A spatial study of the relationships between streamwater acidity and geology, soils and relief (Vosges, northeastern France)

We have used a geographic information system (GIS) to study the relationshi ps between streamwater pH and alkalinity at base flow with geology, soils a nd relief in 100 forested catchments located in the sandstone portion of th e Vosges mountains, where atmospheric deposition and forest cover can be co nsidered homogeneous. At base flow, streamwater acidity depends primarily o n bedrock and soil content of weatherable minerals whose dissolution neutra lizes acidity. Catchments are developed on three main stratigraphic levels, consisting of two sandstone layers rich in weatherable minerals, called '' rich'', at the upper and lower extremity of the stratigraphic sequence, and a quartzitic sandstone, called ''poor'', in between. Catchments were class ified into three major groups in relation to the stratigraphic sequence: (1 ) only ''poor'' bedrock, (2) ''rich'' upstream and ''poor'' downstream, (3) ''poor'' upstream and ''rich'' downstream. Results showed that streamwater pH differed depending on the group. Within each group, the pH was related to bedrock, soil and relief characteristics of the catchments. The relative surface covered by the bedrock located in the lower part of the catchment explained more than 50% of the variability of pH. More than 20% of variabil ity could be explained by soil types in catchments composed of "poor" bedro ck only. Soil type apparently did not influence streamwater pH in the other groups. The catchment area, probably related to the increase of soil volum e and water residence time, explained up to 20% of pH variability, dependin g on the catchment type. (C) 1999 Elsevier Science B.V. All rights reserved .