Redox sensitive metal cycling in mountain streams: Manganese and iron transport

Manganese and iron are two commonly associated metals with acid mine drainage. In order to examine their transport and cycling in stream environments, several different tracer injection experiments were performed in 2 different mountain streams. In one of the streams, manganese is the dominant metal within the system; in the other stream, iron is the dominant metal. The tracer experiments were used in conjunction with a stream solute transport model to identify and understand the transport of these 2 redox sensitive metals. In the tracer experiment involving the iron dominated system, organic acids were co-injected into the stream to access the effects of an increase in organic acids. Laboratory experiments were also performed to help elucidate the chemical mechanisms affecting manganese in the natural system. The solute transport models were calibrated using an inverse modeling approach, which provided the best-fit to the conservative solute's observations. Results indicate that manganese is dominated by oxidative removal mechanisms, which are increased in the presence of light. This increase in oxidation in the presence of light may be due to photosynthetically-catalyzed oxidation on the streambed by attached algae. The laboratory experiments indicated that manganese oxide photoreduction may be important in the natural system, but the field results suggest that photoreduction may only be affecting the solution-phase speciation of manganese. The organic acid addition experiment into the iron-dominated system indicates that the rate of solution-phase and Fe-oxide photoreduction is greatly enhanced by the addition of organic acids, presumably by forming highly photoreactive complexes with Fe(III). Hydrogen peroxide, an organic acid photoproduct, appears to be controlled by the photooxidation of organic acids and by oxidation reactions that remove H₂O₂ from the system.