Dissolved and particulate transport of arsenic and chromium in the Po River (Italy)

The modes of transport of arsenic and chromium have been investigated, as part of a larger study, on the most important Italian river, the Po. Dissolved, particulate and total concentrations are reported and compared with reference background values. The main factors affecting dissolved concentrations appear to be two interrelated variables, flow and suspended matter concentration, and biological activity. The predominant dissolved species were found to be As(V) and Cr(VI). The ratio between oxidized and reduced species appears in the case of arsenic to be significantly influenced by iron and manganese oxides (Fe p and Mn p); the ratio for chromium shows no significant dependence on particulate manganese concentration (Mn p), possibly because its effect is masked by other variables (H 2O 2 and Fe 2+) connected with photochemical reactions. Particulate transport was found to be more important for chromium than arsenic. As and particularly Cr concentrations in suspended matter are strongly influenced by major particulate components including Fe p, Al p and Mg p. It is suggested that adsorption is probably not the only mechanism responsible for Cr p levels; the presence of mixed CrFe or CrMg hydroxides may better explain this dependence. Previously published laboratory work supports the existence and role of these hydroxides in natural waters. The dissolved fraction of total transport was found to be strongly dependent on suspended matter concentration for As and Cr and equations describing this dependence are proposed.