Improving understanding of hydrological and biogeochemical processes controlling the effectiveness of two-stage ditches in reducing eutrophication

Combating eutrophication requires holistic mitigation measures aimed at reducing agricultural losses of nitrogen (N) and phosphorus (P) from field sources to aquatic systems. This need will become critical in the future as increased flashiness, expected from changing climate and growing food demand, will further accelerate N and P pollution. Agricultural headwater streams are the main entry point of diffuse nutrient and particulate losses to stream networks. These streams are often channelized with a trapezoidal design, which effectively convey excess water from fields but also nutrients and soil particles to recipient water bodies. In addition, trapezoidal ditches with steep stream banks are sensitive to erosion and require routinely dredging to maintain their drainage function.

This project will advance the knowledge of processes governing nutrient and sediment retention in agricultural streams in Sweden by focusing on two-stage ditches (SDs) which are new type of mitigation measure. In SDs, the stream channel is surrounded by incised floodplains that increase the hydrological connectivity between stream and riparian zone during high flows. When floodplains are inundated, water residence time increases which allow deposition of suspended particles and biogeochemical processing of nutrients and organic matter. With slower water velocities in SDs at high flows, bank erosion may also be minimized. Existing studies in the US and Finland have found that SDs can mitigate N, P and sediment losses, compared with traditional trapezoidal ditches. This project is the first of its kind to evaluate the efficiency and stability of 10 different SDs in Swedish conditions over 3 years, situated in catchments with diverse agricultural land use and soil characteristics.

In this presentation, we show the details of experimental setup and preliminary results. A sampling has been setup in SD reaches to monitor seasonal sedimentation rate and aggradation since construction. This is coupled with both low- and high-frequency water quality monitoring and measurements of nitrogenous gas emissions from denitrification in sediments. We discuss the success factors in terms of placement and design of SDs to enhance ecosystem functions (self-purification, erosion and flood prevention). These factors determine SDs’ effectiveness in retention of water, N, P and sediments and their suitability as mitigation measure.