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Acid Mine Drainage prevention and mitigation techniques |
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Controlled placement (special handling) is a preventative measure involving the placement of pyritic or alkaline material during mining to minimize or neutralize the formation of AMD. According to the generally accepted chemical equations for pyrite oxidation, oxygen and water are necessary to initiate acid formation. Exclusion of either reactant should preclude or inhibit acid production. Placement of pyritic material encompasses either an attempt to exclude oxygen, usually by complete submergence below the water table; or an attempt to isolate the material from water contact to avoid leaching of acid salts. Placement of alkaline materials has a twofold role:
Submergence relies on several physico-chemical phenomena for success. Oxygen diffuses very slowly and has limited solubility in water. For this approach to succeed, a stagnant or no flow condition and relatively thick saturated zone appears critical. Stagnant flow conditions leading to the development of anoxic (oxygen free) conditions and a saturated thickness on the order of several tens of feet appear to effectively curtail oxygen diffusion. This approach is most successful in large mines in flat terrain where ground-water gradients are low, the saturated zone is thick, and aquifers are of large areal extent. Hammack and Watzlaf (1990) concluded that a water cover to maintain oxygen below a partial pressure of one percent is necessary to inhibit pyrite oxidation.
Submergence is generally not used in the hilly terrain of Appalachia, where gradients and flow velocities are too great to achieve stagnant, anoxic conditions. In these situations, submergence may be counterproductive and actually enhance the production and leaching of acid products.
Submergence or flooding is also applied to prevent AMD from underground mines. Key considerations include:
In general, flooding to prevent AMD is believed to be more successful in below drainage mines. It is assumed that complete flooding eliminates oxygen and halts or severely curtails acid generation, the mine pool is stable and little or no discharge occurs (Kim and others, 1982).
Flooding of above drainage mines is also practiced typically through the use of "wet" seals, which allow water to drain but exclude air entry. Kim and others, 1982 concluded that sealing and flooding above drainage mines does reduce acid loading but is technically more difficult and less effective than other methods in AMD prevention. Monitoring studies of sealed mines indicate a general decrease in pollutant loading 10 to 25 years after mining (Borek and others, 1991), but it is unclear if the decreases were due to mine sealing or "natural phenomena". While pollutant loading decreased, water quality remained well outside accepted water quality standards for mine drainage.
OSM's rules do not specifically address criteria for outcrop barrier thickness for flooding underground works. A consensus "standard" engineering design approach to outcrop barriers and seals is also lacking. A contract report to the U.S. Bureau of Mines (Dames and Moore, 1981) discusses the factors affecting stability of outcrop barriers. Outcrop barriers should be wide enough to prevent seepage and have sufficient overburden to prevent failure (blowout). Curtain grouting, relief wells and compartmentalized barriers are several of the techniques suggested for controlling AMD discharges.
Placement of pyritic material above a water table is an attempt to isolate the material from contact with water, and preclude leaching of acid weathering products. Compaction and capping with clay or other materials may be also be employed to reduce permeability. In practice, it has proven very difficult to completely isolate spoil materials from water contact. Clay caps and other flow barriers are prone to leakage, and the sporadic infiltration of rain or snowmelt may periodically leach the spoil. The capping approach can be extended to complete encapsulation on top, bottom and sides as a further effort to isolate the materials from water contact. Skousen and others, 1987 give a general review of isolation and capping and other preventive techniques for handling pyritic spoil.
Alkaline placement strategies involve either mixing directly with pyritic material or concentrated placement to create a highly alkaline environment. Direct mixing places alkaline materials in intimate contact with pyritic spoil to inhibit acid formation and neutralize any generated acidity in situ. Alkaline addition case studies has been reported by Brady and others, 1990.
"Alkaline recharge" employs trenches loaded with alkaline material, usually a combination of soluble sodium carbonate and crushed limestone. The strategy is to charge infiltrating waters with high doses of alkalinity sufficient to overwhelm any acid produced within the backfill. This approach is highly dependent on the placement of the alkaline trenches to provide maximum inflow to the acid producing zones. An alkaline recharge case study has been reported by Caruccio and Geidel (1989).
A third variant of the alkaline placement technique is encapsulation with alkaline material above and below the acid- producing zone.
Water management strategies both during and after mining are another option for reducing acid generation. Water management can include the following:
Special handling, alkaline placement and water management strategies alone or in combination can substantially reduce or mitigate generation of acid drainage. Optimal strategies are site-specific and a function of geology, topography, hydrology, mining method and cost effectiveness.