Phosphorus transport in a Dutch lowland area: the role of high flow storm events and suspended matter

Abstract

Phosphorus (P) is an essential macronutrient, which also has the potential to cause severe eutrophication issues in freshwater systems. Research done over the last decades has revealed the importance of the transport of particulate-P (PP) during high flow storm events to the annual total-P loading. The present study applied P speciation, by means of chemical sequential extraction, to analyse the temporal variability of P fractions during a high flow event and thereby allow a more detailed characterization of P mobility and the impact on eutrophication.

A sampling campaign was conducted in the Noordplas polder, a former lake in the Netherlands, featuring significant groundwater exfiltration. The polder exports large quantities of PP to downstream areas, often exceeding the regulatory limits on P concentration. A controlled high flow event was forced using the polder’s water pumping station and a weir, thereby eliminating the interference of overland and subsurface drain flow that would have occurred during a natural high flow storm event in a free flowing system. Suspended matter samples collected by centrifugation and filtration were analysed to determine the P speciation, as well as the suspended matter (SM) concentration and particle size distribution.

The results showed that variation in the P concentration in the water column was significantly related to changes in the SM concentration. Changes in other parameters, including particle size distribution and the relative P content of SM, had no noticeable influence on the P concentration. Dissolved-P concentrations were at all times stable and only a minor component of the total P pool. Large increases in SM and P concentrations were associated with erosion during the highest flow velocities, the SM concentration remained largely unaffected if the flow velocity did not reach critical shear stress levels. The relative contribution of P fractions to PP (% PP) was a stable parameter, seemingly independent of other parameters like flow velocity or water quality. Iron-bound P (Fe-P) was the most dominant fraction (about 80% PP), at all locations and for all flow velocities. Organic P was the only other prominent fraction (about 15% PP on average). An important finding is that the P speciation of SM collected by centrifugation underestimated the exchangeable P and authigenic calcium-bound P fractions. This is explained by the inability of the centrifuge to capture the lightest particles. The mentioned %PP values were therefore based on filter samples only. The Fe-P contribution and relative P content of SM are high compared to values found in literature, most likely due to the influx of high concentrations of dissolved P and iron through groundwater, causing authigenic production of particulate Fe-P in the water column.

The findings of the present study implicate the necessity for water management to assess the effect that different pumping rates by pumping stations have on P transport. In this study, doubling the pumping rate led to twice higher PP concentrations in the channel near the pumping station. Spot samples might not be able to detected this spike during standard monitoring programs, therefore, estimations of P export that do not include a full range of pumping activity are prone to underestimate P export. Literature dictates that the dominant Fe-P fraction has to be considered biologically available; an eutrophication effect by the exported P is thus plausible.