REDOX SPECIES IN A REDUCING FJORD - EQUILIBRIUM AND KINETIC CONSIDERATIONS

In June 1977 ten redox sensitive chemical species were determined in the oxidizing and reducing waters of Saanich Inlet, British Columbia. An oxygen-hydrogen sulfide interface was found at a depth of 130 m. The concentrations of the oxidized species: oxygen, nitrate, iodate, and chromate, and the reduced constituents: hydrogen sulfide, ferrous iron, and ammonia reached background or near background levels at this depth. Three reduced constituents, manganese (II), chromium (III), and iodide existed metastably in the oxic layer. Thermodynamic calculations of pe indicate that the redox couples in the anoxic waters approach equilibrium with each other while those in the oxygenated waters are far from equilibrium. A time-dependent diffusion model is used to determine the removal rate constants for Mn2+ and Cr(III) in the linear temperature-salinity region surrounding the oxygen-hydrogen sulfide interface (130 to 115 m). In this region I- is conservative within the error of our measurements over the period of anoxia in the inlet (estimated to be 3 to 6 months). The residence times of Cr(III) and Mn2+ in the oxic layer are one to three weeks and about two days, respectively. We argue that the dominant manganese removal mechanism is by oxidation, resulting in a calculated oxidation rate constant for Mn2+ five orders of magnitude greater than published laboratory values and widely different from previous field estimates. The rapid oxidation kinetics in Saanich Inlet are probably caused by bacterial catalysis near the oxygen-hydrogen sulfide interface. © 1979.