In the future, the Netherlands will have to deal with increasing salinization of the coastal areas. Salinization is already underway in the Haarlemmermeerpolder. The origin of the salt goes back thousands of years, when the coastline was still deep inland. PhD candidate Joost Delsman argues for smarter water management.
The groundwater in our coastal areas contains more and more salt. Contrary to popular belief, this water does not come from the current North Sea; the groundwater flows too slowly for that. The current salinization is largely caused by much older salt, which infiltrated the soil thousands of years ago. This salty water, which is heavier than fresh rainwater, has sunk several hundred meters into the ground and is now finding its way back up.
Hydrologist Joost Delsman will soon be awarded a PhD on salinization of ditches by saline groundwater and conducted research in the Haarlemmermeerpolder. Farmers in this polder are already suffering from saline groundwater, is his firm conviction. “It is expected that we will increasingly have to deal with a shortage of fresh (river) water due to drier summers. These shortages now occur once every ten years. Due to a lack of fresh water, the polders cannot be flushed as well and the saline groundwater penetrates further to the surface.”
Cracks in the basal peat
“The saline groundwater now mainly finds its way up in places where there are cracks in the 10 cm thick basal peat,” says Delsman. “These cracks, through which the groundwater seeps to the surface, are the so-called 'wells'.” The basal peat is the oldest peat layer in the subsoil of the Netherlands. It formed when the groundwater level rose in the early Holocene and created peat swamps. These were later submerged by rising sea levels.
“The problem of salinization is typical for the coastal region of the west of the Netherlands,” says Delsman. “We find the saline groundwater up to a few tens of kilometers inland, up to the old transgression boundary:where the North Sea reached during high sea water levels. Between 8000 and 2000 years ago the sea reached its maximum inland. The salt water infiltrated the soil. In the Northern Netherlands and the Zeeland delta, the seawater has come far inland until even later.
Bad harvest
Although it is difficult to prove, Delsman is convinced that farmers are already dealing with a poorer harvest due to irrigation from salty ditches. “Salinity is not measured systematically and it is also not easy to directly relate the exact cause of a poorer yield to the salt load. A bad harvest can also be related to excessive rainfall, periods of drought or disease pressure.” Many potatoes are grown in the Haarlemmermeerpolder, but also flowers such as peonies, which are a lot more sensitive to salt.
Smarter water management
The Haarlemmermeerpolder is being flushed with Rhine water, and it is precisely this fresh river water that will be scarce in some periods in the future. Moreover, the flushing of the polder does not take place very efficiently because the flushing water only enters a limited number of ditches and quickly becomes too salty for use. Traditionally, water management in the Netherlands has been focused on quickly discharging excess water and less attention was paid to water shortage situations. Delsman therefore argues in favor of taking a closer look at where and when inlet water is needed, in order to limit the intake of fresh river water.
The availability of freshwater for the Haarlemmermeer is not only determined by the supply from the Rhine, but also by the extent to which the salt water from the North Sea penetrates directly via the river (the saltwater tongue).
“In principle there is little you can do about salinization of the soil and it will only get worse,” says Delsman. It's a natural process. “We must therefore move towards smarter water management”, is his message. “We will have to flush smarter so that you save water. This means that you do not flush the entire period from April 1 to October 1, but select the periods in which it is really necessary. Or to look more consciously at where certain salt-sensitive crops can best be grown within a polder. You could also introduce higher water level management in the ditches into which wells emerge.”
Animation
Delsman took samples from the canals and ditches in the Haarlemmermeerpolder and used these for his groundwater models. In collaboration with geologist Peter Vos, among others, he made an animation of the salinization of the groundwater from 8500 years ago to the present. The animation (click on the video below) came about because each water particle got its own color, depending on its age and origin (such as rainwater or infiltrated during an old transgression). Dating of water is done, among other things, with the C14 method because of the presence of CO2 that is dissolved in the water. This makes it possible to determine whether the water particle entered the groundwater before or after 10,000 years ago.