Evaluation of the hydraulic exchange in the hyporheic zone: A depth-oriented analysis focusing on upstream and downstream conditions.

The groundwater-surface water interface, which is known as hyporheic zone, is a dynamic system that plays an important role in the hydrological functioning of catchments. During flow exchange, there are many chemical attenuation processes due to the variation of physical and chemical properties including surface water flow and the morphology of the riverbed. The fluctuations may depend on the seasonality, climatic zone, as well as due to the local variations such as soil structure, geology, land use, water sources etc.

The goal of the study was to gain a depth orientated insight into the hyporheic exchange functioning, mainly focusing on the difference between upstream and downstream conditions. We collected water samples from 4 different depths until 0.5 m below the stream bed surface of the stream Ahna in Kassel, Germany, using multi-level interstitial probes. The samples were taken at different locations in the stream during a whole year. Water samples were analyzed for stable isotopes of water (δ²H and δ¹⁸O) and some major ions. Results indicate that the variations following the depth, sampling time and the location. Isotopic signatures show the summer enrichment and the winter depletion, mirroring the seasonality of stable isotopes in rainfall. We also observed an isotopic enrichment at the downstream sites and significantly higher ion concentrations, especially for K⁺, Na⁺, Mg²⁺, Cl^-, NO₃^-, than the upstream. Water extraction flow rates decreased along the depth profile due to the changes in effective porosities and the hydraulic conductivities which were controlled by the sediment structure and clogging processes.