Insight into fresh, brackish and salt surface water: joint measurements in Zeeland

The campaign was based on the idea that self-measurement not only provides figures, but above all creates more insight and awareness among farmers about their water system as a whole. We are seeing this reflected in practice: by measuring EC, farmers are thinking more broadly about water quality, water management and the dynamics of their environment.

The analysis of the collected data reveals the most important freshwater-saltwater transitions and provides tools for measures that can improve the structural availability of freshwater.

The broad participation also strengthens the understanding of the local water system and contributes to support for policy and practice. The experiences in Zeeland show that this approach can also be applied in other areas to better characterise waterways and water systems.

Background and objective

The Province of Zeeland is working on the task of structurally securing the freshwater supply in the salinising delta. To date, relatively little data is available on the distribution of fresh, brackish and salt water in the surface water system.

In order to gain faster, more detailed and widely supported insight into the space-time pattern of salt and fresh water in surface and drainage water, the Province has asked farmers, nature organisations, citizens and students to carry out EC measurements using the Deltares Aquality App.

After more than four years of intensive measurement, a dataset has been created that provides new insights into the occurrence of fresh and salt water in Zeeland's surface water.

Quick scan results

The quick scan analysis of nearly 6,000 EC measurements shows that the salinity of Zeeland's surface water varies greatly in space and time. There is a clear seasonal pattern, with higher values in summer and lower values in winter.

The measurements also show a correlation with weather conditions, altitude and the influence of groundwater. Lower-lying measurement locations show higher average EC values due to greater (saline) seepage pressure, and higher EC values are also measured in surface water at locations where the fresh-salt interface is shallow.

Farmers use the EC meters to measure the salt content in the ditches around their farms themselves. In addition to providing figures, this mainly provides insight and awareness about their own water system. Very valuable! Moreover, “measuring together” and the same observations ensure an accessible start to the dialogue between farmers and the government, enabling us to arrive at targeted measures together

Arno Vael, Member of the Provincial Executive Zeeland

Characterisation of water bodies

The EC measurements from the Aquality App have been used to characterise the waterways in Zeeland. Based on the median EC values in summer and winter, waterways can be distinguished as structurally fresh, brackish or salty, or as waterways that change character seasonally (see Figure).

By zooming in locally on the characterised measurement locations and taking into account the flow direction of the waterways, it is possible to identify transition zones between permanently fresh and non-permanently fresh waterways. These transition zones can reveal opportunities for taking measures to keep fresh and salt water separate.

It also provides starting points for locations where fresh surface water may be available for above-ground or underground storage.

Insight at plot level

The analysis shows that EC measurements of drainage water provide a direct picture of the salt content in the shallow groundwater system. They thus provide insight into the location and dynamics of the freshwater-saltwater interface and into the influence of saltwater seepage, precipitation and drought.

Recording these measurements via the Aquality App creates a valuable dataset that enables farmers to better assess how salt pollution arises on their plots and what measures can help to retain fresh water.