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 km2). Sampling was carried out monthly during the period July 1998 – July 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 Dogà site B).\ud
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.\ud
As regards particle size distribution, there was great variability among sampling sites. The percentage of the <2 µm grain-size fraction was higher in the southern and northern ends of the Lagoon. \ud
Small differences were found among sites for major elements, whereas higher variability was observed for inorganic and organic micropollutants, 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-2yr-1), Hg (41 vs. 15 µg m-2yr-1), PCB (~2500 vs. ~500 ng m-2yr-1) and HCB (~8000 vs. ~1000 ng m-2yr-1). Comparisons with previous data, collected in the periods 1993-94 and 1995-97, 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-2 in 1996/97 and 1995/96 respectively,  to  ~5 mg m-2 in the present study.\ud
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, ~200 kg of As, ~100 kg of Cd and PAH, 0.7 to 1.3 tons of Cr, Ni and  V, more than 2 tons of Cu and Pb, 17 of Zn, 55 of total P, ~200 of Al, and 3900 of DIN. \ud
Total fluxes of organics inside the lagoon were: PAH ~100 kg; HCB ~ 1 kg; DDT  ~ 0.4 kg. PCB and PCDD/F fluxes were ~ 500 g and ~10 g, corresponding respectively to 0.1 and 0.4 g I-TE. \ud
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 < 1 µm particle size fraction. Relations between levels of EPCDDF, EPCDD, 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 EPCDDF, EPCDD and particle size fraction < 2 µm, and between benzo(a)pyrene and PCB 167 and particle size fraction 4-8 µm