Spatial Distribution of PCDD/F in Surface Sediments of Lake Maggiore (Italy)

Lake Maggiore (LM) is the second largest Italian lake in terms of volume, surface and depth, with the northern third residing in Swiss territory. The most important tributaries are River Ticino (Northern inlet) and River Toce (Western inlet). The River Ticino leaves LM at the southern end and connects LM to the River Po. Particular concern arose from the contamination with DDT and its metabolites, which exceeded Italian and EU levels for consumption in fish, and was detected in bird eggs from fish feeding species as well. As a consequence the local Health Authorities prohibited commercial fishing of a series of contaminated fish species. The source of DDT contamination was a DDT production site, closed down in 1996. This plant was located on the Marmazza River, a tributary of River Toce entering LM from the westernmost part of LM, close to Baveno. Besides the contamination with DDT, PCBs, including some dioxin like congeners, were found at concentrations exceeding 3-7 times the levels of other sub alpine lakes in Italy. Levels of HCB and HCH in the LM ecosystem instead gave no rise to concern. The aim of the present study is to supplement the existing data on POPs with sediment levels of PCDD/Fs, where so far only few data are available for LM. The PCDD/F concentration in the 50 surficial sediment samples taken at water depths between 5 and 60 m in LM display a broad range of concentrations, from 0.13 to 32 pg WHO TEQ g-1 d.w.. The PCDD/Fs show a consistent congener pattern. In the sample taken at the inlet of River Bardello the congener distribution was dominated by Octachloro- followed by Heptachloro-dioxins; the Heptachloro- and Octachloro –furans occurred at a level of only 0.80 pg WHO TEQ g-1 d.w.. The spatial homogeneity of the pattern along the whole lake (Figure 2) underlines the absence of local sources and important riverine inputs into LM. Moreover, the pattern is similar to that of PCDD/Fs in atmospheric particulate matter and bulk deposition collected locally from LM3, and is typical for long range transport. This suggests that atmospheric deposition is the dominant vector for the introduction of PCDD/Fs into LM sediments. The spatial distribution of the PCDD/Fs shows two principal tendencies. First, a north/south gradient of the PCDD/F concentrations probably deriving from the flushing of sediment from the north to the south due to intermittent resuspension in the shallower north basin. Second, sediments close to incoming rivers show lower concentrations than sediments in settling basins where no direct riverine discharges occur. A similar observation can be made at the southern outlet (River Ticino) of LM where the PCDD/F concentrations decrease considerably compared to the southern section of the lake. This indicates that the settling conditions for particulate organic carbon (OC), the dominant transport vehicle for lipophilic compounds like PCDD/Fs in aquatic systems, are an important parameter for the PCDD/F burden in sediments. Consequently, lower concentrations of PCDD/Fs occur in the sediments close to river in and outlets, where turbulences inhibit the settling of OC. The spatial distribution of the total OC content in the sediments reflects the concentration distribution of the PCDD/Fs to some extent, however, not as pronounced as seen for PCDD/Fs. Only part of the spatial variations of PCDD/Fs in LM sediments can be explained by variations in total OC content of the sediments: The normalization of the PCDD/F concentration on the OC content of the sediments reduces the spatial variations from a factor of 246 (d.w. basis) to a factor of 85 (OC weight basis).

JRC.H.5 - Rural, water and ecosystem resources