## 4 Results and Discussion
### Assessment of Hg Delivery to the Augusta Harbor: Direct Inputs or Resuspension?
In addition, such a surface value (0.25 mg kg\({}^{-1}\); Figure 3a) was still higher than the consensus-based threshold effect concentration of 0.18 mg kg\({}^{-1}\) reported by [52], posing a serious threat to biota and people. The same trends and events were recorded with more detail at site L, as the last 50-60 years of industrial history in the area had been represented by a higher number of measurements. This chronological reconstruction was impossible for core D, due to a highly disturbed sedimentary record, but its proximity to the MCCAP in the southern sector of the Augusta Harbor explained the very high Hg values at depth, still high even at the surface (92 mg kg\({}^{-1}\); Figure 3a). These high Hg values in surface sediments demanded further investigations because they could have been determined either by: (1) the presence of active industrial sources or  
Figure 3: (**a**) Hg concentration vs. year of deposition in cores A, L, and D. HCB concentration profile for core L is also reported. The dotted lines encompass the period from the beginning of industrialization in the area (early 1960s) to the promotion of the first remediation measures (late 1970s); (**b**) Hg and HCB correlation in surficial samples collected in 2005 and in surficial layers of cores collected in 2003. The product moment and partial correlation coefficient R is 0.93, significant at \(p<0.0005\), and calculated with Statistica 7.0 StatSoft (). Modified from [27]. (2) the deposition of resuspended old, contaminated sediment layers. The first option is a serious violation of current regulations and, if verified, would have justified legal actions against the last managers of the industrial plants. Indeed, the Italian anti-pollution law 319 of 1976 came into effect in 1979 and from that moment graphite anodes present in Hg cells of the chloralkali plant were removed. This happened because the demercurization plant could not work with the sludge deriving from the graphite anodes [53]. No active source of Hg in the Augusta Harbor could then be tolerated from that moment on. The second option was equally critical as it assigns the responsibility for the contamination of surficial sediments primarily to dredging activities and their management by the authorities in charge. The new scientific challenge was then to establish which of the two was the process at work in the Augusta Harbor. In order to provide a reliable answer, an alternative tracer of industrial pollution with a perfectly time-constrained history in the study area had to be found. After further historical research, the choice fell on HCBs, whose input chronologies in the Augusta Bay were well known and documented, as these compounds are formed by graphite anodes in chloralkali plants, and as by-product of the manufacture of tetrachloroethylene (TCE) [54]. In the industrial area of Augusta, TCE production started in 1959 and ceased definitively in 1979, taking the following nine years for complete decommissioning. In addition, production wastes were discharged in the Augusta Harbor with no treatment or disposal procedures until 1976. In the last 40 years, no HCB sources have been active in the area [54]. The HCB temporal record for core L (Figure 3a) was consistent with the timing of TCE production in the Augusta Bay until the 1980s; values were close to detection limits in the preindustrial period, then increased from the early 1960s, reached the maximum around the beginning of the 1970s, and then abruptly decreased at the end of the 1970s. However, if sediments were to deposit undisturbed from that moment on, measured values had to be close to zero, since HCB inputs ceased definitively in the 1980s. On the contrary, measured values in the topmost section remained quite constant around 25 \(\upmu\)g kg\({}^{-1}\) (Figure 3a). The highly significant correlation observed between Hg and HCB in surficial sediment samples (Figure 3b) was interpreted as evidence that the two contaminants were controlled in surface sediments by the same depositional process.