Efficacy of source-control, use-related and end-of-pipe regulations on river water quality in a large German river catchment: a 50-year trajectory

River water quality changes have been shown to follow typical trajectories, often characterized by sequential phases of accelerated degradation, environmental recovery, and responsive management. However, the relationships between anthropogenic mitigation measures such as regulatory interventions and their effects on water quality remain rarely studied and poorly understood.

In this study, we evaluate the effectiveness of three types of regulations: source-control, use-related and end-of-pipe regulations. Using phosphorus (P) as a model substance for water quality, we analyse a 50-year trajectory of measured total phosphorus (TP) concentrations in the river Ruhr, Germany, and link this with a comprehensive analysis of water quality related laws and regulations being enforced at the national and European level over the same time period. We categorized the regulations according to the aforementioned types and re-analysed the infrastructure developments and operation modes in a literature review and based on research in the archives of the responsible river basin management authority.

The strong decline of TP concentrations from a maximum of 0.59mg/l TP in 1977 to around 0.05mg/l TP in the early 21^st century resulted dominantly from source control by banning of P in detergents, the parallel construction of wastewater treatment plants and their sequential upgrade to treatment stages incorporating P removal. Thus, while point source pollution decreased, the share of agricultural and other diffuse sources of riverine TP concentrations increased to around 50%, making them the focus of attention nowadays. As source control and end-of-pipe measures have reached a level at which a further reduction of TP concentration in the river through those means would be marginal, use-related measures gain importance, especially for agricultural practices.

Our results show that source-control was the most effective and fastest way of reducing TP pollution, whereas end-of-pipe measures were a necessary, complementary way to reduce P related water quality impairment. Given the current dominance of diffuse pollution sources resulting from agricultural inputs, where the effectiveness of regulation is likely to be limited, additional measures such as awareness, economic incentives and support for agricultural best management practices need to be addressed. These findings may provide important insights into understanding the effectiveness of different regulatory measures, in particular with regard to the increasing introduction of (new) pollutants and associated impacts on the environment and human health.