N leaching to groundwater from dairy production involving grazing over the winter on a clay-loam soil

► A dense network of piezometers was installed to determine GW flows and N variation. ► Vertical travel time allowed correlation of management with groundwater N. ► Grazing over winter had no effect on N concentrations and losses to groundwater. ► Any losses from the production systems were reduced by denitrification and DNRA. ► Concentrations were influenced by point sources and GW-surface water interactions.