Ideas and perspectives: Land–ocean connectivity through groundwater

Abstract. For millennia, humans have gravitated towards coastlines for their

resource potential and as geopolitical centres for global trade. A basic

requirement ensuring water security for coastal communities relies on a

delicate balance between the supply and demand of potable water. The

interaction between freshwater and saltwater in coastal settings is,

therefore, complicated by both natural and human-driven environmental

changes at the land–sea interface. In particular, ongoing sea-level rise,

warming and deoxygenation might exacerbate such perturbations. In this

context, an improved understanding of the nature and variability of

groundwater fluxes across the land–sea continuum is timely yet remains out

of reach. The flow of terrestrial groundwater across the coastal transition

zone and the extent of freshened groundwater below the present-day

seafloor are receiving increased attention in marine and coastal sciences

because they likely represent a significant yet highly uncertain component

of (bio)geochemical budgets and because of the emerging interest in the

potential use of offshore freshened groundwater as a resource. At the same

time, “reverse” groundwater flux from offshore to onshore is of prevalent

socio-economic interest, as terrestrial groundwater resources are

continuously pressured by over-pumping and seawater intrusion in many coastal

regions worldwide. An accurate assessment of the land–ocean connectivity

through groundwater and its potential responses to future anthropogenic

activities and climate change will require a multidisciplinary approach

combining the expertise of geophysicists, hydrogeologists, (bio)geochemists

and modellers. Such joint activities will lay the scientific basis for

better understanding the role of groundwater in societally relevant issues

such as climate change, pollution and the environmental status of the

coastal oceans within the framework of the United Nations Sustainable

Development Goals. Here, we present our perspectives on future research

directions to better understand land–ocean connectivity through groundwater,

including the spatial distributions of the essential hydrogeological

parameters, highlighting technical and scientific developments and briefly

discussing the societal relevance of that connectivity in rapidly changing coastal oceans.