Search

Acid mine drainage (AMD) is acidic, metal-rich water produced from the oxidative dissolution of sulfide minerals exposed to air and water during mining. 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 streams as dissolved ions, colloids, 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 (1) examine metal transformations as surface water mixed with contaminated groundwater and intermittent flow from the treatment system, and (2) determine the unique concentration-discharge behaviors of dissolved ions, colloids, and particles at the watershed outlet. Size-fractions of contaminant metals were measured along the stream during three different synoptic sampling campaigns and in surface water collected from the watershed outlet once a month from March to December of 2018 and during storm events. High concentrations of dissolved and colloidal Fe were present where contaminated groundwater upwelled into the stream. Dissolved and colloidal Fe concentrations in the stream then decreased with increasing distance from the upwelling zone, and we infer that oxidation of ferrous iron and aggregation of colloidal Fe removed Fe as contaminated groundwater mixed with oxygenated surface water. In contrast, Mg, Mn, Ca, K, Na, and Si were consistently present as dissolved ions. At the watershed outlet, unique concentration-discharge behaviors were observed for different Fe species. Dissolved Fe concentrations decreased during times of higher discharge, colloidal Fe concentrations displayed no clear trend with changes in discharge, and suspended particles (likely composed of Fe-oxides) increased with increasing discharge. Our results indicate that hydrologic mixing in the stream generates Fe colloids and larger Fe-oxide aggregates that eventually settle out of the water column and become a part of streambed sediments, ultimately influencing metal export from the watershed. This work will help address a knowledge gap regarding the fundamental understanding of metal transport through streams and has implications for policy regarding the effectiveness of AMD treatment systems.