Groundwater-surface water interfaces (e.g., hyporheic and benthic zones and riparian corridors) are integral components of the aquifer-river or aquifer-lake continuum. Interactions between groundwater and surface water lead to strong bi-directional influences between surface waters, aquifers and interconnecting hyporheic zones. A rapidly expanding number of research projects are now investigating the implications of hyporheic exchange on the transport and transformation of nutrients and contaminants within river networks, and on controls to heat, oxygen, and organic matter budgets available to microorganisms and macroinvertebrates in streambed sediments. However, there is still a need to better understand the links between physical, biogeochemical, and ecological process dynamics in groundwater-surface water interfaces and their implications for fluvial ecology or limnology, respectively. Furthermore, it is important to consider the response of hyporheic exchange fluxes to environmental and climatic controls at different spatial and temporal scales (e.g. river channel, alluvial aquifer, regional groundwater flow). We consider up- and downscaling and the development of a general conceptual framework and improved process understanding for groundwater-surface water interfaces as among the most urgent challenges of hyporheic zone research. Consequently, we particularly welcome contributions that aim to close these knowledge gaps and solicit both experimental and modelling studies with a focus on:

- The development and application of novel experimental methods to investigate physical, biogeochemical and ecological conditions at the groundwater-surface water interface in rivers, lakes, riparian corridors, and wetlands;

- Investigations of the role of hyporheic processes for the retention and natural attenuation of nutrients and pollutants, particularly with respect to impacts on surface water and groundwater quality;

- Hydrological, biogeochemical and ecological modelling approaches (e.g. transient storage models, coupled groundwater-surface water models etc.);

- Investigations of the implications of groundwater-surface water interactions for management and risk assessment frameworks with regard to the European Water Framework Directive.

Solicited contribution: Kevin Roche, Institute of Environmental Assessment and Water Research, Spanish National Research Council (CSIC)