Spatial and temporal changes in soil water and groundwater chemistry in the Studibach catchment

Stream chemistry depends strongly on the flow pathways to the stream and the chemistry of the inflowing water. Changes in stream chemistry during rainfall events are assumed to reflect variations in solute concentrations with depth below the surface because shallower flow pathways become more important during rainfall events. Many studies have looked at the relative contribution of soil water and groundwater to streamflow but the spatial and temporal variability in their chemical composition is rarely assessed. This thesis addresses the lack of knowledge on the spatial and temporal variability in the stable isotope composition and solute concentrations in soil- and groundwater in pre-alpine headwater catchments. Samples were taken before and after four rainfall events in autumn of 2021 at seven different sites at four depths: 12.5, 20 and 30 cm below the surface and from the groundwater. In addition, samples were taken from precipitation and stream water. The samples were analysed for their isotopic composition and anion and cation concentrations. The results of the sampling showed that the spatial variability of the dissolved substances was large and for almost all elements larger than the temporal variability. There was no consistent change in mean concentration from before, to after the four rainfall events in the soil or the groundwater and changes over time were very small. The correlations between the mean concentration and the topographic wetness index or the slope was weak. Only the correlation between the mean solute concentration and the groundwater level was slightly better (R2: 0.01-0.66). The change in concentration with depth below the soil surface depended on the source of the solute. For some solutes, the concentrations stayed relatively constant (e.g. lead), for others it decreased (e.g. deuterium, potassium) or increased (nitrate, calcium). Stream chemistry reacted strongly during all sampled rainfall events. Stormflow always had slightly to significantly higher concentrations than precipitation or baseflow, which indicated that new source areas of soil water and groundwater with different solute concentrations contributed to the stormflow. The concentration changes in the soil- and groundwater with depth was reflected in stream chemistry for the majority of the solutes. Only for Mg was there an unexpected change in the stream water, as it went down with an increased water level, while the concentrations in the soil were higher near the surface. This indicates that in addition to shallower flow paths contributing to streamflow during events, other areas must start to contribute as well. The results of this thesis broaden the understanding on the intricate changes in concentration in the soil-, ground- and stream water in headwater catchments. The thesis proves that a better understanding of hydrochemistry in the soil is useful for hydrological studies of headwater catchments and that making assumptions on the spatial variations in concentration solely on stream chemistry is difficult and can cause many uncertainties. The large spatial variation in solute concentrations and changes in hydrological connectivity also affect stream chemistry. Sampling of soil water and groundwater at several locations in the catchment is therefore needed to assess the spatial variability in solute chemistry