Application of DGT technique for labile Hg species determination in urbanized surface freshwater environment (Zenne River, Brussels)

Mercury (Hg) is one of the primary health concerns in urban environments due to past and present anthropogenic emissions. Its environmental cycle is controlled by redox, organic matter and microbial reactions, which may facilitate the formation of bioaccumulative organic Hg species; methylmercury (MeHg), with damaging consequences to human and ecosystem health. Knowing the concentration of the labile (proxy for bioavailable) Hg species is particularly important to understand processes leading to enhanced exposure and bioaccumulation.

For the first time, we have investigated Hg species lability with Diffusive Gradient in Thinfilms (DGT) technique in surface water and sediment of the Zenne River (Brussels, Belgium). Our results indicate seasonal behaviour of labile Hg and MeHg species in both environmental compartments. The seasonality depends on the proximity to urbanized areas. Higher labile Hg concentrations (8.2 ng/L) in surface water in colder season was due to increased emission/deposition via incineration for transport and household heating. Higher MeHg concentration (2.3 ng/L) was observed in the beginning of warmer season, and was more pronounced away from heavily urbanized area. This suggested that although urban-released Hg is very labile, its conversion to the MeHg is likely more microbially controlled by temperature and nutrient availability and not by precursor availability. It also appears that the is a source of Hg to be dispersed and distributed in Zenne River. Sediment pore waters featured an active labile MeHg profile in the warmer season with a subsurface peak (5 ng/L), but is absent in the colder months. Sediments also appear to be a source of MeHg and THg to the water. Finally,our results indicate also the lack of data and understanding of labile Hg urban biogeochemical cycling, including the sources, spatial distribution and fate of Hg species. It is clear there is an intensive transformation occurring in the Zenne and more further work will reveal its potential for interaction andd anger it may pose to the biota.