Subsurface particulate deposit

Metal release from tidal Elbe river sediments by a process of oxidative remobilization has been described by Kersten (1989) (Figure 8.2). Short (30-cm) sediment cores were taken from a site, where diurnal inundation of the fine-grained fluvial deposits take place. In the upper part of the sediment column, total particulate cadmium content was 10 mg kg whereas in the deeper anoxic zone the total particulate concentration of Cd was 20 mg kg h Sequential extractions indicate that in the anoxic zone 60 -80% of the Cd was associated with the sulfidic/organic fraction. In the upper (oxic and transition) zone, the association of Cd in the carbonatic and exchangeable fractions simultaneously increase up to 40% of total Cd. This distribution suggests that the release of metals from particulate phases into the pore water and further transfer into biota is controlled by the frequent downward flux of oxygenated surface water. From the observed concentrations, it would be expected that long-term transfer of up to 50% of the Cd from the sediment subsurface would take place either into the anoxic zone located further below the sediment-water interface or released into the open water. 

As discussed earlier, most of the phosphorus entering wetlands accumulates within the system. Surface soils in nutrient-impacted wetlands are often enriched as a result of recent accumulation, decomposition processes, and remobilization of phosphorus from subsurface soils to surface through plant uptake and deposition as detritus material. Thus, total phosphorus content of surface soils is higher than that of subsurface soils. Similar total phosphorus profiles have been seen for many wetlands and aquatic systems. In the impacted site, subsurface total phosphorus content can also represent the background levels of phosphorus for these soils, assuming that the surface material is the result of recent accumulation. Much of this phosphorus accumulation is due to organic matter accretion (detrital matter deposition) associated with phosphorus sorption to particulate matter. 

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