PERMAFROST and ACID ROCK DRAINAGE

Bruce W. Downing, M.Sc., P.Geo.

INTRODUCTION

PERMAFROST: Layers of soil in which a temperature below freezing exists continuously.

Acid rock drainage has become a major topic with respect to mining and the environment in the permafrost regions of Canada. This essentially has become very important in the recent mine development review of the potential diamond mines in northern Canada. Natural acid rock generation and drainage professes resulting in natural gossan development is very slow in permafrost areas. Some rocks may be quite permeable, such as kimberlites and hence water movement will occur through these kimberlite bodies, even though they may occur within permafrost areas. The studies of ARD and its impact upon the permafrost environment have become quite noticeable within the last five years.

There are several websites dealing with permafrost i.e.:

· Geological Survey of Canada: sts.gsc.nrcan.gc.ca/permafrost,

· International Permafrost Association: www.geodata.soton.ac.uk/ipa/f_main.html,

however there are essentially no articles dealing with permafrost and acid rock drainage except for a MEND report that does discuss permafrost and acid rock drainage, the summary and report details of which can be found at mend2000.nrcan.gc.ca.

Water Flow in Permafrost

There is the assumption that permafrost is frozen and that essentially there is no ground water movement. The question of discontinuous and continuous permafrost is important, as this will allow ground water flow. There are active layers within the permafrost, specifically near the surface, as well as scattered areas of ‘taliks’, which are essentially unfrozen ground near surface and which is a conduit for groundwater movement. There will be disturbance of permafrost during construction and dumping of waste rock which has the potential to break down the permafrost at and near the surface. If the proponent builds a ‘protective’ layer on top on the permafrost so as to keep this material frozen, then surface water percolating through the waste dump must flow somewhere during mining operations and be collected. If the layer is not continually frozen, then this water will percolate into the ground and flow as ground water (which needs to be monitored for contamination).

ARD Prediction

The aspect of ARD prediction in permafrost will depend upon local climatic conditions for long term mitigation and engineering, however the tools for ARD prediction are similar for non-permafrost areas. The long term effects that should be determined include global warming that may aid in the destruction of the permafrost, and hence water treatment may become a bigger issue at that time. The use of field test pads for ARD prediction is definitely warranted in conjunction with laboratory test work. The duplication of field conditions in the laboratory for kinetic and column leach tests are not really viable, thus the need for field test areas.

The recent kimberlite discoveries in northern Canada has had an impact in understanding acid rock drainage in areas of permafrost, especially for mine permitting and ultimate water quality studies. The onset of ARD in these areas is relatively slow at present and natural ARD sites are not common. There are very few published cases histories dealing with ARD in permafrost areas.