Characterising N and P concentration patterns in farmyard-connected drainage ditches and the potential of leaky dams as an in-ditch mitigation option

Agricultural drainage ditches can act as important pathways for the delivery of phosphorus (P), nitrogen (N) and sediment to surface waters. However, spatial and temporal variability in nutrient concentrations within these systems remains poorly characterised, despite their role as direct hydrological connections between farmyards and receiving waters. In addition, these networks provide opportunity to facilitate targeted in-ditch mitigation. This study characterised N and P concentrations within farmyard-connected drainage ditches on two dairy farms (W and X) in south-east Ireland and evaluated the effectiveness of wooden leaky dams as an in-ditch mitigation measure. Water quality was monitored along ditch networks over a two-year period, including pre- and post-dam installation and at upstream and downstream locations. Flow was modelled and combined with nutrient concentration data to estimate loads. Sediment retained behind dams was analysed for available P. Distinct differences in N and P concentrations were observed between the two ditch systems, reflecting variation in hydrological connectivity and ditch configuration, highlighting the importance of considering these factors with respect to in-ditch mitigation. Phosphate concentrations in drainage water were significantly lower after interception by dams (studied at Farm W only), and modelled P loads exported from the farm were also reduced, thereby showing potential to reduce impact. Sediment accumulated behind the dams and often had elevated available P levels, indicating retention of sediment-bound nutrients. Overall, leaky dams showed potential as a low-cost mitigation option, but their performance should be further validated, and factors such as design and maintenance require consideration.