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The natural cap concept involves the application of natural elements to cover and degrade toxic materials. Wetland development stimulates peat formation and peat layers serve as a natural barrier to infiltration. The peat layer absorbs almost all the precipitation, so almost no water infiltrates in the ground and through the waste material. This results in no or less leakage. Degradation of pollutants can take place in the peat layer through bacterial processes.
In traditional designs, waste dumps are capped with metal or concrete sheets, bottom sealing and foil that has to be replaced over time. This is a very costly, inefficient and unnatural method.
General Project Description
Title: Natural capping in the Volgermeerpolder
Location: Volgermeerpolder, Netherlands
Companies: Advies Combinatie Volgermeerpolder (ACV, consisting of Witteveen+Bos and Tauw), Bouw Combinatie Volgermeerpolder (BCV, consisting of Boskalis, De. Vries & Van de Wiel and Cofra) and the City of Amsterdam
Abstract: Applying a growing peat layer for natural capping of toxic waste.
Topics: Toxic waste remediation, natural cap, peat formation, ecosystem services, building with nature.
The Volgermeerpolder is an area of approximately 105 ha, located 5 km north of the city of Amsterdam in a marshy polder with shallow groundwater, open water and peaty soils. In the twentieth century the Volgermeerpolder was used as a dump site for domestic and industrial waste. This included large amounts of chemical waste, making it one of the most heavily contaminated areas in Western Europe.
In the 1970’s the discussion started about the risk of uncontrollable spreading of the toxic waste into the environment, which was thought to be only a matter of time. The heavily polluted landfill was considered a ‘chemical time bomb’. The Volgermeerpolder was put on the political agenda and closed in 1981. After closure, the local authorities took the initiatives towards remediation, under pressure of the ‘Burgerkomitee’ (a group of organized citizens). Starting point for the remediation was complete isolation of the site, in order to prevent the spreading of the pollution.
Over a period of more than 30 years, with a growing intensity in the last 5 years before remediation, the quality of the groundwater surrounding the Volgermeerpolder was monitored. This led to the insight that the waste did not pollute the groundwater in the surrounding polder system (peat meadows). It was concluded that the peat efficiently acted as a natural barrier and prevented spreading of the contaminants. This triggered the question whether the same material could effectively be used for capping.
For the capping an 'Eco-variant' was chosen, which should reduce contact risks and prevent waste spreading, but would also create conditions for developing a natural wetland. In 2001 the consortium ACV - Advies Combinatie Volgermeerpolder - started remedial planning and design for the Volgermeerpolder. The cap construction started in 2005 by covering the landfill with several surface sealing layers of soil and HDPE (high-density polyethylene) foil and was completed in 2010. The capping is now in the management phase.
After the discovery that peat acted as a natural barrier, it was decided that for remediating the Volgermeerpolder it would be sufficient to only cover the landfill and that complete isolation of the landfill or placement of a leachate-interception system was not necessary. The HDPE foil (used to cover the landfill) will normally have to be replaced after some time, due to leaks in the foil. To avoid this expensive operation, ACV developed the ‘natural cap’ concept, which boils down to a gradual, functional replacement of the standard cover (of soil and a synthetic foil) by a natural layer of peat that develops over time. In the Volgermeerpolder, the foil will not be removed because its leaking will not be a problem as its function is taken over by the developing peat layer. To start the peat development, swamp-like conditions are created on top of the cover in so-called sawas (paddy-fields), terrace-like baisins inspired by Asian rice fields. These sawas facilitate water storage and accurate water level control.
The ‘natural cap’ concept has great potential for landfill remediation. In suitable areas, it is considered a sustainable and cost-effective remediation for a standard synthetic or concrete cap and thus a valuable contribution to the remediation of landfills in the Netherlands and abroad. It is based on the hypothesis that one can use the nominal lifespan of the initial synthetic foil (approximately 30 – 100 years) to let a natural peat layer develop which takes over the cap function. This hypothesis is based on research such as in the 'Westbroekse Zodden' in the Netherlands, where after 30 years a thick peat layer was formed.The 'natural cap' concept will not be applicable for every landfill remediation, it is for example very important that the soil can be kept in a wet condition. The circumstances for developing peat need to be present.
The use of natural processes to develop peat on top of the Volgermeerpolder has a lot of advantages. The peat landscape that will develop over the years is considered very attractive from an ecological and a landscape point of view. The many ponds initially present match well within the surrounding water-abundant landscape of 'Waterland' and the same goes for the future peat fields. The landfill can be considered an oasis of ecologically valuable nature within agricultural land use, with very high biodiversity and robust nature. Because of the ecological value, the landfill may fullfil many recreational purposes. Moreover, peat formation binds a significant amount of CO2. If succesful, the 'natural cap' makes replacement of the standard cover unnecessary, thus avoiding considerable costs and disruption of newly developed nature. Therefore, this investment in natural capping is expected to yield a long-term return in the form of ecosystem services.
An effect which is difficult to value is that this project serves as a large-scale experiment on peat formation. It will produce knowledge that may enable scientists to tackle the problem of peat oxidation and the ensuing subsidence of lowland agricultural areas, like in the western part of the Netherlands.
In summary, the project is executed to provide the following ecosystem services:
- Seapage blocking
- Water buffer (because of the sawa (paddy field) structure)
- Nature values (species, habitat)
- Corridor for species such as the grass snake (Natrix natrix) (the grass snake is present in adjacent areas and was present in the Volgermeerpolder in the past)
- Degradation of toxic substances
- CO2 sequestration
Planning and Design
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 pobably 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.
The natural cap construction was built up layer by layer. The first layer consisted of soil from ground works in the region, which is then covered with HDPE (high density polyethylene) foil. On top of this HDPE foil a second soil layer was placed and sawa dikes were built. The sawas were not filled artificially, but were left to fill up with rainwater (see figure).
The bottom and the banks of existing waterways were also covered with HDPE foil and a soil layer. On the 60 ha low-lying area a wetland is created via a system of 59 sawas surrounded by dikes on top of the soil/HDPE cover. In some sawas reed rhizomes and mown reed were added to speed up reed growth. The remaining 45 ha is considered a dry area. As part of the remediation a buffer zone is created around the site in which groundwater quality is intensively monitored.
Operation and Maintenance
For the stimulation of nature development an ecological maintenance plan was made. This plan focuses on several nature values such as reed marshes, sawa development (see figure) and the central waterway. As indicated before, apart from creating an impermeable layer, peat formation itself is also of great ecological value. Peat marshes function as habitats for many animal species, such as birds, amphibians and reptiles. Because in systems like the Volgermeerpolder, ecology always influences related factors such as water quality and vice versa, it is important that management for nature is always synchronized with other goals and types of management. Therefore, the most important maintenance activities are selective vegetation control, water level control (actively pumping water from the buffer sawas to the sawas), and reed control.
To speed up the nature development, periodic management is applied to guide natural development with a minimum of intervention. The goal is to create a robust system that can be maintained with as little management and maintenance activities as possible, in terms of ecology as well as pollution prevention.
‘Do not engineer against nature. Work with the capacity of nature itself to solve environmental problems’
A key finding from two decades of intensively searching for ways to tackle the environmental threats of the Volgermeerpolder was that ‘the natural peat underground of the polder has a viable capacity for contaminant migration control’. This insight has been the main driver for developing an alternative concept for hazardous waste management based on natural capabilities of peat. Artificial isolation of the landfill or installing a leachate-interception system was not necessary, which reduced the costs of remediation significantly.
Furthermore, to avoid replacement of the standard cover, the ‘natural cap’ concept was developed with great potential for landfill remediation. Not only can it act as a ‘natural barrier’, but also as a filter for contamination, as a pollutant degrader, as a hydraulic control system and for carbon sequestration, water storage and restoration of the natural landscape. In suitable areas, it is considered a sustainable and effective alternative for standard capping and thus a valuable contribution to the cost-effective remediation of landfills in the Netherlands and abroad.
Integration of design, remediation and management
One of the lessons learned from the remediation of the Volgermeerpolder is that further integration of design, remediation and management is necessary to make a wetland less sensitive to nutrient loading. Because of the agricultural land use of the surrounding polders and the high nutrient load in the surrounding polders, the water from outside the Volgermeerpolder is not suitable for use in the sawa system. For the wetland, the ideal situation would be an exclusively rain-fed system, which is feasible in the Netherlands.
During capping of the Volgermeerpolder it was necessary to take a critically evaluate the quality of the surface layer and that of the soil applied in the sawas. To prevent a negative effect on the quality of the surface water in the new water system, the nutrient content of the soil in direct contact with the surface water should be limited.
The long term development and validation of the natural cap
The challenge after capping is to manage the area in such a way that peat can form on top of the soil layer. Time is needed for forming a sufficiently thick and dense layer of organic material that can take over the functions of the HDPE foil. In order to know whether the peat layer is ready to do so, monitoring is necessary.
Validating the ‘natural cap’ as a sustainable solution requires a good understanding of the processes involved, particularly at the boundary layer between the landfill and the surrounding peat soil. In this respect the question as to whether a peat layer is capable of sufficiently absorbing the range of contaminants is particularly relevant. In cooperation with the Centre for Wetland Ecology, Deltares and the municipality of Amsterdam ACV is conducting further research to continue the development of the ‘natural cap’ concept, and further optimize the design for the Volgermeerpolder. This is necessary because peat has not been developed yet, this takes several years. Therefore, we still don't know the best management strategy.
- ACV (2010), Aanpassen watersysteem Volgermeerpolder (in Dutch, not public)
- ACV (2010), Ecologisch Beheerplan Volgermeerpolder (in Dutch, not public)
- Website of the ACV
- W.H. Clemens, Wijk, van der M., Stook P. & Pal ,van der J.C.N. (2009), Natural capping of the landfill Volgermeerpolder.
- Centre for Wetland Ecology (2011), Research project PeatCap. Volgermeerpolder: from waste dump to research area for peat development
- Article in magazine 'de Ingenieur' (in Dutch)