The rates of chemical oxidation and gas transfer were determined for two passive abiotic systems for the treatment of mine drainage. One system (HB) is a series of ponds whose discharge was net acidic. The second treatment system (CK) has shallow channels and net alkaline discharge. The O2 mass transfer coefficient was 2 cm h−1 for HB Pond #2 and ranged from 4 to 40 cm h−1 for the CK channels, depending on channel velocity and depth. The CO2 mass transfer coefficient was 1.3 cm h−1 for HB Pond #2 and ranged from 1.4 to 15 cm h−1 for the CK channels. Oxygen transfer appeared to be the rate-limiting step for oxidation of Fe(II) at HB Pond #2. The oxidation rate for Fe(II) adsorbed to already-formed ferric oxides (heterogeneous oxidation) appeared to control the rate of oxidation of Fe(II) at the CK channels. Based on field data, the heterogeneous rate constant for oxidation of Fe(II) was 6.0×10−9 mg l−1 s−1 at 17°C or k2 of 5.5×10−8 mg l−1 s−1 at 25°C. This value was consistent with previously reported values for k2 based on laboratory experiments. The annual-averaged iron removal rates were 18 g d−1 m−2 for HB and 42 g d−1 m−2 for CK, compared to empirical design expectations of 10 to 20 g d−1 m−2. Heterogeneous oxidation accounted for >80 of the oxidation of Fe(II) at HB and >90% of the oxidation of Fe(II) at CK.