| Summary: | A study was undertaken on the Bowden Close (BCl) Passive mine water Treatment System (PTS). BCl consists of two parallel working Reducing and Alkalinity Producing Systems (RAPS, reactive substrate = limestone, compost, manure) that are followed by a polishing wetland. Thereafter, the water is discharged into a local burn. The main purpose of the PTS is to decrease metal concentrations (Fe (≤177mgL-1), Al (≤85mgL-1), Zn (≤2.8mgL-1), Mn (≤20.5mgL-1)) and increase alkalinity (≥0mg L-1 CaCO3 eq) and pH (≥3.2) in two coal mine drainages. The aim of this study was to assess the treatment performance and the dominant (bio)geochemical processes promoting metal removal and alkalinity generation, particularly in the RAPS. Over nearly six years of operation, BCl performed well with regards to the removal of iron (-84%) and aluminium (-87%) and the generation of alkalinity (+74%). Zinc (-51%), manganese (-23%) and sulfate (-29%) were partially removed. The effluent pH was raised to ~6.9. However, a long-term decrease in alkalinity generation has been observed, which could threaten the treatment performance over the short term and might eventually lead to metal remobilization. Against expectations, Bacterial Sulfate Reduction (BSR) is not a driving process in the removal of the main contaminant, iron. Only ~5% of iron was removed as di-sulfide mineral (i.e. pyrite). Rather, removal processes such as observed in aerobic treatment systems predominate (i.e. retention in (hydr)oxides). It is suggested, that the reoxidation of hydrogen sulfide by Fe(III)hydroxides is limiting the generation of mineral sulfides. Carbon isotope ratios of total dissolved inorganic carbon indicate that anaerobic microbial respiration, including BSR, has considerable influence on the generation of alkalinity. Two mass balances suggest, that more than 52% of bicarbonate generated by the RAPS derives from the oxidation of organic matter, thereby safeguarding the limestone in the reactive substrate and increasing the overall lifetime of the RAPS. Analyses of sulfur and oxygen isotope ratios of dissolved sulfate and sulfide, together with solid phase sulfur and water isotopes suggested: i) mine waters are of meteoric origin, ii) and have one single sulfate sulfur source (potentially oxidation of coal derived iron sulfide), iii) sulfide oxidation in the mine waters is dominated by anaerobic oxidation, iv) in both RAPS, BSR is occurring year round, v) sulfate concentrations might be limiting BSR in RAPS 1 during the summer months and vi) pyrite seemed to form via the hydrogen sulfide pathway without solid phase iron mono-sulfide intermediate. Overall, sulfur and oxygen isotope fractionation suggest that BSR kinetics are slow and bi-directional. Detailed studies, including the microbial ecology in the RAPS are proposed to enhance understanding about the functioning of the system. Key words: coal mine drainage, microbial sulfate reduction, alkalinity generation, isotopes
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