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.
Highlights
1.1 Importance of mixing at the riparian zone40 The importance of riparian zones for regulating water quality in stream corridors has long been recognized (Bernhardt et al, 2017; Gu et al, 2012; Hill, 1996; Jencso et al, 2010; Mayer et al, 2006; McClain et al, 2003; Vidon et al, 2010)
290 3 Results we will focus on the results of simulations for the years of 2013-2016 since it was the period used for the validation of Hydraulic Mixing Cell (HMC) results
A small gaining portion was observed for the simulated reach only at a localized deep pool downstream in the domain 300, whereas the majority of infiltrating stream water (SW) exited the domain via the downstream subsurface boundary
Summary
1.1 Importance of mixing at the riparian zone40 The importance of riparian zones for regulating water quality in stream corridors has long been recognized (Bernhardt et al, 2017; Gu et al, 2012; Hill, 1996; Jencso et al, 2010; Mayer et al, 2006; McClain et al, 2003; Vidon et al, 2010). It has been proven that the highest potential for mixing-dependent turnover of groundwater-borne solutes is at the fringe of the HZ, where mixing between infiltrating SW and local flowing GW might develop to a larger degree (Hester et al, 2014, 2017, 2019; Sawyer and Cardenas, 2009; Trauth et al, 2015; Triska et al, 1989), Fig.. It has been proven that the highest potential for mixing-dependent turnover of groundwater-borne solutes is at the fringe of the HZ, where mixing between infiltrating SW and local flowing GW might develop to a larger degree (Hester et al, 2014, 2017, 2019; Sawyer and Cardenas, 2009; Trauth et al, 2015; Triska et al, 1989), Fig.1 These mixing-triggered processes could 65 represent the last natural protection before harmful groundwater-borne solutes such as NO3- enter a stream. This is partly due to the significant effort required to identify different water sources and their dynamics at high spatio-temporal resolution at the river corridor scale
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