Bradley A. Weymer (first and corresponding author), Associate Professor of the School of Oceanography, Shanghai Jiao Tong University, along with his colleagues Mark E. Everett (Texas A&M University), Amir Haroon (GEOMAR), Marion Jegen-Kulcsar (GEOMAR), Aaron Micallef (University of Malta), Christian Berndt (GEOMAR), Holly A. Michael (University of Delaware), Rob L. Evans (WHOI), Vincent Post (Edinsi Groundwater) publish a Perspective paper in Communications Earth & Environment, titled “The coastal transition zone is an underexplored frontier in hydrology and geoscience.”

What is the coastal transition zone and why is it important?

The transition between land and sea, or the so-called “coastal transition zone” is a very poorly studied frontier resulting from limitations of technology and logistical barriers. Sometimes referred to as the “white ribbon,” this region is a gradational boundary comprising an elusive information gap spanning the onshore to shallow-water offshore region (Fig. 1). It is one of the most challenging environments of the solid Earth system to study because water depths in nearshore areas are too deep for remote sensing from aircraft and satellites, but too shallow for deep-sea and ocean geophysical and drilling technologies. The region contains, amongst many intriguing targets, the transition from continental to oceanic crust. As an underexplored frontier, we cannot yet grasp its full importance for certain fundamental Earth processes.

Figure 1. The coastal white ribbon spans the transition between land and sea; it is defined as the nearshore shallow water region below which there exists a paucity of data constraining deep structure and processes. The image shows breaking waves resembling a white ribbon in the nearshore zone at Padre Island National Seashore, Texas, USA (Photo credit: B. Weymer. 29 November, 2013).

Although strides are being made to bridge the information gap, most work to date has utilized interpolation between disparate datasets crossing the shoreline and/or has focused on the challenges associated with integrating terrestrial and marine datasets to map surface morphology and shallow (~500 m) subsurface features. In other words, we have only begun to scratch the surface, and a challenge moving forward is to characterize the deep (> 500 m) white ribbon region with seamless data coverage. Improved technologies are needed to fill the blank slate lying deep beneath the coastal white ribbon.

In this Perspective, we highlight several examples that identify the region deep beneath the coastal white ribbon as an important frontier for a variety of research endeavors related to; tectonics, hydrocarbon exploration, geohazards and particularly, coastal hydrogeology. In each case geophysical imaging complemented by scientific drilling is envisioned to play a primary role.

• We focus on the connections between onshore-offshore aquifers as an illustration of the important need for white ribbon characterization. The lack of deep subsurface information beneath the coastal white ribbon impedes an accurate accounting of the partitioning and volume transfers between terrestrial groundwater and sub-ocean reservoirs. These reservoirs are important to study because of their potential as an unconventional resource that may help alleviate water stress for coastal communities worldwide.
• We discuss some current challenges and propose possible solutions in developing and/or adapting existing exploration geophysical and drilling technologies and integrating them with advanced subsurface process modeling. Many large offshore groundwater reservoirs are located beneath the maximum depth of investigation of conventional terrestrial, marine, airborne or amphibious hydrogeophysical methods. Several geophysical techniques have the potential to be adapted for deep investigations beneath the coastal white ribbon including; controlled-source electromagnetic systems, semi-airborne electromagnetic systems, and seismic methods. Sediment coring into the coastal white ribbon and ideally installing long-term subseafloor observatories could result in a step-change understanding of how coastal aquifers function and the overall importance of connections between the onshore and offshore systems.
• The paper concludes with a motivation for future research with the aim of developing capabilities to seamlessly bridge the transition from land to sea. Due to the inherent multi-disciplinary nature in coastal hydrogeological research, understanding the connections, storage and fluxes of groundwater beneath the white ribbon is likely to benefit from data collected by deep-probing geophysical and drilling technologies. A lack of data often forces groundwater modelers to make simplified assumptions about the offshore groundwater salinity distribution (e.g. all groundwater is saline) that may not be in line with the more complex reality. Researchers focused on either side of the coastline face similar challenges, but often approach solutions from different perspectives. It is important to bridge the information gap across and beneath the white ribbon to bring these two scientific communities together to expand the global inventory of deep coastal groundwater systems and model their dynamics and evolution in response to climate change and possible future resource exploitation.

About the Journal:

Communications Earth & Environment is a new open access journal from Nature Portfolio publishing high-quality research, reviews and commentary in all areas of the Earth, environmental and planetary sciences. Research papers published by the journal represent significant advances that bring new insight to a specialized area in Earth science, planetary science or environmental science. Perspective articles are intended to provide a forum for authors to discuss models and ideas from a personal viewpoint. They are more forward looking and/or speculative than Review articles and may take a narrower field of view. They may be opinionated but should remain balanced and are intended to stimulate discussion and new experimental approaches.

The article is an invited contribution to the journal. It is the outcome of fruitful discussions on the state of the art of offshore groundwater research held during a two-day international workshop organized and led by Bradley Weymer as part of the SMART project* through the Helmholtz European Partnering Initiative. The workshop brought together leading scientists in the fields of marine and applied geophysics, coastal geomorphology, hydrogeology, and geochemistry. The workshop was hosted at Texas A&M University, College Station, Texas, USA in December, 2019.

Article information and related links:

Weymer, B.A., Everett, M.E., Haroon, A. et al. The coastal transition zone is an underexplored frontier in hydrology and geoscience. Commun Earth Environ 3, 323 (2022). https://doi.org/10.1038/s43247-022-00655-8

This Perspective is an open access article and can be accessed here: https://www.nature.com/articles/s43247-022-00655-8

*Link to the SMART project homepage: https://www.offshoregroundwater.com/