Atmospheric bulk deposition to the lagoon of Venice. Part I. Fluxes of metals, nutrients and organic contaminants

First available data on atmospheric fall-out were provided by sampling monthly bulk depositions in four sites inside the Lagoon of Venice (550 sq km). Sampling was carried out monthly during the period July 1998¨CJuly 1999, in one site near an industrial area (Porto Marghera; site D), another site in the city of Venice (site A), and the remaining two in the southern- and northernmost ends of the Lagoon (Valle Figheri, site C; Valle Dog¨¤ site B). The following determinations were carried out for each samples: pH, conductivity, grain-size, particulate load, and dissolved nutrients (N, P). Samples were then subdivided into soluble and insoluble fractions, and Al, Ca, Na, K, Mg, Si, Mn, Fe, Zn, Ni, Cr, Cu, Pb, Cd, As, Hg, Ti, V, S, P, Se and Sb were analysed on both fractions. Total organic micropollutants (PAH, PCB, HCB, DDT, PCDD/F) were measured. As regards particle size distribution, there was great variability among sampling sites. The percentage of the¡ d 2 micrometers grain-size fraction was higher in the southern and northern ends of the Lagoon. Small difference were found among sites for major elements, whereas higher variability was observed for inorganic and organic micro-pollutants, with standard deviations between 20% and 60% of the fluxes measured. Major differences in annual fluxes between the most polluted sites (mostly D and A) and background (site B) were seen for Cd (0.26 vs 0.06 mg m (to the minus 2) year (to the minus 1), Hg (41 vs 15 micrograms m (to the minus 2) year (to the minus 1), PCB (about 2500 vs about 500 ng m (to the minus 2) year (to the minus 1) and HCB (about 8000 vs about 1000 mg m (to the minus 2) year (to the minus 1). Comparisons with previous data, collected in the epriods 1993-1994 and 1995-1997, were only available for a few trace metals. A definite decline in the annual Pb flux in the city of Venice was detected, from 18 to 13 mg m (to the minus 2) in 1996/1997 and 1995/1996, respectively to about 5 mg m (to the minus 2) in the present study. Total annual deposition was calculated by means of two different methods, which gave very similar results: (i) the mean value of deposition in the four sites was multiplied by lagoon area (550 km2); (ii) the monthly rain isopleths were combined to normalize deposition values. The figures are: 15–34 kg of Hg and Sb, about 200 kg of As, about100 kg of Cd and PAH, 0.7–1.3 tons of Cr, Ni and V, more than 2 tons of Cu and Pb, 17 of Zn, 55 of total P, about 200 of Al, and 3900 of DIN. Total fluxes of organics inside the lagoon were: PAH about100 kg; HCB about 1 kg; DDT about 0.4 kg. PCB and PCDD/F fluxes were about 500 g and about10 g, corresponding respectively to 0.1 and 0.4 g I-TE. The correlations between fluxes of inorganic micropollutants and grain-size were significant. Multivariate statistical analysis was applied to investigate more accurately relationships between the insoluble and dissolved fractions of inorganic micropollutants and grain-size fractions. In particular, significant correlations were highlighted between the dissolved fraction of As and the d1 micrometers particle size fraction. Relations between levels of the total amount of PCDDF, total amount of PCDD, PCB and PAH congeners and grain-size revealed significant correlation coefficients for the remote sites (B, C), and none in the urban and industrial sites (A, D). In particular, significant correlations were highlighted between the total amount of PCDDF, total amount of PCDD and particle size fraction d2 micrometers, and between benzo(a)pyrene and PCB 167 and particle size fraction 4–8 micrometers.