Differentiating Particulate, Nanoparticle, and Dissolved Metal Speciation and Transport in an Acid Mine Drainage Impacted Stream

Acid mine drainage (AMD) is acidic, metal-rich water produced from the oxidative dissolution of sulfide minerals exposed to air and water during mining. In the eastern United States alone, more than 7,000 km of streams are degraded by AMD. AMD can continue to contaminate groundwater and surface water years after mining operations end, damaging affected ecosystems, and requiring costly reclamation. Metals associated with AMD can be transported through water systems as dissolved ions, nanoparticles, and larger suspended particles; however, most AMD studies either do not measure or differentiate between these phases. Yet, it is important to differentiate between metal phases as they differ in bioavailability/toxicity, transport, and sorption of trace contaminants. Here, we examined metal speciation in a stream that remains impaired by AMD despite mixing with flow from a passive limestone treatment system. The objectives of this study were to examine metal transformation as surface water mixes with contaminated groundwater and intermittent flow from the treatment system, and to determine the unique concentration-discharge behaviors of dissolved ions, nanoparticles, and particulates at the watershed outlet. Size-fractions of contaminant metals were measured in synoptic samples taken along the stream and in surface water collected from the watershed outlet during storm events. High concentrations of Fe nanoparticles were found where contaminated groundwater upwells into the stream, suggesting oxidation of ferrous iron during mixing. In contrast, Mg, Mn, Ca, K, and Na were consistently present as dissolved ions. At the watershed outlet, nanoparticulate Fe concentrations decreased relative to dissolved Fe during times of higher discharge, indicating unique concentration-discharge behaviors for the same metal depending on metal speciation. Our results indicate that hydrologic mixing in the stream generates nanoparticles and influences metal export from the watershed. This work will help address a knowledge gap regarding the fundamental understanding of metal transport though streams and has implications for policy regarding the effectiveness of AMD treatment systems.