Character of submarine groundwater discharge around islands
Abstract
Submarine groundwater discharge (SGD) from oceanic islands has been estimated to contribute over a third of the global SGD due to the high shoreline-to-land area ratio, orographic precipitation, short aquifer pathways and poorly developed surface drainage. Relatively few islands have been studied, but SGD is typically found to be an important, and often the only, source of nutrients to coastal waters, enhanced by the short distances between the areas of recharge inland and discharge at the shoreline. An inverse correlation between salinity and the concentrations of nitrate plus nitrite ([NOx]) found in northern Guam, Barbados and the Balearic Islands demonstrate the freshwater source of NOx. Inputs can be strongly modulated seasonally; in Barbados, the low-salinity end-member [NOx] was 1.8 times greater in the dry season than the wet season. Observations on islands can be challenging not only because of extremes seasonal precipitation but also because of the inhomogeneous nature of aquifers, and strong, oceanic influences. On islands with fractured rock (Mauritius) or karstic (Barbados, Guam, or the Balearic Islands) aquifers, discharge is often into shallow coastal lagoons where a thin veneer of sand often covers the highly permeable aquifer rock; the rapid, but heterogeneous, pathway makes the distribution of SGD variable both in time and space. In Mauritius, SGD exceeded 4.9 m/d near a known spring, but large variations in SGD rates were seen over distances of a few meters. In Guam, SGD was four times greater around the northern, permeable limestone aquifer, than it was along the southern, semi-impermeable volcanic terrain; [NOx] was sevenfold greater along the coast of northern Guam while, in the south, silicate concentrations were two times greater, phosphate concentrations were four times greater and ammonia concentrations were 1.3 times greater than those in the north. Oceanic influences, such as the recirculation of ocean water through fringing reefs, also can exert a strong influence on an island’s SGD. SGD in Mallorca showed little or no dilution from the ambient, open-water salinity being dominated by the recirculation of sea water through the aquifer, driven by oceanic processes. SGD at one site in Barbados had a 33% freshwater component of SGD, while an adjacent watershed’s SGD was composed entirely of recirculated seawater. ([NOx] was an order of magnitude greater with the freshwater component of SGD). Even tiny Mauritius could be supplying almost 80 million cubic meters per day to carry the composition of the subterranean estuary into the adjacent, deep ocean. Other investigators have found the geochemical signature of SGD widespread offshore of Puerto Rico at depths up to 3.5 km. High population densities and limited freshwater resources also make islands especially sensitive to the impacts of climate change. SGD provides not only a link between development and water quality in the coastal zone but can be expected to be one bellwether of climatic changes. Forecast changes in precipitation, temperature, growing season and the intensity of wet and dry periods should all alter SGD.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.H13H..03B
- Keywords:
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- 1828 HYDROLOGY / Groundwater hydraulics;
- 1830 HYDROLOGY / Groundwater/surface water interaction