Acid Mine Drainage Flow Research Articles

The effects of acid mine drainage from an old mine in the Witbank Coalfield, South Africa

Acid mine drainage seeping from an abandoned mine in the Witbank Coalfield has flowed into a nearby stream, the Blesbokspruit, which is a tributary of the Olifants River. The amount of precipitation over the mine area which sinks into the mine has been greatly increased by associated subsidence features. There is a time lag between the maximum precipitation and the maximum flow of acid mine drainage from the mine of several months. The water issuing from the mine has a low pH, high electrical conductivity and high total dissolved solids. It has severely impaired the quality of the waters of the Blesbokspruit. The heavy metals concentrations of the stream water are well above the levels set by the South African standards. In addition, there is a high concentration of heavy metals in the stream sediments and in algal mats which occur at the seepage points. Salts have been precipitated from the acidified waters, one of the chief of which is jarosite. Formation of ferrihydrite is associated with the algal mats. Four decantation ponds constructed in the headwaters of this stream have had no effect on the pollution. Indeed, the concentration of aluminium in the waters exiting from the ponds is higher. The Blesbokspruit flows through a wetland and on leaving the concentration of some heavy metals has been reduced. This presumably is attributable to the sorption capacity of the high organic content of the sediments in the wetland. Nonetheless, the quality of the water is not improved sufficiently for it to be acceptable, even with the dilution effect of the entry of water from an uncontaminated stream downstream of the wetland.

Acid Mine Drainage Treatment with Armored Limestone in Open Limestone Channels

Much attention has been devoted to developing inexpensive, limestone-based systems for treating acid mine drainage (AMD) with little or no maintenance. Treatment of AMD with limestone results in a surface coating of metal hydroxides, a process known as limestone armoring. Once armored, limestone is assumed to cease dissolution and acid neutralization. Laboratory and field experiments determined acidity changes in AMD when contacted by armored and unarmored limestone and investigated the implications of armoring on the construction of open limestone channels for treating AMD. Results of a laboratory titration study indicated armored limestone was only 2 to 45% less effective in neutralizing a hydrochloric acid solution as unarmored limestone. A laboratory container study showed that armored limestone was 90% as effective in neutralizing AMD as unarmored limestone. The field study surveyed 2- to 8-yr-old, rock-lined channels constructed of sandstone or limestone, and measured water quality changes down the length of the channel. Open limestone channels, though armored, reduced more acidity in AMD (4-62%) than the sandstone channel (2%). The results from open limestone channels were compared to an acid neutralization kinetics model that predicts the rate of acid neutralization for a specified channel size, and AMD flow and acidity concentration. The open limestone channels in the field neutralized more acidity than the model predicted. Open limestone channels show promise for neutralizing AMD in watershed restoration projects and abandoned mine land (AML) reclamation projects where one-time installation costs are incurred, little to no maintenance is required, and systems do not have to meet specific water quality standards.

A survey of constructed wetlands for acid coal mine drainage treatment in the Eastern United States

The oxidation of pyritic minerals, exposed to oxygen and water during the mining of coal, results in the formation of acid mine drainage (AMD), which is characterized by low pH and high concentrations of dissolved sulfate, iron, and other metals. Federal and State regulations require that discharges from coal surface mines meet water quality criteria. Toward that end, chemical treatment of AMD, usually with soda ash briquettes, lime, limestone, or sodium hydroxide, is effective but expensive. Recently, man-made wetlands have been proposed as a low-cost, low-maintenance alternative to chemical treatment of AMD. To assess the status of man-made wetland treatment of AMD in the eastern U.S., a survey was conducted by the U.S. Office of Surface Mining, Reclamation, and Enforcement. As of May 1988, 142 wetlands had been constructed for AMD treatment. In 50% of the constructed wetlands, treatment efficiencies (reductions in concentration) for H+, acidity, Fe, Al, Mn, and SO4 2 of at least 68, 67, 81, 48, 34, and 8%, respectively, were obtained. However, over 11% of the constructed wetlands yielded greater concentrations in the effluent from the wetland than were present in the influent AMD for one or more of these 6 chemical parameters. Treatment efficiency generally was not correlated with design criteria (e.g., area of wetland, depth of the organic substrate in the wetland, AMD flow rate, metal loading rates). Also, treatment efficiency was generally not affected by either the type of organic substrate used in wetland construction or the addition of lime and/or fertilizer to the constructed wetland. The effectiveness of wetland treatment of AMD is not only extremely variable, but also presently not predictable.