English

Abstract

The authors present a numerical modeling study of unsaturated water flow and acid mine drainage in idealized (but representative) waste rock piles and using observed climatic recharge data. The simulations were used to help understand the long term hydrogeological behaviour and to help design and assess in situ groundwater monitoring methods. The flow simulations showed that when the same annual cycles of average monthly recharge are applied each year at the top of the piles, the water content profiles become periodic after a few years. The water distribution within the piles then becomes independent of the preceding hydraulic conditions for the cases considered here. Also, the results indicate that the amplitude of water content variations inside the pile between humid and relatively dry seasons is generally small (a few percent). Consequently, typical measurements of the water content variations can, in some cases, be limited for in situ monitoring because their level of precision is often of same order of magnitude as the expected changes. Long term simulations of oxygen diffusion and acid mine drainage through the waste rock piles showed that oxygen is generally not a limiting factor in the unsaturated zones of these types of systems and that preferential flow, moisture content and grain size can have a significant influence on oxidation rates and pH distribution. ______________________ 1 Paper presented at the 7 International Conference on Acid Rock Drainage (ICARD), March 26-30, 2006, St. Louis MO. R.I. Barnhisel (ed.) Published by the American Society of Mining and Reclamation (ASMR), 3134 Montavesta Road, Lexington, KY 40502 2 Ecole Polytechnique, PO Box 6079, Stat. Centre-Ville, Montreal, QC, Canada, H3C 3A7, 3 Universite du Quebec en Abitibi-Temiscamingue, 445 boul. de l’Universite, RouynNoranda, Qc, Canada, J9X 5E4, Tel. 514-340-4711, ext. 4046 ; fax. 514-340-4477 ; michel.aubertin@polymtl.ca