Assessment of the risk of phosphorus loading due to resuspended sediment

Resuspension is a multiphase phenomenon where suspended solids encounter wa ter layers differing in physico-chemical properties that affect the reactio ns of phosphorus (P). The role of resuspended sediment as a sink or source of dissolved P was determined in a laboratory study of P desorption-sorptio n equilibria. Gradual mixing was simulated using decreasing solid concentra tions and varying environmental conditions (pH, redox, ionic strength). To describe the P exchange when the particles encounter dissimilar water layer s, the extent of P sorption to or desorption from solids was expressed as a function of P concentration in the bath solutions. The equilibrium phospho rus concentration (EPC), at which there is no net P release from or retenti on to the particles, proved to be a suitable parameter for assessment of P load risk. Under oxic conditions at pH 7, commonly prevailing in lakes, the EPC values ranged from 11 to 27 mug P L-1. The larger the water volume the suspended material was mixed with, the higher the P concentration, allowin g desorption to occur. As for chemical factors affecting P mobilization, EP C followed the order: pH 7 < pH 7 anoxic < pH 9. A separate extraction expe riment revealed that elevated pH enhanced P mobilization more as the concen tration of solids decresed. The results demonstrate that high pH (a common characteristic in eutrophic lakes during summer), when linked with intensiv e resuspension, may markedly increase the internal P loading risk. As for t he risk assessment, the quantification of the internal P loading would be i mproved by isotherm studies combined with field observations.