A fresh look at protecting coastal water resources Extraction wells
(fresh and brackish)
Treatment facility
(Candle filter & RO)
Figure 2b: Treatment of the brackish groundwater.
Monitoring Well
Figure 1: Overview of the demonstration pilot: well field and water treatment facility.
Europe’s coastal freshwater resources are under intense pressure, with overexploitation leading to salinization of coastal aquifers (groundwater bodies). The team behind the LIFE FRESHMAN project has developed an innovative technique to protect coastal aquifers from salinization, while also increasing the volume of readily available freshwater, as project leader Dr. Gertjan Zwolsman explains. Many
coastal zones around Europe rely on freshwater reserves in aquifers for drinking water production, agriculture or industry. These freshwater reserves depend on rainfall or infiltration from local rivers, and if more freshwater is extracted than is replenished on an annual basis, the aquifers will be overexploited, risking serious long-term consequences on groundwater quantity and quality. “Aquifers (sandy or gravel layers in the soil profile) are the basis of underground freshwater storage. If coastal aquifers are overexploited, seawater will intrude into them, leading to salinization”, explains Dr. Gertjan Zwolsman, senior strategist on water resources at Dunea, a public drinking water utility which supplies drinking water across the Southwest Netherlands. “In the coastal dunes around The Hague there are four aquifers, separated by confining layers of clay and peat,” he outlines. “Based on previous drilling of groundwater wells, we know precisely what the subsurface looks like in the dunes.” 40
FRESHMAN project The coastal dunes around The Hague play an important role in drinking water supply to the city. Below the surface, a freshwater lens is present of 60-90 meters depth, underlain by brackish groundwater. This brackish water could provide an additional source of drinking water after extraction and treatment. This is a topic that Zwolsman and his team are
exploring in the EU-funded LIFE FRESHMAN project. “Our pilot project is being conducted at Dunea’s main drinking water production site in the coastal dunes around The Hague, where population growth is leading to increased drinking water demand, underlining the wider importance of the project’s work. The pilot consists of a well field and a water treatment facility (Figures 1 and 2). We have
Figure 2a: Location and layout of the demonstration pilot.
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installed two extraction wells. The first is for extraction of brackish groundwater and has three separate well screens between 90 and 110 meters below ground level. The other well is for extraction of deep fresh groundwater and has two well screens between 60 and 90 meters below ground level,” he explains. “We wanted to investigate the sensitivity of this deep freshwater well to salinization. We have installed six monitoring wells around the extraction wells (Figure 2a), to follow the impact of groundwater extraction on the salinity distribution in the aquifers.” The project team started by exploring the geohydrological conditions at the pilot site, by drilling an exploration well of 200 meters deep. The soil profile of the exploration well and the vertical distribution of the chloride concentration (and related variables) were recorded (Figure 3). Fresh groundwater, with a chloride concentration of less than 150 milligrams per liter (mg/l), is present up to a depth of 90 meters. In the 20-meters thick brackish zone below, the chloride concentration increases to 10.000 mg/l (50% seawater). The well field was installed in the third quarter of 2021. The soil profile of the wells and the vertical salinity distribution were carefully recorded (Figure 4). “We started brackish water pumping in January 2022. Our first aim was to investigate the operational stability of the process of extracting and purifying brackish groundwater. We used a pumping rate of 50 m3/hour, divided over three well screens.” A side stream of the extracted brackish groundwater (20 m3/hour) was purified by reverse osmosis (RO; Figure 2b). The brackish groundwater passes a cartridge filter, which removes any particles, and is then pressurised to 30 bar. The water passes through reverse osmosis membranes, leading to the removal of salts and other natural substances, such as nutrients and trace metals. “In the RO tubes, the water is split into two streams, ultrapure freshwater and a saline waste stream. In the field pilot, this saline water stream is discharged to a sewer, passes a waste water treatment plant, and finally ends up
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in the North Sea. The freshwater is added to our existing drinking water treatment train,” says Zwolsman. This reverse osmosis process works similarly for both brackish and saline water, although Zwolsman says the freshwater yield (recovery) is lower for seawater (ca. 30%) than for brackish water (ca. 50-60%). Moreover, higher pressures are required to treat water with a higher salt concentration. “The higher the pressure you
apply, the more energy-intensive the process is. Also the quality of the resulting freshwater is affected and disposal options are limited with a more concentrated saline waste stream”, he says. The reverse osmosis process has been applied on water with varying salt concentrations, from almost fresh to heavily saline, and it produces high quality freshwater in all cases. “The salt concentration in the freshwater we produce of course depends on the salt concentration of the extracted brackish groundwater, but all our freshwater complies with Dutch drinking water quality standards,” stresses Zwolsman. After 18 months of operation, with only a few days downtime, the project team now has proven the operational stability of the production process. “We were initially a bit concerned about potential clogging of the well screens, because of the presence of small clay particles (colloids) in the
“The coastal zones around Europe are densely populated with high freshwater demand for drinking water, agriculture and industry.”
Figure 3:Vertical profile of chloride, resistivity and electrical conductivity in the exploration well. Soil composition is also indicated: aquifers are yellow to brown and confining layers (aquitards) are in green.
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