Charting water quality improvements and practice reversion with pesticide interventions at catchment scale

Freshwater quality, and the impacts of farming practice on drinking water supplies, are of concern in many countries and time-limited catchment management interventions are commonly used to improve water quality. However, ending such schemes may result in practice reversion. This study adopts an interdisciplinary approach combining evidence from water quality monitoring data with a behavioural study of farmers to explore changes in land use practice with reference to the pesticide MCPA (2-methyl-4-chlorophenoxyacetic acid) following a catchment-based management scheme delivered in the cross-border Derg catchment in Northern Ireland/Ireland between 2018 and 2021. Analysis of over 10,000 water samples demonstrated that, compared to the Finn (Control), the scheme achieved a 15.2 % and a 5.5 % reduction in the frequency with which MCPA concentrations in the Derg (Treatment) exceeded the total (0.5 μg L^-1) and individual (0.1 μg L^-1) EU Drinking Water Directive limits for treated drinking water respectively. The post-intervention flow-weighted mean concentration (FWMC) of MCPA for Peak usage season (April-October) was 19 % lower than pre-intervention in the Derg when compared to the Finn, although the during-intervention Peak season FWMC was lower in the Derg than post-intervention, suggesting practice reversion. The farmer survey and workshops provided further evidence of changes in pesticide usage, but also subsequent practice reversion due to a strong status quo bias for MCPA and other, mainly financial, barriers inhibiting a shift to the alternative pesticide, glyphosate. This study concludes that catchment approaches can be successful, but sustained investment in catchment measures is essential to effect meaningful long-term behavioural change.