Mineral Leeching

Types of Engineering involved: Mining, Chemical, Environmental

Turtle Island is rich in mineral and metal deposits including gold, silver, zinc, diamonds and coal; all of great interest to mining companies. Many of these deposits lie deep within traditional lands. In many places they have been mined to the point where it is no longer economical for companies to continue operations, and so mines are abandoned.

Rock contains sulphides - a combination of sulfur and other minerals. When rock is exposed to air or water through natural weathering processes, the sulphides chemically react to produce sulfuric acid. The acid can move (leech) into the nearby soil or watershed but, generally, there isn't enough sulfuric acid produced through weathering to significantly raise their acidity. During and after mining, however, large quantities of rocks that are normally underground become exposed. Much more sulfuric acid is produced than through natural conditions. In addition, the acid can dissolve the traces of metal left in waste rock, dragging poisonous heavy metals such as lead, zinc, copper, arsenic, selenium, mercury, and cadmium into the ground and surface water. Left unchecked, this mineral leeching can destroy local aquatic life and habitat.

While steps are being made to avoid the negative effects of mining, damage is still being done, particularly by abandoned mines. For example, a salmon breeding river in northern BC, the Taku, is threatened due to the mineral leeching from the abandoned Tulsequah-Chief Mine. Mineral leeching is a potential long-term problem of mining. There are ways to minimize and even avoid leeching, but they work best when put in place before operations begin. Clean-up is always harder: some abandoned Roman coal mines in Britain still leech minerals today, almost 2000 years after being abandoned.

For more information visit Mining Watch Canada and Natural Resources Canada