It is one of the largest historic garbage dumps in the Netherlands, the Diemerzeedijk, east of Amsterdam. Until 1982, this old seawall, which traditionally protected Amsterdam against high water from the Zuiderzee, was used for dumping household and chemical waste. Despite the remediation, the polluted groundwater will have to be pumped up here forever.
The area outside the dykes of the Diemerzeedijk was used as a landfill half a century ago. The remediation was completed exactly ten years ago. This former landfill will forever remain a concern for the municipality of Amsterdam, as it turned out this month on the Day of Practice, organized by the Soil and Subsurface Expertise Network.
Inverted shoebox
The old landfill site was remediated sixteen years after its closure. Between 1998 and 2001 the area was isolated by driving sheet piles around it and covering the whole area with a sand-bentonite polymer gel (Trisoplast). On top of it is a 'living layer' of one meter thick, in which plants take root.
The excess rainwater - pre-calculated at about ten percent - that seeps through the sealing structures must be continuously pumped out to prevent the water level inside the box from rising to the point where the inverted 'shoe box', which is referred to as this insulating device, goes down. leaking or overflowing.
“The pumped up water is aerated and kept in storage cellars for as long as possible,” explains Jacques de Jong, soil remediation consultant and Aftercare Coordinator Diemerpark of the Municipality of Amsterdam Development Company. “The air that is released is purified with a carbon filter. The waste water is then discharged into the Amsterdam sewerage system. This pumping up is perpetual. There is no discernible trend that the water is becoming cleaner.”
Clay layer from an interglacial period
In the case of the Diemerzeedijk, remediation involves packing the contaminated landfill material. Screen walls have been placed around the dump, which connect at a depth of 26 meters to a naturally present clay layer, the so-called Eemklei from the Eemian, an interglacial around 120,000 years ago. This clay layer is very poorly permeable and closes everything at the bottom. “Pumping has to be done from a depth of approximately fifteen meters to bring the water level inside the box down,” says Gerald Bockting, who is a remediation expert involved in the aftercare of the Diemerzeedijk. However, the amount of water that was pumped turned out to be disappointing.
Bockting:“Basic peat is located at a depth of about ten meters, which we initially thought would function as a kind of natural filter and purify the water from the landfill, but this layer allowed too little water to pass through so that the water level in the landfill could not be maintained. fell”. This Basic Peat is a peat layer that has been compressed by the weight of the overlying soil into a compact layer of a few tens of centimeters thick, a substance that resembles a shoe sole.
Black box
The precise locations of the different types of (chemical) waste have never been accurately mapped. That is why the underground of the Diemerpark is still a black box, also for soil specialists. At a distance of five meters the composition of the soil can change significantly. “It would be a huge improvement if we knew more about which substances are where”, says De Jong.
“We could then divide the area into natural – historically created – sub-areas and adjust the intensity of the pumping accordingly. After all, the acid tar area deserves different attention than the central area, where there is quite a lot of rubble, or the area of the sports fields that is used intensively.” In the future, helophyte filters may also be used, whereby nature (the reed) is used for water purification.
Pumping into eternity
On average, between ten and fourteen cubic meters of water is pumped up and processed per hour. Why not a total remediation of the Diemerpark, you wonder, because this pumping up 'into eternity' does not really seem sustainable. After all, the definition of 'sustainable' is that future generations are not burdened with it. Moreover, it costs the municipality of Amsterdam at least one million euros annually to prevent the shoebox from overflowing.
De Jong responds:“It would be extremely expensive to completely remove the contamination. This cost assessment has been made in the past. At that time it was expected that most of the contamination would have to be stored twenty or thirty kilometers away, in the same kind of mountain. What it comes down to is that you just move the waste mountain, not counting the necessary transport and other nuisance and necessary additional measures, such as sheet piling.”
De Jong:"At the moment we think from the concept of remediation and how you can implement this as sustainably as possible. We do not discuss the concept itself. Sustainability for us is about energy saving and the use of solar and wind energy, in combination with the concept of monitored natural attenuation, where you let nature do some of the work itself and you use measurements to check whether that is sufficient.
Which new management plans will be chosen will partly depend on future infrastructural projects, such as the construction of a new metro line from Diemen via IJburg to Almere.