The main lessons-learned during this project were:
- ‘Do not engineer against nature. Work with the capacity of nature itself to solve environmental problems’
- Integration of design, remediation and management
- Long-term development and validation of the natural cap
‘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.