Sediment ponds; an effective mitigation measure for reducing nutrient export in agricultural drainage ditches?

The agricultural sector is a large contributor to poor water quality in our freshwater systems. One potential pathway for agriculturally sourced pollution to enter the freshwater environment is through drainage ditches, which can be either open surface drains or subsurface pipes, or a combination of both. While large scale tile-drainage systems with a central output point are common in some countries, Irish agricultural drainage networks tend to be comprised of a complex network of drains, each with varying levels of connectivity to freshwater systems. Recent research by Moloney et al. (2020) categorised these drains in terms of their connectivity, finding that those with a direct connection between a farmyard and river/stream were at the greatest risk for transporting the highest concentrations of nutrients (phosphorus and nitrogen). Therefore, to target the most ideal location to mitigate against nutrient transport through drainage ditches, drains with direct farmyard connectivity provide the most resource- and cost-effective option.

This study investigated the effectiveness of sediment ponds installed in drainage ditches which had a direct connection between a farmyard and a river. Three case study farms were selected, two in the south (Cork) and one in the south-east (Wexford) of Ireland. All ponds were installed by 2021, with sampling commencing in December 2022. Grab water samples were collected weekly (Wexford farm) and fortnightly (Cork farms) at multiple points upstream and downstream of the ponds, and were analysed for nitrogen, phosphorus and dissolved organic carbon. Sediment samples were extracted from within the drainage ditches in summer 2023 and analysed for Mehlich-3 P, pH, Morgan’s P and particle size distribution. Initial results suggest ponds provide limited attenuation of nutrients, with no significant decreases at the downstream sample points. While sediment phosphorus concentrations are marginally elevated downstream, suggesting potential accumulation in the soil, further sampling is needed to confirm this trend. This study provides valuable insights into nutrient dynamics within agricultural drainage ditches and contributes to a better understanding of the effectiveness of sediment ponds as a potential mitigation measure for nutrient retention.