Impact of Climate and Hydrological Variability on Drinking Water Production and Trihalomethane Levels: A Case Study in Barcelona, Spain (2010-2024).

Surface water-based utilities increasingly face challenges in drinking water production during prolonged droughts and heavy rainfall events. We assessed the impact of climate and hydrological variability on trihalomethane (THM) levels in two drinking water treatment plants in Barcelona: one river-based (Llobregat plant) and one reservoir-based (Ter plant). We examined data from 15 years (2010-2024) using generalized additive models (GAMs) to evaluate the change (β) in chloroform, bromodichloromethane, dibromochloromethane, bromoform, and total THMs (THM4), by extreme (≤percentile 10, ≥percentile 90) hydrometeorological predictors, including temperature, river flow, or reservoir level relative to normal conditions (P10-P90), and the Standardized Precipitation Evapotranspiration Index (SPEI 1). In the Llobregat plant, THMs were unaffected under low river flow events (≤P10), while THM4 decreased by -1.41 (confidence interval (CI) 95%: -2.77, -0.05) during high river flow events (≥P90), mainly driven by bromoform (β: -2.64, CI 95%: -3.61, -1.67). In the Ter plant, THM4 increased by 1.64 (CI 95%:0.09, 3.19) and 4.08 (CI 95%:0.83, 7.33), respectively, under high (≥P90) and low (≤P10) reservoir levels. Overall, moderate effects of extreme weather events on THM levels were observed, attributed to climate-resilient water management strategies. Further research is needed in other settings with diverse water sources and management.