Seasonal Distribution of PCDDs/PCDFs in the Small urban Reservoirs

Abstract

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