Towards a sustainable peat meadow. Cost-benefit scenarios of soil subsidence in the jurisdiction of water board de Stichtse Rijnlanden.
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Large parts of the west of the Netherlands consist of peat meadows. In these areas, soil subsidence occurs, mainly as a result of the oxidation of the peat layer above the groundwater level. This subsidence is causing sagging of houses and infrastructure, greenhouse gas emissions and eutrophication of the surface water. Therefore, governments are currently looking for a sustainable approach to combat soil subsidence in the peat meadows. Water board De Stichtse Rijnlanden (HDSR) is responsible for setting the water levels in about 25.000 hectares of peat meadows. Since the soil subsidence rate is depending on the groundwater levels, the water board is facing the choice whether to lower the water levels, following the soil subsidence, in order to maintain a freeboard that is suitable for agriculture (current practice) or stop this lowering of the water levels in order to slow down the subsidence of the soil. The first part of study aims at filling some of the knowledge gaps that are currently still obstructing an inclusive assessment of the different management options available. These are the lack of a comprehensive overview of the outcomes of previous studies on this subject and the lack of knowledge concerning the relationship between soil subsidence and the costs for road and sewer management. The second part of the study aims at indicating what scenarios for future management may contribute to a sustainable development of the peat meadows. For the first part, a literature study and interviews with municipal road and sewer managers were carried out. For the second part, four scenarios were compared, of which two assumed a continuation of the lowering of the water levels, and in the other two, water level lowering is ceased. Of both scenario sets, the first assumed no additional measures to mitigate the negative consequences of the scenario, and the second did include such measures. It turned out that, when contemplating soil subsidence and measures to mitigate soil subsidence, the following costs and benefits are relevant to take into account: pumping costs, initial investment costs, agricultural yield, foundation damage, damage to roads, sewers, pipelines and cables, high water level facilities, sagging of dikes, inundation damage, water quality, property prices, recreational benefits, greenhouse gas emissions, landscape and cultural heritage, biodiversity/non-use value of nature and further emotional aspects. Furthermore, it became clear that municipal road and sewer managers indeed need to take additional measures as a result of soil subsidence. The costs per m2 for road management in peat meadow areas are about 25% to 35% higher than in areas without soil subsidence. Concerning sewers, the major difference was that the depreciation periods of sewers in peat meadow areas were much shorter (40 years instead of 60 years for gravity drained sewers) and that there are additional costs for cleaning the sewers. The scenario study indicated that it is not possible to maintain the current use of peat meadows in a way that is physically sustainable. Agricultural use of the peat meadows will always entail some soil subsidence. Although the scenario study showed that the total costs of soil subsidence will likely be the lowest in scenarios where the lowering of the water levels is ceased and additional measures are taken to mitigate loss of agricultural yield, this will still have a negative impact on the agricultural yield. Therefore, the choice whether to put a strategy like this in practice will be mainly a political one.