Chromium speciation in a river system in Veneto (ltaly) affected by tannery effluent

Abstract
Tannery effluent has been discharged for decades in small tributaries of the River Guà and River Fratta, which in turn discharge into the River Brenta near of the Lagoon of Venice (Veneto, Italy). Sequential extraction data have shown that at most sites, higher values of total Cr are typical of the finer sediment (pelite, <63 m). In bulk sediment ( <2 mm) most of the Cr is in the residual fraction whereas in pelite, the oxidisable fraction accounts for about 50% of total chromium. Cr in water is significantly correlated with dissolved Cl ^-, Fe and Mn, whereas in sediments it is significantly correlated with Fe. In the river stretch downstream of the main effluent discharge, the total Cr levels in water can be more than 300 g L ^-1, and can be still above 40 g L ^-1 20 km downstream of the discharge point. Also, though most chromium is present as Cr (111) species, Cr (VI) is in the 0.2-63 g L ^-1 range, i.e. above environmental and legal limits. The concentration of Cr in the sediments exceeds the levels known to produce adverse biological effects. Suspended sediments carry a significant chromium load towards the delta area.