Is Sustainable Extraction from Small Freshwater Lenses Possible?

Water for agriculture
Netherlands

In the Dutch province Zeeland, many areas depend on rainwater stored in the soil for their fresh water supply. Water may be extracted from large freshwater lenses using a deep drain. This is not permitted for small freshwater lenses. In a pilot, we are investigating whether small freshwater lenses can also be sustainably managed so that they can be a source of water supply.

Drone image from pilot location in Zeeland, the Netherlands

Key Takeaways

  • The pilot demonstrated potential for extraction from small freshwater lenses: Over two years, around 4,500 m³/year was extracted, without immediate signs of salinization.
  • Multiple factors influence sustainability: Model studies showed that sustainable extraction depends on various conditions like the resistance of separating layers, groundwater levels, and proximity to surface water.
  • The results of the pilot study indicate that sustainable use of small freshwater lenses may be possible under certain conditions.
  • Need for expanded and continued monitoring, including more EC measurements, extended geophysical surveys, and deeper wells, is crucial to understand long-term changes and support sustainable management.

Project Details

Started: 2021 – Completed: 2024
Ref: 211268 | Status: Completed
Title: Samenwerken voor zoetwater: Duurzaam gebruik zoetwaterbellen

Background

Due to climate change, extreme weather conditions are becoming more frequent in the Netherlands. Agriculture is experiencing problems as a result; both drought and salinization in coastal areas are already recurring challenges for farmers. Smart usage of the available freshwater for agriculture is therefore important, now and in the future.

Water is extracted from large freshwater lenses at least 15 meters thick, for irrigation of agricultural land. However, the deep drains used to do this, may not be applied to small freshwater lenses. This is because so far, it is not clear whether water can be sustainably extracted from these small lenses.

It is known from large freshwater lenses, that the extracted water is sufficiently replenished, so that the lens doesn’t shrink. If freshwater lenses with a thickness of less than 15 meters can be managed sustainably, new opportunities arise, for example for farmers who have land on top of such a freshwater lens.

Activities

Acacia Water is conducting an exploration of the possibilities of sustainable extraction of water from small freshwater lenses. For this research, we extract water from the small freshwater lens with a deep drain and we apply water with an underground drip irrigation system.

Besides testing whether sustainable management of the water from a small lens is possible, in this pilot we are also looking at the possibility of drawing up directives and preconditions within which small freshwater lenses can be managed sustainably. This could then serve as input for renewing policy.

Finally, this pilot should give insight into the possibility (or impossibility) of administering water with underground drip irrigation in well-drained soil.

To visualize the effects of the deep drain abstraction, we use monitoring wells to monitor water abstraction, groundwater level change and electrical conductivity (EC). EC is the measure of salinity in water. We measure the amount of water in the upper part of the soil with soil moisture sensors. We also conduct CVES measurements (geophysics measurements) with which we map the fresh-saline water interface and the composition of the subsoil. In total, over two years, about 4,500 m3/year was extracted from the deep drain.

Findings

The main measurement results indicate that during the growing season, the maximum lowering of the groundwater level at a distance of 60 m from the lens is about 15 cm.

After the first growing season with extraction from the deep drain, based on the EC measurements in the deep monitoring wells, no salinization effects were observed at the bottom or at the edge of the freshwater lens.

At the end of the second abstraction season, based on the geophysical measurements, the EC of the groundwater at 11 meter depth increased. This is possibly an indication that the freshwater-saline boundary has been raised as a result of the abstraction.

Considering the wet years 2023 and 2024, the measurement data are strongly influenced by exceptional conditions. Further measurement in 2025 is strongly recommended to get a better picture of the behavior in the system.

Modeling study: scenarios for sustainable abstraction
Within the project, we also performed model calculations. With the model we can calculate several scenarios, so that it becomes clear under which physical conditions sustainable abstraction can be achieved from small freshwater lensen.

The results of the modeling study show that several boundary conditions together determine whether a given extraction level will lead to salinization effects, and then to what extent.

An initial conclusion toward guidelines for sustainable management of freshwater lenses are, that the resistance of any separating layer is important to the calculated upconing. With higher resistance, a larger volume can be extracted. Please, refer to the report for further conclusions and recommendations.

Project Area

The location where we are conducting this pilot is in the fields of vegetable grower Op ‘t Hof in Sirjansland, in the municipality of Schouwen-Duiveland in the province of Zeeland. There is a small freshwater lens here and deep drains are connected to this lens for the pilot.

Geographic scope: Local

Downloads and Links

The final report (Dutch only) ‘Duurzaam omgaan met kleine zoetwaterbellen’. Eindrapport van het project Samenwerken voor zoetwater: duurzaam gebruik zoetwaterbellen (maart 2025). [PDF]

Download

Presentation of the project for the Dutch Hydrological Society:
‘Sustainable use of shallow freshwater lenses​’ (November 2024).

Download

In Cooperation With

Commissioned by:

European Commission

Funded by:

European Commission Provincie Zeeland

Project partners:

Acacia Water Partnership Op 't Hof

Contact

Arjen Roelandse

Hydrologist

+31 6 15 19 86 20
arjen.roelandse@acaciawater.com
arjenroelandse

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Drainage, Salinization

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