Exchange of Inorganic Phosphate between River Waters and Bed-Sediments

The kinetics of the release of Soluble Reactive Phosphorus (SRP) in oxic and anoxic conditions and uptake in oxic conditions by contaminated river sediments (River Blackwater in Southern England) were measured using a fluvarium channel operated to mimic environmental conditions. Release rates (from 1 to 10 nmol m-2 s-1) and uptake rates were modeled successfully using a Parabolic equation and Diffuse Boundary Layer model. A SRP release experiment over 61 days showed that large gradients in SRP developed in the porewater as a result of diffusion, sorption, and in-situ generation of SRP in the anoxic zone. This was modeled using a new Triple Zone Model that incorporated diffusion through a liquid boundary layer and sorption/desorption in oxic and anoxic zones. The results highlighted the importance of the oxic zone in controlling the exchange of SRP between the sediment and water column. The model was also applied to explain why the Equilibrium Phosphate Concentration (EPC0) of the sediment measured in oxic conditions was constant (and equal to the value calculated from sorption isotherm measurements) during 2-day release experiments, and also why it increased in the uptake experiments. Measurements in anoxic conditions showed the importance of the sediment temperature in controlling the flux at the interface.