IJsseloog, or the “Eye of IJssel”, is an enormous circular pit in the middle of Ketelmeer lake in the mouth of the river IJssel, in the province of Flevoland, the Netherlands. The pit is one kilometer across and 45 meters deep and functions as a storage tank for heavily contaminated silt that is still being dredged from the bottom of the lake.

Between 1950 and 1990, lake Ketelmeer became heavily polluted from toxic industrial run-offs carried by Rhine and IJssel river from industries and factories located upstream. These pollutants were released into the river by industries not only in Holland but in Germany, Switzerland and France as well. The polluted sediments settled to the bottom of Ketelmeer in a thick layer of contaminated sludge. It was feared that the pollutants could contaminate the ground water or spread to the connected fresh water lake of Ĳsselmeer, which is the largest lake in the Netherlands and a major source of fresh water for both agriculture and drinking. The Ĳsselmeer also offers a number of opportunities for recreational activities such as yacht sailing.

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In the 1994 a decision was made to remove the polluted sediments from the lakebed. Removing the silt will also deepen the channel leading to the mouth of the IJssel, thus improving access to the river for navigation. There was, however, the problem of disposal. The silt couldn’t be disposed on land without being a nuisance to agriculture and for local residents, and it also posed a serious health hazard due to presence of toxic substances and metals such as cadmium, nickel, lead, arsenic, and mercury. Therefore it was decided to store the contaminated sludge in an enormous pit called IJsseloog, situated on the lake itself.

IJsseloog was constructed between 1996 and 1999. It has a diameter of 1 kilometer and depth of 45 meters and a capacity to hold 20 million cubic meters of sludge. The pit is surrounded by a ten-meter-high embankment. To prevent leakage, the floor of the depot is sealed with clay, the dike is lined with foil, and the water level in the pit is held below that of the lake.

An island was created surrounding the pit that has processing facilities to separate pollutants from the dredged sand and peat. The cleansed sand will be used for construction at other locations.

The dredging operation started in 2000 and is expected to take 20 years to complete. When the depot is full, it will be sealed with layers of clay and sand, and the island and marshland will be used for recreation purposes, allowing nature to take over. Two artificial island created immediately east of the sludge depot have already been taken over by waterfowl like swans, geese, spoonbills, mountain ducks and grebes.

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Outside the rim of IJsseloog island, overgrown with trees in just 15 years. Photo credit

The eastside of IJsseloog. Photo credit

Sources: Slope Indicator / Wikipedia / Boskalis / “Realisation of the Ketelmeer Storage Depot” by Dirk C. Roukema, Jan Driebergen and Adri G. Fase [PDF]

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