Mobilisation and Transport of Sediment-Associated Phosphorus by Surface Runoff

Surface runoff transporting sediment with high phosphorus (P) concentrations has been identified as a major hydrological pathway for sediment-associated P delivery to surface waters and is considered a major threat to water quality, due to the ability of P to cause eutrophication in fresh water. Not all P-rich sediment that is mobilised by erosion will however be delivered directly to the channel. Some may instead be deposited in intermediate storage away from its source area. The aim of this contribution was to determine the influence of land use and soil type on the P content of surface runoff sediment and sediment deposited in intermediate storage and was undertaken in the largely agricultural and rural catchments of the Rivers Frome and Piddle in Dorset, UK. The study formed part of a larger investigation of hydrological and hydrogeochemical processes and fluxes in lowland permeable catchments in the UK (LOCAR). Soil samples were collected from the main land use types; freshly deposited sediment was sampled from ditches, hedge boundaries and depressions in fields, and sediment-laden runoff was collected during heavy rainfall events. The concentrations of total phosphorus (TP) and the P fractions found in the surface runoff sediment were significantly different from those measured in the original source soils, with a greater degree of enrichment associated with surface runoff sediment from cultivated land than from pasture land. For cultivated land, concentrations of TP and the P fractions in deposited sediment were higher than those in the original source material, while for pasture soils, concentrations of TP and the P fractions tended to be lower than in the original source soils. The relative importance of the P fractions associated with surface runoff sediment and sediment deposits also differed from that for the original soil samples. Surface runoff sediment was finer than source pasture and cultivated soils, reflecting the particle size selectivity of sediment mobilisation and transport. Soil physical properties and land use can both influence the P content of surface runoff and deposited sediment.