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

Abstract

We have used a geographic information system (GIS) to study the relationships between streamwater pH and alkalinity at base flow with geology, soils and relief in 100 forested catchments located in the sandstone portion of the Vosges mountains. where atmospheric deposition and forest cover can be considered homogeneous. At base flow, streamwater acidity depends primarily on bedrock and soil content of weatherable minerals whose dissolution neutralizes 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 classified 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 variability could be explained by soil types in catchments composed of poor' bedrock only. Soil type apparently did not influence streamwater pH in the other groups. The catchment area, probably related to the increase of soil volume and water residence time, explained up to 20% of pH variability, depending on the catchment type.