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According to a recent summary, estimates of the global magnitude of SGD based on hydrological and water balance calculations range from 0.3-16% of river discharge, with most estimates in the range of 6-10% [62]. These values refer only to the discharge of fresh, terrestrially-derived water via SGD and, therefore, ignore the contribution of seawater discharged to the coastal zone following mixing with fresh groundwater. As this mixing of fresh water and seawater within the aquifer has important ramifications for the chemistry of the discharged groundwater, these freshwater-only estimates must be considered the lower boundary.
The increased use of radioisotope tracers over the past ten years has allowed researchers to more accurately constrain groundwater discharge fluxes. By pairing multiple tracers and improved seepage meter technologies, groundwater flux estimates can be made more accurate yet [62]. Improved accuracy in groundwater flux calculations will yield better nutrient flux data. Furthermore, as multiple tracers, including dissolved nutrients and stable isotope signatures, are added to the existing radioisotope tracers, we can increase our understanding of how SGD transports dissolved nutrients to the coastal zone.
The number of investigations into the transport of nutrients to the coastal zone via SGD is growing, but this remains a relatively new field of research. Most of the existing research has been performed in the eastern United States and in limited regions of Europe, Japan, and Oceania [458]. There is a conspicuous lack of SGD studies in Asia, India, Africa, South America, and the western United States. An expansion of SGD studies, including assessments of SGD-nutrient fluxes to estuaries and the coastal ocean, will provide insight into anthropogenic impacts on, and non-point source nutrient fluxes to, the coastal zone.
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