Photodegradation of methylmercury in the water body of the Three Gorges Reservoir

Abstract

Biogeochemical cycling of mercury in the young Three Gorges Reservoir (TGR), China, is strongly considered. Although methylmercury (MMHg) photodegradation (PD) is an important process involved in mercury cycling in this zone, little is known about this process. In situ incubation experiments were therefore performed to quantify the effect of different wavelength radiations and environmental factors on the PD process of MMHg in the water bodies of TGR. It was found that the effect of solar radiation on MMHg PD was highly dependent on wavelength and water depth. All PD-rate constants resulting from each wavelength range decreased rapidly with water depth. For surface water, UV-A radiation (320–400 nm) was the key driver, accounting for 49%–62% of MMHg PD. For the entire water column, both photosynthetically active radiation (PAR, 400–700 nm) and UV-A were responsible for MMHg PD. MMHg PD fluxes peaked in summer (7.5−18 ng m⁻² d⁻¹), followed by spring (3.3–8.0 ng m⁻² d⁻¹), autumn (1.0–2.7 ng m⁻² d⁻¹), and winter (0.060–0.15 ng m⁻² d⁻¹). The annual fluxes of MMHg PD were estimated to be 1.1–2.8 μg m⁻² a⁻¹. Filtering the reservoir water and amending it with chemicals (i.e., Cl⁻, NO ⁻₃ , and dissolved organic matter (DOM)) showed significant effects on MMHg PD rate constants. Stepwise regression analysis showed that intensity of solar radiation, suspended particulate matter (SPM), DOM, Cl⁻, and NO ⁻₃ were involved in the PD process. Path analysis clarified the relationship between MMHg PD rate constants and environmental variables, as well as the comparative strength of direct and indirect relationships among variables. The results are of great importance for understanding MMHg cycling characteristics in TGR and also facilitate the understanding of the underlying process, MMHg PD, in natural waters.