HYDROGEOCHEMISTRY OF A STREAM DRAINING SULFIDE-BEARING POSTGLACIAL SEDIMENTS IN FINLAND

The influence of hydrological conditions and overburden type on the st ream-water quality in a catchment effected by fine-grained sulfide-bea ring sediments was studied. Water samples for measurement of pH and el ectric conductivity were collected at 16 sites along the main course o f the stream during autumn and spring high water flow and during low a nd intermediate water flow. On one occasion (spring high-water flow) s amples for analysis of total organic carbon were collected at these si tes. Seventeen water samples for analyses of chemical elements (ICP-MS and ICP-AES) were taken within the catchment basin during autumn high water flow. The pH and conductivity of snow, and the geochemistry of the sulfide sediments were also studied. The occurrence of fine-graine d sediments within the catchment was estimated from topographic and ge ologic maps and by visit to areas were the overburden type could not b e determined from the maps. The hydrological conditions had a strong i nfluence on the water quality. During high water flow in spring and au tumn the concentrations of ionic components (conductivity) acid hydrog en ions in the stream water were high, while the concentrations were m uch lower during low and intermediate water flow and when the ground w as frozen. Extensive leaching of the sulfide-bearing sediments, which had higher contents of S and metals than the fine fraction of till in the study area, resulted in elevated contents of Li, Ni, Zn, Co, B, Al , Mg, U, Sr, Ca, K and Na in the stream water during autumn high water flow. The contents of Cr, Ba, Pb, Si and Fe in stream water were not much influenced by the type of overburden, while V was depleted in dra inage waters from the sulfide sediments. Humic substances flocculated in the water in the middle section of the stream, which may have been caused by di- and trivalent metal cations leached in large quantities from the oxidizing sulfide sediments.