Seasonal stratification in temperate lakes greater than a few meters deep provides conditions favorable to pronounced vertical zonation of redox chemistry. It often leads to seasonally anaerobic hypolimnion and profound changes in geochemical conditions. In this study, we investigated the formation of the thermocline and trace element behavior in the water column of a seasonally stratified lake. We collected two consecutive years of data from Pond B at the US Department of Energy Savannah River Site, S.C. Pond B is a man-made, monomictic reservoir that received reactor cooling water from R reactor between 1961–1964. A strong thermal stratification forms a distinct, seasonal thermocline in May and then progresses downwards through November. Trace and major elements were measured from water samples collected at different depths to demonstrate the occurrence of highly correlated profiles of arsenic, iron, aluminum, and dissolved organic matter, all of which increased in concentration by 1–2 orders of magnitude within the anaerobic hypolimnion. This sampling period captured the seasonal progression from winter mixing, through summer thermal stratification, to autumn overturn. Thermodynamic modelling indicates that Fe(III) hydroxides were likely responsible for controlling dissolved Fe and As concentrations in surface mixed layer.

Depth-Time Isomap of temperature, dissolved oxygen, total suspended solids and conductivity in pond B water column at the deepest location (average of 2 years of survey).

Dissolved redox species concentration in the deepest locations, sampled in summer and winter

Nimisha Edayilam

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