The influence of parent material on small-scale spatial changes in streamwater chemistry in Scottish upland catchments

Abstract

Spatial changes in streamwater chemistry in seven upland catchments in NE Scotland have been studied using samples collected at 250 m intervals from the stream source to the catchment outlet. The catchments contain a number of different parent materials including granites and diorite from the Lochnagar Complex, and metasediments and metabasites from the Argyll and Appin Groups of the Dalradian Supergroup. Samples were collected under low flow conditions between June 1989 and January 1990 and ten component ions were determined; this paper concentrates on pH, Ca, Mg, Na and K. All seven catchments show major changes in stream solute concentrations along their length. Samples collected immediately downstream of the source typically show a rapid increase in pH with distance, reflecting degassing in the stream of CO 2 from the soil solution. Further down the catchment small-scale spatial changes in streamwater chemistry appear to be strongly influenced by the underlying geology. As the stream flows over parent materials with distinctly different geochemical characteristics, the pH, and Ca and Mg concentrations in the stream show rapid changes, reflecting changes in the underlying lithology. In a number of the catchments K also exhibits significant spatial variability along the length of the stream. The underlying geology appears to have little effect on Na concentrations, which typically increase from the source to the bottom of the catchment. These data suggest that small-scale spatial changes in streamwater chemistry may provide a useful tool for identifying changes in the underlying geology in poorly exposed areas. In addition, they also have implications for regional critical load mapping and prediction of streamwater sensitivity to acidification.