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 o ver a particulate (> 1 mum), a colloidal (size < 0.45 mum and molecular wei ght > 10 kD) and an ultrafiltered fraction (< 10 kD) was determined at seve ral 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 conve ntional filtrate (< 0.45 mum) was compared using a ligand-exchange/CSV meth od. 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 downstrea m. The major part of Cu, Zn, Ni and Co usually occurred in the ultrafiltrat e fraction at all sites, whereas Fe and Mn were mostly in the particulate f raction, 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 appr oximate to Ni(median 5-6%) > Mn approximate to Co (median < 5%). DOC also c onsisted mostly of molecules in the < 10 kD range. Cu was strongly complexed by natural organic ligands in all filtrate and ul trafiltrate samples. A large part of the strong Cu binding ligands consiste d of compounds in the < 10 kD range, but colloidal ligands with similar pro perties also occurred. Cu was distributed among the dissolved and the collo idal ligands, roughly in proportion to organic carbon. The colloidal fracti on (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.