Improving water quality at the catchment scale through process-informed management and mitigation

Improving water quality at the catchment scale proves difficult as indicated by shifting deadlines of the Water Framework Directive for achieving good chemical and ecological status in European freshwaters. Low-hanging fruits of reducing point sources of pollution have already been targeted in most catchments, with more challenging and persistent diffuse pollution jeopardising widespread water quality improvements. Diffuse pollution originates from a range of anthropogenic activities such as agriculture, forestry and mining and leads to gradual accumulation of pollutants in impacted catchments. These abundant pools of pollution, also known as legacies, can control water quality and limit the effectiveness of mitigation measures in the long term. Yet, our understanding of hydrological and biogeochemical processes controlling mobilisation, transformation, transport and impact of pollution legacies in catchments is still limited. Here, we present results from multiple projects focusing on identifying processes controlling eutrophication and erosion in headwater agricultural catchments to support management and mitigation. We use a suite of experimental and modelling tools from high-frequency stream chemistry data, in situ measurements of dominant processes, laboratory assays, and process-based models to quantify hydrological and biogeochemical processes. Our results show that despite common pollution pressures, agricultural catchments differ in how they modulate pollution legacies. Their resilience/sensitivity to pollution depends on the interplay between hydrological and biogeochemical processes. Biogeochemical processes such as sorption, sedimentation and denitrification show potential for pollution retention and removal; however, their capacity is rapidly exhausted during hydrologic events mobilising large fluxes of legacy pollutants. As the importance of hydrological accumulation increases with catchment size, the cumulative impact of mitigation measures on water quality declines along the stream network. Finally, our modelling approach shows that only large-scale mitigation interventions are likely to bring the required water quality improvements, particularly under future climatic conditions with accelerated biogeochemical and hydrological processes. Understanding these processes is key to effective mitigation and reducing potential pollution swapping in heavily impacted catchments.