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PROGRESSIVE IRON REMOVAL WITHIN THE INITIAL OXIDATION CELL OF A PASSIVE TREATMENT SYSTEM 1

Iron oxidation, hydrolysis and settling are key processes promoted in passive treatment system to remove iron from influent acid mine drainage (AMD). For net alkaline mine waters, an initial oxidation cell is typically used to remove and store large amounts of precipitated iron oxyhydroxides prior to water flowing through additional wetland treatment cells. Regular monitoring of influent and effluent is suitable to determine overall decreases in iron concentrations due to treatment. However, a detailed analysis of iron removal within specific zones of the oxidation cell provides additional information about performance and functionality. The purpose of this study was to investigate progressive iron removal with increasing distance from the influent AMD discharge and depth in a U-shaped oxidation pond receiving ferruginous lead-zinc mine drainage at the Tar Creek Superfund Site. It was hypothesized that iron concentration changes would be greatest in the shallow waters nearest the mine water inflows. Three catwalk structures were used with a discrete horizontal sampler to collect samples along depth profiles at progressively greater distances through the treatment system. In-situ measurements included pH, temperature, specific conductance, dissolved oxygen, and oxidation-reduction potential; turbidity and total alkalinity were determined immediately upon collection. Water samples were collected for laboratory determination of total and dissolved metals (EPA methods 3050 and 6010), and sulfate (EPA method 300) concentrations. The oxidation pond had a mean iron mass loading of 106 kg/day and an average removal rate of 25 g m -2 day -1 based on one year of system operation. The majority of iron removal in Cell 1 occurs within the first section of the cell (82.2% total iron removal) with only an additional 5.4% removal in sections 2 and 3. Precipitated iron oxyhydroxides undergo sedimentation in all three sections of the oxidation pond rather than only in sections 1 and 2 as predicted. Finally, Changes in water quality measurements with increasing depth may have implications on long-term storage of iron precipitates within the system. Additional