Environmental impacts of groundwater extraction at the Drentsche Aa brook valley. Vegetation development and greenhouse gas emission reductions of rewetting measures.

Summary The Drentsche Aa brook valley contains various valuable ecosystems and harbours many rare species. Groundwater is extracted in the area to produce drinking water. This affects the local ecosystems by reducing groundwater exfiltration, which can cause a series of negative environmental effects, such as desiccation, acidification, eutrophication and greenhouse gas emissions. Parts of the Drentsche Aa brook valley catchment are drained for agriculture, while other areas have been rewetted to improve environmental conditions. Rewetting can potentially mitigate the environmental problems associated with groundwater extraction. The ecohydrological system of the Drentsche Aa is complex, but most of it is understood in a qualitative sense. In this learning thesis several aspects of the system have been quantified. The main research question was: ‘How does groundwater extraction for drinking water production affect the natural environment of the Drentsche Aa brook valley, and is rewetting a suitable compensation strategy to mitigate these negative environmental effects?’. To answer this question, the effects have been quantified by studying the vegetation and greenhouse gas emissions. With this knowledge, management decisions can perhaps be better substantiated, which would allow for a more optimal environmental management. To study the effects of groundwater extraction on the environment, vegetation maps of six nature reserves spanning three decades were analysed. The vegetation can indicate different environmental conditions which can be affected by groundwater extraction. After the new groundwater pumping stations in Annen and the Breevenen have been opened, the Eexterveld reserve severely acidified. This suggests that the new groundwater pumping stations might partly be responsible for this development. The groundwater pumping station in Assen recently reduced its extraction rate. Smalbroek, which lies within its impact zone, has acidified the least of the six nature reserves. Three other reserves lie within the former impact zone of the closed pumping station of Zuidlaren. These areas show an increase in mesotrophic vegetation types, suggesting the development of groundwater-dependent ecosystems. The Kappersbult is affected by groundwater extraction and drainage for agriculture. The vegetation developed to more acidic types and it is expected that this will have negative consequences for the reserve in the future. Overall, the vegetation in the Drentsche Aa brook valley has developed towards wetter and less eutrophic, but more acidic types. Further improvements to the environment of the Drentsche Aa brook valley are still possible, especially to reduce acidification and promote exfiltration-dependent vegetation. However, the groundwater pumping facilities in Assen, De Punt, Annen and the Breevenen appear to remain among the obstacles that prevent this in parts of the catchment area. To improve the environmental conditions in the Drentsche Aa brook valley, 635 hectares of natural area have been rewetted since 1996. As a secondary benefit, rewetting measures can reduce greenhouse gas emissions from peat mineralization. These emissions reductions were calculated from peat maps and the emission factors of CO2 and CH4 in relation to the groundwater level rise. About 400 tonnes of CO2-equivalent emissions are prevented each year as a result of these rewetting measures. Such a magnitude of emission reduction can be profitable to sell on the voluntary carbon market and this could become an additional source of income for nature management. The Breevenen project is a combination of wet nature development and groundwater extraction. Agricultural lands have been rewetted and converted into nature areas. Greenhouse gas emissions due to peat mineralization were not reduced substantially because rewetting appeared insufficient. It is expected that the whole peat stock of ca. 341 kilotonnes of CO2-equivalent will be emitted eventually. The main environmental benefit of the Breevenen project is the conversion of agricultural land into natural area. This has led to a reduction of about 3.3 kilotonnes of CO2-equivalent emissions per year, due to termination of agricultural activities. The project does not mitigate the negative environmental effects of groundwater extraction, as originally planned. However, the project does compensate for habitat degradation because of the development of a large semi-natural nature area. Groundwater extraction appears to promote acidic and eutrophic vegetation types in the Drentsche Aa brook valley. Proper rewetting can not only be a suitable compensation strategy to mitigate these negative environmental effects; it can also be an effective measure to reduce greenhouse gas emissions.