Salinity and total suspended solids control mercury speciation in a tidal river: Comparisons with a photochemical mercury model.

Daytime volatilization of gaseous elemental mercury (Hg(0)) is a significant mechanism for mercury removal from aquatic systems and potentially limits the production and bioaccumulation of methylmercury. Changes in incoming solar radiation (in the ultraviolet range), dissolved organic matter, salinity, and total suspended particles were investigated concurrently with several mercury species (Hg(0), dissolved total mercury (THg), easily reducible mercury (ERM), and mercury associated with total suspended solids (THg)) during daylight hours near the mouth of a hypertidal river. There were no predictable temporal changes observed for Hg(0) in unfiltered surface water. Hg(0) ranged from 0 to 12 pg L, THg ranged from 0 to 492 pg L, ERM ranged from 13 to 381 pg L, and THg ranged from <1.58 ng g to 261.32 ng g. The range of Hg(0) predicted by the empirical model was similar to measured ERM concentrations, but it was shown that ERM did not significantly predict in-situ photoreducible Hg(II) (Hg(II)). Production of Hg(0) appears to largely be suppressed by suspended solids, which limits ultraviolet radiation transmission through surface water. Comparison of these results to an empirical model developed for this site to predict Hg(0) indicates that significantly more mercury is available for photoreduction near the mouth of the tidal river, and that Hg(II) will likely photoreduce quickly when TSS levels decrease with ocean mixing.