poor quality drainage on Phase 2 was attributed to weathering of partly reclaimed material during cessation and poor adherence to the special handling plan. Median water quality data for the two sites is summarized in Table 14.1.

A cap refers to an overlying "impermeable" zone created through placement of compacted, fine grained soil material, combustion byproducts (flyash, fluidized bed wastes), kiln dust, or synthetic (plastic or geotextile) fabric. The cap is significantly less permeable (two orders of magnitude or more difference) than the surrounding material. Caps restrict or prevent the infiltration of water into acidic material from above.

The term liner is normally used in the context of an underlying "impermeable" zone created through placement of an earthen or synthetic material which is significantly less permeable than the surrounding units. However, materials used for liner construction can also be used as a cap over the specially handled pod. Liners restrict or prevent the adjacent groundwater flow system from encountering the acid-forming material. Caps and liners also restrict diffusion of oxygen; a key component of acid generation.

Acid material is isolated by construction of low permeability caps or liners. Rose et al. (1995) concluded that test pods at the Kauffmann mine were made less permeable with percolating water tending to flow around rather than through the pods. Compaction and cementation of lime layers by gypsum formation were identified as the likely capping mechanism.

A detailed study of special handling at a Montana surface coal mine included the construction of a 3 ft thick (0.9 m) clay cap over special handled material (Dollhopf et al., 1982). Construction of the cap required several pieces of equipment, including pans and bulldozers. Maintaining clay at optimum moisture content for maximum compaction was difficult; water sometimes had to be added to the clay material. Cost of special handling with the clay cap was about 1.5 times "normal" operations due in large part to idling the dragline at certain stages of cap construction. An experienced mining engineer was needed on-site to supervise operations and schedule equipment. Groundwater conditions were monitored for several years after cap construction. Special handled material was maintained in a dry state, and the investigators concluded that capping was successful.

On a larger scale, a cementitious cap constructed of fluidized bed combustor (FBC) ash mixed with waste lime is near completion on a 97 ac (39 ha) reclaimed mine site in Clearfield County, Pennsylvania. Hellier (1998) reports on the successful efforts of the operator. Surface mining on the lower and middle Kittanning coal seams began in the 1940s on this site. Upon completion of the mining, circa 1991, the operator was required to pump and treat an acidic postmining discharge. Treatment costs threatened to bankrupt the operator. Mining on the site predated special handling techniques. The operator removed the top 3 ft (1 m) of material and spread a 3 ft (1 m) layer of FBC ash mixed with 10% waste lime. Water was added to increase the moisture content. The ash/lime mixture set up to form a low strength cement. The top material was then replaced and revegetated. The cap served to inhibit infiltration, which was thought to be the primary source of water at this site. The cap would also inhibit oxygen from entering the backfill. At 80% completion, the operator no longer has to provide chemical treatment, pumps significantly less water, and the chemistry of the water remaining in the backfill has improved. A passive treatment system, which is in place, is adequate to mitigate the reduced flows of AMD.

Synthetic liners (plastic or geotextile) are a technology borrowed from the waste management industry. Thick, high-strength plastics of 20, 30, 40 or even 80 mil thickness are used to isolate acid forming material from infiltrating precipitation and groundwater interflow. The liners are designed to be resistant to a wide range of leachate conditions. They are laid in sheets with the seams heat or solvent welded or stapled.

Synthetic liners require a smooth, firm base to avoid puncture or stretching. Some plastics can also deteriorate as a result of ultraviolet radiation. A potential area of weakness is the seams which must be joined properly to avoid leakage or failure. The cost of synthetic liners is also very high, resulting in probably the most expensive technology in comparison to other methods. Refuse piles may be amenable to capping with liners, due to their engineered structure, and more controlled particle size distribution

Meek (1994) reported that a plastic cap reduced acid load about 70 percent compared to no special handling. A cap was one of the most effective treatment measures evaluated in that study.

Caruccio and Geidel (1983) used a 20 mil liner at a 40 ac (16 ha) site in West Virginia as an infiltration barrier. The acid load from two highly acidic seeps was reduced such that the liner would pay for itself in 6 years. Because of a steep outslope, the liner only covered the flatter, upper portion of the mined area. Recharge along this outslope probably accounted for most of the remaining flow to the seeps.

Earthen materials can be placed and compacted to form relatively impervious flow barriers. Cap thickness is frequently an issue, but a rule of thumb from the solid waste industry is 2 ft (0.6 m). Little information directly applied to mining is available to determine if 2 ft (0.6 m) is adequate. Permeability of a cap is affected by grain size, mineralogy, and moisture content of the earthen material, the degree of compaction, and the thickness of the lifts (lifts of 6 in (15 cm) are frequently required). Bowders et al. (1994) tested mixtures of flyash, sand, and clay as candidate hydraulic barriers in minespoil. They found that a mix of particle sizes and materials provided the highest packing density and lowest permeability, rather than flyash alone. Hydraulic conductivity varied about 2 orders of magnitude from 10^-5 to 10^-7 cm/sec over different mixes and moisture contents.

Design geometry of the cap may enhance or reduce the volume of water passing through the cap. A dome shape tends to "shed" water while flat caps could retain additional water. Construction of caps or liners has similar requirements to building refuse piles or other engineered disposal structures. Rubber tired equipment or a sheepsfoot roller are required for good compactive efforts. Caps constructed of earthen material can shrink and crack if allowed to dry out. Caps can also be damaged by differential settlement of spoil.

Success of segregation and isolation methods to prevent acid drainage off the mine site is directly related to proper construction of the caps and management of the hydrologic system at the site. Some documentation exists that demonstrates that this technique is effective. Rapid disposal of the material will necessitate concurrent reclamation, which may be difficult for large mining operations such as those using draglines.

Several site conditions need evaluation when using segregation and isolation disposal. Sites with small recharge areas, that are located on topographically steep slopes, and that are primarily composed of shales may pose the best conditions because higher surface water runoff and limited groundwater movement might be achieved. Within the overburden or soil material there must also be sufficient "impermeable" material to form the protective caps. The volume of acid forming material must be low enough to allow for placement of the material in the designated zone of the backfill.

Placing acid materials in a contour surface mine backfill usually invokes the following criteria:

• Isolation above the reestablished water table, 10-20 ft (3.0 to 6.1 m) above the pit floor,
• Separation from the highwall (25 ft (7.6 m)) and out of the recharge zone at the highwall-spoil interface,
• Placement below the root zone, and
• Out of surface drainage channels.

Submergence involves the placement of special handled material below a static water table. This method is expected to exclude oxygen from pyrite and is similar in concept to sealing and flooding of underground mines to reduce acid generation. Submergence or "dark and deep" generally requires a relatively flat area with a thick saturated zone and stationary water table to produce a near stagnant or no flow condition. The technique is not widely used in Pennsylvania or other Appalachian states because of thin and seasonally variable saturated zones. It is used in Canada and elsewhere for tailings disposal at hard rock mines (Fraser and Robertson, 1994; Robertson et al., 1997) and in the Interior coal basin of the United States where thick and stable saturated zones are more conducive to this method.

In Canada, tailings disposal in lakes usually involves water bodies with minimal circulation and anoxic conditions at depth. Tailings may also be buried on the lake floor by accumulating sediment and organic debris, providing a further barrier to oxygen. In the US midcontinent, topographic relief is low, water tables tend to be near ground surface, and flow gradients are small. Surface mining is conducted mainly by area mining methods, and the final cut is often allowed to flood at reclamation, leaving a relatively deep narrow lake incised into the terrain.

Leach and Caruccio (1991) characterized backfill materials as consisting of three broad hydrologic zones. The first zone is the vadose zone or zone of high oxygen concentration. Next is the zone of water-table fluctuation with alternately high and low oxygen concentration. The final zone is saturated with very low oxygen concentration. Leaching experiments representing the three zones showed acid load under saturated conditions to be about 5 percent of that produced in the unsaturated zone. Acid-forming material should be in the saturated portion of the backfill to restrict oxygen diffusion. Thiobacillus ferroxidans, a significant factor in the acid-generation process, can remain active at oxygen levels well below atmospheric conditions.

Submergence has not been widely documented as a disposal technique in the Appalachian coalfields. Perry et al. (1997) found that submergence of acid material buried on the pit floor produced very poor quality drainage at one Appalachian surface mine. In the Interior Coal Basin of the central United States, flooding of final pits and development of a thick saturated zone occurs on many sites. The water quality of most flooded last cut lakes is alkaline; some also have elevated concentrations of dissolved solids and sulfate (Gibb and Evans, 1978). Some alkaline lakes are located in calcareous spoil derived from glacial till, loess, and shale.

Submergence requires a relatively flat area with a stable (minor seasonal fluctuations) water table in order to achieve saturated conditions. Flat topography enhances recharge, reduces surface runoff, and allows development of a low gradient water table. A "nonflowing" or stagnant (very low hydraulic gradient) groundwater regime reduces the effects of dissolved oxygen being brought into the system. Flushing, or transport of weathering products from mine spoil, can be minimized in a "no flow" regime. A typical submergence scenario for the Interior Coal Basin is shown in Figure 14.9.

Submergence entails risk. If postmining hydrology is not correctly anticipated, acid may be generated. Weathering products are leached or mobilized by flowing groundwater. Therefore, it is imperative that the site hydrology be well understood. Information necessary to characterize the groundwater flow system include:

• Estimates of available groundwater recharge to ensure a permanent water table.
• The site must be capable of being hydrologically isolated from the rest of the groundwater system.
• Backfill must be constructed to produce a reservoir containing the acid-forming material.
• This type of disposal during the mining operation should involve handling the acid-forming material only one time before permanent placement (probably on the pit floor of a previously excavated pit). Material handling for submergence is not as complex as segregation and isolation methods.

Some disadvantages to this method are that pyrite oxidation may have already begun before material is submerged, forming ferric-sulfate salts. Upon dissolution these salts release ferric iron that can oxidize pyrite and sustain acid generation. If material handling is unsuccessful; i.e., the water table is not stagnant or thick enough, resultant drainage problems will be large scale. This technique might require a relatively long lag time before success/failure can be determined and large areas can be impacted before the results are known.

Blending is the mixing of rocks on a mine site to promote the generation of alkaline drainage. Blending maximizes the contribution of carbonates by mixing them with acid-forming rock to inhibit the oxidation of pyrite. In theory, it is possible to blend rocks from virtually any position in the overburden column, but the actual practice is dependent on the mining method and spoil handling equipment. A spoil mixing experiment with dragline mining was conducted in Montana where saline or "toxic" overburden was present in varying amounts across a mined area (Dollhopf et al., 1982). Premining distribution and properties of the "toxic" material were determined from overburden analyses. Systematic drilling and sampling of the reclaimed spoil after mining showed:

• When the "toxic" material constituted about 5 % or less of the overburden, the material was undetectable in the regraded spoil.
• When the "toxic" material constituted 5 to 15 % of the overburden, partial to complete mixing occurred.
• At concentrations greater than 15% "toxic" material, partial mixing occurred.

Special handling and spoil mixing were conducted on this mine to protect both root zone material and groundwater quality.

Alkaline redistribution is special handling of alkaline material that is subsequently disposed of with the acid material. Alkaline redistribution uses material that frequently is from a different portion of the mine site, thus requiring haulage from one part to another. Skousen and Larew (1994) describe the redistribution of alkaline material from separate but adjacent mine sites. Calcareous rock was hauled from a mine extracting Bakerstown coal to a mine on the upper Freeport coal. Alkaline redistribution consisted of placement of about 3 ft (0.9 m) of calcareous shale on the pit floor, partial backfilling, then placement of acidic material about 20 ft (6.1 m) high in the spoil, followed by capping with more calcareous shale. A pre-existing mildly acidic discharge (acidity about 75 mg/L CaCO₃) was ameliorated and is now alkaline.

The term "blending" has been used widely in the past to refer to the mixing that occurs during the routine mining process. This technique has been recognized since at least the mid 1970s. Anecdotal information exists to suggest that it is an effective practice.

General considerations for use of alkaline redistribution include :

• Position of alkaline materials within the overburden section and volume present at the mine site:

• Cost, availability, volume, and quality of off-site alkaline materials:

• Mining and equipment:

An important aspect of the mining plan is a reasonable estimate of the minimum and maximum final highwalls expected. A special handling plan may require that alkaline material, which is present at high cover, be recovered. If economic conditions change, it may be impossible to continue mining if the alkaline material needed to fulfill the special handling plan is present only at higher cover.

When the overburden contains significant alkaline material and potentially acid forming material, the location of these two materials relative to each other may provide conditions favorable to alkaline redistribution. For example, if the alkaline section lies adjacent to the potentially acid-forming section, these two materials may become mixed without additional effort during the overburden removal operation, and separation of the potentially acid-forming strata may not be needed. If the alkaline material is located near the upper seam to be mined, and the potentially acid-forming material is located near the lower seam in a two seam operation, a special handling plan can be implemented to place the alkaline material on top of the acidic material in the backfill.

An excess of neutralizers inherent to the site and dispersed throughout the overburden profile is generally necessary to offset both acid production and imprecise mixing. Mixing alkaline strata should balance or exceed the potential acidity. The acid producing rock should be present in discrete and defined zones because this favors effective mixing with a larger volume of calcareous rock. A simple blending plan is shown in Figure 14.10

Figure 14.10 Blending and alkaline redistribution does not require the isolation of acid-forming materials in isolated pods.

Alkaline addition/redistribution strategies can include:

• Determining the proportions of alkaline material to be placed on the pit floor, mixed into the spoil, and added to the spoil/soil interface,
• Determining the methods for incorporating the alkaline material into the backfill,
• Choosing the best pit orientation to minimize haulage of the alkaline material,
• Designing a multiple pit operation to facilitate redistribution of alkaline material, and
• Ripping the pit floor to expose alkaline material beneath the coal.

These techniques are discussed below:

Once a choice has been made to add alkaline material from off-site sources or to redistribute on-site material, the mode of material placement within the backfill must be determined. Past alkaline addition plans focused on placement of alkaline material largely on the pit floor (Surface Mine Drainage Task Force, 1979).

Recently, alkaline addition has focused on the placement of alkaline material at three locations within the backfill; on the pit floor, mixing into the spoil, and placement at the soil/spoil interface. Field evidence suggests that the best chance for the production of alkaline drainage exists when the bulk of the alkaline material is mixed into the spoil. It is believed that infiltrating precipitation will become buffered alkaline water by the time it reaches the pit floor, leaving little opportunity for acid mine drainage production to begin.

Pit floor liming is useful as best management practice and to supplement alkaline addition to the backfill, particularly where the pit floor is composed of high-sulfur material. For complete coverage of the pit floor, at least 44.8 mt/ha (7 t/ha) must be applied. Higher application rates may be appropriate provided that sufficient alkaline material is applied within the backfill. Similarly, supplemental alkaline material can also be placed at the spoil/soil interface. However, because of solubility constraints, this should constitute a relatively small proportion of the total alkaline material to be applied. Placement of alkaline material in backfills is covered more fully in Chapter 13 (Alkaline Addition).

Deposition of the alkaline material onto the pit floor can be accomplished by the use of trucks or by a dragline. The materials are then spread to cover the pit floor by dozers or loaders. Redistribution of the material into the spoil is more difficult, but can be accomplished by several techniques.

Blasting can be used to help mix the alkaline material into the newly created spoil. One method is to spread the alkaline material around the blast holes so that when the blast is set off, the alkaline material mixes directly into the spoil. This may be especially useful if the operator is using blast casting to facilitate overburden removal. An operator in Lycoming County, Pennsylvania, uses waste limestone, sized at approximately ¼ inch as stemming in the blast holes. The blast holes drilled are 9 inches in diameter and, for the blast pattern used at the site, limestone was added at a rate of 165 mt/ha (27.2 t/ha) for each of the three coal seams removed (Smith and Dodge, 1996). The main benefit of this method is that the alkaline material is thoroughly mixed into the spoil.

Alkaline material can also be added to the blasted rock and spoil. Alkaline material can be dumped onto the spoil by a truck and distributed on the spoil surface by a dozer. This can be done at different depths in the spoil to ensure that the alkaline material is spread throughout the backfill. This method can also be used for a site with blasting to avoid double handling the alkaline material.

A dozer can spread the alkaline material that is to be located at the spoil/soil interface during the rough grading of the backfill.

Several operational constraints arise as a result of the location of the alkaline materials. The orientation of the pit has an important role where alkaline material is located under high cover in the overburden section and is missing or minimal under low cover. In general, the block cuts need to be oriented perpendicular to the slope to intercept both high and low cover overburden sections. Using this method, within each cut, alkaline material is available for placement in the backfill. If the mining were conducted along the contour, the lower cover mining would result in alkaline deficient spoils.

A coal mine operator in Clearfield County, Pennsylvania, approached the high alkalinity/high cover problem in a slightly modified manner. The first cut was taken along contour. Spoil from the cut was stockpiled near the final highwall. The ensuing mining was done with block cuts oriented perpendicularly to the contour. Spoil generated from these cuts contained the alkaline material present at higher cover as well as some lower cover overburden deficient in alkalinity. A portion of this spoil was then used to fill that part of the initial contour cut located below the block cut, as well as the first block cut. A portion of the spoil from the contour cut was used to fill the higher cover part of the block cut, thus effectively spreading the alkaline spoil throughout both the block cut and the contour cut, with the added advantage of producing low cover coal on the first cut.

A second operational constraint arises when multiple mining pits are used to blend spoil. Use of multiple pits is an integral part of a special handling plan when the alkaline unit must be mined separately from the coal. This may occur during single seam mining if an alkaline zone is isolated and will be excavated and redistributed throughout the site. It may also occur on a multiple seam operation, where it is necessary to move alkaline material associated with the overburden of one or more seams to another pit or pits where mining is being conducted on a seam with no alkaline strata.

A mine in Westmoreland County, Pennsylvania used alklaine redistribution to amend carbonate deficient rocks. Acid-forming materials were laterally continuous and had 0.5 to over 2% total sulfur. A zone of calcareous materials, with carbonate content exceeding 20% was present in a few acres at the updip end of the site. The calcareous materials were absent or thin elsewhere. Two of the four acid base accounting drillholes included acidic strata and had NNP's less than zero. Special handling consisted of moving excess calcareous strata from the upper end of the mine and redistributing it in the alkaline deficient areas. Three pits were operated simultaneously. Operations were timed so alkaline material was available and cut and fill balances could be maintained. Material placement and backfilling included crushed limestone on the pit floor, "neutral" spoil backfill, placement of potentially acid material in lifts covered by more "neutral" spoil, and finally topsoil.

Wells and springs have been monitored for four years after reclamation at the alkaline redistribution site (Table 14.2). In Well MW-6, located downgradient of the site, median sulfate concentration decreased about 70%, and net alkalinity rose above zero after reclamation was completed. MP-10, a spring located downgradient of the mine, is representative of shallow groundwater conditions and contains negligible alkalinity. Overburden rocks in the recharge area for MP-10 and well MW-6 were likely acid formers. Postmining water quality for MP-10 and MW-6 show a small but significant increase in net alkalinity. Sulfate concentrations indicate some oxidation and leaching is occurring within the spoil.

Key factors influencing postmining water quality are the redistribution of calcareous rock to alkaline deficient areas, and rapid completion of mining and reclamation. Responses in water chemistry are attributed to placement of acid forming materials above the water table to minimize leaching, while the calcareous rocks are dissolving and producing alkalinity.

Another operational constraint occurs when the alkaline material is located beneath the coal being mined. Ripping the pit floor can be done to incorporate alkaline material into the mine backfill at sites where alkaline strata exist below the lowest coal seam to be mined. This method involves removing the coal and ripping the pit floor to expose the alkaline strata for contact with groundwater on the pit floor. It is a suitable practice if the pit floor or underclay is not acid forming. The operator must have equipment capable of ripping the pit floor to the needed depth and sufficiently breaking up the alkaline zone. Typically an average size dozer can rip to a depth of about 3 ft (1 m), while a D-11 dozer is capable of going to greater depths. If the alkaline material is at a depth greater than the depth accessible by ripping, the overlying material may need to be removed prior to ripping.

Limestone is generally a durable rock and is resistant to abrasion. When ripped, the rock tends to be of a much larger size than that normally associated with alkaline addition or redistribution. Therefore, this method is adequate for mines where alkaline deficiencies are small, as it may have a limited effect on groundwater quality when compared to alkaline addition of fine-grained material or its redistribution in the spoil.

Special handling is one of a group of tools for managing acid materials, and is often used in combination with alkaline addition and water management practices. Placement above the postmining water table is the predominant special handling practice on Pennsylvania surface mines. Plans are site specific. Key concepts include:

• Special handling practices used in Pennsylvania include: blending of acid and alkaline materials, the segregation and isolation of acidic materials (high and dry), management of groundwater, and alkaline addition. Submergence (dark and deep) is seldom used in Pennsylvania.
• Special handling in the absence of alkaline materials cannot produce alkaline drainage.
• Special handling usually involves both acid and alkaline materials and may also include clay materials for capping and lining pods of acidic materials, and boulders for use in drains, trenches, and ditches.
• Special handling is most effective in conjunction with alkaline addition, surface and groundwater management, and other techniques.
• The volume of the material to be special handled should generally be less than 20% of the mine backfill volume because of the need to keep acidic materials away from the surface, water table, highwalls, etc.
• Special handling is not necessary on all mine sites.
• Identification and segregation of acid material is extremely difficult if multiple zones exist in the stratigraphic section unless: 1) blasting is not used to remove the overburden, 2) the acid zones are persistent (laterally and vertically), uniform in thickness, and distinctive in appearance.
• Special handling requires that the proper equipment be used at the mine site.
• Monitoring during and after mining is necessary to evaluate special handling techniques

Effectiveness of special handling has been evaluated in a few case studies and the results are frequently beneficial. Special handling may not totally prevent postmining water quality impacts, however. Special handling cannot be relied upon by itself to prevent acid generation, but should be used as one of a collection of best management practices. Many questions remain as to the effectiveness of special handling, the most advantageous placement of materials, how to best isolate acidic materials, how to best promote dissolution of calcareous materials, and so forth.

David Maxwell

This section describes the approach of one mining company to management and prevention of acid materials and drainage, from exploration through mine planning and operating procedures.

Premining determination of groundwater and surface water quality and quantity, and overburden quality are the most important factors in determining a site's mineability and potential production of AMD. Each of these items can be documented and understood prior to mining. A special handling program that addresses these factors can be designed and implemented during mining. The result should be a successful surface mined site with no adverse affects to the environment.

After each potential mine site is core drilled and the coal has been determined to be marketable, an extensive amount of engineering is done to determine groundwater and surface water quality and quantity and overburden analysis. To expand on these three factors, the following sections look at how each is handled premining and followed up in the field for implementation.

In order to form AMD in minespoil, groundwater must be present. Therefore, it is important to determine the amount and chemical composition of groundwater that exists prior to mining an area. This step is simply accomplished through measuring static water levels, and drawdown and recovery testing of core holes. Certain holes are left open and capped after the initial drilling has taken place. These holes are then pumped completely dry and then measured for the time it takes to recharge to a static water level. If a hole recharges quickly, it can be assumed that there exists a good possibility of appreciable groundwater inflow during mining and reclamation on the site. Likewise, if a hole takes much more time to reach its equilibrium, if at all, then little or no groundwater inflow may occur during mining and reclamation.

One must also take note that drawdown and recovery tests are indicators and not full scale aquifer tests. Strike and dip of the formation, stratigraphy, and the presence of fractures affect the magnitude and direction of hydraulic flow and head pressures.

It is also important to know the groundwater quality prior to mining. Water in the open core holes can be sampled after pumping, and tested for its chemical composition. The presence of alkalinity in premining groundwater can be useful for indicating the presence of "neutralization potential", or carbonate minerals, in the overburden.

As stated previously, groundwater can be handled during mining to help prevent the formation of AMD. Highwall drains and anoxic drains can be constructed to carry groundwater safely away from mine spoils to natural surface drainage ways. This must first be accomplished prior to mining by determining the strike and dip of the coal seam to allow for the engineering of the drains for easy construction during mining. Once the mine site is laid out, the actual construction of the drains should be a simple procedure.

After all coal is loaded away from the highwall and the coal waste is cleaned up from the pit floor, an impervious barrier is placed on the pit floor. The area against the highwall is then lined with 4 in (10.2 cm) limestone approximately 6 to 10 in (15.2 to 25.4 cm) thick, which will act as a french drain. A 4 to 6 in (10.2 to 15.2 cm) flexible perforated pipe can also be used in conjunction with the aggregate. This procedure is carried out throughout the life of the mine site in each successive block cut to insure the groundwater draining from upslope areas will be safety conveyed through an alkaline drain past the mine spoils and discharge to a natural drainageway without encountering acid-forming materials.

Another procedure Amerikohl has concluded to be effective in preventing AMD has been the elimination of coal crop barriers. Contrary to past industry standards, the elimination of crop barriers allows free flow of water from mine spoils, preventing "pooling" in the backfill, thus preventing chemical reactions in the spoil.

As with groundwater, the more surface water which can be redirected from the mine spoils, the better chance you will have in elimination of postmining AMD. At Amerikohl mine sites, the field personnel try to redirect as many natural springs and wet weather drainage courses as possible. This is simply done by constructing upland diversion ditches and redirecting the water, which would normally flow into our mine sites, away from the site and to a natural drainageway. Not only are upland diversions beneficial in the reduction of surface waters influencing the mine spoils, but they are also relied upon to reduce the erosion, which can take place after reclamation.

Finally, another important step in prevention of AMD is the determination of overburden quality, the recognition of acid bearing rock strata, and the special handling of those strata.

During initial core drilling operations, strategically placed overburden holes are drilled to collect one-foot intervals of rock chips using an air rotary drill. In addition, Amerikohl likes to core the areas above and below the coal seam to further define the thickness of possible acid-forming rock and to prevent contamination of those units by coal chips. These overburden chips are then logged and sent away for lab analysis. Using this lab work, mass balance computations can then be made through the recognition of acidic and alkaline rock formations. For the most part, if the mass balance ratio of alkaline material to acidic material does not come out to a 2 to 1 ratio, the site most likely will no longer be evaluated as part of Amerikohl's future mine plans. If you cannot predetermine potential AMD through the accumulation of strong data, then the site is without merit regardless of the economic value of the coal.

Once a proposed site is deemed minable based on analysis of good data; the implementation of the site specific special handling plan becomes important. While there have been a few different special handling techniques used at various Amerikohl mine sites to be discussed later, there still exists many commonplace procedures at each site which contribute to the overall success of a mine.

Amerikohl employs the modified block cut method of mining at its mine sites. This entails simply hauling the initial two cuts of overburden from the coal outcrop to final highwall away to a place where it will be deposited in the final open pit. Subsequent overburden cuts are then placed in the open cut created before it, and so forth. Topsoil and subsoil are saved for final reclamation. The benefit of block cut mining is it automatically lends itself to concurrent reclamation. This ultimately helps prevent the generation of AMD and promotes environmental protection.

Normal equipment fleets located on each site consist of no less than one Caterpillar DION dozer, one Caterpillar 988B front end loader, two 40 to 50 ton rock trucks, and various coal loading, coal crushing, and grading equipment.

After the coal is mined from each block cut, the operators take special care to make sure that all coal waste and inferior or "boney" coals are either trucked or carried out of the pit area to a location high in the backfill, but away from the final highwall. This eliminates the possibility of the coal waste coming in contact with groundwater. The cleaned pit area is then lined and compacted with an impervious material such as clay. Anywhere from 6 in (15.24 cm) to 5 ft (1.5 m) of material is placed on the pit floor depending on the quality and availability of the material on site. Once the pit floor is sealed off from water contact, a limestone waste or other calcareous material is then placed at an average rate of approximately 448 mt/ha (73.4 t/ha) of 100 % CaCO₃ equivalent.

As with the coal waste, potentially acidic rock strata and alkaline strata as documented by the overburden analysis, are also special handled. These overburden zones are recognized in the field by highwall observation in relationship to the drill log data, and also by field fizz testing of the alkaline zone with the use of 10 % HCl acid. When those mine sites with special handling procedures are being developed, extra care is taken to educate the mine supervisor and equipment operators on their awareness of those zones to be special handled. Without exception, acidic materials are trucked or carried high in the backfill for much the same reasons as the pit cleanings are hauled. This procedure allows for the potentially acidic material to be placed at a location high in the backfill away from surface and groundwater infiltration. In conjunction, the alkaline material, if present, is also trucked or placed in lifts with the acidic materials.

The typical Amerikohl surface mine has a job life of approximately 5 months from initial development through final reclamation. Within this average there have been mine sites which have lasted less than 1 month, to an uncommon 18 months. This type of short lived mine site lends itself to a continuously aggressive acquisition and development program to find not only coal which is of economic value, but also a site which is environmentally conducive to surface mining.

One of the most beneficial of all the procedures at an Amerikohl mine site in the prevention of AMD is its concurrent reclamation process. Never is there a time when an area which has been mined and reclaimed to approximate original contour, gone past a growing season without the placement of subsoil and topsoils, then seeded and mulched. This simple and necessary function helps to eliminate and seal off surface water infiltration in addition to the reduction of liability.

Finally, while the logic behind the special handling of acidic and alkaline material seems to be a constant, the varying techniques implemented at Amerikohl mine sites differ with the location of each area as it relates to the regulated receiving stream classification. The three specific special handling techniques for alkaline materials have been:

• Alkaline amendment (purchased)
• Alkaline amendment (within overburden column)
• Alkaline redistribution

Certain times when the overburden indicates that the mass balance equation would show that the mine site would be capable of alkaline post mining discharges, it is still a good idea to provide alkalinity to the backfill to ensure positive results. Therefore, in those cases it is necessary to incorporate alkaline material in the mine spoils which is purchased and brought into the mine site. At a few Amerikohl mine sites, an alkaline amendment has been trucked in and incorporated in lifts within the mine spoils.

Some sites possess the ability to produce alkaline waters through their own inherent overburden characteristics. In these cases, there exists certain zones of alkaline material within the rock strata which will be mined as a course of uncovering the coal.

In these cases, operator awareness is necessary due to the importance of singling out the alkaline rock to segregate and special handle. The operator is first given a general location of the alkaline rock based on the overburden analysis, then a field fizz test is administered to tie down the location. Then the alkaline rock is segregated and trucked to the area where it is to be placed in lifts within the backfill and incorporated with the acid bearing materials.

The final and certainly the most broad-minded approach to special handling is alkaline redistribution. In simple terms the redistribution of alkaline material is no more than special handling alkaline material within the permit area, but not an actual part of the coal removal area, to a place where coal overburden is taking place for use as an alkaline amendment. In this particular case, the alkaline material can originate from below the coal seam to be mined, or from an area above the final highwall.

Using a more specific description; alkaline redistribution consists of incorporating alkaline material within mine spoils to meet or exceed a minimum mass balance ratio or 2 parts alkaline material to 1 part acidic material.

Actual implementation of alkaline redistribution is impossible without the use of rock trucks, since the alkaline amendment is not an integral part of coal overburden removal. Once the amount of alkaline amendment per acre affected is calculated via overburden analysis and mass balance equations, it is important that a sufficient stockpile is maintained. Once a stockpile has been built, the actual implementation of alkaline redistribution is the same as all of the special handling techniques (i,e. place alkaline material in lift with acid bearing material).

In conclusion, past experience with the success of the mine sites Amerikohl has completed has brought the realization that as much homework that can be completed prior to a particular property being surface mined, the better chance that site has to produce alkaline, low metal discharges. That is, after all, what everyone involved in the industry is striving to achieve.