Legacy tailings produced in taconite iron ore mining in Northeastern Minnesota are rich in iron oxides and have strong carbonate-derived pH buffering capacity. These properties make taconite tailings an attractive candidate for attenuating trace metals and metalloids via surface complexation to iron oxides and/or co-precipitation of solid solutions. The proposed development of copper-nickel resources in the Duluth Complex of Minnesota represents an opportunity for beneficially re-using legacy taconite tailings to control metal mobility from newly created Cu-Ni tailings.

To probe the geochemical interactions between Cu-Ni and taconite tailings, a unique set of laboratory-scale kinetic tests were conducted. The test program placed humidity cells in series with leachate from Cu-Ni cells being fed as influent into taconite cells. The tests remained in operation for thirteen years. The data show distinctive geochemical signatures of (i) surface area normalized release for conservative ions such as SO₄^2-, (ii) approach to an equilibrium solubility limit for carbonate forming cations such as Ca²⁺, and (iii) sorptive attenuation for trace metals and metalloids such as AsO₄^3-. The sorptive attenuation was observed to increase over time as the taconite cells aged over the course of the test.

A geochemical model was developed and calibrated to the test data. The model was used to forecast water quality in a hypothetical full-scale tailings co-management scenario. Simulations with the model demonstrate the potential for non-linear scaling of water quality with respect to water-to-rock ratio and highlight the critical importance of assumptions that influence model predictions of pH on metals mobility. Results highlight the role of the in-situ pH on geochemical processes in both kinetic tests and in field-scale tailings facilities.

Both kinetic test data and modeling results suggest an opportunity for beneficial reuse of byproducts from the established taconite and developing Cu-Ni mining industries in Northeastern Minnesota. The beneficial geochemistry depends on the in-situ pH of the two interacting tailings and, therefore, a need exists for better tools for monitoring and predicting in-situ pH of both kinetic tests and field-scale mine waste.

During the mining of kyanite from a pyritiferrous quartzite-kyanite ore body near Lincolnton, Georgia, USA, several tailings ponds (TP) were created; the largest were 31 ha (77 ac) (East TP) and 10.5 ha (26 ac) (West TP). The acidic interstitial water (pH < 2.7), coupled with the acid producing tailings, hindered site reclamation. A new surface topography was established in the TP materials by the installation of ridges and furrows (R&F) to create a surface for successful reclamation and improved surface water quality runoff (RO).

Successful test plot studies led to establishing a R&F topography across the entirety of the ETP to divert surface RO into vertical infiltration. Initially, 0.15 m (6”) of organic material (straw) was incorporated into the top 0.3m (1 ft) of tailings and fine grained CaCO₃ was applied at rates of 90-135 mton/ha (40-60 tons/ac). Ridges, approximately 0.5m (1.5 ft) high and 0.6m (2.5 ft) wide with corresponding sized furrows, were created using farm equipment. The function of the remodeled topography was to allow for ponding of RO within the furrows and the leaching of acidity from the ridges, creating a more ecologically improved surface for vegetation. The R&F were further amended by 673 kg/ha (600 #/ac) of 10-10-10 fertilizer and seeded with an 11 seed mix. Lime and fertilizer were reapplied for two seasons and vegetation successfully (>90%) established in three years. Improved surface water quality, plant succession and lowering of the ETP water table have occurred during the subsequent 20 years. The smaller WTP, had R&F topography installed in the lower ¼ of the WTP and the remainder had been previously covered with a 40mm (2”) veneer of soil and reclaimed with limited success. Additional lime, fertilizer and seed were applied to the WTP simultaneously with the ETP.

The surface topographic technique can be used to reclaim an acidic tailings pond as indicated by: 1) successful growth of the planted species, 2) invasion of volunteer species, 3) an increase in the pH of the RO water (to pH 5.5-7.5) and 4) a decrease in the surface water discharge due to increased evapotranspiration as supported by a hydrologic study. In the portion of the WTP, without R&F and only a surface veneer of soil rather than incorporated mulch and changed topography, there were similar results related to decreased RO and improved water quality, but there was little plant succession and no tree establishment even after 20 years.

This paper demonstrates the importance of mineralogical and geochemical analyses for comprehensive and accurate assessment of the potential of mine wastes to produce AMD. It includes water balancing of a tailing storage facility (TSF) and the modeling of its long-term sulfate release to adjacent groundwater. It expands our understanding of the environmental effects of storage facilities several decades after their decommissioning.

The once famous mining of lead and zinc ores in the Stolberg and Mechernich regions (Western Germany) left behind former mine sites and mine waste storage facilities that are still affecting ecosystems. The Beythal TSF was one of these sites, classified as a potentially AMD releasing site with risk to ecosystems. We used scanning electron microscopy, QEMSCAN for a quantitative analysis of minerals, and some static geochemical tests to analyze acid-producing and neutralizing reactions. This was done on freshly drilled cores treated in a glove box. Furthermore, we set up a water balance model for the storage facility and implemented a numerical transport model to calculate sulfate flux to the local groundwater. These modeling tools were used to test various remediation measures.

Our detailed mineralogical and geochemical investigations have shown that the mine waste remains neutral and has a positive neutralization capacity, with dolomite being the main buffering carbonate. The neutralization potential ratio is three to ten. Metal concentrations are locally high but remain below threshold values in the seepage. However, the mine waste has a potential of approx. 2,000 metric tons of sulfate. The transport modeling showed a sulfate flux of 12 t a^-1 to the groundwater. Concentrations were 7 times higher than regional values and well above threshold values.

The comprehensive approach enabled to test different remediation measures and enabled decisions about effective approaches to mitigate the influences to downstream ecosystems. Three measures proved to be the most promising. Altering the vegetation on top of the stored material can reduce infiltration and seepage by 5 to 25 %. Along the storage facility’s toe an up to 8 m deep drain would be required to contain the sulfate plume. In addition, infiltration of collected and treated leachate on top of the waste could substantially reduce sulfate concentrations and the potentially 170 years of sulfate flux from this tailing storage facility.