Reducing diffuse pollution in agricultural catchments: retention of sediment and nutrients in field wetlands

Ockenden, Mary and Deasy, Clare and Quinton, John and Favaretto, Nerilde and Stoate, Chris (2012) Reducing diffuse pollution in agricultural catchments: retention of sediment and nutrients in field wetlands. In: Hydrology for a Changing World : Proceedings of British Hydrological Society National Symposium, Dundee, Scotland. July 2012. UNSPECIFIED, GBR.

Abstract

Diffuse water pollution continues to be an important environmental concern, with only 28% of surface waters in the UK currently classified as ‘good ecological status’ in a recent update of Water Framework Directive classifications. Agriculture has been identified as the largest sector associated with diffuse sources of nutrients and sediment. Field wetlands have been used for mitigation of diffuse pollution in Scandinavia but there is currently little quantitative evidence of their effectiveness in the UK. Ten field wetlands have been built at four sites in the UK, covering a range of soil types, runoff sources, wetland sizes and wetland designs. The wetlands, small (< 350 m2), unlined ponds constructed along runoff pathways, slow the connection between the pollution source and the waterways, and provide more opportunity for sediment and nutrients to settle out or be taken up by aquatic organisms. This paper describes sediment retention in all ten wetlands and nutrient retention at one highly polluted site. Sediment retention was estimated from annual surveys of the sediment build-up in each wetland. Sediment trapping rates were highest on a sandy soil site (0.5 – 6 t ha-1 yr-1), compared to a silty soil site (0.02 – 0.4 t ha-1 yr-1) and a clay soil site (0.01 – 0.07 t ha-1 yr-1). Nutrient retention was estimated from samples collected at the inlet and outlet of each wetland. At Whinton Hill, Cumbria, a wetland system with a long hydraulic residence time, concentrations of both particulate and dissolved nutrients were observed to decrease between the inlet and outlet of the wetland system. The average concentration of some nutrients was observed to reduce by up to 80%. Overall, small field wetlands have shown good potential for mitigation of diffuse pollution and should be considered alongside in-field measures as part of an integrated solution for catchment management.