Preprints
https://doi.org/10.5194/hess-2021-573
https://doi.org/10.5194/hess-2021-573

  07 Dec 2021

07 Dec 2021

Review status: this preprint is currently under review for the journal HESS.

Spatio-temporal variations of water sources and mixing spots in a riparian zone

Guilherme E. H. Nogueira1, Christian Schmidt2, Daniel Partington3, Philip Brunner4, and Jan H. Fleckenstein1,5 Guilherme E. H. Nogueira et al.
  • 1Department of Hydrogeology, Helmholtz-Centre for Environmental Research - UFZ, Leipzig, Germany
  • 2Department of Aquatic Ecosystem Analysis, Helmholtz-Centre for Environmental Research - UFZ, Magdeburg, Germany
  • 3National Centre for Groundwater Research and Training, & College of Science and Engineering, Flinders University, Adelaide, Australia
  • 4Centre for Hydrogeology and Geothermics, University of Neuchâtel, Neuchâtel, Switzerland
  • 5Bayreuth Centre of Ecology and Environmental Research, University of Bayreuth, Bayreuth, Germany

Abstract. Riparian zones are known to modulate water quality in stream-corridors. They can act as buffers for groundwater borne solutes before they enter the stream at harmful, high concentrations, or facilitate solute turnover and attenuation in zones where stream water (SW) and groundwater (GW) mix. This natural attenuation capacity is strongly controlled by the dynamic exchange of water and solutes between the stream and the adjoining aquifer, creating potential for mixing-dependent reactions to take place. Here, we couple a previously calibrated transient and fully-integrated 3D surface-subsurface, numerical flow model with a Hydraulic Mixing Cell (HMC) method to map the source composition of water along a reach of the 4th-order Selke stream and track its spatio-temporal evolution. This allows us to define zones in the aquifer with similar fractions of surface- and groundwater per aquifer volume (called “mixing hot-spots”), which have a high potential to facilitate mixing-dependent reactions and in turn enhance solute turnover. We further evaluated the HMC results against hydrochemical monitoring data. Our results show that on average about 50 % of the water in the aquifer consists of infiltrating SW. Within about 200 m around the stream the aquifer is almost entirely made up of infiltrated SW with nearly no other water sources mixed in. On average, about 9 % of the aquifer volume could be characterized as “mixing hot-spots”, but this percentage could rise to values nearly 1.5 times higher following large discharge events. Moreover, event intensity (magnitude of peak flow) was found to be more important for the increase of mixing than event duration. Our modelling results further suggest that discharge events more significantly increase mixing potential at greater distances from the stream. In contrast near the stream, the rapid increase of SW influx shifts the ratio between the water fractions to SW, reducing the potential for mixing and the associated reactions. With this easy-to-transfer framework we seek to show the applicability of the HMC method as a complementary approach for the identification of mixing hot-spots in stream corridors, while showing the spatio-temporal controls of the SW-GW mixing process and the implications for riparian biogeochemistry and mixing-dependent turnover processes.

Guilherme E. H. Nogueira et al.

Status: open (until 01 Feb 2022)

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  • RC1: 'Comment on hess-2021-573', Anonymous Referee #1, 13 Jan 2022 reply

Guilherme E. H. Nogueira et al.

Guilherme E. H. Nogueira et al.

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Short summary
In near stream aquifers, mixing between stream water and ambient groundwater can lead to dilution and removal of substances that can be harmful to water ecosystem at high concentrations. Here, we used a numerical model to track the spatio-temporal evolution of different water sources and their mixing around a stream, which are rather difficult in the field. Results show that mixing mainly develops as narrow spots, varying on time and space, and mainly affected by magnitudes of discharge events.