b'Assessing the role of colloidal P in P delivery to surface water in two contrasting agricultural catchmentsFresne M1,2,3, Jordan P2, Daly K3, Fenton O3 and Mellander P-E1,31Agricultural Catchments Programme, Teagasc, Johnstown Castle Environment Research Centre, Wexford, Co. Wexford, Ireland2School of Geography and Environmental Sciences, University of Ulster, Coleraine, UK3Crops, Environment and Land Use Programme, Teagasc, Johnstown Castle Environment Research Centre, Wexford, Co. Wexford, IrelandBelowground pathways can be important carriers of nutrients and other pollutants to surfacewater in groundwater-fed river catchments. The high sorbing capacity of soil colloids canenhance transport of phosphorus (P) from soils to groundwater but also the delivery of P tosurface water via groundwater pathways.The role of colloidal P in P delivery to surface water was studied in two agricultural catchments with well drained soils but with contrasting land use (arable and grassland) of the Agricultural Catchments Programme. Free soil solution, stream water and shallow groundwater were sampled on a weekly basis to monitor background concentrations of particulate, dissolved and coarse colloidal (0.200.45 m) P fractions and P-related parameters (metals, TOC).In addition, high-frequency (2h) monitoring was carried out during rainfall events in stream water and in near-stream shallow groundwater to provide evidence on P fractions delivery to surface water via groundwater pathways. An automated sampler was deployed in the stream and anautomatic and disturbance-free sampling set up was developed for groundwater.Multi-parameter probes were also deployed to monitor stream water and near-stream shallow groundwater physico-chemical parameters (DO, temperature, conductivity, ORP). Streamdischarge was measured using a flow velocimeter coupled with a water level logger and stream gauging campaigns in order to quantify the different P fractions loads and apportioned pathways.Recent findings in the arable catchment showed that background stream P concentrations were comparable to shallow groundwater P concentrations, typical of groundwater-fed streams. The relative importance of the coarse colloidal P fraction in the total P fraction was low and increased quickly but briefly during a rainfall event. Changes in P concentrations during rainfall events were also associated with changes in metals (Al and Fe) and TOC concentrations. Further monitoring in the grassland catchment is expected to show higher P concentrations and a larger colloidal P fraction.10'