The Hill Annex Mine Pit (Hill Annex) is a pit lake is formed in a complex of numerous inactive iron ore pits in Itasca County, Minnesota. The Hill Annex is located north of the city of Marble and Calumet.
Open pit iron ore mining commenced in the Hill Annex in 1913, and continued into the late 1970s. After mining operations ceased, the mine infrastructure was sold to the Department of Iron Range Resources and Rehabilitation (DIRRR), formerly Iron Range Resources and Rehabilitation Board (IRRRB).
Adopted in 1980, the Mineland Reclamation Rules for ferrous mining require mining operations to plan for closure and reclamation, which may require water level management. For operations that ceased prior to the adoption of the Mineland Reclamation Rules, such as the Hill Annex, reclamation was not required. Therefore, the Hill Annex is considered a legacy mine pit and no party is responsible for water level management.
As water levels continued to rise within the Hill Annex, the full extent of potential hydrologic impacts to surrounding communities was unknown. Recognizing the importance of a data record from which water level predictions can be made, the DNR began monitoring for pit water level and surrounding groundwater changes.
An engineered outlet structure is needed for the Hill Annex to prevent future flooding and impacts on nearby communities. While this area may still be a viable option for future mining operations, which may include pumping water from the pit, an engineered outlet structure is the only guaranteed method to control water levels.
Hill Annex water level management
In 1981, the IRRRB obtained a lease for the land to operate an interpretive center and facilitate mine site tours. In 1988, the Minnesota Legislature made the Hill Annex Mine a State Park. To facilitate mine pit site tours, the State periodically pumped water from the Hill Annex to reduce water levels between 1988 and 2008. During this time, water was generally pumped at the beginning of each year to lower water levels enough to facilitate these in-pit tours for the remainder of the year. After pumping stopped in 2008, the pit water level began to continuously rise from precipitation, surface water inflow, and groundwater inflow.
Water level monitoring
Water level elevations through October 2022
The DNR Lands and Minerals Division 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 materials. Hydrologic information gathered near the pit informs assessment of the need for an engineered outlet structure to prevent future flooding and other impacts. The Hill Annex water level trend shown represents a combination of manual measurements (dashed line) and continuous logger measurements (solid line).
As of October 2022, the Hill Annex water level is approximately 1,324, 44 feet below the natural overflow elevation. There are three natural outflow locations at elevation 1368 feet along the pit rim. The pit water level has risen approximately 75 feet since pumping ceased in 2008. The rate of water level rise from 2009 through 2014 was approximately 7 feet per year. However, this rate has declined to approximately 5 feet per year from 2014 to present. As Hill Annex continues to gain water, additional work is needed to understand potential hydrologic impacts to nearby communities.
Engineered outlet planning
Between 2018 and 2020, State agencies worked collaboratively to gather water quantity and quality information from the Hill Annex, as well as from nearby surface waters that may serve as receiving waters for any constructed outlet.
Groundwater monitoring data are limited in the cities of Marble and Calumet. Increasing the spatial distribution of groundwater wells along the southern pit rim will aid in assessment of water outflow through the subsurface materials and potential impacts to the local groundwater system. The DNR recently coordinated the installation of ten additional groundwater monitoring wells in the area to increase available data needed to further discussions and planning for a potential outlet.
In September 2022, DNR conducted a bathymetric survey of the Hill Annex Pit to provide pit volume data. Additional data collection and work will assist in future modeling efforts, inform potential outlet route decisions, and aid in the assessment of possible impacts from rising water levels. Potential work may include pit wall stability analysis, wetland delineations, and groundwater modeling. Following the necessary data collection the DNR will start the predesign, design, and engineering at the Hill Annex. If the Hill Annex water level continues to rise at a rate similar to those in recent years, water may begin to naturally overflow by 2028-2031.
DNR Lands and Minerals Division
St. Paul, MN