AN INVESTIGATION OF THE EFFECTS OF WATER VELOCITY ON INORGANIC PHOSPHORUS INFLUX TO A SEDIMENT

Measurements of the net influx of soluble reactive phosphorus (SRP), to a river bed-sediment, illustrate the importance of the water velocity and hydrodynamics in controlling the transfer rates. Experiments are reported using a characterised bed-sediment, with associated fauna, contained in a flowing-water channel. The results show a systematic increase in the net influx of SRP with increasing water velocity. A mathematical description of the influx was sought by modelling the experimental results using the Elovich equation, a boundary-layer model and a parabolic rate equation. In fact all three kinetic equations produce a good representation of the experimental data and it is concluded that further research is needed, in well-defined hydrodynamic conditions, to distinguish between the boundary-layer model and the parabolic equation, The boundary-layer model leads to an inverse relationship between the boundary-layer thickness (z/mu m), and the water velocity (v/cm s(-1), viz z approximate to 2500/v). In comparison, the parabolic equation of the form: influx of SRP (mu mol m(-2)) = k(p) [SRP-EPC(0)](2), where EPC(0) is the concentration at which the influx is zero prior to the sorption of phosphorus by the sediment and k(p) is the rate constant which leads to a velocity dependence, k(p)* = 0.714v + 1 where k(p)* is the reduced rate constant, k(p)* = k(p)(v)/k(p)(0). The semi-empirical Elovich equation in the form: influx of SRP (mu mol m(-2)) = (1/b) ln(1 + abt) where a and b are the Elovich parameters and t the time, gives a convenient description of the net influx of SRP to bed-sediments downstream of a point-source of pollution. The parameters calculated from the I results obtained from the experimental channel are used to estimate the SRP flux to the sediment for a distance of up to 5 km downstream of a point-input of SRP.