Inferring groundwater influences on surface water in montane catchments from hydrochemical surveys of springs and streamwaters

SummaryStreamwaters and emergent groundwaters in springs and seeps were sampled over the 2003–2004 hydrological year in a geologically complex 31 km2 catchment. Samples were analysed for Gran alkalinity and chloride; tracers that would respectively indicate the provenance and residence times of water. Streamwaters were sampled at the catchment outfall and in nine sub-catchments. Streamwater Gran alkalinity showed predictable fluctuations with flow, with high flows and baseflows exhibiting low and high alkalinity respectively. During storm flow conditions the nine monitoring points exhibited similar levels (0–50 μeq l−1), whilst under baseflows alkalinity was highly variable (300–1000 μeq l−1), depending upon catchment geology. Comprehensive spatial surveys of springs and seeps in 6 of the sub-catchments during a typical summer low flow period revealed marked differences in groundwater chemistry. This broadly related to sub-catchment geology and geochemistry, but local variability implied marked differences in groundwater flow paths, residence times and geochemical reactions. Chloride data indicated a high degree of synchronicity between concentrations in precipitation and streamwaters. In contrast, concentrations in groundwaters were more consistent. This implies that Cl concentrations in the stream depend upon the relative contribution of groundwaters and soil waters where Cl concentrations are respectively more stable and more dynamic. In general, at the catchment scale, mean water residence times appear to be relatively short which appears to relate to the low permeability of soils and bedrock.