The Hughes Borehole is located in the bituminous coal region of southcentral Pennsylvania and drains an underground mine complex of ca. 2,950 ha. The borehole discharge averages approximately 63 L/s of acidic (pH 4, 230 mg/L acidity) mine drainage that contains elevated concentrations of Fe(II) (100 mg/L) and numerous trace metals. Long-term monitoring of the site showed that biological Fe(II) oxidation occurred without human intervention and produced a 0.6 ha iron mound up to 2 m deep. On-mound channel reactors along with laboratory-scale "gutter" reactors were constructed to determine conditions that can exploit biological Fe(II) oxidation for passive treatment. Dissolved Fe(II) was much more efficiently oxidized from gutter reactors that contained iron mound sediment than ones without any sediment. Residence times of 5-10 h were shown to oxidize ~100% of dissolved influent Fe(II) and remove 75% of dissolved total Fe. The Lower Red Eyes discharge (pH 4, 1,420 mg/L acidity, 540 mg/L Fe(II)) is also located in south-central Pennsylvania and is unique in that it emerges in the middle of a state forest and produces a spectacular series of iron terraces without ever being "captured" by any adjacent stream. Similar laboratory-scale experiments were conducted with intact sediment "chunks" to measure the kinetics of Fe(II) oxidation at various locations on the iron mound. Residence times of 10 h were shown to oxidize 55% of dissolved influent Fe(II) and remove 27% of dissolved total Fe. Rates of Fe(II) oxidation were not strongly correlated to dissolved oxygen but were affected by the water column height overlying the sediment chunk. While the water chemistry was distinctly different at these two sites, both showed that Fe(II) oxidation was strongly dependent on hydraulic residence time. Therefore, a consistent design approach may be feasible for sites that wish to exploit low-pH Fe(II) oxidation for treatment.