b'8Assessing the risk of colloidal phosphorus transfer togroundwater in relation to soil chemical and physical properties Fresne M1 , 2,3, Jordan P2, Daly K3, Fenton O4 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 EnvironmentResearch Centre, Wexford, Co. Wexford, IrelandColloid-facilitated phosphorus transport is an important mechanism of phosphorus loss from soils to groundwater due to the high sorbing capacity of colloids. In order to better localise critical source areas of colloidal phosphorus loss to groundwater, it is important to better understand the role of soil properties in the transfer pathways of colloidal phosphorus. For this purpose, soil chemical and physical properties were examined in three Agricultural Catchments Programme sites with contrasting soil properties and land use. The study included ten sites adjacent to long-term groundwater chemistry monitoring wells. Composite soils were analysed to determine their chemical properties such as soil Water Dispersible Colloidalphosphorus and metals fractions, phosphorus sorption properties, degree of phosphorussaturation or soil solution phosphorus. Undisturbed soil cores were also taken to determine soil hydraulic properties through the determination of Soil Water Retention Curves. Physicaland hydraulic parameters were then incorporated into Hydrus-1D to model solute transport to groundwater at two targeted sites. Results showed a strong effect of the soil chemical properties on colloidal phosphorus source and mobilisation in soils. In particular, the labile inorganic iron fraction positively influenced the colloidal phosphorus fraction whereas the soil organic matter correlated negatively withcolloidal phosphorus suggesting competition for sorption sites. Mobilisation of colloidalphosphorus was also positively influenced by weak soil sorption properties and a high degree of phosphorus saturation. Soil physical and hydraulic properties also influenced transfer to groundwater. Higher long-term groundwater phosphorus concentrations were measured at a site with a higher soil saturated hydraulic conductivity, total porosity and macroporosity suggesting transport limitation. Lower concentrations were measured at a site with a higher soil bulk density. However, the high spatial variability in soil properties highlight the need for larger scale studies with consideration of below ground colloidal phosphorus transfers.52'