Fresh Submarine Groundwater Discharge to the Near‐Global Coast

Abstract

The flow of fresh groundwater to the ocean through the coast (fresh submarine groundwater discharge or fresh SGD) plays an important role in global biogeochemical cycles and coastal water quality. In addition to delivering dissolved elements from land to sea, fresh SGD forms a natural barrier against salinization of coastal aquifers. Here we estimate groundwater discharge rates through the near‐global coast (60°N to 60°S) at high resolution using a water budget approach. We find that tropical coasts export more than 56% of all fresh SGD, while midlatitude arid regions export only 10%. Fresh SGD rates from tectonically active margins (coastlines along tectonic plate boundaries) are also significantly greater than passive margins, where most field studies have been focused. Active margins combine rapid uplift and weathering with high rates of fresh SGD and may therefore host exceptionally large groundwater‐borne solute fluxes to the coast.

Quantifying Regional Fresh Submarine Groundwater Discharge With the Lumped Modeling Approach CoCa‐RFSGD

“…Hence, Java can be expected to be a hot spot for FSGD. This correlates with the global models, which have located SE Asia and Central America as hot spots for FSGD (Beusen et al, ; Dürr et al, ; Zhou et al, ).…”

“…The island of Java, Indonesia, was selected to test the model because global (Beusen et al, ; Dürr et al, ; Seitzinger et al, ; Zhou et al, ) and regional (Adyasari et al, ) studies pointed out that high precipitation fluxes and anthropogenic changes result in high groundwater and nutrient fluxes, which suggests the importance of FSGD in that region. Java has a total area of 127,000 km 2 .…”

“…However, transient Fresh Submarine Groundwater Discharge (FSGD) is difficult to measure and regional models that quantify FSGD are only available for the Unite States (Befus et al, 2017;Sawyer et al, 2016) and Sweden (Destouni et al, 2008), which are well-monitored regions. Scientific field and modeling studies on FSGD at ungauged basins with sparse hydrogeological data availability are underrepresented, although the potential relevance of these regions has been pointed out in the past (Beusen et al, 2013;Destouni et al, 2008;Dürr et al, 2008;Seitzinger et al, 2005;Zhou et al, 2019). SGD and its effects on the ocean have been increasingly researched since the year 2000 (Moore, 2010).…”

“…FSGD has been studied by different types of models. Global models estimated the regional importance of FSGD for, for example, discharge volume or nutrient fluxes (see, e.g., Beusen et al, 2013;Dürr et al, 2008;Zhou et al, 2019). Local models were applied theoretically to study either drivers of groundwater seepage Water Resources Research 10.1029 into the ocean, such as oceanic oscillations like waves (Bakhtyar et al, 2011;Li et al, 2002;Xin et al, 2010) and tides (Abarca et al, 2013;Li et al, 2009;Robinson et al, 2007) as well as terrestrial groundwater oscillations (Michael et al, 2005) and the spatial distribution of SGD (Greskowiak, 2014).…”

Fresh groundwater discharge insignificant for the world’s oceans but important for coastal ecosystems

“…Global-scale estimates of fresh SGD vary by four orders of magnitude, and range up to 10% of the river discharge towards the oceans 1,2,10 . A recent estimate for the USA 11 and a near-global estimate 12 suggest that fresh SGD is at the lower end of most earlier estimates. However, these have been based on surface water budget calculations only, and their consistency with groundwater flow processes in the subsurface is uncertain.…”