MERCURY SPECIATION IN FLOODPLAIN SOILS AND SEDIMENTS ALONG A CONTAMINATED RIVER TRANSECT

A novel mercury-specific sequential extraction procedure (SEP) for the assessment of mercury (Hg) speciation in soils and sediments, with em phasis on studying the interaction between Hg and organic matter (OM), was developed and tested. It was applied to determine Hg speciation i n floodplain topsoils and surface sediments dong the Hg-contaminated p art of the river Elbe, and to simultaneously derive some information o n the (re)mobilization potentials for Hg from these matrices. The majo rity of the total Hg in the ecosystem today is bound in the floodplain s, which also still geographically reflect the historic emission recor d. Most of the Hg in both matrices is bound strongly to OM, suggesting low availability. However, distinct differences between Hg speciation in the floodplain soils and sediments were also discovered. Mercury d eposited in the floodplains shows speciation patterns that indicate st ronger fixation compared with Hg in the sediments. This difference is attributed to the association of Hg with larger quantities of OM, whic h presumably also has higher molecular weight (MW). By comparison, Hg in the sediments was distributed among weaker binding forms, which are more likely to liberate Hg. Particularly, sediments showed a total la ck of sulfidic binding forms for Hg. Pronounced geographical trends we re detected in the Hg speciation along the river transect, with a gene ral downstream shift from weaker to stronger binding forms, probably d ue to increased association with OM. These studies indicate that Hg sp eciation in riverine ecosystems is dynamic and reflects the chemical m echanisms underlying (bio)geochemical processes like distribution and transport.