The influence of geology and land-use on inorganic stream water quality in the Oslo region, Norway

Thirty-nine stream and river water samples were collected along a 120 km long transect through the Oslo Rift and the city of Oslo. All samples were analysed for 59 elements (Ca, Fe, K, Mg, Na, Si, Al, As, B, Be, Bi, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Gd, Ge, Hf, Ho, La, Li, Lu, Mn, Mo, Nb, Nd, Ni, P, Pb, Pr, Rb, Sb, Se, Sm, Sn, Sr, Tb, Th, Tl, Tm, U, V, W, Y, Yb, Zr, the anions Br,Cl-,F-,NO2-, NO3-,PO43-,SO42-), and the additional parameters pH, alkalinity, colour, turbidity and electrical conductivity. The transect crosses four different lithologies, ranging from Precambrian gneisses, Cambro-Silurian sedimentary rocks (including black shales and limestones) to Permian syenites and granites. Parts of the transect are covered by glacial Drift or marine clays (especially in the south), others are Drift-free. Although varying forms of land-use occur throughout the transect, forestry is predominant in higher elevation, Drift-sparse areas, while agriculture and urban development are more characteristic of low lying areas with clayey Drift. Differences in stream water chemistry are likely to arise from the interrelationship between lithology, Drift cover, landscape, land-use and climate-related factors (different evaporation rates). It is possible to tentatively identify the impacts of marine influence, water–rock interaction, pH-related solubility and lithological influence (e.g. black shales). Even a city of the size of Oslo, which is a major diffuse source of contaminants in southern Norway, and intense agriculture have a limited influence on inorganic stream water quality. Nitrate, however, does appear to indicate human impacts, occurring in some streams in concentrations higher than can be explained by rainfall chemistry. Overall, “natural” element sources and processes dominate surface water chemistry at a short distance from any point source of contamination.