Geochemical and hydrodynamic phosphorus retention mechanisms in lowland catchments

Abstract

The release of phosphorus (P) to surface water from heavily fertilised agricultural fields is of major importance for surface water quality. The research reported in this thesis examined the role of geochemical and hydrodynamic processes controlling P speciation and transport in lowland catchments in the Netherlands. This study showed that the mobility and ecological impact of P in surface waters in lowland catchments or polders like those in the Netherlands are strongly controlled by the exfiltration of anoxic groundwater containing ferrous iron. Results of an experimental field set-up showed showed that the formation of Fe precipitates upon oxygenation of anaerobic Fe-bearing groundwater is a key process affecting the P speciation in surface waters in lowland catchments. We found that Fe oxidation rates are much slower in winter than in summer, but that removal of P by oxidising Fe(II) was highly efficient in both seasons, resulting in low dissolved P concentrations in the surface water throughout the year. Our observations cannot be explained by surface complexation of PO4 to freshly formed Fe oxyhydroxides but indicates that Fe(III) phosphate precipitates are formed preferentially. In a series of aeration experiments, we explored the relationship between solution composition, reaction kinetics and the characteristics of the produced Fe(III) precipitates. The aeration experiments with Fe(II)-containing solutions demonstrated that dissolved PO4 can be effectively immobilised in the form of a homogeneous Fe hydroxyphosphate. Fe(II) oxidation was generally slower in the presence of PO4 than in the absence of PO4, but the decrease of the PO4 concentration during Fe(II) oxidation due to the formation of Fe hydroxyphosphates retarded the reaction rate even more. The progress of the Fe(II) oxidation reaction could be described using a pseudo-second-order rate law with first-order dependencies on PO4 and Fe(II) concentrations. The results from the field and laboratory experiments were confirmed with data from field surveys in catchments. We showed that the suspended particulate matter (SPM) sampled during various conditions in six lowland catchments in the Netherland had high P contents. The average content was about 2-10 higher than the P contents of SPM reported in international studies. Iron-bound P was, with an average value of 61%, the dominant P species in the TP concentration in surface water bodies in the lowland catchments. After the turnover of dissolved P to iron-bound particulate P, transport of P in catchments or polders is controlled by sedimentation and erosion of particles. Hydrodynamic resuspension of P stored in bed sediments is commonly considered as being the main transport mechanism in lowland catchments. A combination of erosion experiments with undisturbed bed sediment cores and a hydrodynamic model that calculates bed shear stresses in water courses revealed contrasting insights for polder catchments. Polder catchments typically have a dense network of drainage ditches and a water flow strongly controlled by weirs and pumping stations. This geographic setting tempers peak flow and thus results in lower maximum hydrodynamic forces that generally may not be strong enough to cause erosion of bed sediment.