Investigating River Water/Groundwater Interaction along a Rivulet Section by 222Rn Mass Balancing

Abstract

Investigation of river water/groundwater interaction aims generally at: (i) localizing water migration pathways; and (ii) quantifying water and associated matter exchange between the two natural water resources. Related numerical models generally rely on model-specific parameters that represent the physical conditions of the catchment and suitable aqueous tracer data. A generally applicable approach for this purpose is based on the finite element model FINIFLUX that is using the radioactive noble gas radon-222 as naturally occurring tracer. During the study discussed in this paper, radon and physical stream data were used with the aim to localize and quantify groundwater discharge into a well-defined section of a small headwater stream. Besides site-specific results of two sampling campaigns, the outcomes of the study reveal: (i) the general difficulties of conducting river water/groundwater interaction studies in small and heterogeneous headwater catchments; and (ii) the particular challenge of defining well constrained site- and campaign-specific values for both the groundwater radon endmember and the radon degassing coefficient. It was revealed that determination of both parameters should be based on as many data sources as possible and include a critical assessment of the reasonability of the gathered and used datasets. The results of our study exposed potential limitations of the approach if executed in small and turbulent headwater streams. Hence, we want to emphasize that the project was not only executed as a case study at a distinct site but rather aimed at evaluating the applicability of the chosen approach for conducting river water/groundwater interaction studies in heterogeneous headwater catchments.

Seasonal variation and release of soluble reactive phosphorus in an agricultural upland headwater in central Germany

“…The section-specific quantification of groundwater discharge was more difficult than their localization, due to considerable uncertainties in parameterizing radon degassing (i.e., in defining a degassing coefficient k (m d −1 )) for the FINIFLUX model in such a small stream. The uncertainties of the model results are mainly correlated to the uncertainty of the applied degassing coefficient (for a detailed discussion, see Schubert et al, 2020). The k values that resulted from our FINIFLUX model (for details, see Frei and Gilfedder, 2015) for the summer and winter campaign were 4.2 and 2.3 m d −1 , respectively.…”

“…A number of experimental and empirical methods are available to estimate radon degassing from a stream. A detailed discussion of the approach applied specifically to the Schäfertal stream is provided in Schubert et al (2020). Furthermore, Raymond et al (2012) have published a comprehensive review of scaling gas transfer velocities in streams and small rivers.…”

Natural Radionuclides as Aquatic Tracers in the Terrestrial and the Coastal/Marine Environment

“…Groundwater/river interactions are the focus of two studies by Schubert et al [4,5] In one of the papers, the problems of radon degassing and radon groundwater endmember estimation, aiming at groundwater discharge localization and quantification, are discussed based on data collected at a small and turbulent stream. The second paper discusses groundwater discharge localization and quantification along a section of the Elbe River (Germany).…”