No abstract

An experiment was set up to assess the factors affecting metal mobility in five wetland soils of the Scheldt estuary at different sampling depths when subjecting the soils to various water table levels. Pore water metal concentrations were monitored for 10 months at four sampling depths (10, 30, 60 and 90 cm) upon adjusting the water table level to 0, 40 and 80 cm below the surface of the soils. Nickel (Ni) release is facilitated by reductive conditions. These reductive conditions mainly occur below the water table. The fate of chromium (Cr) under reductive conditions seems to be promoted by the presence of dissolved organic matter. However, Cr fate seems to be inconsistent between the soils, as it is affected by a series of counteracting mechanisms. Copper (Cu), zinc (Zn) and especially cadmium (Cd) are all primarily released above the water table under high salinity conditions. These elements are also released below or just above the water table when organic matter is being decomposed, resulting in calcium (Ca), manganese (Mn), Ni and/or iron (Fe) release upon CO 2 accumulation and Fe/Mn oxide reduction, without being accompanied by sulphide production. Their mobility is low under reducing conditions, i.e. in the presence of sulphides, whereas the complexation by soluble organic matter especially seems to promote Cu mobility.

We aimed to assess the presence and availability of arsenic (As) in intertidal marshes of the Scheldt estuary. Arsenic content was determined in soils sampled at 4 sampling depths in 11 marshes, together with other physicochemical characteristics. Subsequently, a greenhouse experiment was set up in which pore water arsenic (As) concentrations were measured 4 times in a 298-day period in 4 marsh soils at different sampling depths (10, 30, 60 and 90 cm) upon adjusting the water table level to 0, 40 and 80 cm below the surface of these soils. The As content in the soil varied significantly with sampling depth and location. Clay and organic matter seem to promote As accumulation in the upper soil layer (0-20 cm below the surface), whereas sulfide precipitation plays a significant role at higher sampling depths (20-100 cm below the surface). The As concentrations in the pore water of the greenhouse experiment often significantly exceeded the Flemish soil sanitation thresholds for groundwater. There were indications that As release is not only affected by the reductive dissolution of Fe/Mn oxides, but also by e.g. a direct reduction of As(V) to As(III). Below the water table, sulfide precipitation seems to lower As mobility when reducing conditions have been sufficiently established. Above the water table, sulfates and bicarbonates induce As release from the solid soil phase to the pore water.