Chemodynamics of Mercury (Hg) in a Southern Reservoir Lake (Cane Creek Lake, Cookeville, TN, USA): I-Estimation of the Kinetics of Photochemical Reduction of Aquatic Hg(II) Using Field-Measured Data of Hg Water/Air Exchange and Dissolved Gaseous Hg

An alternative, independent estimation of the kinetics of aquatic Hg(II) photochemical reduction featuring dissolved gaseous mercury (DGM) emission from water in consideration was obtained by using a mass balance box model. An interactive Excel spreadsheet was constructed to implement the model equations to yield the rate constants and the rates of the Hg(II) photoreduction. The model calculations used field-measured data of DGM paired with its emission flux coupled with the corresponding field sampling times. This data set came from a previous, separate, year-long field study conducted at a southern reservoir lake (Cane Creek Lake, Cookeville, Putnam County, TN). The mean value of the model-calculated rate constants (k(DGM)) of the Hg(II) photoreduction for the warm season (June-August) (4.5 fM h(-1)/pg L-1) is higher than that for the cold season (October-January) (2.2 fM h(-1)/pg L-1). The rate constants were found to be the highest (22.5 fM h(-1)/pg L-1) in August whereas the lowest (0.03 fM h(-1)/pg L-1) in January. The model-calculated rate constants are clearly higher in value than but comparable in order of magnitude to the published kinetic data. The model-calculated rates (r(DGM)) of the Hg(II) photoreduction are significantly higher, by one order of magnitude (10(2) vs. 10(1)) than the apparent rates calculated using the same field DGM data without consideration of the Hg emission from the water. A sensitivity analysis of the model parameters points to a high sensitivity of Hg emission flux to the rate constant under modeled realistic environmental conditions. The initial Hg(II) concentration is also a sensitive model parameter under certain conditions. The results of our model study support the conclusion that DGM emission from water has a strong impact on the kinetics of aquatic Hg(II) photoreduction and the model calculation can provide an independent, valuable approach for estimating the kinetics of aquatic Hg(II) photoreduction.

Automated summary

An alternative method for estimating the kinetics of Hg(II) photoreduction in water was developed using a mass balance box model that took into account the emission of dissolved gaseous mercury (DGM) from water. The model equations were implemented in an interactive Excel spreadsheet to calculate the rate constants and rates of Hg(II) photoreduction. The model calculations used field-measured data of DGM and its emission flux, obtained from a previous year-long field study conducted at Cane Creek Lake in Tennessee. The results showed that the rate constants and rates of Hg(II) photoreduction varied with the seasons, with higher values in the warm season compared to the cold season. The model calculations also indicated that DGM emission had a significant impact on the kinetics of aquatic Hg(II) photoreduction and that the model approach could provide valuable independent estimations.