Effects of Fe-rich acid mine drainage on percolation features and pore structure in carbonate rocks

Abstract

Acid mine drainage (AMD) poses significant ecological, environmental and geological challenges worldwide, especially in more vulnerable karst areas. However, the impact of AMD on percolation features and pore structure of carbonate strata is poorly understood, despite its importance for the evolution of carbonate aquifer as well as karst groundwater quality. To fill this knowledge gap, field investigations, core-flooding experiments, reactive transport modeling and pore structure studies [mercury intrusion porosimetry (MIP) and nitrogen physisorption analysis] were conducted to understand the geochemical processes and associated pore development in carbonate strata exposed to Fe-rich AMD. Our field investigation shows that the AMD is characterized by low pH values (2.4–3.8), high contents of sulfate (1457–4217 mg/L), Fe (100–830 mg/L, Fe3+/Fe2+) and heavy metals, that can potentially leak into carbonate strata. The major findings of this study include: (1) core-flooding experiments reveal that the interaction of carbonate rock with AMD causes a rapid elevation of solution pH and simultaneous precipitation of Fe3+/Fe2+ as iron (oxy)hydroxides or (oxy)hydroxy sulfates within the rock; (2) the results of reactive transport modeling suggest that ferrous iron has a longer transport distance than ferric iron in the carbonate core plug, and then it precipitates to fill the pore space; and (3) despite the precipitation, the flooding of AMD increases the overall permeability of cores by creating large numbers of pores with throat diameter between 0.001 and 10 μm. We also found that the interaction of AMD with the carbonate rock might stimulate the formation of preferential flow channel(s). Our results have been incorporated into a conceptual model, built to illustrate the geochemical processes and mechanisms involved. Our research has important applications in AMD treatment for coal-mining industries, and for local governments devising strategies for karst water resource protection and management.