Drivers of nitrogen and phosphorus dynamics in a groundwater-fed urban catchment revealed by high-frequency monitoring

[摘要] Eutrophication of water bodies has been a problem causing severe degradation of water quality in cities. To gain mechanistic understanding of the temporal dynamics of nitrogen (N) and phosphorus (P) in a groundwater-fed low-lying urban polder, we applied high-frequency monitoring in Geuzenveld, a polder in the city of Amsterdam. The high-frequency monitoring equipment was installed at the pumping station where water leaves the polder. From March 2016 to June 2017, total phosphorus (TP), ammonium ( NH 4 ), turbidity, electrical conductivity (EC), and water temperature were measured at intervals of less than 20 min. This paper discusses the results at three timescales: annual scale, rain event scale, and single pumping event scale. Mixing of upwelling groundwater (main source of N and P) and runoff from precipitation on pavements and roofs was the dominant hydrological process governing the temporal pattern of the EC, while N and P fluxes from the polder were also regulated by primary production and iron transformations. In our groundwater-seepage controlled catchment, NH 4 appeared to be the dominant form of N with surface water concentrations in the range of 2–6 mg N L −1 , which stems from production in an organic-rich subsurface. The concentrations of NH 4 in the surface water were governed by the mixing process in autumn and winter and were reduced down to 0.1 mg N L −1 during the algal growing season in spring. The depletion of dissolved NH 4 in spring suggests uptake by primary producers, consistent with high concentrations of chlorophyll  a , O 2 , and suspended solids during this period. Total P and turbidity were high during winter (range 0.5–2.5 mg P L −1 and 200–1800 FNU, respectively, where FNU represents Formazin Nephelometric Unit) due to the release of P and reduced iron from anoxic sediment to the water column, where Fe 2+ was rapidly oxidized and precipitated as iron oxides which contributed to turbidity. In the other seasons, P is retained in the sediment by sorption to precipitated iron oxides. Nitrogen is exported from the polder to the receiving waters throughout the whole year, mostly in the form of NH 4 but in the form of organic N in spring. P leaves the polder mainly during winter, primarily associated with Fe(OH) 3 colloids and as dissolved P. Based on this new understanding of the dynamics of N and P in this low-lying urban catchment, we suggested management strategies that may effectively control and reduce eutrophication in urban polders and receiving downstream waters.

[学科分类] 妇产科学