Hydrogeochemical Processes Limiting Aqueous and Colloidal Fe Export in a Headwater Stream Impaired by Acid Mine Drainage

Contaminant metals
derived from acid mine drainage (AMD) are transported
through stream networks in aqueous, colloidal, and/or particulate
phases; however, hydrogeochemical processes that regulate export of
these phases from headwater catchments are not fully resolved. We
investigated metal speciation and transport along redox and pH gradients
and as a function of discharge in an AMD-impaired stream. Contaminated
groundwater upwelling into the stream mixed first with oxygenated
surface water and then with alkaline effluent from a treatment system.
Contaminant Fe was effectively removed from the stream as Fe2+ oxidation generated Fe­(III)-bearing colloids that rapidly aggregated
and accumulated in streambed sediments. Iron precipitated first as
Fe­(III) (oxyhydr)­oxides but then as oxyhydroxysulfates as Fe hydrolysis
lowered stream pH. Smaller amounts of Fe2+ were incorporated
into framboidal pyrite in oxygen-poor sediments. Conversely, AMD-derived
Mn2+ and Al3+ were only minimally removed from
the stream during seasonal mixing with treated effluent. Although
natural attenuation limited Fe export from the watershed, Fe stored
in stream sediments has the potential to be remobilized due to oxidative
dissolution of Fe-sulfides and/or physical scouring of the streambed.
This work demonstrates how colloid formation and mobility respond
to geochemical gradients, with implications for how sediment–water
interactions influence metal transport through streams.