Investigation of the factors controlling hyporheic exchangeat multiple spatial scales

Hyporheic exchange is the mixing between stream water and sediment pore water occurring vertically through the riverbed and laterally through the riverbanks. This mixing between water coming from the river and water coming from the aquifer, with very different physical and chemical characteristics, creates a unique environment in which important biogeochemical reactions occur and rich communities of microorganisms and macroinvertebrates flourish. The occurrence of the hyporheic exchange significantly influences the quality of the stream water and the nutrient cycling, playing a crucial role in hydrological, biogeochemical, and ecological processes. Although hyporheic fluxes are driven by the local morphology of the streambed, they are strongly affected by the large-scale groundwater system, which obstructs the penetration of stream water into the sediments and limits the intensity of hyporheic exchange. The present thesis aims to: i) investigate the role of the regional groundwater flow system on hyporheic exchange, analyzing the factors controlling the spatial variability of groundwater discharge patterns along the river corridor and ii) study the effect of microbial growth on exchange fluxes and nutrient reactions within the hyporheic sediments. The work is divided into five Chapters. Chapter 1 presents a general overview on groundwater-surface water interactions, with a description of the multiple scales involved in these processes. The main aspects for which these interactions are important are recalled and a brief review on the modeling,of river-aquifer interactions is presented. The attention is then focused on hyporheic processes, analyzing the main hydraulic and biogeochemical features characterizing these processes. The impact that the groundwater flow system has on these local processes is discussed. Finally, the main research topics investigated in the thesis are outlined. In Chapter 2, the role of groundwater table structure at basin scale on the spatial patterns of groundwater discharge to the stream network and, consequently, on hyporheic exchange was investigated. Specifically, we determined the spatial structure of the groundwater upwelling along the stream network in order to investigate the effect of large-scale groundwater flow on local hyporheic flow velocity. A semi-analytical method for the estimation of the three-dimensional groundwater flow field was adopted, based on an approximation between the groundwater head distribution and the landscape topography. Results highlight that the complex topographic conformation of a basin determines a strong spatial variability of the groundwater flow field that, in turn, translates into a fragmentation of the hyporheic zone. Chapter 3 is in line with the study developed in Chapter 2, looking at the groundwater-surface water interactions induced by large-scale hydrogeological characteristics. A more complex numerical model was adopted, allowing us to remove some simplifications on which the previous semi-analytical model was based on. The influence of some topographic and hydrogeological factors on determining the spatial variability of groundwater discharge patterns was investigated. Results indicate that the geological heterogeneity of the aquifer is the main control of river-aquifer exchange patterns and the structure of subsurface flow patterns is marginally affected by other modeling assumptions. Chapter 4 shifts the focus on biogeochemical processes occurring at smaller scales and deals with the existing coupling between hydrodynamic processes, solute transport, and microbial metabolism within the hyporheic zone. A flow and reactive transport model was coupled with a microbial biomass model where two microbial components representing autotrophic (nitrifying) bacteria and heterotrophic (facultative anaerobic) bacteria were considered. The aim was to investigate how the filling of sediment pore space induced by biomass growth (i.e. bioclogging) alters hyporheic flow patterns and transformation rates of nitrogen, oxygen, and organic carbon within hyporheic sediments. Results show how the bioclogging-induced biogeochemical zonation of hyporheic zone strongly influences coupled nitrogen, carbon, and oxygen dynamics. Finally, Chapter 5 presents general conclusions of the work.