Water, Vol. 17, Pages 2931: Chemodynamics of Mercury (Hg) in a Southern Reservoir Lake (Cane Creek Lake, Cookeville, TN, USA): II—Estimation of the Hg Water/Air Exchange Coefficient Using the Two-Thin Film Model and Field-Measured Data of Hg Water/Air Exchange and Dissolved Gaseous Hg

Water doi: 10.3390/w17202931

Authors:
Hong Zhang
Lesta S. Fletcher
William C. Crocker

This paper reports a novel effort to estimate and evaluate the coefficients of Hg transfer across the water/air interface in lakes such as Cane Creek Lake (CCL, Cookeville, TN, USA). This was accomplished by calculating the coefficients (kw) using the Two-Thin Film (TTF) Model for Hg transfer together with the field-measured data of Hg emission flux (F), dissolved gaseous mercury concentration (DGM), air Hg concentration (Ca), and water temperature for Henry’s coefficient (KH) obtained from a separate field study at the CCL. The daily mean kw values range from 0.045 to 0.21 m h−1, with the min. at 0.0025–0.14 and the max. at 0.079–0.41 m h−1, generally higher for the summer, and from 0.0092 to 0.15, with the min. at 0.0032–0.033 and the max. at 0.017–0.31 m h−1, generally lower for the fall and winter, exhibiting an apparent seasonal trend. The highest kw values occur in August (mean: 0.21, max.: 0.41 m h−1). Our kw results add to and enrich the aquatic interfacial Hg transfer coefficient database and provide an alternative avenue to evaluate and select the coefficients for the TTF Model’s application. The kw results are of value in gaining insights into the Hg transfer actually occurring across the water/air interface under environmental influences (e.g., wind/wave, solar radiation). Our kw results do not show a clear, consistent correlation of kw with wind/wave effect, nor sunlight effect, in spite of some correlations in sporadic cases. Generally, the kw values do not exbibit the trends prescribed by the model sensitivity study. The comparisons of our kw results with those obtained using wind-based transfer models (the Liss/Merlivat Model, the Wanninkhof Model, and the modified linear model) show that they depart from each other. The findings of this study indicate that the TTF Model has limitations and weaknesses. One major assumption of the TTF Model is the equilibrium of the Hg distribution between the air and water films across the water/air interface. The predominant oversaturation of DGM shown by our DGM data evidently challenges this assumption. This study suggests that aquatic interfacial Hg transfer is considerably more complicated, involving a group of factors, more than just wind and wave.

Source link

Hong Zhang www.mdpi.com