A Comparison of Field Techniques for the Analysis of Groundwater-Surface-Water Interactions: Porewater Sampling and Hyporheic Temperature and EC Time Series

The interactions between streamwater and hyporheic or riparian porewater are tightly linked to the biogeochemical and ecological processes within fluvial ecosystems. The analyses of hyporheic biogeochemical cycles or hyporheic exchange fluxes often involves manual sampling of porewater or measurement of temperature time series in the streambed. Here, we compare these two techniques with electrical conductivity time series from a cluster of small, hyporheic EC sensors and discuss their implications on the interpretation of groundwater – surface-water interactions in a first-order boreal stream.

Based on repeated measurements and the co-located small, hyporheic EC sensors, we found that even small sampling-rates of sediment porewater alter the hyporheic flow at some locations significantly. However, since porewater samples are necessary for the analysis of hyporheic biogeochemical cycles and water source partitioning, we recommend for future experiments to either co-locate small, continuous sensors with the sampling ports or to conduct experiments quantifying the induced flux.

Calculated 1D fluxes based on profiles of temperature time series are often integrated over many centimetres to a few decimetres, if the sensors spacing is not very small. This might be the reason, why fluxes based on our temperature and EC measurements do not always have the same direction, especially if the fast hyporheic exchange fluxes are very shallow.

We conclude, that using several, co-located techniques together can compensate for the limitations of each technique and reduce the risk of misleading conclusions.