GEOCHEMICAL MODELING OF FE(II) OXIDATION RATE IN A WATERSHED SEVERELY AFFECTED BY COAL-MINE DRAINAGE, PENNSYLVANIA, U.S.A

In the West Branch Schuylkill River, which drains a 61.8-km²watershed in eastern Pennsylvania, base flow consists mainly of coal-mine drainage (CMD) from two large outfalls. During 2005–12, the CMD had elevated concentrations of sulfate (190–450 mg/L) and dissolved iron (2.8–21 mg/L) but was net alkaline with slightly acidic to circumneutral pH (5.3–7.3). Downstream of the CMD outfalls, pH and sulfate concentrations decreased with increased flow because of dilution by low-ionic-strength, low-pH runoff. However, dissolved iron concentrations in the river did not vary with flow, indicating control by other processes.

Dissolved iron in the river is attenuated by the gradual oxidation of Fe(II) and consequent precipitation of hydrous Fe(III) oxides. Evaluation of a 5-km reach downstream of the CMD outfalls over a range of flow conditions indicated decreased attenuation of iron with decreased transport time, decreased temperature, and decreased pH. These trends are consistent with kinetic control of Fe(II) oxidation as described by the Singer-Stumm homogeneous rate model:

-d[Fe(II)]/dt = k_H·[O₂]/[H⁺]²·[Fe(II)]

where at pH > 5 and 20 °C, k_H = 3 x 10^-12 mol/L/min. Along this reach, pH increased by as much as 1 unit and dissolved iron concentration decreased by 30 to 95 percent because of CO₂ degassing and Fe(II) oxidation, respectively. Kinetic modeling with PHREEQC was used to evaluate interdependent changes in dissolved CO₂ (degassing), pH, and Fe(II) oxidation rate downstream of the CMD outfalls. After temperature correction, the Fe(II) oxidation rate indicated by the PHREEQC models was 1 to 1.3 times the reference value of k_H = 3 x 10^-12mol/L/min.

Given kinetically controlled, pH-dependent iron attenuation, elevated concentrations of dissolved CO₂ and expected decreases in pH (< 6.4) during high-flow events should be considered in treatment strategies to remove iron from CMD. For aerobic treatment of marginally net alkaline CMD: (1) mechanical aeration may be incorporated to degas CO₂, thereby increasing pH and the rates of iron attenuation; and (2) during high-flow conditions, a supplemental source of alkalinity may be needed to maintain or increase pH and/or additional storage capacity may be needed to increase hydraulic retention time.