Sulfate release and transport from taconite tailings basins

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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.

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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 (δ²H_H2O, δ¹⁸O_H2O) and sulfate (δ¹⁸O_SO4 and δ³⁴S_SO4), 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.

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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.

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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.

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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.