Lower Red Eyes is an
acid mine drainage site in Pennsylvania where low-pH Fe(II) oxidation has created a large, terraced
iron mound downstream of an anoxic, acidic,
metal-rich spring. Aqueous chemistry,
mineral precipitates, microbial communities, and laboratory-based Fe(II) oxidation rates for this site were analyzed in the context of a depositional
facies model. Depositional
facies were defined as pools, terraces, or microterracettes based on cm-scale sediment morphology, irrespective of the distance downstream from the spring. The sediments were composed entirely of Fe precipitates and cemented organic matter. The Fe precipitates were identified as
schwertmannite at all locations, regardless of
facies. Microbial composition was studied with fluorescence in situ hybridization (FISH) and transitioned from a microaerophilic, Euglena-dominated community at the spring, to a Betaproteobacteria (primarily Ferrovum spp.)-dominated community at the upstream end of the
iron mound, to a Gammaproteobacteria (primarily Acidithiobacillus)-dominated community at the downstream end of the
iron mound. Microbial community structure was more strongly correlated with pH and geochemical conditions than depositional
facies. Intact pieces of terrace and pool sediments from upstream and downstream locations were used in flowthrough laboratory reactors to measure the rate and extent of low-pH Fe(II) oxidation. No change in Fe(II) concentration was observed with (60)Co-irradiated sediments or with no-sediment controls, indicating that abiotic Fe(II) oxidation was negligible. Upstream sediments attained lower effluent Fe(II) concentrations compared to downstream sediments, regardless of depositional
facies.