Attenuation and diel cycling of coal-mine drainage constituents in a passive treatment wetland: A case study from Lambert Run, West Virginia, USA

This study reports changes in coal-mine drainage constituent concentrations through a passive treatment wetland and over diel cycles. The purpose of the study was to determine what physiochemical mechanisms control attenuation of metals and if they varied by location and through time. The source water was slightly acidic (average pH 5.43); downstream degassing of CO 2 contributed to an increase in pH prior to discharge from the site (average pH 7.05). Aluminum, Fe, rare earth elements (REE) and Y were removed to a greater extent than were Mn, Co and Ni. At acidic pH, the REE and Y were generally complexed by SO 4. At higher pH, carbonate complexes became more important. The REE and Y concentrations were normalized to the North American Shale Composite standard; the normalized patterns were coherent near the source but anomalies of Ce and Y were present further downstream indicating oxidation and sorption processes. Four sets of diel-based samples were collected, one from a shallow, surface-flow wetland and three from a deeper, newly constructed wetland. Well-defined diel cycles were observed for concentrations of Si, Mn, Fe, Co, Ni, As, REE and Y in the shallow wetland. In all cases, the concentrations increased (up to 863%) during night and decreased during the day. These cycles had an inverse relationship with the temperature cycle (pH had no discernable cycle). The consistency of concentration cycles suggests a common mechanism, most likely associated with the formation of Fe oxyhydroxides. The increased rate of Fe 2+ oxidation in warm water can account for the cycles in Fe; scavenging of the other elements by the Fe precipitate can account for the consistency of the cycles even though the elements include cations, anions ( H 2 AsO 4 - or HAsO 4 - 2 ), and neutral species ( H 4 SiO 4 0 ). While REE and Y had clear cycles in the constructed wetland, the other elements did not. This is partially due to the lower elemental concentrations (including Fe) but the cycles may also be damped by the deeper, slower-moving water. This study illustrates the dynamic nature of metal removal in passive treatment systems. Furthermore, it suggests that grab samples collected during daytime hours may underestimate the concentrations and flux of metals in these systems.