A Review of On-farm Roadway Runoff Characterisation and Potential Management Options for Ireland

Runoff from farm roadways within farm boundaries are acknowledged as a year-round source of pollutants discharging to surface water, particularly during the main grazing season (Feb-Nov) when their usage is high. These losses are considered to be a significant catchment scale pressure and have led to recent legislation in Ireland to prohibit direct runoff from farm roadways to waters with similar guidance in the UK and New Zealand. However, roadway runoff (RR) remains an undocumented and understudied part of the transfer continuum where knowledge gaps remain in relation to its quantity and composition and in relation to the impacts of RR management options on adjacent surface waters. Indeed some information on mitigation measure design and efficacy is only available from non-agriculture land uses e.g. forestry and needs to be presented in a farm specific context. The present review brings together knowledge on RR in terms of content on- or off-roadway management options and proposes alternative mitigation measures that may require pilot scale testing. Studies show that RR contains a mix of legacy (surface materials) and incidental (fresh urine and faeces) constituents such as phosphorus (P), Escherichia coli (E. Coli) and sediment which become temporarily mobilised during rainfall events. Once mobilised, the roadway network can quickly transport pollutants providing connectivity between farmyards, hard standings, underpasses where attached to a watercourse, fields and even public roadways to watercourses. Its contents are not dissimilar to dilute slurry or dairy soiled water with loads being highest where animals congregate (within 100 m of the farmyard or where roadway configuration or quality impedes stock movement along the network). On-roadway management options include regularly spaced diversion structures that divert runoff for passive treatment in a field or drainage ditch or the application of chemical amendments to roadway surfaces. Off-roadway options can include natural or enhanced features, which attenuate flow leading to settlement of suspended sediment and in some cases treatment of a percentage of the load before delivery to a watercourse. It should be noted that the options presented in this review need to be retrofitted to site specific conditions. Therefore, a number of factors, including their effectiveness, cost and management needs on a particular farm, as well as local regulation requirements will need to be considered prior to implementation. Therefore, only the measures that divert RR away from waters are applicable to Ireland. Future research should develop and validate a farm roadway risk assessment tool to identify vulnerable roadway sections that need management and/or re-design, and identify means of incorporating RR into catchment scale risk assessment models. In addition, design of on- and off-RR management options should consider the wide variations in farm configuration and management with regard to size, topography, soil type/drainage class, enterprise, intensity, scale, stage of development and the potential impacts of climate change.