Geochemical cycling of phosphorus in rivers

Abstract

Concentrations of contaminants in fresh waters are controlled, not only by point and diffuse sources within the catchment, but by a multitude of biogeochemical reactions (riverine processes) that may occur in the water column during transport. As an example, the results of mass-balance studies using “snapshot” field sampling techniques for P transport in a large river in Northern England (R. Swale) are summarised to identify the processes that contribute to internal cycling within the river, and their relative size compared with the total export of P. The interaction of P with sediments is an important factor controlling concentrations of soluble P in the water column, during both low and high river-flows, with in-river storage in some instances amounting to >30% of the load throughput. Antecedent weather conditions are identified as important in controlling the pool of fine sediment and associated P available for remobilisation during storm pulses. Examples of P reactions with sediments that have been identified from field studies are briefly reviewed. Firstly, a mass-balance approach to elucidate the net interaction of P with river bed-sediments is presented and then two specific interactions of P with sediments are discussed: (a) the coprecipitation of phosphate with calcite in lakes during phytoplankton blooms and in benthic algal biofilms on river sediments and (b) the formation of vivianite in the anoxic zone of bed-sediment in eutrophic lakes and rivers. Phosphate coprecipitation with calcite is an important mechanism controlling P concentrations in some hardwater lakes, and has been found to contribute as much as 30% of the dry mass associated with the colonisation and development of algal biofilms on hardwater river beds. Vivianite has been detected in anoxic river sediments (up to 20% by mass) receiving treated sewage, although little is known about the conditions of formation and how widespread the occurrence of vivianite is in sediments.