Water quality is a widely discussed topic in the Netherlands. Governments and water boards invest a great deal of effort in measuring water quality, but sediment—the layer of soil material where pollutants accumulate—is hardly taken into account. According to Wieringa, this is a problem: ‘Toxic chemicals that settle here, whether for a short or long period, affect life in and on the sediment. When these substances are released back into the water, they further degrade water quality.’

Living organisms and sediment samples

Wieringa studied the impact of sediment on aquatic ecosystems in drainage ditches across the Netherlands. One important technique she used was the so-called bioassay, a test in which living organisms, such as midge larvae, are brought into contact with sediment samples. ‘Bioassay show how a polluted environment affects the development and survival of organisms that live in it. It measures the actual impact of harmful substances on life in the sediment,’ she explains.

A more realistic picture

Wieringa also took several steps to improve the bioassay’s accuracy. First, she collected entire layers of sediment by extracting them from the ditches using a special tube. ‘Harmful substances can accumulate in different layers. By preserving the sediment’s original layering, the test becomes much more realistic,’ Wieringa explains.

She also used specialised techniques to measure the concentrations of substances that organisms actually absorb. ‘Not all pollutants in sediment are immediately harmful. Some remain trapped in the sediment and do not easily become bioavailable.’ Additionally, she developed a method to isolate a specific group of pollutants and conduct a targeted bioassay with them. ‘This allowed me to determine whether the effects I observed were truly caused by contamination and not by other factors.’

Finally, Wieringa tested whether other types of organisms, with varying sensitivities to pollution, could also be used in bioassays. ‘Sediment research often relies on just a few species, such as worms or midge larvae. By studying a broader range of species, I was able to get a more complete picture of how sediment pollution impacts different organisms.’

Mud matters

Wieringa concludes that sediments play a major role in the ecological health of Dutch drainage ditches. ‘The negative effects of polluted sediments are quite severe,’ she says. ‘They are harmful to animals and plants living in the water, even in places where the water itself did not appear to be polluted.’

According to Wieringa, the lack of reaching water quality goals may indeed, at least to some extent, be attributed to contaminated sediments. ‘If we do not consider sediment in water quality assessments, we are overlooking a crucial factor.’ She therefore advocates for making sediment monitoring mandatory, just like water monitoring.

Thesis Details

Nienke Wieringa, 2025, "Mud Matters. The Significance of Effect-Based Sediment Quality Assessment." Supervisors: Prof. Dr. M.H.S. Kraak and Prof. Dr. Ir. P.F.M. Verdonschot. Co-supervisors: Dr. S.T.J. Droge and Dr. T.L. ter Laak.

Time and Location

Friday, March 14, 11:00–12:30, Aula, Amsterdam.