Reactive transport modelling of porewater geochemistry and sulfur isotope fractionation in organic carbon amended mine tailings

Highlights: • Integrated simulation of sulfide oxidation, reduction and $\delta$³⁴S–SO₄ fractionation. • Sulfate reduction and metal-sulfide precipitation promote S, Fe, Mn and Zn removal. • Simulated$\delta$³⁴S–SO₄ fractionation demonstrates microbial control on SO₄ reduction. • Simulation of observed geochemistry quantitatively supports conceptual model. • Carbon amendments evaluated for in situ treatment of tailings porewater. Field experiments previously conducted to assess organic carbon (OC) amendments for in situ biological treatment of tailings porewater at the Greens Creek Mine (Alaska, USA) showed sulfate reduction, metal-sulfide precipitation, and decreased fluxes of sulfate and dissolved metals. Here, we develop two reactive transport models using the reactive transport code MIN3P to simulate hydrogeochemical processes and $\delta$³⁴S–SO₄ isotope fractionation over four years in test cells containing unamended (control) and amended (5 vol % OC) tailings. These models successfully simulate observed data including pH, SO₄, $\delta$³⁴S–SO₄, Ca, Fe, K, Mg, Mn, Si, and Zn. The models also indicate that dissolution of carbonate and, to a lesser extent, aluminosilicate minerals neutralize acidic porewater generated from sulfide mineral oxidation and sulfate reduction reactions. Application of a constant kinetic fractionation factor of 0.9820 to simulate measured $\delta$³⁴S–SO₄ trends confirms that sulfate removal principally results from microbially-mediated sulfate reduction in conjunction with OC oxidation and subsequent metal-sulfide precipitation. Gypsum precipitation/dissolution and thiosulfate disproportionation have negligible effects on modelled porewater $\delta$³⁴S–SO₄ signatures. Our simulations are consistent with the previous findings that metal-sulfide precipitation controls Fe and Zn attenuation in amended tailings and that coprecipitation reactions contribute to metal removal. Overall, these simulations demonstrate that coupled reactive transport modelling incorporating stable isotope fractionation can improve the understanding of hydrogeochemical and biogeochemical controls within in situ treatment systems, further illustrating the benefits and limitations of this technique for improving water quality of mine drainage.