Iron-bearing groundwater makes phosphate less harmful

Published: 7 June 2017

New research by doctorate student Bas van der Grift of Deltares and Utrecht University shows that agricultural phosphate affects the ecological quality of surface water less than previously thought. This is due to the groundwater that seeps into ditches and streams, which almost always contains iron in the Netherlands.

Most of the phosphate present in surface water is not directly available for uptake by algae or other aquatic organisms. This factor should be taken into effect in the standards for phosphate in surface water and when calculating the effects of the fertiliser policy on water quality. The results show that, in the case of Dutch regional river basins or polders, the behaviour of phosphate cannot be viewed separately from the behaviour of iron. Soluble iron and phosphate from groundwater are converted in the surface water to the mineral iron hydroxyphosphate. The Dutch water system – with its polders, streams full of sluices, and gently-flowing ditches and canals – helps to retain this phosphate in the water bed. So adequate levels of iron-bearing groundwater in our ditches and the man-made structure of our Dutch water system mean that the eutrophication problems caused by intensive farming are much less than might be expected on the basis of the phosphate load alone.

Phosphate settles to the water bed

The low flow rates in the ditches and canals in our low-lying country allow the phosphate to settle to the water bed. That was the conclusion of Bas Van der Grift after an extensive field campaign in six river basins dominated by agriculture. The conversion to iron hydroxyphosphate takes between minutes and several hours, depending on the temperature, pH and available oxygen. So it can take slightly longer in the winter.
The Netherlands is fortunate in that, in many places, the water system can render soluble phosphate harmless and store it in ditches. If this were not the case, the eutrophication problems would be much worse.

field measurements phosphate by Bas van der Grift

Bas van der Grift will defend his PhD thesis coming Friday, 4.15 pm at the University of Utrecht.