Assessing the variations in hyporheic flow exchange by isotopic and chemical analysis

The hyporheic zone plays a critical role in nutrient cycling, biogeochemical processes, and overall stream ecosystem health. Variation of physical and chemical properties in the hyporheic zone affects the quality and quantity of the exchange process. To gain a depth-oriented insight into the hyporheic functioning, isotopic (¹⁸O and ²H) and chemical analysis (major ions such as K⁺, Na⁺, Mg²⁺, Ca²⁺, Cl^-, SO₄^2-, NO₃^-) was carried out focusing on the differences between upstream and downstream conditions. Multi-level interstitial probes were used to take subsurface water samples up to 0.6 m depth from two streams named Ahna and Losse in North Hesse, Germany.

Fast downward flow and higher mixing rates were observed for Losse compared to Ahna and no differences were detected between the two locations in Losse. In Ahna, due to changes in effective porosities and the hydraulic conductivities, the water extraction rates were decreased along the depth. Downstream water was more isotopically enriched than upstream being subject to more evaporation and mixing with other waters. Higher concentrations of  K⁺, Na⁺, Mg²⁺, Cl^-, and NO₃^- were found in downstream than the upstream due to the addition of wastewater, agricultural runoff, pollutants, and road salt while flowing. Denitrification and thus decrease in nitrate and increase in nitrite concentrations were more dominant in the upstream due to the high oxygen consumption. By combining isotopic and chemical analyses with detailed hydrogeological measurements, this study can provide valuable insights into the spatial dynamics of hyporheic flow exchange in a stream ecosystem.