A catchment engineering approach to nutrient and sediment management: A case study from the UK

A rural catchment in the UK is being monitored in a multi-scaled, nested experiment to identify the parts of the landscape that contribute Diffuse Water Pollution from Agriculture (DWPA) by measuring phosphorus (P), nitrate (N) and suspended sediment (SS) losses across the river network. Results have been analysed spatially and temporally, with a focus on storm events and high flows, and it is conceptualised that the majority of polluted runoff is delivered via a relatively low number of diffuse flow pathways. It is therefore hypothesised that by targeting these distributed point sources and 'engineering' catchments, it is possible to mitigate the impacts of DWPA cost effectively without compromising agricultural productivity. Runoff Attenuation Features (RAFs) are examples of soft engineering measures, including small ponds, wetlands, sediment traps, filters, which are designed to intercept polluted flow pathways in order to slow, store and filter the runoff. A number of RAFs have been designed and constructed and their ability to trap SS and remove and recover P and N has been quantified. The interventions have demonstrated the ability to substantially reduce certain pollutant concentrations (up to 90% SS and 65% total P), particularly during storm events, as well as attenuating flood flows and providing wider ecosystem services. RAFs function most effectively when applied to farm ditches and small streams, relatively close to pollution sources, and we propose that this is the most cost effective Best Management Practice methodology. Our work has shown, however, that P-rich fine sediment (<0.45μm) is difficult to remove from runoff when relying on settlement alone. Therefore, a series of physical filter features have been developed, comprising a range of filter media installed in ponds and ditch networks to test their efficacy. Catchment engineering provides the ability to sustainably manage water quality and quantity using a proactive, interventionist approach. It is cost-effective, dynamic and involves stakeholder participation to ensure that any catchment modifications integrate with everyday agricultural practice whilst protecting, or even enhancing, the freshwater environment.