Sediment and nutrient trapping in field wetlands.

Ockenden, Mary and Deasy, Clare and Quinton, John and Favaretto, Nerilde and Stoate, Chris (2012) Sediment and nutrient trapping in field wetlands. In: Workshop on Diffuse Agricultural Pollution Methods, 2012-09-252012-09-26.

Abstract

Agriculture has been identified as the largest sector contributing to diffuse water pollution. Field drains have been identified as a fast pathway for dissolved nutrients to reach the waterways, potentially bypassing conventional buffer zones. Field wetlands, (small, unlined ponds (< 500 m2) constructed in unproductive agricultural areas and designed to intercept runoff), are a simple and effective option for diffuse pollution mitigation, addressing both sediment-associated and dissolved pollutants. This paper describes the results of sediment and nutrient trapping from ten field wetlands built at four sites in the UK, covering a range of soil types and runoff sources. Sediment and nutrient retention in the wetlands was estimated from samples collected at the inlet and outlet of each wetland, as well as from an annual survey of the sediment retained in each wetland. Sediment masses trapped within the wetlands ranged from 0.3 – 1 t yr-1 at a clay soil site to 25 – 40 t yr-1 at a sandy soil site. At the sandy site, 30 – 70 kg yr-1 total phosphorus (TP) and 60 – 200 kg yr-1 total nitrogen (TN) was trapped with the sediment. Rainfall was a confounding factor, with much lower rainfall at the clay site during the monitoring period. Concentrations of both particulate and dissolved nutrients were observed to decrease between the inlet and outlet of wetland systems. At a sandy soil site, the average concentration of some nutrients was reduced by up to 80% in a wetland system with a long hydraulic residence time. Overall, these multi-functional edge-of-field features have shown good potential for reduction of sediment and nutrient input to the waterways and should be considered alongside in-field measures as part of an integrated solution for catchment management.