VERTICAL-DISTRIBUTION OF ACID-VOLATILE SULFIDE AND SIMULTANEOUSLY EXTRACTED METALS IN A RECENT SEDIMENTATION AREA OF THE RIVER MEUSE IN THENETHERLANDS

The bioavailability of heavy metals in sediments may be strongly decre ased by the presence of AVS (acid-volatile sulfide). In contaminated f reshwater sediments, however, AVS levels may not be sufficiently high to scavenge all reactive heavy metals. In different seasons, the verti cal distribution of AVS and SEM (simultaneously extracted metals) was studied in sediment cores of a recent sedimentation area of the river Meuse. In the suboxic layer of the sediments, the amount of AVS is alw ays too low to bind all reactive metals as sulfides (SEM/AVS > 1). SEM /AVS decreases with depth in the sediments, going from ratios higher t han one in the surface sediments to ratios on the order of one or even less than one at greater depths. This large vertical variation in AVS levels is attributed to diagenetic processes i.e., sulfide oxidation in the suboxic top layer and sulfate reduction in the underlying anoxi c sediment layer. The depth of the boundary between the suboxic and th e anoxic layer changes seasonally. An important implication of these f indings is that SEM/AVS ratios in mixed homogenized sediment samples a re generally not suited for the assessment of potential metal toxicity of sediments.