Potential Physical and Chemical Environmental Impacts of Mining the Ironwood Iron Formation in the Bad River Watershed
by Tom Fitz
Gogebic Taconic, LLC, (G-Tac) is studying a four-mile segment of the Ironwood Iron Formation east of Mellen, Wisconsin to assess the potential for developing an open-pit iron mine. A lot still needs to be studied to assess the feasibility of a mine and the potential environmental impacts. Every geologic setting is different, but based on studies of existing iron mines, it is known that mining can have physical, chemical, and resulting biological impacts.
Physical impacts are those that involve changes in the physical environment such as modification of Earth’s surface, and changes in the flow of water -- both surface water and groundwater. A mine would certainly have a physical impact at the site because removal of the ore rock and storage of the waste rock would reshape Earth’s surface. These influences would be limited to the mine site and waste rock storage areas. However, changes in the flow of water can have impacts at the mine site and down gradient as well.
The quantity and flow of surface water could be impacted by changes in the topography of Earth’s surface and resulting effects on the drainage of surface water. The proposed mine site is in the headwaters of Tyler Forks river, which flows into the Bad River at Copper Falls State Park. If a mine were developed, headwater streams of the Tyler Forks could receive either more or less water than they do today. The amount of change in stream flow, and the impact of that change, still needs to be assessed and addressed.
One potential impact that changes in land use and stream flow can have is by affecting the sediment carried by rivers. The amount of suspended sediment clouding the water (increased turbidity) could be affected because of changes in stream flow, and increased loose sediment available to the flowing water. Proper engineering controls used in a mining operation could decrease this affect by minimizing the availability of loose sediment available to rivers.
Groundwater would have to be pumped to lower the water table in order for the mine to operate, but how much water would be pumped, and how much would be discharged versus used at the mine is unknown. Pumping would certainly draw down the water table in the area, so it needs to be determined how large the area of groundwater draw-down would be, how much the water table would be lowered, and what influence these changes would have on wells and surface water in the area.
Mining can also have environmental impacts from changes in the chemical composition of water from the area of a mine and waste rock piles. Of particular concern is the potential effect of the water interacting with minerals from the mine -- dissolving metals that impact the quality of surface water. The two dominant minerals in the Ironwood Formation, magnetite and quartz, are nearly insoluble and do not influence the chemistry of water they are in contact with. However, the detailed mineralogy still needs to be studied thoroughly, because bedrock geology is complex everywhere, and there is always the possibility of the presence of minerals that could impact water quality.
The rock on the north side of the Ironwood Formation, the Tyler Slate, is known to contain small amounts of the mineral pyrite (iron sulfide) which can influence the chemistry of water. Part of the Tyler Formation would have to be removed in order for the mine to access the ore of the Ironwood Formation, so there is potential for water in the area to have an increased concentration of metals from interaction with pyrite in waste rock piles. How much pyrite is present still needs to be determined, and if there is much pyrite, what could be done to prevent it from impacting water quality.
At this point in the process there still needs to be scientific studies to assess the basic physical and chemical setting of the site. The potential impacts on the flow of groundwater and surface water, and the chemistry and turbidity of the water flowing from the area, are among the many important questions that need to be addressed in the initial studies. Reasonably accurate answers to these questions can be obtained through the right scientific studies of the rocks, the groundwater, the surface water, and the biological communities that live in those environments. These topics need to be well understood in order to assess the feasibility of a mine, the potential environmental impacts, and what measures would be used to minimize environmental impacts if a mine is developed.