The Water Framework Directive (WFD) calls for various modes of public. These participation and involvement. These are judged as key factors to support the successful implementation in terms of attaining a good water status. This paper aims to explore the role of the ‘active involvement’ of stakeholders for the effective implementation of the WFD regarding the specific problem of reducing agricultural nitrate pollution of groundwater. Our case of reference is the Hase river catchment in northwest Germany, which is a paradigmatic example of an intensive livestock farming region with high nitrate intakes in groundwater. Emphasis is placed on the various forms of involvement that have recently been or will soon be established in northwest Germany at different spatial and administrative scales. We argue that although the WFD refers to whole river basins as the central unit of governance, it is particularly the regional and local scales that will strongly influence the implementation process. We identify different influencing factors and scenario paths, demonstrating both the uncertainties at stake and the range of possible effects that different outcomes of participatory processes will have. These, in turn, are closely linked to the interests, perceptions and strengths of different actors. Identification of critical paths and decision points enables corridors to be mapped out regarding the anticipated success or failure of regional public participation to reduce diffuse agricultural groundwater pollution.