title: Flow Dynamics and Reactivity in the Transition Zone between Streams and Riparian Aquifers
Creators: name: Nogueira, Guilherme E. H.
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: groundwater
subject: Nitrate
subject: numerical modelling
subject: biogeochemical reactions
subject: riparian zone
subject: water quality
subject: hydrogeology
subject: 500
subject: 550

description: The stream water (SW) and groundwater (GW) domains are two parts of one hydrologic continuum. The riparian zone (RZ) is a transition zone between both domains, which connects the aquatic and terrestrial ecosystems. The RZ has been shown to have a potential for regulating water quality in stream-corridors. This potential is mainly related to SW-GW interactions and the associated hydrological and biogeochemical processes occurring at different spatio-temporal scales within RZ. Mixing of different solutes, and high microbial activities are only some of the factors responsible for the high potential for contaminant attenuation and general solutes turnover within RZ. However, an adequate representation of these coupled processes, and their variations in space and time is challenging due to the multiple interactions between the hydrological and biogeochemical processes. This PhD addresses this challenge by aiming at an evaluation of the effects of flow dynamics on riparian reactive potential at different scales. The three consecutive studies comprising this thesis were carried out in a well-instrumented RZ located at the low-land portion of the Selke River catchment, a 4th-order stream, central Germany. The location is well suited for this type of research since it is well instrumented and has a long history of agricultural activity within the riparian corridor with associated inputs of nitrate (NO3-) into the riparian aquifer. This thesis combines data-driven and numerical modelling in order to explore and disentangle the different factors and processes shaping water quality at the different scales within the RZ. As dissolved oxygen (DO) is a key-component regulating the redox state of the system, in the data-driven analyses (Study 1), a suite of tracer-tests were carried out and combined with high-resolution hydrological and chemical data to characterize the near stream system (appx. 20m from stream bank) for aerobic respiration. For that, Damköhler numbers for DO (DADO) were employed. Results showed that seasonal and short-term variations in temperature are major controls shaping the reactive state of the system. Seasonal temperature variations in GW induce a shift on reactive state from transport-limited (DADO&gt;1) in summer to reaction-limited conditions (DADO&lt;1) in winter. On the other hand, short-term events had only minor impacts on the system, resulting in slightly less transport-limited conditions due to decreasing temperature and transit-times associated with the events. The study also shows that assuming a constant water temperature along a SW infiltration flowpath could lead to an over- or underestimation of reaction rates by a factor of 2-3 due to different infiltrating water temperature at the SW-GW interface. Also assuming constant water transit-times throughout the hydrological year results in an underestimation of NO3- removal (40%-50% difference). The numerical modelling of Study 2 focused on the simulation of water flow and mass (DO and NO3-) transport using the measured data from Study 1 but extended the spatial scale. The modelling concept combined a fully-integrated 3D transient numerical flow model with a temperature-dependent reactive transport along subsurface flow paths. Results revealed that temperature variations shift the reactive zone for NO3-, whereas this zone is near the stream under warmer conditions. Even under limited carbon availability (as an electron donor) and low-temperatures, NO3- removal fractions (RNO3) were greater further from the stream than along short hyporheic flow paths (RNO3=0.4 and RNO3=0.1, respectively). Conversely, transit-times and DO concentrations constrained nitrate removal at the near stream region. Additionally, with increasing temperature, the effects of stream flow and solute concentrations on biogeochemical turnover and the redox zonation around the stream strongly decreased. The modelling concept of this study provides an adaptive framework to quantify reach-scale biogeochemical turnover around hydrological dynamic streams. In Study 3, the flow model is coupled with a Hydraulic Mixing Cell method for mapping the source composition of water and tracking their spatio-temporal evolution within RZ. This allowed the identification of mixing hotspots which can be defined to have nearly equal fractions of SW and GW per aquifer volume. These mixing hotspots can facilitate mixing-dependent reactions and solute turnover. Only about 9% of the total simulated domain could be identified as mixing hot-spots (mainly at the fringe of the geochemical hyporheic zone), but this value could be 1.5x higher following large discharge events. Such events increase mixing further away from the stream, whereas 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. The study also provides an easy-to-transfer approach to assess spatio-temporal patterns of mixing processes and mixing-dependent turnover reactions in riparian zones. In summary, findings from the three studies elucidated the relationships and controls among hydrology and biogeochemistry at different scales in the RZ. By combining innovative methods and using coupled, mechanistic models, this thesis advanced the understanding of reactive potentials within the RZ, which can be useful to devise further research and actions for integrated aquatic ecosystem management and recovery.