R&D highlights edition 2018

PROJECTS Ecosystems and environmental quality The release of phosphorus (P) to surface water from heavily fertilised agricultural fields is of major importance for surface water quality. New PhD research financed by Deltares showed that the effects of phosphate losses from agricultural sources on the ecological quality of surface water are smaller than previously thought. This is the result of groundwater that flows into ditches and streams which contains ferrous iron in many places in the Netherlands. When groundwater flows into surface water, chemical precipitates are formed due to the oxidation of dissolved ferrous iron. If dissolved phosphate is present during this oxidation process, it is converted to the mineral iron hydroxyphosphate. This process takes minutes or several hours depending on the temperature, pH and available oxygen. As a consequence, most of the phosphate in the surface water is not readily available for uptake by algae or other aquatic organisms. A comprehensive field campaign in six agricultural-dominated lowland catchments revealed that the total phosphorus concentration is strongly dominated by iron-bound particulate phosphorus. After the dissolved phosphorus is converted into iron-bound particulate phosphorus, the transport of phosphorus in catchments or polders is controlled by the sedimentation and erosion of suspended sediments. Due to the low flow rate in the ditches and canals in our low-lying country, the particulate phosphorus settles into the sediment. Research in two polders has shown that this settled phosphorus pool will not be readily remobilised. The Dutch water system – with polders, streams full of weirs and gently-flowing ditches and canals –helps to keep it in the sediment layer. However, in the summer, when there may be a shortage of oxygen in the ditch water, iron-bound phosphorus can be converted back into dissolved phosphorus that can leach back into surface water. This implies that ongoing sedimentation of phosphorus into the sediment must be prevented, and it is also an incentive for sediment clean-up operations. The conclusion from this research was that that Netherlands is fortunate that the water system in many places immobilises large amounts of dissolved phosphorus and stores it in ditches. If this were not the case, the eutrophication problems would be much more serious. Moreover, these properties of the water system also give possibilities for reusing this phosphorus pool. The findings of the study led to parliamentary questions for the State Secretary of Agriculture and the Minister of Environment about the impact of iron-bearing groundwater on the fate and behaviour of phosphorus in surface water. In his response, the State Secretary stated that the iron-phosphorus interaction should be taken into account in assessments of the effects of the manure policy on water quality and that such processes should be included in the update of the water quality models used to support the implementation of the EU Nitrate Directive and the Water Framework Directive. Contact Bas van der Grift, Bas.vanderGrift@deltares.nl , t +31 (0)6 2347 8769 Further reading: B. van der Grift (2017) Geochemical and hydrodynamic phosphorus retention mechanisms in lowland catchments. PhD thesis Utrecht University. Available at: https://dspace.library.uu.nl/ bitstream/1874/350533/1/Grift.pdf IRON-BEARING GROUNDWATER MAKES PHOSPHATE LESS HARMFUL 44