Is the Hyporheic Zone Relevant beyond the Scientific Community?

Rivers are important ecosystems under continuous anthropogenic stresses. The hyporheic zone is a ubiquitous, reactive interface between the main channel and its surrounding sediments along the river network. We elaborate on the main physical, biological, and biogeochemical drivers and processes within the hyporheic zone that have been studied by multiple scientific disciplines for almost half a century. These previous efforts have shown that the hyporheic zone is a modulator for most metabolic stream processes and serves as a refuge and habitat for a diverse range of aquatic organisms. It also exerts a major control on river water quality by increasing the contact time with reactive environments, which in turn results in retention and transformation of nutrients, trace organic compounds, fine suspended particles, and microplastics, among others. The paper showcases the critical importance of hyporheic zones, both from a scientific and an applied perspective, and their role in ecosystem services to answer the question of the manuscript title. It identifies major research gaps in our understanding of hyporheic processes. In conclusion, we highlight the potential of hyporheic restoration to efficiently manage and reactivate ecosystem functions and services in river corridors. View Full-Tex

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

Impact of hyporheic zones on nutrient dynamics

Riverine sediments play a fundamental role within the fluvial system, since they represent potential removal zones of stream-borne pollutants and, in particular, nutrients derived by anthropogenic activities. The region of sediments where the exchange and mixing of surface and subsurface waters occurs is the hyporheic zone. This region is also a place of intense biogeochemical activity, influencing both the flora and the fauna living in the fluvial environment. In the last decades several works were focused either on the water exchanges or the biochemical reactions in the hyporheic zone but just few considered the interactions of both hydraulic and biochemical processes. In this thesis the reactive transport of oxygen and the most common water-borne nutrients (i.e., dissolved organic carbon, nitrate and ammonium) in a duned streambed is investigated. In particular, a numerical model is employed to simulate the flow field, the biogeochemical reactions and the solute spatial distribution in the hyporheic zone. Sensitivity analyses are also performed to study the influence of different hydrological and chemical properties of the system on the net solute fluxes across the streambed. Finally, the effect of sediment heterogeneity on substance reaction rates and, specifically, on nitrate source/sink role played by the sediments is also analyzed for a rippled streambe

Nutrient cycling in bedform induced hyporheic zones

The hyporheic zone is an ecotone connecting the stream and groundwater ecosystem that plays a significant role for stream biogeochemistry. Water exchange across the stream-sediment interface and biogeochemical reactions in the streambed concur to affect subsurface solute concentrations and eventually nutrient cycling in the fluvial corridor. In this paper we investigate the interplay of hydrological and biogeochemical processes in a duned streambed and their effect on spatial distribution of solutes. We employ a numerical model to simulate the turbulent water flow and the pressure distribution over the dunes, and then to evaluate the flow field and the biogeochemical reactions in the hyporheic sediments. Sensitivity analyses are performed to analyze the influence of hydrological and chemical properties of the system on solute reaction rates. The results demonstrate the effect of stream velocity and sediment permeability on the chemical zonation. Changing sediment permeability as well as stream velocity directly affects the nutrient supply and the residence times in the streambed, thus controlling the reaction rates under the dune. Stream-water quality is also shown to influence the reactive behavior of the sediments. In particular, the availability of dissolved organic carbon determines whether the streambed acts as a net sink or source of nitrate. This study represents a step towards a better understanding of the complex interactions between hydrodynamical and biogeochemical processes in the hyporheic zon

Organizational Principles of Hyporheic Exchange Flow and Biogeochemical Cycling in River Networks Across Scales

Hyporheic zones increase freshwater ecosystem resilience to hydrological extremes and global environmental change. However, current conceptualizations of hyporheic exchange, residence time distributions, and the associated biogeochemical cycling in streambed sediments do not always accurately explain the hydrological and biogeochemical complexity observed in streams and rivers. Specifically, existing conceptual models insufficiently represent the coupled transport and reactivity along groundwater and surface water flow paths, the role of autochthonous organic matter in streambed biogeochemical functioning, and the feedbacks between surface-subsurface ecological processes, both within and across spatial and temporal scales. While simplified approaches to these issues are justifiable and necessary for transferability, the exclusion of important hyporheic processes from our conceptualizations can lead to erroneous conclusions and inadequate understanding and management of interconnected surface water and groundwater environments. This is particularly true at the landscape scale, where the organizational principles of spatio-temporal dynamics of hyporheic exchange flow (HEF) and biogeochemical processes remain largely uncharacterized. This article seeks to identify the most important drivers and controls of HEF and biogeochemical cycling based on a comprehensive synthesis of findings from a wide range of river systems. We use these observations to test current paradigms and conceptual models, discussing the interactions of local-to-regional hydrological, geomorphological, and ecological controls of hyporheic zone functioning. This improved conceptualization of the landscape organizational principles of drivers of HEF and biogeochemical processes from reach to catchment scales will inform future river research directions and watershed management strategies

Biofilm-induced bioclogging produces sharp interfaces in hyporheic flow, redox conditions, and microbial community structure

Riverbed sediments host important biogeochemical processes that play a key role in nutrient dynamics. Sedimentary nutrient transformations are mediated by bacteria in the form of attached biofilms. The influence of microbial metabolic activity on the hydrochemical conditions within the hyporheic zone is poorly understood. We present a hydrobiogeochemical model to assess how the growth of heterotrophic and autotrophic biomass affects the transport and transformation of dissolved nitrogen compounds in bedform-induced hyporheic zones. Coupling between hyporheic exchange, nitrogen metabolism, and biomass growth leads to an equilibrium between permeability reduction and microbial metabolism that yields shallow hyporheic flows in a region with low permeability and high rates of microbial metabolism near the stream-sediment interface. The results show that the bioclogging caused by microbial growth can constrain rates and patterns of hyporheic fluxes and microbial transformation rate in many streams

Influence of Stream-Subsurface Exchange Flux and Bacterial Biofilms on Oxygen Consumption Under Nutrient-Rich Conditions

The lack of a complete understanding of the complex reciprocal interactions between hydrological processes and the structure and function of microbial communities limits our ability to improve the predictions of microbial metabolism in streams. We report here on how overlying water velocity and losing and gaining flux interact with bacterial community structure and its activity to control oxygen consumption in a sandy streambed under high nutrient levels. We used an experimental flume packed with natural sediment and measured the bacterial biomass distribution and oxygen profiles in the streambed and across bed forms. Local oxygen consumption rates were calculated with a 1-D numerical model (GRADIENT). Bacterial abundance and production rates varied across the bed form within 1 order of magnitude, while their taxonomic classes were similar across bed forms despite variations in flow conditions and sediment disturbance events. However, bacterial production rates were not directly correlated with bacterial abundance. On the other hand, oxygen consumption rates ranged over 4 orders of magnitude across the bed forms and were highly correlated with the vertical exchange flux between the water and the streambed. The results strongly suggest that under high nutrient levels, the system is, in general, transport limited and that predicting oxygen consumption rates depends on local vertical exchange fluxes