GEOCHEMISTRY OF STREAM WATER IN A CATCHMENT IN FINLAND AFFECTED BY SULPHIDIC FINE SEDIMENTS

The impact of sediment type on stream water geochemistry was studied i n a catchment in Finland affected by sulphidic fine-grained sediments. Stream water samples for general characterisation of water quality (p H, electrical conductivity) were taken at the basin outlet during vari ous hydrological conditions, while samples for detailed geochemical an alysis were collected at 119 sites in the catchment on one single occa sion during high-water flow in autumn. The occurrence of sulphidic fin e sediments was estimated based on data from an airborne electromagnet ic survey carried out by the Geological Survey of Finland. Growing-sym bol maps, which were prepared for each of the studied variables in wat er, and statistical calculations including factor analysis and Spearma n correlations show that the concentrations of Al, Ga, U and Tl, all t he lanthanides and several alkali and alkaline earth metals (K, Mg, Na , Li, Ca, Rb, Sr), transition metals (Cd, Co, Cu, Zn, Sc, Mn, Ni, Y, H f) and non-metals (S, Pr, I, Si) increase in water when the proportion of the catchment cover of sulphidic fine sediments increases. It is t herefore argued that these elements are released and mobilised in cons iderable amounts by the oxidation and subsequent acidification and wea thering of this type of sediment. Other elements are either slightly d epleted in streams in areas of sulphidic fine sediments (V, Nb, Pb, Zr ), have a distribution unrelated to sediment type (Fe, Cr, Cs, Mo), or are only weakly impacted by the occurrence of sulphidic sediments in the catchment (As, Ti, Ba). It is argued that these elements are not l eached extensively from the oxidising sulphidic sediments, and that th eir distributions at least partly may be controlled by the contents of dissolved humic material and/or suspended organic and inorganic phase s in the water. (C) 1997 Elsevier Science Ltd.