USING REACTIVE TRANSPORT MODELING TO QUANTIFY LONG-TERM REACTION RATES IN PERMEABLE REACTIVE BARRIERS FOR THE TREATMENT OF MINE DRAINAGE

Organic-carbon based permeable reactive barriers are useful for the treatment of mine drainage impacted waters, which are rich in SO₄ and Fe(II). In order to quantitatively assess the long-term the performance of these systems, it is of importance to understand the processes controlling rates of SO₄-reduction and organic matter consumption. Reactive transport modeling of a permeable reactive barrier for the treatment of mine drainage was used to integrate a comprehensive field data set including pore water chemistry, solid phase and dissolved gas data from several sampling events over a >7-year time period. The simulations consider the reduction of sulfate by the organic carbon-based treatment material and the removal of sulfate and iron by precipitation of reduced mineral phases including iron monosulfides and siderite. Additional parameters constraining the model include dissolved gases (H₂S, CO₂, CH₄, N₂, Ar), alkalinity and pH, as well as a suite of solid phase S-fractions identified by extractions. Influences of spatial heterogeneity necessitated the use of a 2-D modeling approach. Simulating observed seasonal fluctuations and long-term changes in barrier reactivity, required the use of temperature-dependent rate coefficients and a multimodal Monod-type rate expression accounting for the variable reactivity of different organic carbon fractions. Simulated concentrations of SO₄, Fe, alkalinity, pH, dissolved gases, and solid phase accumulations of reduced sulfur phases generally compare well to observed trends. Spatial variations, seasonal fluctuations and the time-dependent decline in reactivity were also captured. The modeling results indicate that only a relatively small fraction of the treatment material is consumed by methanogenesis. Overall sulfate reduction and S-accumulation rates are constrained with confidence within a factor of 1.5.