Planning and design
The natural cap concept design is built up as follows. On top of the contaminated soils an artificial cap of soil layers and synthetic foil (about 1 meter deep) is made. The soil layer on top is below the water level and reed development is stimulated. Over time, reed residues and other organic debris will form a peat layer. By the time the foil should normally be replaced, peat will have formed an impermeable layer.
One of the most important prerequisites for peat development is water. Because a peat system is highly sensitive to pollution, good water quality and good environmental conditions are necessary. In the natural cap design of the Volgermeerpolder, one of the big challenges was therefore to create a peat development site with water of good quality.
Due to agricultural land use and intense fertilization of the meadows, the surface water in the surroundings of the Volgermeerpolder is of poor quality. Using this water to inundate the sawas would negatively affect growth conditions and proper peat formation would probably not occur. Instead of using expensive alternatives such as water filtration systems, incoming rainfall is used as the only source of water. Within these sawas swamp-like conditions are created that are most effective for peat development. To be able to use only rainfall as a source of water and keep wet conditions year round, water balances were made to simulate the water levels in the sawas. It was necessary to create buffer sawas where rainfall is stored. Water from these buffers can be used to maintain the minimum water levels in the sawas (by actively pumping the water). A certain amount of water level fluctuation is allowed in all sawas.
Another important aspect of peat development is time. Because the formation of a proper peat layer takes several decades, the ‘natural cap’ is only applicable to projects where enough time is available. In the Volgermeerpolder, a period of at least 30 years is foreseen during which the HDPE (high-density polyethylene) foil will slowly decay. Active management will be needed to maintain peat development.
In the long run, once a natural cap has developed, the management of the area will be less time and energy consuming than conventionally covered waste sites, as the HDPE foil does not have to be replaced or removed. Thus, the natural cap design becomes cheaper and more effective over time. Note, however, that the natural cap concept remains to be proven, so monitoring in the Volgermeerpolder is very important to do so and to rule out risks in the meantime.
Because a wetland in an agricultural landscape is highly attractive for many animal and plant species, the natural cap design also adds to the nature value of the area. Already many bird species seek refuge in the Volgermeerpolder and it is expected to become an ecological hotspot with many uncommon and rare species.