In voicing support for copper-nickel mining in northern Minnesota, a recent letter argued that mining sulfide ore would be no worse than natural processes already exposing these minerals at the surface ("Modern science means mining safe," July 15). The letter contained factual errors and misleading statements that are worth highlighting.

The letter described how harmful minerals naturally "pump up from the ground." This is false. There are indeed places where naturally occurring minerals contaminate local watersheds. However, rather than by an upward movement of solid minerals through the Earth, rocks containing such minerals are slowly exposed to the surface by continuous weathering and the erosion of overlying material over long periods of time (thousands or millions of years). In relatively flat regions like Minnesota, this is a very slow process.

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Once minerals are exposed to oxygen and water at the surface, elements within them may be leached chemically and released to the environment. For sulfide minerals, biologically toxic elements and other products of weathering - such as arsenic, sulfuric acid, and dissolved sulfate - typically are highly localized near the source of natural mineral weathering. When potentially harmful minerals are disseminated, they represent a nonpoint source type of contamination. The situation is very different when mining activity rapidly exposes concentrated ore deposits and leaves behind mine waste.

The minerals themselves are also quite different than the letter portrayed. Pyrite is not "usually made up of copper and nickel" but combines iron and sulfur. The most common iron sulfide in mineralized zones of the Duluth Complex is actually pyrrhotite, however, and important copper and nickel ores include chalcopyrite, bornite, and pentlandite. Pyrite is normally benign because most rocks contain only small amounts and natural leaching processes occur over a long time at a slow rate. It is misleading to equate this situation to the exposure of highly concentrated sulfide ore minerals found in mineral deposits.

Mining rapidly brings concentrated sulfide minerals to the surface where oxidation and hydrolysis can produce toxic substances. The crushing of ore increases the surface area of minerals relative to their volume, which enhances chemical reactions and reaction rates. It is therefore mistaken to compare slow natural processes of mineral weathering to the rapid exposure of crushed rock at the surface where we expect it to remain inert for long periods.

It is misleading to advocate that copper-nickel mining is safe because it will be "well outside of the Boundary Waters (Canoe Area Wilderness)." It is true that targeted ore deposits lie outside the BWCAW, but most are within the same watershed as the BWCAW. Mines and tailings stockpiled within a watershed have potential to contaminate both surface water and groundwater downstream.

PolyMet's proposed Northmet project is the only one in the Duluth Complex that lies within the St. Louis River watershed, which drains into Lake Superior through sensitive bog, stream, and lake habitats. But other proposed projects lie within the Rainy River drainage, which flows to Lake of the Woods through the BWCAW and Quetico Provincial Park. All of these projects carry environmental risks that extend beyond their perimeters.

Mining companies propose to maintain tailings in sealed and/or treated surface and underground storage facilities, including a site near Babbitt that is outside of the Rainy River watershed. Mine backfill may be safe because the natural fracture permeability (the ability of water to flow through a material) of the bedrock at the mine level is very low. However, a surface tailings pile of crushed rock has a small net footprint on the ground. The pile itself becomes a localized (point) source of potential contamination that is quite different from natural situations.

Sulfide mining in northern Minnesota is not comparable to traditional iron mining on the Mesabi Range. The mining of an already-oxidized iron formation creates minimal environmental harm. But sulfide mining has the potential for fundamentally different outcomes.

Whether for or against sulfide mining, it is important to examine with informed skepticism proposals that have potential for long-term environmental impact.

As we seek solutions that support our economy while protecting our environment, science can best inform our decision-making - if we check facts and apply scientific principles correctly.

John Goodge is a professor of earth and environmental sciences at the University of Minnesota Duluth.