Size fractionation (dissolved, colloidal and particulate) of trace metals in the Thur River, Switzerland

The distribution of some trace metals (Cu, Zn, Ni, Co, Fe, Mn) and of DOC over a particulate (> 1 mum), a colloidal (size < 0.45 mum and molecular weight > 10 kD) and an ultrafiltered fraction (< 10 kD) was determined at several sites on the Thur River, Switzerland, at various times of the year. The complexation of Cu by strong ligands in the ultrafiltrate and in the conventional filtrate (< 0.45 mum) was compared using a ligand-exchange/CSV method. The <0.45 mum concentrations of Cu (from an average of 7 nM to 24 nM), Zn (<5-23 nM), Ni (5-13 nM), Co (1.5-3 nM) and Mn (7-92 nM) increased downstream. The major part of Cu, Zn, Ni and Co usually occurred in the ultrafiltrate fraction at all sites, whereas Fe and Mn were mostly in the particulate fraction, under conditions of low suspended matter content (< 10 mg L(-1)) in the river. The percentage of metal in the colloidal fraction, with respect to the 0.45-mum filtrate, decreased in the order: Cu (median 11%) > Zn approximate to Ni(median 5-6%) > Mn approximate to Co (median < 5%). DOC also consisted mostly of molecules in the < 10 kD range. Cu was strongly complexed by natural organic ligands in all filtrate and ultrafiltrate samples. A large part of the strong Cu binding ligands consisted of compounds in the < 10 kD range, but colloidal ligands with similar properties also occurred. Cu was distributed among the dissolved and the colloidal ligands, roughly in proportion to organic carbon. The colloidal fraction (as defined here) did not increase in its proportional amount downstream and was only of limited significance in transporting trace elements in the Thur River under low discharge conditions.