Forecast of organic matter content in the waters of the Danube river (Kilia)

Purpose. To assess and forecast the risk of water quality deterioration in the Danube River within the city of Kiliya based on the biochemical oxygen demand (BOD₅) indicator. Methods. Statistical analysis methods were applied, including the lognormal distribution to estimate the probabilities of exceeding threshold concentrations, and extrapolation of the trend line to forecast the dynamics of the BOD₅ indicator. Data visualization was performed by constructing graphs for various time intervals. Results. The comparison between actual and forecasted BOD₅ values showed a satisfactory level of discrepancy (up to 17% for risk assessment and up to 5% for actual values), confirming the effectiveness of the chosen approach even under conditions of irregular monitoring data, while taking into account ecological standards and EU Directive requirements. It was found that the concentration of organic substances in the water at the 10% probability level significantly exceeds the ecologically permissible value of 3 mg/dm³ (according to EU standards) and shows a tendency for gradual increase. Conclusion. The results of the study confirm the effectiveness of the applied approach to water quality forecasting under conditions of irregular monitoring. The use of the lognormal distribution combined with trend line extrapolation allowed for reliable estimations of both the actual state of the water and the probability of exceeding ecologically hazardous concentrations.