Chen-Tung Arthur Chen1,2*, Ting-Hsuan Huang1, Hon-Kit Lui1 and Jing Zhang3
1Department of Oceanography, National Sun Yat-sen University, Taiwan
2Institute of Marine Chemistry and Environment, Zhejiang University, China
3Environmental Biology and Chemistry, Graduate School of Science and Engineering, University of Toyama, Japan
Submission: September 16, 2019; Published:October 09, 2019
*Correspondence author: Chen-Tung Arthur Chen, Department of Oceanography, National Sun Yat-sen University, Kaohsiung 80424, Taiwan
How to cite this article:Chen-Tung Arthur Chen, Ting-Hsuan Huang, Hon-Kit Lui, Jing Zhang. Unheralded Submarine Groundwater Discharge. Oceanogr Fish Open Access J. 2019; 10(5): 555797. DOI: 10.19080/OFOAJ.2019.10.555797
Land meets the ocean in the coast zone, which is of great importance for aquaculture, fisheries, tourism and transportation, among other things. As fifty percent of the world population lives within 100 km of the coast, human activities have put tremendous pressure on the coastal environment. For instance, excess nutrients that are generated by agriculture and present in domestic waste have been discharged into the oceans by rivers, making many coastal regions hypoxic or even anoxic. Most people do not realize that a substantial fraction of the freshwater on land enters the oceans directly from the seabed, unseen by human eyes. This process is called submarine groundwater discharge (SGD) [1-5]. Burnett and coworkers  were among the first to provide a ballpark estimate that the amount of SGD is 0.2-10% of global river discharge. Since groundwater has been frequently in contact with soil and bedrock for hundreds of years, if not longer, some of the dissolved organic carbon (DOC) in the groundwater as well as particulate organic carbon and CaCO3 in the soil and bedrock are expected to have decomposed or dissolved.
These processes release nutrients, alkalinity (TA), and dissolved inorganic carbon (DIC) into the groundwater while reducing its dissolved oxygen (DO) content and pH [7-11]. The partial pressure of CO2 (pCO2) in the groundwater normally exceeds the saturation level. The variation of DOC concentration depends on the balance between its decomposition in groundwater and its generation from soil and bedrock. Groundwater is isolated from the atmosphere but when it enters the oceans, supersaturated CO2 may be released to the atmosphere . Excess nutrients that are carried by the SGD promote primary production, which reduces pCO2. Whether the SGD eventually functions as a carbonsource or a sink into the receiving coastal waters cannot be determined a priori. Only recently did two investigations provide a preliminary answer. First, Chen and coworkers  estimated that the freshwater component of the SGD in Taiwan equals around 5.2% of the annual river discharge in Taiwan, where the over pumping of groundwater has been a serious problem.
Even in such an environment, the SGD is substantial. Additionally, Wang and coworkers  indicated that in Taiwan, the SGD exports N, P, Si, TA, and DIC in amounts equivalent to 12.7, 0.9, 9.3, 21.1 and 19.6%, respectively, of those in riverine fluxes (Table 1). Finally, Wang and coworkers found that primary production, supported by nutrient outflows from the SGD in Taiwan and from the Jiulong River across the Taiwan Strait, do not suffice to compensate for the DIC that is supplied by the SGD. Consequently, the SGD helps to make the coastal waters in Taiwan and Jiulong River heterotrophic, releasing CO2 to the atmosphere. Unfortunately, such a conclusion cannot be generalized to a region- let along to all coastal seas around the world - because the total
area of Taiwan (35,873km2) and the Jiulong River watershed
(14,740km2) is only 0.034% of the area of all land on earth.
Whereas rivers have been extensively studied, only a handful of
reports concerning nutrients, TA and DIC fluxes have focused on
Worse, the relevant literature is mainly concerned with the
flux per unit area of discharge into oceans. Such data cannot
be extrapolated to the global oceans as the discharge area is
unknown. On the other hand, the yield, which is the flux per unit
area of the river basin, can be extrapolated from readily available
river basin data. According to the latest tabulation of Wang et al.
 yield data for N, P, Si, TA and DIC are available only for Taiwan,
the Jiulong River and the Pearl River. An accurate estimate of the
SGD flux is essential to predicting coastal environments in an era
of rapid global change but most - if not all - coastal biogeochemical
models have no SGD component. Substantial errors may arise if
SGD is ignored, especially in estuaries, bays, inland seas, deltas
and salt marshes. Since the areas of thousands of river basins
worldwide are known, with a knowledge of the yield the global
amount of material flux by SGD can be reasonably estimated and
its effect on the coastal biogeochemistry better evaluated.
Liu Q, Dai M, Chen W, Huh C A, Wang G et.al. (2012) How significant is submarine groundwater discharge and its associated dissolved inorganic carbon in a river-dominated shelf system? Biogeosciences 9: 1777-1795.
Hong Q Q, Cai P H, Shi X M, Li Q and Wang G Z (2017) Solute transport into the Jiulong River estuary via pore water exchange and submarine groundwater discharge: New insights from Ra-224/Th-228 disequilibrium. Geochimica et Cosmochimica Acta 198: 338-359.