A little history of Submarine Groundwater Discharge (SGD)

People have known about submarine groundwater discharge for many centuries. Etruscan citizens already made use of coastal springs for “hot baths” in the 2nd century A.D.. The earliest scientific documentation was recorded by a Roman geographer by the name of Strabo who lived from 63 B.C to 21 A.D. He illustrated a visible submarine spring 2.5 miles offshore of Latakia, Syria.

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Location of Latakia, Syria

At that time water from the spring was collected directly, transported to the city and used as a source of fresh water. Other historical material accounts for water vendors in Bahrain making a living of selling potable water from offshore submarine springs.

Although people already used SGD as an important water source for a long time, scientific research on groundwater discharge developed slowly. There are hardly any research results before the 19th century. Many hydrologists tried to understand the relationship between groundwater discharge and the movement of the seepage face but hardly any results were published since the majority of research was based on unrealistic hydrostatic situations where only the gravitational balance between the fresh water lens and brackish water was considered.

The Dupuit approximation was the first approach to incorporate the dynamic equilibrium which essentially illustrated the flow of submarine groundwater in its entire horizontal scale.

jules-dupuit
Jules Dupuit

Over the following years scientists started modelling groundwater movement in coastal areas. However, the stationary and sharp boundary proposed by this theory was not consistent with the fluctuations in interactions between groundwater and sea water along the shoreline. Finally, in 1940, M. King Hubbert introduced the concept of outflow gaps to explain these deviations and created the basis for the Potential Theory and the Glover Solution which provided independent means to calculate the width of the outflow gap, as well as the interface position. However, while these representations simplified calculations, researchers still mistook submarine groundwater discharge for fresh groundwater input only.

After the Second World War, with the development of new techniques and increasing knowledge of hydrology, natural tracers and computer simulations started playing an important role in hydrological research. In 1999 W.S. Moore, a researcher with experience in radioactive tracers, first established the concept of the Subterranean Estuary (SE) which describes a mixing zone between groundwater and sea water by comparing the similarities between subsurface and surface. In 2003 W.C. Burnett systematically illustrated SGD and defined it as “any and all flow of water on continental margins from the seabed to the coastal ocean, regardless of fluid composition”. Since then, SGD research has become a significant branch of hydrological research. More and more scientists from different areas showed interests in SGD and SE, the word “SGD” appeared in journals and attracted public attention.

Authored by Shan Jiang, PhD student, Biogeochemistry Research Group, TCD

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