Sulfate release and transport from taconite tailings basins
A series of studies were conducted in 2014 and 2015 focusing on the generation and fate of sulfate at five taconite tailings basins and processing plants in northeastern Minnesota. Work was carried out as part of the Mine Water Research Advisory Panel (MWRAP), funded jointly by DNR’s Iron Ore Cooperative Research (IOCR) and Cooperative Environmental Research (CER) programs and the MWRAP participating mines.
MWRAP Project Reports
Provided below is information and links to recent MWRAP projects.
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- Physical and Hydrologic Properties of Taconite Tailings
Project Summary: Physical and hydrologic properties were determined for coarse and fine tailings cores collected from four Minnesota taconite operations in 2014 and 2015. Hydrus 1D, a computer program that can model water flow and solute transport in variably saturated media, was also used to estimate tailings infiltration rates applicable for the study period. The study found that coarse tailings ranged from sand to gravel in size and textures were similar across all cores and operations. In contrast, fine tailings particle sizes ranged from fine sand to silt/clay in size with the coarsest particles being deposited near tailings discharges and the finest being deposited near tailings cell outlets. Coarse tailings water contents were low for all cores and the newer coarse tailings cores still contained some process water. Older fine tailings cores generally had lower water contents and contained no process water whereas newer tailings cores had higher water contents and still contained process water that was entrained in the tailings during deposition, indicating that fine tailings dewatering is very slow. A transient seepage model estimated the average yearly evaporation (for 2014 and 2015) for a freely drained fine tailings core with a fine sand texture to be 47 percent of total annual precipitation or approximately 10.5 inches. Net infiltration was estimated to be 12.0 inches. This research helps the Minnesota Department of Natural Resources better understand the hydrologic properties of tailings which can be used to improve water balance calculations and models for taconite tailings basins.
- Geochemical Tracer Based (GTB) Sulfate Balance Models for Active Tailing Basins on Minnesota’s Iron Range (I) Process waters
Project Summary: This report is the first of a three part series evaluating sulfate release and transport associated with taconite tailings basins using geochemical tracer based (GTB) methods. Part I of the series focused on the development of water and chemical mass balance models that utilize GTB methods to estimate water and sulfate fluxes for five operating Minnesota taconite facilities. Geochemical parameters including bromide and chloride concentrations, and isotope ratios for water (δ2HH2O, δ18OH2O) and sulfate (δ18OSO4 and δ34SSO4), were analyzed at key locations at each taconite processing plant and tailings basin. This data, along with available hydrologic information, was incorporated into steady state GTB mass balance models to estimate sulfate and water fluxes for each of the five facilities studied. The relative amounts of sulfate generated and released into process waters at each facility was found to be dependent on the reactivity of the tailings and the specific water management practice at each facility. While more work is needed to fully understand water and sulfate fluxes at taconite facilities, this study demonstrates how GTB methods can be incorporated in to chemical and water mass balance models to improve understanding of these complex facilities and aid decision making for permit actions.
- Geochemical Tracer Based (GTB) Sulfate Balance Models for Active Tailing Basins on Minnesota’s Iron Range (II) Field-Based Sulfate Release Rates for Coarse and Fine Tailings
Project Summary: This report is the second of a three part series evaluating sulfate release and transport associated with taconite tailings basins using geochemical tracer based (GTB) methods. Part II of the series focused on pore water chemistry measurements and physical properties for tailing samples collected at four Minnesota Taconite mining operations. This research helps the Minnesota Department of Natural Resources better understand how much sulfate is generated when taconite tailings are placed in above ground storage facilities. Chemical and physical measurements were combined with modeled infiltration rates from Bavin et al. (2016) to estimate sulfate release rates for several tailings types at each operation. Calculated release rates were highly variable and dependent on factors such as drainage characteristics, age of tailings, plant cover, and tailings type. While more work will be necessary to gain a full hydrologic understanding of the tailing basin environment, the results of this study provide a preliminary estimate of the relative impacts of tailings on sulfate in waters that penetrate tailings on Minnesota’s Iron Range.
- Geochemical Tracer Based (GTB) Sulfate Balance Models for Active Tailing Basins on Minnesota’s Iron Range (III) Well waters, seeps, and downstream surface waters
Project Summary: This report is the last in a three part series evaluating sulfate release and transport associated with taconite tailings basins using geochemical tracer based (GTB) methods. Part III of the series focused on the chemistry of waters sampled from wells, seeps, and streams at locations outside five Minnesota taconite tailings basins during 2014 and 2015. The geochemical and isotopic tracer method applied in this study was used to estimate the amount of sulfate added to and/or removed from process waters seeping out from taconite tailings basins into the surrounding environment, through processes including dilution, oxidation of sulfide minerals in mine tailings, and microbial sulfate reduction. Results demonstrate that sulfate is nonconservative in waters downstream of all five tailings basins, though the relative impact of sulfide oxidation and sulfate reduction varies considerably from site to site. This research helps the Minnesota Department of Natural Resources to determine the extent to which a mining operation contributes sulfate to the environment and informs decisions where reduced loading may be necessary.
- Development of a Reactive Sulfate Transport Model for a Minnesota Taconite Tailings Basin (Minntac). A University of Minnesota Research Report.
Project Summary: A suite of 2D numeric models were developed to represent water and sulfate transport at the edge of a tailings basin under existing conditions. These preliminary geochemical and flow models help to evaluate factors impacting the transport of sulfate from tailings basins into the surrounding environment. The models were constructed using available site-specific hydrologic and geochemical observations, which served as both model inputs and calibration targets. The report outlines additional data needs and model development necessary to advance our understanding of geochemical mechanisms and subsurface physical properties. With improvements, however, the reactive-transport models described here could be further adapted to test different operation scenarios as a basin ages and approaches eventual closure.
Sulfate and Methylmercury Relationships in the St. Louis River Watershed
The following publications resulted from a collaborative research effort led by the DNR during 2012-2013, focused on gaining a better understanding of sulfate and methylmercury relationships in the St. Louis River Watershed.
- Bailey, L. T., Mitchell, C. P., Engstrom, D. R., Berndt, M. E., Coleman-Wasik, J., Johnson N. W.
Influence of porewater sulfide on methylmercury production and partitioning in sulfate-impacted lake sediments. Science of the Total Environment. In Press. Corrected Proof Available Online 23 Dec 2016.
- Berndt, M. E., Rutelonis, J. W., and Regan, C. P. (2016)
A comparison of results from a hydrologic transport model (HSPF) with distributions of sulfate and mercury in a mine-impacted watershed in Northeastern Minnesota. Journal of Environmental Management. 181, 74-79.
- Jeremiason, J. D., Reiser, T. K., Weitz. R. A., Berndt, M. E., Aiken. G. R. (2016)
Aeshnid dragonfly larvae as bioindicators of methylmercury contamination in aquatic systems impacted by elevated sulfate loading. Ecotoxicology, On-Line Publication: DOI: 10.1007/s10646-015-1603-9
- Jeremiason, J. D., Portner, J. C., Aiken, G. R., Hiranaka, A. J., Dvorak. M. T., Tran, K. T., and Latch, D. E. (2015)
Photoreduction of Hg(II) and photodemethylation of methylmercury: the key role of thiol sites on dissolved organic matter. Environmental Science: Processes & Impacts, 17, 1892 – 1903.
- Johnson, N. W., Mitchell, C. P., Engstrom, D. R., Bailey, L. T., Kelly, M. J., Coleman-Wasik, J., and Berndt., M. E. (2016)
Methylmercury production in a chronically sulfate-impacted sub-boreal wetland. Environmental Science: Processes & Impacts.
- Rutelonis, J. W., Berndt, M. E., Fox, D. L. (2016)
Linking Riparian Flow-Concentration Integration Modeling and HSPF to Predict Background Methylmercury Concentrations in Northeastern Minnesota Streams. A University of Minnesota Research Report. 18 pages.
Mercury Methylation and Sulfur Cycling in NE Minnesota Streams
The following links to studies were conducted by or for the DNR - Division of Lands and Minerals, Reclamation Section. These reports relate to methyl mercury and sulfur cycling in NE Minnesota streams and in the St. Louis River estuary.
- Bailey, L. et al. 2014a
Geochemical factors influencing methylmercury production and partitioning in sulfate-impacted lake sediments. University of Minnesota Duluth Research Report, 63 p.
- Bailey, L. et al. 2014b
Seasonal and spatial variation in methylmercury in the water column of sulfate-impacted lakes. University of Minnesota Duluth Research Report, 64 p.
- Berndt, M.E. and Bavin, T.K., 2009
Sulfate and mercury chemistry of the St. Louis River in Northeastern Minnesota: A Report to the Minerals Coordinating Committee. Minnesota Department of Natural Resources, Division of Lands and Minerals, St. Paul, MN, 83 p.
- Berndt, M.E. and Bavin, T.K., 2011
Sulfate and Mercury Cycling in Five Wetlands and a Lake Receiving Sulfate from Taconite Mines in Northeastern Minnesota: A Report to Iron Ore Cooperative Research Program., Minnesota Department of Natural Resources, Division of Lands and Minerals, St. Paul, MN, 77p.
- Berndt, M.E. and Bavin, T.K., 2012a
Methylmercury and dissolved organic carbon relationships in a wetland-rich watershed impacted by elevated sulfate from mining. Environmental Pollution 161, 321-327.
- Berndt, M.E. and Bavin, T.K., 2012b
On the cycling of sulfur and mercury in the St. Louis River watershed, northeastern Minnesota, Minnesota Department of Natural Resources. An Environmental and Natural Resources Trust Fund Final Report. 91 p.
- Berndt, M. et al., 2014
Hydrologic and Geochemical Controls on St. Louis River Chemistry with Implications for Regulating Sulfate to Control Methylmercury Concentrations. A Minnesota Department of Natural Resources Research Report. 33 p.
- Jeremiason, J. et al. 2014a
Dragonfly larvae as bioindicators of methylmercury contamination in aquatic systems impacted by elevated sulfate loading. Gustavus Adolphus College Research Report. 24 p.
- Jeremiason, J. et al. 2014b
Binding of methylmercury to dissolved organic matter. Gustavus Adolphus College Research Report. 23 p.
- Jeremiason, J. et al. 2014c
Methylmercury photodegradation: Laboratory and field studies in the St. Louis River. Gustavus Adolphus College Research Report. 17 p.
- Johnson, N. W. and Beck, B. F., 2013
Sulfur and carbon controls on methyl mercury in St. Louis River Estuary sediments Phase II. University of Minnesota Duluth Research Report. 48 p.
- Johnson, N. W. and Zhu, X., 2012
Carbon and iron additions to stimulate in-pit sulfate reduction and removal. University of Minnesota Duluth Research Report. 35 pages.
- Johnson, N. W. et al. 2014 Methylmercury production and transport in a sulfate-impacted sub-boreal wetland. University of Minnesota Duluth Research Report. 55 p.
- Kelly, M., and Berndt, M. 2015
An updated isotopic analysis of sulfur cycling and mixing processes in the St. Louis River Watershed. 67 p.
- Kelly, M., et al. 2014
Use of sulfate and water isotopes to improve water and chemical balance estimates for water seeping from tailings basins (focus on US Steel’s Minntac Basin). A Minnesota Department of Natural Resources Research Report. 31 p.
- Theriault, S.A., 2011
Mineralogy, Spatial Distribution, and Isotope Geochemistry of Sulfide Minerals in the Biwabik Iron Formation, Geology. University of Minnesota, 165 p.
- Von Korff, B. and Bavin, T., 2014
Short term sulfate release rates and sulfide oxidation mechanisms for taconite tailings from the Minntac and Keetac mining facilities. A Minnesota Department of Natural Resources Research Report. 48 p.
Please also see reports on DNR studies of mercury in taconite stack emissions.