Comparison between a variety of territorial contexts is often useful to explain the differing implementation of public policies. At the same time, however, such comparisons often come at the cost of detailed knowledge of each context and a precise understanding of the causal mechanisms at stake. In this article, we present a methodology developed in order to benefit from both a medium-N comparison and from a substantial understanding of causal mechanisms, arrived at through close observation in the field. This research design combines a process tracing approach with a qualitative comparative analysis (QCA) of 15 cases of (non-)implementation of public polices in France. This article highlights two important factors that can help to explain how these policies were implemented: the existence of a coalition of local actors that supported the protection of water resources and a reduction in uncertainties with regard to the source and effects of water pollution.