EVALUATION OF HEAVY METAL POLLUTION IN VENICE LAGOON SEDIMENTS BY CALCULATING ELEMENT-SPECIFIC ENRICHMENT FACTORS

Venice Lagoon (VL) is a transitional environment on the Adriatic coast influenced by such human activities as agriculture, industry, and tourism. Industrial wastes of Porto Marghera, the sludge of Venice, and rivers from the hinterland pollute the lagoon in which sediments record the pollution. Quantitative comparison of heavy metal (HM) concentrations between 30 grab samples (1996) and 9 samples from 4 cores (3-9 m depths; dated pre-1900) in the north basin of VL allows calculating element-specific enrichment factors (EF) that estimate recent pollution. Grain-size, mineral composition (by XRD), and chemical composition (47 elements by ICP and INAA) were determined for all samples. Grain-size varies from sandy to clayey silt; dolomite, quartz, calcite, and clay minerals (dominant in the <2µm fraction) are the principal minerals. We have used “box-plot”, a statistical graphical technique to display HM distribution in a dot-map of the basin. This method describes the main features of the distribution removing any variability due to sampling and analytical procedures and other random inconsistencies. Highest values for Cu, As, Zn, and Pb, occur near the municipal dump of Mestre and the mouth of the Dese River, and vary up to 9-fold within a small area (~ 2-3 km²) presumably depending on grain size and clay mineral content (GRCM) of the sediments. Because Al is well correlated with GRCM (r>0.9), Al is used as a proxy-normalizer to eliminate GRCM effects on heavy metal distribution in the sediments.  HM concentrations of unpolluted sediments from the cores were used to calculate baseline values with linear regression with Al. The ratio between actual and predicted baseline value for an HM is its EF. Near the Mestre dump EF of Zn, Pb, Cu exceed 3 (e.g., EF_[Zn]=13.5) indicating high pollution. Elsewhere, including the area near the mouth of the Dese River, EF values are below 1.5, i.e., not significantly different from the background. Therefore, the high absolute HM concentrations near Dese river mouth are natural and not due to pollution. Thus, EF and not absolute values indicate pollution increase.