European stakeholders engaged in combatting the eutrophication of the North Sea welcomed three Water Framework Directive innovations: a more holistic approach to quality, the binding nature of WFD objectives, and greater public participation. Twenty years later, however, there has been a disappointing amount of progress in the reduction of diffuse pollution. In the Seine River Basin, the amount of livestock rearing is low; yet the basin is subject to significant diffuse pollution due to agriculture. This paper reports our study of this case; we examine the literature on WFD implementation policy in order to identify the physical and social causes of this failure to reduce diffuse pollution. We show that the nitrates, phosphorus, and pesticides that affect ground, surface and marine waters are attributable to structural changes in agricultural production rather than to inefficient farming practices. We describe how a series of instruments that were designed to combat the diffuse agricultural origins of pollutants have had little effect. We identify the main obstacles to improvement as being the dispersion of the targeted public and the dispersion of benefits, given the current nature of legitimacy in the European Union. This case illustrates the fact that intensive agricultural production has an impact on water quality far beyond the problem of excess manure from livestock production.