## 4 Results and Discussion
### The History of PCDD/F Contamination in the Venice Lagoon  
As already evidenced in Section 4.1, also in the Venice Lagoon the acquisition of reliable sediment chronologies was a mandatory prerequisite for the correct definition of PCDD/F temporal trends. Once sedimentary levels are correctly associated to a reasonable range of deposition years (i.e., an error confined below 5 years in the most recent levels, increasing to 20-30 years at higher depths) it is possible to \"read\" sediment profiles as historical documents.  
Salt marshes are coastal wetlands that are regularly flooded and drained by seawater following the tidal cycle. They are found mostly at middle to high latitudes and are common habitats in the Venice Lagoon, where they are called \"barene\". Among their many ecosystem services [42], Venetian salt marshes represent an important historical and scientific archive that is able to record important environmental parameters over many decades [23; 47; 48], when their average accretion rates are comparable to the long-term average rate of sea level rise (SLR) in the lagoon (\(\sim\)0.25 cm year\({}^{-1}\); [23]).  
Figure 6 shows the pattern of PCDD/F contamination recorded by salt marsh core M3 (Figure 1b). It is evident that PCDD/F concentrations reached the highest values between 1940 and 1980, and then decreased significantly. This \({}^{210}\)Pb\({}_{\text{ex}}\) dating is highly reliable, since it was verified by Cs-137 peak activity at 4 cm depth, following the Chernobyl accident in 1986. The recent decreasing trend of PCDD/F concentrations in salt marshes was attributed to a strong reduction of inputs, due to the closure of the most polluting activities within the first industrial area, and to the promulgation of the first environmental regulations in Italy in the mid 1970s. In particular, the already cited Law 319/1976 regulated the composition of industrial effluents, and the Ministerial Decree 23 April 1998 (\"Water Quality in the Venice Lagoon\" published on the Official Gazette n. 140 and further revised in 1999) established very strict limits for industrial effluents to lagoon waters, banning the discharge of several toxic, persistent, and bioaccumulative contaminants (As, Cd, PCDD/Fs, PAHs, Hg, Pb, PCBs, chlorinated pesticides, cyanides, and tributyltin compounds, TBTs). During this period, several wastewater treatment plants became operative [28].  
The definition of these contaminant input chronologies in salt marshes and lagoon sediments [73] has permitted assessment of the risks for both the environment and humans, with the use of models that were combined to create several past and future scenarios [74; 75; 76]. They have highlighted the predominant role of deep sediments as secondary sources of contamination through dredging [74; 75] and the varying distribution pathways under different wind scenarios (i.e., bora and scirocco) [76]. Input chronologies were also used to implement a human intake and a physiologically-based pharmaco-kinetic model [77] used to simulate human internal exposure and provide a preliminary ecological and human health risk assessment for selected chemicals. Moreover, contaminant concentrations in sediments were at the base of a dynamic multimedia model applied to selected PCB and PCDD/F congeners, simulating the effects of climate change on their distribution and fluxes over the next 50 years in the Venice Lagoon (Italy) [78]. This model's results suggested that global warming would probably enhance PCB and PCDD/Fs mobility and hence their potential for long range atmospheric transport, reducing the environmental levels of these chemicals in the lagoon.