Nitrate (N03) contamination is a significant global concern in many agricultural catchments. This research undertook a national scale and a catchment scale to investigate the influence of hydrogeological setting the fate and transport of N03 in agricultural catchments. In this researc statistical analyses of a national database showed that groundwater N03 concentrations are controlled by a combination of factors including the hydrogeological. setting, which incorporates transmissivitY ' and flow path length, landuse pressure, soil type, subsoil thickness and" permeability, groundwater oxidation reduction potential (ORP) conditions. A catchment scale study was undertaken of two study catchments containing contrasting hydrogeological characteristics. In the Nuenna catchment underlain by an Rkd aquifer, point sources are not a significant influence on the water quality in terms of N in the Nuenna River. In the Glen Burn catchment, underlain by a Pl aquifer, point sources have a significant influence on the Glen Burn River water quality. This contribution from sources increases as the groundwater level decreases and discharge from shallow groundwater reduces. Greater storage and persistence of N03 in the Nuenna Rkd aquifer indicate that lag time for a decrease in groundwater N03 concentrations to occur more significant factor in Rkd aquifers. In contrast, lag time is not as significant in the Glen Burn Pl aquifer where N03 concentrations in the shallow bedrock are influenced by groundwater table fluctuations, the presence of preferential pathways through the till and seasonal changes pressure loadings. Denitrification with depth in this aquifer is shown through suitable hydrochemical conditions, an isotopic enrichment ratio 1.8:1 between 015N and 0180 and the presence of bacteria containing the gene. In contrast, hydrochemical, isotopic and microbial analyses have that denitrification is not significant in the Nuenna catchment, but the dominant biogeochemical process is nitrification.