Assessing iron dynamics in the release from a stratified reservoir

Abstract

Field and laboratory studies were conducted to describe the fate of total, dissolved, and ferrous (Fe 2+ ) iron in the release from a stratified reservoir with an anoxic hypolimnion. Concentrations of total iron in the tailwater indicated a first order removal process during a low flow release (0.6 m 3 sec −1 ), yet negligible loss was observed during a period of increased discharge (2.8 m 3 sec −1 ). Dissolved and ferrous iron concentrations in the tailwater were highly variable during both release regimes and did not follow responses based on theoretical predictions. Ferrous iron concentrations in unfiltered samples were consistently greater than concentrations observed in samples filtered separately through 0.4, 0.2, and 0.1 μm filters. Total iron removal in laboratory studies followed first order kinetics, but was twice that rate (0.077 mg L −1 hr −1 ) observed during low flow discharge in the tailwater (0.036 mg L −1 hr −1 ). Dissolved and ferrous iron losses in laboratory studies were rapid (~75% in the first 15 minutes and 95% within 1 hour), followed theoretical predictions, and were much faster than observations in the tailwater (~30% within the first hour). The presence of particulate forms of ferrous iron in the field and differences in removal rates observed in field and laboratory studies indicate a need for improved field assessment techniques and consideration of complexation reactions when assessing the dynamics of iron in reservoir releases and downstream impacts as a result of operation regimes.