Size and settling velocities of phosphorus-containing particles in water from agricultural drains

Sedimentation and filtering of suspended solid particles are commonly suggested as removal mechanisms for particle-borne nutrients such as phosphorus, but little information on the particles themselves exists. A procedure to physically and chemically characterise suspended solids in drainwater was developed. Drainflow samples from clay soils were fractionated by successive centrifugation corresponding to sedimentation rates in natural water bodies of 0.68 and 0.0034 cm day(-1), filtered through membrane filters with a pore size of 0.2 mum, and then analysed for the phosphorus content. Another set of drainflow samples was characterised with respect to particle size by photon-correlation spectroscopy (PCS). From the fractionation, on average 35% of the total phosphorus (TOTP) was found to be in colloidal form in accordance with the definitions, which means that it only settles very slowly or hardly at all. A significant proportion (45%) of the particulate phosphorus (PP) fraction settled at a very slow sedimentation rate of between 0.68 and 0.0034 cm day(-1). The majority of particles from the clay soils were of colloid size, and as idealised spheres would have an average settling rate of 0.08 cm day(-1). Catchment clay mineralogy is suggested to be an important influence on phosphorus retention. The data suggest that sedimentation of suspended material in tile drains may remove little phosphorus.