ACID-MINE DRAINAGE TREATMENT WITH ARMORED LIMESTONE IN OPEN LIMESTONECHANNELS

Much attention has been devoted to developing inexpensive, limestone-b ased systems for treating acid mine drainage (AMD) with little or no m aintenance, Treatment of AMD with limestone results in a surface coati ng of metal hydroxides, a process known as limestone armoring, Once ar mored, limestone is assumed to cease dissolution and acid neutralizati on, Laboratory and field experiments determined acidity changes in AMD when contacted by armored and unarmored limestone and investigated th e implications of armoring on the construction of open limestone chann els for treating AMD, Results of a laboratory titration study indicate d armored limestone was only 2 to 45% less effective in neutralizing a hydrochloric acid solution as unarmored limestone, A laboratory conta iner study showed that armored limestone was 90% as effective in neutr alizing AMD as unarmored limestone. The field study surveyed 2- to 8-y r-old, rock-lined channels constructed of sandstone or limestone, and measured water quality changes down the length of the channel, Open li mestone channels, though armored, reduced more acidity in AMD (4-62%) than the sandstone channel (2%), The results from open limestone chann els were compared to an acid neutralization kinetics model that predic ts the rate of acid neutralization for a specified channel size, and A MD now and acidity concentration. The open limestone channels in the f ield neutralized more acidity than the model predicted, Open limestone channels show promise for neutralizing AMD in watershed restoration p rojects and abandoned mine land (AML) reclamation projects where one-t ime installation costs are incurred, little to no maintenance is requi red, and systems do not have to meet specific water quality standards.