Spatial and temporal variations in aquatic organic matter composition in UK surface waters

Drinking water is becoming more difficult to treat, especially in the UK, due to the changing concentration and composition of aquatic dissolved organic matter (DOM). The spatial and temporal variations in the DOM composition are not well understood. This study investigated how DOM composition varies along a north/south gradient in the UK, over four years, and between headwaters and reservoirs. There were trends in DOM composition metrics from north to south; carbohydrate and peptide-like compounds were lower in northern sites, while lipid-like compounds were lower further south, suggesting different sources of DOM in north/south catchments. DOM collected in Autumn 2021, after a Summer of low rainfall, was more aromatic, less oxidized, and more diverse than DOM collected in 2018–2020. Decreased lipid content and increased oxy-aromatic content occurred in Autumn, at the end of the plant growing season, when increased rainfall rewets catchments and mobilizes soil OM into surface waters. These seasonal changes in DOM composition coincide with increased DOM concentrations in raw drinking water, leading to more challenges for drinking water treatment, especially as climate change alters rainfall distribution in the UK.