Iron (Fe) is a geogenic, redox-active metal that can degrade water quality when accumulated in drinking water reservoirs during seasonal stratification. In water supplies with elevated Fe²⁺, water quality managers are increasingly using hypolimnetic oxygenation (HOx) to promote Fe oxidation and precipitation of Fe minerals. Many studies confirm HOx effectiveness, but few investigate how oxygenation, and other water column conditions (pH, Fe²⁺, dissolved oxygen), may affect Fe mineralogy. Rapid oxidation results in poorly crystalline Fe oxyhydroxides via homogeneous Fe oxidation (oxidation by O₂), but heterogeneous Fe oxidation (oxidation by electron transfer between Fe²⁺ and Fe³⁺) may transform them to more stable Fe oxides, like goethite and lepidocrocite. We hypothesized that HOx would decrease Fe²⁺ and thus change the relative proportions of the heterogeneous Fe oxidation products (goethite and lepidocrocite). To test these hypotheses, we collected solids fortnightly that precipitated in two freshwater reservoirs - one with HOx and one without. Additionally, we compared samples from when HOx was operating at a limited capacity vs fully operational. We analyzed solids using two methods: acid digestion coupled with ICP-MS to quantify total Fe mass and X-ray total scattering coupled with pair distribution function analysis to determine mineralogic abundance. Pair distribution function data were analyzed using linear combination fitting to determine the relative mineral fractions of the different solids. Additionally, we analyzed water samples for total and soluble Fe concentrations with ICP-MS. Our results suggest that differing Fe²⁺ in each reservoir resulted in different relative amounts of ferrihydrite, goethite, and lepidocrocite in the solids. Low dissolved oxygen and low Fe²⁺ in the reservoir with HOx resulted in higher proportions of lepidocrocite, whereas low dissolved oxygen and high Fe²⁺ in the control reservoir resulted in similar proportions of goethite and lepidocrocite. Additionally, the relative mineral proportions are less variable when HOx is at full capacity. Given the differing stabilities of these minerals, heterogeneous Fe oxidation could impact the long-term stability of HOx-produced Fe oxides against reductive dissolution upon burial in the sediment. Ultimately, our results suggest that both homogeneous and heterogeneous oxidation mechanisms contribute to Fe removal during artificial oxygenation of drinking water reservoirs.

Cecelia Wood

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