Seasonal varability of metals transport through a wetland impacted by mine drainage in the Rocky Mountains.

An annual study of a natural wetland receiving drainage with high concentrations of iron, zinc, and manganese from an abandoned mine tunnel was conducted. During summer, the wetland reduced the mass flow from the tunnel to a receiving stream by more than 90% for iron, by 65% for zinc, and by 25% for manganese. Plant uptake accounted for less than 1% of the total retention. Zinc and manganese mass flows to the stream were greater than mass flows from the tunnel during autumn and winter, indicating a seasonal cycle of the wetland acting as a net sink in the summer and then a net source in the winter. Iron mass flow to the stream was less than the iron mass flow from the tunnel for the majority of the year, with the high retention of iron in the wetland associated with large deposits of iron oxides, especially thick in the upper reaches of the wetland. Spring snowmelt saturated the otherwise dry material in the mine tailings above the wetland, creating an additional source of inflow, with metal concentrations two times higher and three pH units lower than water flowing from the mine tunnel. This period of runoff from the tailings, with the associated high concentration of metals in the stream below the wetland, represented a critical time for stream biota.