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.