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Abstract Title: Preliminary Assessment of Effects of Hypolimnetic Oxygenation on Mercury Cycling in Hodges Reservoir, California, USA
Presenter Name: Marc Beutel
Company/Institution: University of California Merced
Session: Mercury Treatment / Abatement
Co-Authors: Marc Beutel

Abstract Information :

Hodges Reservoir is a hypereutrophic water supply reservoir in northern San Diego, USA with a typical maximum depth of around 22 m, mean depth of 8 m and surface area of 6 km2. Poor water quality has limited use of the reservoir to serve as a regional source of potable water. In addition, the reservoir is listed as impaired for mercury in fish tissue, as are many lakes and reservoirs in the state. In the fall of 2020, the City of San Diego Public Utilities Department installed a 6-metric-ton-per-day hypolimnetic oxygenation system using pure oxygen gas to enhance bottom water dissolved oxygen in Hodges Reservoir, aiming to improve water treatability and lower internal nutrient loading. Intensive water quality monitoring from 2017-2019 in coordination and consultation with the City of San Diego Public Utilities Department revealed extreme hypolimnetic accumulation of ammonia and phosphate, which fueled summertime algal productivity in this modestly deep reservoir. Bottom waters also accumulated high levels of manganese (Mn), which was also found to increase in surface waters, indicating active internal loading of dissolved compounds from the hypolimnion to surface waters. Iron (Fe) accumulation in bottom waters appeared to be limited due to the precipitation of Fe-sulfide. Anoxic bottom waters also accumulated toxic methylmercury (MeHg) coincidental with Mn accumulation. There was also a hot moment of MeHg bioaccumulation in seston and zooplankton in the spring when mildly reduced conditions prevailed. MeHg levels in water were lower later in the season coincidental with elevated sulfide levels in bottom waters. Recent water quality data collected after oxygenation in 2021 shows a dramatic decrease in bottom water Mn (from peak of ~1 mg/L to < 0.1 mg/L) and MeHg (from peak of ~1.0 to < 0.05 ng/L). Fe in bottom waters increased slightly while turbidity decreased substantially, which was attributed to a lack of sulfide and associated Fe-sulfide precipitation. Additional water analyses for nutrients in water and mercury in biota in 2021 are pending, but they will be included in this compelling study of how bottom water oxygenation may suppress mercury bioaccumulation, in addition to decreasing internal loading of nutrients and redox sensitive metals (e.g., Mn and Fe) that complete potable water treatment.

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