Geo-statistical analysis of the contaminant distribution into the sediments of the Venice lagoon

The state of pollution of sediments in the Venice Lagoon has been analysed, taking into consideration the data concerning the contaminant concentration coming from the surveying campaign carried out by the Consorzio Venezia Nuova from 1997 to 1999. The data coming from the analysis of the collected samples have been utilised to determine the distribution of the contaminants in the whole lagoon area by using a geostatistical method. The distribution of the following contaminants have been estimated: heavy metals (Pb, Zn, Ni, Cr, Cd, As, Hg and Cu) and organic compounds (polycyclic aromatic hydrocarbon, polychlorinated biphenyl, hydrocarbons, chlorinated pesticides) in confrontation to the Italian standard limits. Dedicated software, such as Surfer and ArcView, has been applied to produce thematic maps. In particular, Surfer has been utilised, to put in evidence the iso-concentration lines obtained after application of kriging. ArcView, software to implement a Geographic Information System, allowed lying out the contaminant distribution in the real geographic co-ordinates. A statistic analysis of data has been carried out in order to calculate average, variance, standard deviation, minimum and maximum value of the considered variables. The correlation between the contaminants has been computed and the analysis of the principal components of the variables has been carried out. The obtained maps show that the heavy metal pollution (characterised by huge concentrations especially of Cu, As, Zn, Cd, Hg and Pb) is enhanced in the areas close to the industrial site of Porto Marghera. Furthermore, a relevant chromium contamination has been found in the sediments of the south part of the Venice lagoon, probably coming from to leather industries located in the surroundings the Brenta River now flowing in sea close to the mouth of the same lagoon. The distribution of nickel presents a spot concentration, as ascertained by a singular sample. The contamination by polycyclic aromatic hydrocarbon (PAH), polychlorinated biphenyl (PCB) and hydrocarbons is concentrated in the sediments of the central and south area of the lagoon closer the coastline. Finally, the contamination by chlorinated pesticides increases from the continental border to the coast line as a result of the constraints in the use of some herbicides, imposed by the Venice rules in last times, which allowed to obtain less contaminant charge drained from the basin surrounding the Venice lagoon. Starting from measured values it allowed to describe the mathematical structure of the contaminant distribution in such a way to be able to assign a weight to each point of the lagoon. Kriging, a geostatistical tool, has been used to define the structure of the contaminant distribution, taking into consideration the distance between the position of the sampled points in relationship to any others to be evaluated.