The Hill Annex Mine Pit (Hill Annex) is located north of the city of Calumet in Itasca County, Minnesota. The large pit lake is formed in a complex of numerous inactive iron ore pits.
Mineral exploration in the area began around 1892. Open pit iron ore mining commenced in 1913 and continued into the late 1970s with last shipment of iron ore in 1979. After which the mine infrastructure was sold to the Department of Iron Range Resources and Rehabilitation (IRRR), formerly Iron Range Resources and Rehabilitation Board. IRRR obtained a lease to the land for operation of an interpretive center and facilitate site tours in 1981. In 1988, the Minnesota Legislature made the Hill Annex Mine a State Park.
The Mineland Reclamation Rules for ferrous mining were adopted in 1980 and require mining operations to plan for closure and reclamation at the onset of mining. However, because mining in the Hill Annex ceased prior to the Mineland Reclamation Rules, no company is responsible for reclamation of the site, which may include water level management. The Department of Natural Resources’ (DNR) Division of Lands and Minerals currently monitors surface water and groundwater levels in the area and may serve as technical experts regarding current and predicted conditions.
Hill Annex water level management
Water was pumped from the Hill Annex for the purpose of iron ore extraction when mining was active. When mining ceased, the pit began to fill with water from precipitation, surface water inflow, and groundwater inflow. The state periodically pumped water from the Hill Annex between 1988 and 2009 to manage rising water levels to facilitate site tours.
The Hill Annex continues to gain water and may need an engineered outlet structure and route in the future to prevent flooding. To enable informed discussion with state leaders and legislators, a number of state agencies are collaboratively working to gather data. The initial data gathering will be to collect water quantity and water quality information from the pit as well as potential nearby surface waters which may be receiving waters in various designed outlet scenarios.
Future work may include but is not limited to continuation of water level monitoring, bathymetric surveying, installation of groundwater wells, surveying potential outlet routes, and construction of a groundwater model. Additional work may also include a study to evaluate pitwall stability.
Once a preferred outlet location and route are chosen, future phases would include engineering a design and then construction. Future data gathering, engineering design, and construction would need to be funded before these phases of the project could move forward.
Water level elevations through 11/7/19
Water level monitoring
The DNR Division of Lands and Minerals actively monitors the Hill Annex water level and surrounding groundwater levels to understand the rate of pit water level increase and groundwater flow through the subsurface. The Hill Annex water level trend shown represents a combination of manual measurements and continuous logger measurements.
DNR Division of Lands & Minerals
St. Paul, MN