Use of limestone drains in treating acid mine drainage (AMD): Status report
Symposium Rouyn-Noranda 2002
Michel Aubertin, Charles Poirier,
Most systematic studies carried out since 1978 on passive treatment systems incorporating the use of limestone drains under aerobic or anaerobic conditions essentially deal with the treatment of effluents from American coal mines. A limestone drain typically comprises a trench filled with limestone rocks surrounded by impervious materials. In the case of anoxic drains, the trench is generally isolated under a geomembrane liner covered with soil. Contaminated, unaired (anaerobic condition) or aired (aerobic condition) mine effluents flow by gravity towards these limestone drains where the passive treatment consists in bringing the carbonates into solution, thus helping to increase the pH and to create alkalinity. Among the design criteria to be considered are: chemistry of the contaminated mining effluent ([metals, [Al], [sulphates], dissolved oxygen, acidity, pH), desired residence time, nature, density, void index and mass of the limestone rock used, alkalinity, desired life span and hydraulic behaviour of the drain (under load or not). Certain secondary effects (saturation by sulphates, co-precipitation, adsorption) may be both beneficial (lowering the sulphate and metal load) and harmful (coating the limestone rock and clogging of the drain). This article will give a status report on the issue and on the knowledge we have in applying the design criteria for limestone drains for passive treatment of mining effluents heavily loaded with metals. Examples given include the old Lorraine Mine (Ni-Cu) in the Témiscamingue where limestone and dolomite rock drains were installed when a capillary barrier was placed over the tailings site in 1998.