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Predicting biochemical oxygen demand in European freshwater bodies

Abstract: 

Biochemical Oxygen Demand (BOD) is an indicator of organic pollution in freshwater bodies correlated to microbiological contamination. High BOD concentrations reduce oxygen availability, degrade aquatic habitats and biodiversity, and impair water use. High BOD loadings to freshwater systems are mainly coming from anthropogenic sources, comprising domestic and livestock waste, industrial emissions, and combined sewer overflows. We developed a conceptual model (GREEN+) to assess mean annual current organic pollution (BOD fluxes) across Europe. The model was informed with the latest available European datasets of domestic and industrial emissions, population and livestock densities. Model parameters were calibrated using 2008-2012 mean annual BOD concentrations measured in 2478 European monitoring stations, and validated with other 811 stations. The most sensitive model parameters were abatement of BOD by secondary treatment and the BOD decay exponent of travel time. The mean BOD concentrations measured in monitored stations was 2.10 mg/L and predicted concentrations were 2.64 mg/L; the 90th percentile of monitored BOD concentration was 3.51 mg/L while the predicted one was 4.90 mg/L. The model could correctly classify reaches for BOD concentrations classes, from high to poor quality, in 68% of cases, raising to 93% when considering acceptable one class difference. Conversely, high overestimations (incorrect classification by 2 or more classes) were 2% and large underestimations were 5% of cases. Across Europe about 14% of freshwater network was mapped to be failing good status due to excessive BOD concentrations (> 5 mg/L). Dominant sources of BOD to freshwaters and seas were point sources and emissions from intensive livestock systems. Comparison with previous assessments confirms a decline of BOD pollution since the introduction of EU legislation regulating water pollution.