Groundwater-surface water interactions – the least understood part of the water cycle?

On every diagram of a water cycle, interactions between groundwater and surface water reservoirs feature prominently. But just how much do we know about these interactions? Many if not most aquatic systems are significantly impacted by groundwater discharge and hyporheic flow (into and out of the sediment), but water flows across the sediment-water interface to rivers, lakes and the coast arguably remain the least well known component of the water cycle. Likewise, solute fluxes associated with this exchange and ‘downstream’ hydrological, geochemical and ecological consequences remain mostly unknown.

Using primarily geochemical tools, I will present example studies of groundwater processes in tropical wetlands, mangroves and coastal lagoons. For example, in a recent study of coastal Mediterranean lagoons, we demonstrated that (a) the discharge of low-salinity karstic groundwater maintains brackish ecosystem functioning throughout the dry summer months, and (b) equally as important, wind-driven water circulation through lagoon sediments is surprisingly large: the equivalent of the entire lagoon volume is circulating through the sediments in less than one month on average, indicating how these ‘invisible’ fluxes may be important to hydrological, geochemical and ecological processes in aquatic and coastal systems.

Finally, I will brainstorm future projects, and how and where studies of groundwater-surface water processes could interact with current research at GET, in particular in tropical regions (Amazon, West African coastal zone etc).