Upland catchments are important drinking water source areas in Ireland, representing areas with lower potential for contamination and, therefore, treatment costs. However, in recent years, issues with the selective herbicide, 2-methyl-4-chlorophenoxyacetic acid (MCPA), and exceedances of the EU Drinking Water limit (0.1 µg L-1) in the raw surface water supply, have become a concern. MCPA usage is increasingly prevalent in areas of marginal and upland agricultural land where it is applied in areas of rough grazing and pasture to control broadleaf weeds and, in particular, soft rush (Juncus effusus) cover. One compounding factor in the increased use of MCPA is that EU farm subsidies are calculated on an area basis, with land that has a high density of rushes deemed as ineligible for payments. A major initiative in Irish catchments (www.sourcetotap.eu) is investigating whether financial incentives to alter MCPA management practices in drinking water catchments could offset the costs of water treatment. A programme of monitoring has been established across two river catchments (c. 380 km2), with high resolution monitoring being used to characterise MCPA fluxes at catchment scale and provide a baseline from which to gauge the effectiveness of the management measures implemented. Initial analyses have demonstrated that exceedances of the drinking water limit are more frequent (~37% of samples) than indicated by weekly raw water samples at local water treatment works. The findings highlight that the extent of the MCPA issue is far greater than initially thought and that MCPA transfers are strongly driven by short duration storm events occurring over several hours to a day and which also persist in baseflows.