SREL Reprint #3821

Effect of seasonal anoxia on geochemical cycling in a stratified pond: Comparison to cooler pond conditions 40 years ago

Fanny Coutelot1,2, Daniel I. Kaplan3, Annie B. Kersting4, Mavrik Zavarin4, and Brian A. Powell1,2,5

1Department of Environmental Engineering and Earth Sciences, Clemson University,
Anderson, SC 29625, United States
2Center for Nuclear Environmental Engineering and Science and Radioactive Waste Management,
Clemson University, Anderson, SC 29625, United States
3Savannah River Ecology Laboratory, University of Georgia, Aiken, SC 29803, United States
4Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, CA 94550, United States
5Savannah River National Laboratory, Aiken, SC 29808, United States

Abstract: Seasonal stratification in temperate lakes deeper than a few meters creates favorable conditions for pronounced vertical redox zones, often resulting in anaerobic hypolimnions and significant geochemical changes. This study examined thermocline formation and trace element behavior in a seasonally stratified pond amid rising air temperatures. Over two years, data were collected from Pond B at the US Department of Energy Savannah River Site in Aiken, South Carolina. Pond B, a man-made monomictic reservoir, received cooling water from a nuclear reactor from 1961 to 1964. Strong thermal stratification forms a distinct thermocline in May and progresses downward until November. Compared to the 1980s, this study shows a delayed onset and extended duration of stratification. The prolonged summer stratification reduces deep water oxygen replenishment, extending hypoxic conditions. Trace and major elements sampled in the water column revealed strong correlations between As, Fe, and Mn profiles, with concentrations increasing by 1–2 orders of magnitude in the anaerobic hypolimnion. This period captured the seasonal transition from winter mixing to summer stratification to fall overturn. Under anoxic conditions, Fe(III) reduces to Fe(II) in the sediment, releasing dissolved iron into the water column. The extended anoxic periods likely promoted arsenic release from sediments. Prolonged anoxia may enhance arsenic mobilization and solubility in the lake. This study illustrates how climate-induced changes in seasonal stratification of contaminated waters can convert contaminant sinks into sources, offering insights into the cycling of arsenic and other dissolved ions in stratified lakes and their implications for water quality management.

Keywords: Seasonal stratification; Monomictic lake; Arsenic cycling; Iron cycling; Manganese cycling; Anoxic conditions; Climate change impacts; Thermocline; Geochemistry; Trace elements

SREL Reprint #3821

Coutelot, F., D. I. Kaplan, A. B. Kersting, M. Zavarin, and B. A. Powell. 2025. Effect of seasonal anoxia on geochemical cycling in a stratified pond: Comparison to cooler pond conditions 40 years ago. Science of the Total Environment 976(179337).

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).