Seasonal Distribution of PCDDs/PCDFs in the Small urban Reservoirs

Man-made reservoirs constructed on running waters, where a decrease in flow velocity and an increase in flocculent settling occurs, create perfect conditions for the deposition of allohtonic matter and adsorbed pollutants, such as toxic polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). The results obtained from the research on a cascade of five small reservoirs located along an urban river (Lodz, Poland) showed significantly lower concentrations of PCDDs/PCDFs in the spring season in the first three reservoirs; whereas the other two demonstrated an opposite tendency with the lower values observed during the autumn. Also, analysis of the seasonal variations of the TEQ concentrations showed lower values in the reservoirs located at the beginning of the cascade (I, II and III) in spring, while in the lower ponds (IV and V) lesser values were observed during autumn. The winter and spring snow and ice melting, and the accelerated run-off washing out organic and mineral matter with the associated micropollutants accumulated during this period, were recognized as the main reasons for such seasonal distribution of PCDDs/ PCDFs. Intensive rains occurring during the autumn, especially after dry summers, may be in turn responsible for the raised total PCDDs/PCDFs and TEQ concentrations during the autumn sampling period. Furthermore, the discharges of polluted stormwater and illegal domestic sewage through the stormwater outlets located along the river boosted the reservoirs sediments in the PCDDs/PCDFs.