Does the filtering of water in the dunes lead to pollution of the soil?

Drinking water companies are among the largest nature managers in the Netherlands. Large dune areas act as natural water purification systems. A reader wondered whether the purification of drinking water in the dunes does not lead to pollution of the soil. Where is the contamination?

The river water must already meet certain standards before it is allowed into the dunes, says Steven van Duijvenbode, process technologist at Waternet, which manages the Amsterdam Waterleidingduinen. That area south of Zandvoort has been a water extraction area since 1853. « If there is a contamination on the Rhine, we get a signal, Van Duijvenbode says. » Thanks to the Rhine alarm model, we know exactly at what hour that contamination comes to our Intoamepunt at Nieuwegein, then we stop the water intake. Fortunately, that rarely happens.  »

For more than a hundred years, of course, of course, dune water was extracted, but in the long run that led to considerable drying out and salinization. That is why Waternet has been introducing the dunes from the Lekkanaal to supplement the groundwater since the 1950s. Today, the dunes still provide the capital and surrounding municipalities in two -thirds of the drinking water – 180 million liters per day. Other dune areas are also important for drinking water.

Water pollution comes in four forms, Luuk Rietveld explains. He is professor of drinking water and urban water technology at TU Delft. Silk particles that could hide the sand from the dunes, nutrients that promote unwanted algae growth, heavy metals and finally micro -contaminants, such as pesticides, germs and medicine residues. The first three are removed from the river water in advance. And most micro -contaminants simply rinse the dunes again, says Rietveld.

What do the dunes do? In an average of three months the water drops through the sand, Van Duijvenbode says, while naturally occurring bacteria occur to organic micro -infections – roughly 40 percent of them break down. The one water drop only takes sixty days, the other for a year. That is a matter of probability distribution. Van Duijvenbode: « A drop has to go through all the grains of sand. One time it turns left at a grain, then turn right again. » Also nice for a constant water quality, by the way, says Van Duijvenbode. The incoming water can differ in quality, but what comes out the dunes is ‘nice flat’.

Nice flat, but not yet clean enough. The bacteria can do nothing with PFAS. Van Duijvenbode: « These substances are so soluble in water that they cycle through the bottom and are extracted from the dunes again. Cool filters in the post -cleanling take it out. » So most purification is done outside the dunes.

The dunes are also important as a buffer. Van Duijvenbode: « We can do without new water supply for two to three months. But after that it is very dry, though. Fortunately that has never happened. » In any case, it does not lead to pollution of the dunes. The bit of sludge that accumulates in the infiltration streams mainly comes from dead aquatic plants and is removed every 25 years.

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