Fifty years of hyporheic zone research have shown the important role played by the hyporheic zone as an interface between groundwater and surface waters. However, it is only in the last two decades that what began as an empirical science has become a mechanistic science devoted to modeling studies of the complex fluid dynamical and biogeochemical mechanisms occurring in the hyporheic zone. These efforts have led to the picture of surface‐subsurface water interactions as regulators of the form and function of fluvial ecosystems. Rather than being isolated systems, surface water bodies continuously interact with the subsurface. Exploration of hyporheic zone processes has led to a new appreciation of their wide reaching consequences for water quality and stream ecology. Modern research aims toward a unified approach, in which processes occurring in the hyporheic zone are key elements for the appreciation, management, and restoration of the whole river environment. In this unifying context, this review summarizes results from modeling studies and field observations about flow and transport processes in the hyporheic zone and describes the theories proposed in hydrology and fluid dynamics developed to quantitatively model and predict the hyporheic transport of water, heat, and dissolved and suspended compounds from sediment grain scale up to the watershed scale. The implications of these processes for stream biogeochemistry and ecology are also discussed.

Recognizing greywater as a relevant secondary source of water and nutrients represents an important chance for the sustainable management of water resource. In the last two decades, many studies analysed the environmental, economic, and energetic benefits of the reuse of greywater treated by nature-based solutions (NBS). This work reviews existing case studies of traditional constructed wetlands and new integrated technologies (e.g., green roofs and green walls) for greywater treatment and reuse, with a specific focus on their treatment performance as a function of hydraulic operating parameters. The aim of this work is to understand if the application of NBS can represent a valid alternative to conventional treatment technologies, providing quantitative indications for their design. Specifically, indications concerning threshold values of hydraulic design parameters to guarantee high removal performance are suggested. Finally, the existing literature on life cycle analysis of NBS for greywater treatment has been examined, confirming the provided environmental benefits.

A model for the evaluation of the intra‐meander hyporheic exchange fluxes is presented. The method relies on a physically‐based morphodynamic model to predict the characteristics of the flow field in a meandering river and the temporal evolution of its planimetry. The hyporheic fluxes induced at the meander scale by the river sinuosity can therefore be computed. The application of the model to a simulated case has shown the fundamental role of the river planimetry on the hyporheic exchange pattern at the meander scale, and its influence on the long‐term evolution of the hyporheic exchange.

The dynamics of chemicals within a catchment are strongly affected by the exchange between surface water and groundwater. The activity of hyporheic microorganisms plays a key role in these processes, as they are able to oxidize and reduce organic matter and nutrients. The interplay between the residence times in the hyporheic zone and the temporal scales of the microbial reactions must thus be understood in order to improve our understanding of the role of fluvial environments in nutrient cycling. In this paper we focus on the intrameander hyporheic region and investigate the links between river morphology, hyporheic flow field, and biogeochemical processes. We adopt a modeling framework that considers the planimetric evolution of a meandering river, the hyporheic flow field induced by the river sinuosity, and the main biogeochemical reactions of organic carbon degradation. The chemical zonation of the intrameander hyporheic zone clearly emerges as a result of the coupling between hyporheic flow and biochemical activity. The resulting patterns of nutrient concentrations are strongly influenced by the river morphology. We also show how to estimate the characteristic timescales of the redox reactions and that their interplay with the kinematic timescales of the hyporheic flow controls both the biochemical zonation and the overall rate of nutrient transformation.