Hyporheic zone investigations using combined parameters of dissolved radon and oxygen isotopes

David Piatka1, Robin Kaule2, Johannes Barth1
1 Lehrstuhl Angewandte Geologie, Friedrich-Alexander-Universität Erlangen-Nürnberg
2 Lehrstuhl für Hydrologie, Universität Bayreuth

3.5 in Grundwasser-Oberflächenwasser-Interaktionen - vom Flussabschnitt zum Einzugsgebiet

The hyporheic zone (HZ) describes the interface between ground and surface waters. It plays an important role in the ecology of streams and their self-cleaning potential. In this project, we investigate the hypothesis that changing climatic drivers significantly endanger the function of the HZ. Increased temperature leads to increased respiration and formation of oxygen-reduced areas. This can worsen by accelerated deposition of fine sediment via more frequent heavy precipitation. Under conditions of low stream water levels (with high proportion of groundwater) reduced substances, such as Fe2+ and nitrite, enter the upper parts of the HZ and surface water. The contact with atmospheric oxygen leads to formation of areas of preferential oxidation of reduced substances (iron clogging, nitrification).

We combine Radon (Rn) measurements and modeled groundwater influxes with stable isotope measurements of dissolved oxygen (δ 18ODO). 222Rn has a half live of 3.82 days. Therefore, it can serve as a conservative tracer. Because it is a noble gas the only Radon sink in the saturated zone is radioactive decay. Emission and decay of Radon (222Rn) should result in an equilibrium state. However, hyporheic exchange may prevent the formation such equilibria, due to surface water infiltration. With this, Radon is increasingly used as tracer to investigate groundwater-surface-water-exchange.

Stable oxygen δ 18ODO can be used to identify sources and sinks of dissolved oxygen (DO). In atmospheric equilibrium δ 18ODO values should have a value of ~24.6 ‰. The dominance of photosynthesis or respiration alter stable isotope ratios. Besides day-night cycles with dominance of photosynthesis during day and respiration during night, inputs of groundwater with typically very low oxygen contents and reduced substances into the hyporheic zone and stream water lead to an increased respiration signal in δ 18ODO in both compartments.

By combining these two methods we aim to learn more about influences of hyporheic exchange on respiration. We will present first results of this study.