Determining the impact of organic chemicals and nutrients on river ecological quality: a case study from Donegal, Ireland.

A benthic macroinvertebrate indicator known as Q-value shows that the ecological quality of rivers in Donegal, Ireland has declined since biological monitoring began in 2004. Previous studies suggested that the insecticide cypermethrin may be a driving factor. Cypermethrin and three other organic chemicals (MCPA, PFOA and PFOS) have been frequently detected in rivers in Donegal since hazardous chemical monitoring started in 2020. Available monitoring data on those organic chemicals and nutrients, from 15 river sites in Donegal between 2020 and 2024, were analysed to estimate their relative impact on river ecological quality, using Q-values as surrogate of ecological quality. Around 10-30% of cypermethrin, MCPA and phosphate levels were above relevant environmental standards, while PFOA, PFOS and nitrate levels were below. Bivariate analyses showed negative relationships of Q-value with cypermethrin, PFOS and nutrients but not with MCPA or PFOA. Multivariate models confirmed that cypermethrin may be an important driver of ecological decline in Donegal rivers, explaining around 20% of the Q-value trends (partial R squared, pR²: 0.2009). Q-value trends were also explained by PFOS (pR²: 0.2907), nitrate (pR²: 0.1834) and phosphate (pR²: 0.1073). Our results are based on approximately 1,500 datapoints from 15 (∼4%) river sampling sites in Donegal. However, they suggest the need to: 1) better align biological and chemical monitoring programmes; 2) assess the ecological impact of chemicals even at relatively low concentrations, and 3) further reduce surface water pollution from nutrients and PFAS. Future studies are recommended using larger datasets from Ireland or other countries with similar monitoring programmes.