Removal process of phosphorus during the settlement of particulates with runoff and its implication for reservoir management

Substantial particulates and phosphorus (P) loads are carried into the reservoir by flood runoff and the P exchange between water and settling particulates under variable water conditions in the reservoir after flooding is critical to the removal of active P from water. To investigate the impact of particulate sedimentation on P changes in reservoir water, runoff samples were collected at four locations in the upstream channels of two reservoirs after a rainstorm. Two batches of particulate sedimentation simulations were conducted separately in four plexiglass columns to analyze the changes of water P and environmental factors during particulate sedimentation. The results showed that the contents of total P (TP), total particulates P (TPP), and phosphate (PO₄^3-) decreased with the settlement of particulates. The correlation between the environmental factors and the amount of PO₄^3- in water changed from uncorrelated to correlated with particulate settlement, implying that the rapid settling of particulates might weaken the effect of environmental factors on P exchange between water and particulates. Particulates firstly release PO₄^3- rapidly in river or reservoir and then adsorb PO₄^3- slowly during settlement in reservoirs. Coarse particulates release more and adsorb less PO₄^3- during settlement, and fine particulates play an important role in the removal of water PO₄^3- due to the slow sedimentation rates. Therefore, to mitigate the reactive P content of reservoir water, the loss of coarse particulates from the watershed should be controlled, and the discharge of water with particulates downstream should also be avoided during the flood season.