Water pollution control by aquatic vegetation of treatment wetlands

Abstract

Supplying polluted river water to nature reserves in The Netherlands often leads to eutrophication of the reserve. The eutrophication can be caused directly by the high nutrient input (external eutrophication) or indirectly by altering nutrient availability due to changes in nutrient desorption or mineralization. This paper investigates the potential of a ditch system that is tested for its potential to improve the water quality of polluted river water prior to supplying to the wet meadow reserve De Meije in The Netherlands. Concentrations of the macro-ions chloride, sulphate, calcium and bicarbonate in the polluted river water were much higher than original background values, measured in the reserve. During transport of the river water through the ditch system, no decline was observed in the concentrations of these macro-ions. The phosphorus concentration, however, decreased along the flow path and was significantly negatively correlated with the distance from the inlet point. High phosphorus removal occurred in a stretch of the ditch system where submerged and free floating species such as Fontinalis antipyretica and Lemna trisulca were dominant. The N: P ratio of F. antipyretica was especially low (N : P < 5) at sampling stations where high phosphorus concentrations were measured. The high N: P ratio indicated a luxury consumption of phosphorus. With decreasing phosphorus concentrations, the N: P ratio of F. antipyretica increased to a maximum of N: P = 25. The nutrient budget of the ditch system showed that supply of river water was the main input of phosphorus (12 kg P) whereas the main inputs of nitrogen of the ditch system were atmospheric deposition (66 kg N) and leaching from the wet meadows (44 kg N). For both nutrients, harvesting the aquatic vegetation in September was the main removal mechanism from the ditch system with 92 kg of nitrogen (80% of the annual input N) and 14 kg of phosphorus (95% of the annual P input) removed. It was concluded that the ditch system with aquatic vegetation could successfully remove nutrients from polluted river water. The concentrations of macro-ions, however, are not influenced by the ditch systems and internal eutrophication due to changes in adsorption or mineralization may still occur.