Methyl mercury production and distribution in river water-sediment systemsinvestigated through radiochemical techniques

The toxicological consequences of Hg releases to the environment are largel y governed by the conversion of inorganic Hg to the most toxic methylmercur y (MeHg), that is biomagnified through aquatic food chains. To gain further insight on the biological and physico-chemical factors controlling MeHg pr oduction and distribution among freshwater sediments and water, we used a s ensitive and specific radiochemical procedure, developed at the National In stitute for Minamata Disease. Systems containing Hg-203(2+)-spiked sediment cores (0.7 mug total Hg g(-1) d.w.) and overlying water, both from a prist ine mountain stream in Southern Japan, were incubated for 21-38 days in dif ferent conditions. Inorganic Hg and MeHg in sediment and water were extract ed in dithizone-benzene and measured after separation by thin-layer chromat ography. The conversion of added Hg to MeHg was 3.0 to 13.7% in sediments, with a tendency for higher proportions in the top layers. Surprisingly, mor e MeHg was found in the sediment (11.3%) and water (66.5%) of a system bubb led with air than in one bubbled with nitrogen (4.2 and 44.1%). Artificiall y increased levels of bioturbation reduced by half the MeHg concentrations and % of added total Hg in sediment and water. In all systems, 55-68% of to tal Hg and MeHg in water were associated to suspended particles >1 mum. MeH g bioaccumulation factors (BFs) from water ranged 270-8100 and from sedimen t, 0.2-5.7 (wet weight basis). BFs in relation to water where 3 times highe r for MeHg than for total Hg.