aquifer. The biogeochemical reactions in such
underground estuaries are presumed to be similar to
the surface estuary (river water/seawater) counterpart, though few comprehensive studies of chemical
cycling in subterranean estuaries have been undertaken. Drivers of biogeochemical reactions in these
environments include oxidation–reduction gradients,
desorption–sorption processes, and microbially driven diagenesis.
In a study of the Waquoit Bay subterranean estuary, a large accumulation of iron (hydr)oxide-coated
sediments within the fresh–saline interface was encountered. These iron-oxide-rich sands could act as a
geochemical barrier by retaining and accumulating
certain dissolved chemical species carried to the
subterranean estuary by groundwater and/or coastal
seawater. Significant accumulation of phosphorus in
the iron oxide zones of the Waquoit cores exemplifies
this process (Figure 8).
Phosphorous is not the only nutrient that can be
retained/removed via reactions in the subterranean
estuary. The microbial reduction of nitrate to inert
dinitrogen gas, a process known as denitrification, is
known to occur in the redox gradients associated
with fresh and saline groundwater mixing. Conversely, ammonium, which is more soluble in saline
environments, may be released within the subterranean estuary’s mixing zone. While the overall
importance of SGD on the ‘global cycle’ of certain
chemical species remains to be seen, there is little
doubt that SGD is important at the local scale both
within the United States and throughout the world.
Summary
Groundwater discharge to the coastal ocean can be
an important source of fresh water and dissolved
solutes. Although a significant amount of research
into the role of SGD in solute budgets has only occurred in the past decade or so, there is increasing
evidence that solute loading from groundwater can
be significant enough to affect solute budgets and
even ecosystem health. Proper estimation of solute
loads from groundwater requires confident estimates
of both the groundwater discharge rate and average
solute concentrations in the discharging fluid, neither
of which are easily determined.
Estimating groundwater discharge rates is complicated by the spatial and temporal variability of
groundwater flow. A multitude of time-varying driving mechanisms complicate analysis, as does geologic
heterogeneity. Nonetheless, a suite of tools from
hydrogeology, geophysics, and geochemistry have
been developed for sampling and measuring SGD.
The nature of geochemical reactions within nearshore sediments is not well understood, yet recent
studies have shown that important transformations
occur over small spatial scales. This is an exciting
and critically important area of research that will
reveal important process in the near future.
See also
Chemical Processes in Estuarine Sediments. Gas
Exchange in Estuaries. Inverse Modeling of
Tracers and Nutrients. Long-Term Tracer
Changes. Tracer Release Experiments. Tracers of
Ocean Productivity.
Further Reading
Burnett WC, Aggarwal PK, Bokuniewicz H, et al. (2006)
Quantifying submarine groundwater discharge in the
coastal zone via multiple methods. Science of the Total
Environment 367: 498--543.
Burnett WC, Bokuniewicz H, Huettel M, Moore WS, and
Taniguchi M (2003) Groundwater and pore water inputs
to the coastal zone. Biogeochemistry 66: 3--33.
Cable JE, Burnett WC, Chanton JP, and Weatherly GL
(1996) Estimating groundwater discharge into the
northeastern Gulf of Mexico using radon-222. Earth
and Planetary Science Letters 144: 591--604.
Charette MA and Sholkovitz ER (2002) Oxidative
precipitation of groundwater-derived ferrous iron in the
subterranean estuary of a coastal bay. Geophysical
Research Letters 29: 1444.
Charette MA and Sholkovitz ER (2006) Trace element
cycling in a subterranean estuary. Part 2: Geochemistry
of the pore water. Geochimica et Cosmochimica Acta
70: 811--826.
Heath R (1998) Basic ground-water hydrology. US
Geological Survey Water Supply Paper 2220. Washington,
DC: USGS.
Kohout F (1960) Cyclic flow of salt water in the Biscayne
aquifer of southeastern Florida. Journal of Geophysical
Research 65: 2133--2141.
Lee DR (1977) A device for measuring seepage in lakes and
estuaries. Limnology and Oceanography 22: 140--147.
Li L, Barry DA, Stagnitti F, and Parlange J-Y (1999)
Submarine groundwater discharge and associated
chemical input to a coastal sea. Water Resources
Research 35: 3253--3259.
Michael HA, Mulligan AE, and Harvey CF (2005)
Seasonal water exchange between aquifers and the
coastal ocean. Nature 436: 1145--1148 (doi:10.1038/
nature03935).
Miller DC and Ullman WJ (2004) Ecological consequences
of ground water discharge to Delaware Bay, United
States. Ground Water 42: 959--970.
Moore WS (1996) Large groundwater inputs to coastal
waters revealed by Ra-226 enrichments. Nature 380:
612--614.
GROUNDWATER FLOW TO THE COASTAL OCEAN 473
underground estuaries are presumed to be similar to
the surface estuary (river water/seawater) counterpart, though few comprehensive studies of chemical
cycling in subterranean estuaries have been undertaken. Drivers of biogeochemical reactions in these
environments include oxidation–reduction gradients,
desorption–sorption processes, and microbially driven diagenesis.
In a study of the Waquoit Bay subterranean estuary, a large accumulation of iron (hydr)oxide-coated
sediments within the fresh–saline interface was encountered. These iron-oxide-rich sands could act as a
geochemical barrier by retaining and accumulating
certain dissolved chemical species carried to the
subterranean estuary by groundwater and/or coastal
seawater. Significant accumulation of phosphorus in
the iron oxide zones of the Waquoit cores exemplifies
this process (Figure 8).
Phosphorous is not the only nutrient that can be
retained/removed via reactions in the subterranean
estuary. The microbial reduction of nitrate to inert
dinitrogen gas, a process known as denitrification, is
known to occur in the redox gradients associated
with fresh and saline groundwater mixing. Conversely, ammonium, which is more soluble in saline
environments, may be released within the subterranean estuary’s mixing zone. While the overall
importance of SGD on the ‘global cycle’ of certain
chemical species remains to be seen, there is little
doubt that SGD is important at the local scale both
within the United States and throughout the world.
Summary
Groundwater discharge to the coastal ocean can be
an important source of fresh water and dissolved
solutes. Although a significant amount of research
into the role of SGD in solute budgets has only occurred in the past decade or so, there is increasing
evidence that solute loading from groundwater can
be significant enough to affect solute budgets and
even ecosystem health. Proper estimation of solute
loads from groundwater requires confident estimates
of both the groundwater discharge rate and average
solute concentrations in the discharging fluid, neither
of which are easily determined.
Estimating groundwater discharge rates is complicated by the spatial and temporal variability of
groundwater flow. A multitude of time-varying driving mechanisms complicate analysis, as does geologic
heterogeneity. Nonetheless, a suite of tools from
hydrogeology, geophysics, and geochemistry have
been developed for sampling and measuring SGD.
The nature of geochemical reactions within nearshore sediments is not well understood, yet recent
studies have shown that important transformations
occur over small spatial scales. This is an exciting
and critically important area of research that will
reveal important process in the near future.
See also
Chemical Processes in Estuarine Sediments. Gas
Exchange in Estuaries. Inverse Modeling of
Tracers and Nutrients. Long-Term Tracer
Changes. Tracer Release Experiments. Tracers of
Ocean Productivity.
Further Reading
Burnett WC, Aggarwal PK, Bokuniewicz H, et al. (2006)
Quantifying submarine groundwater discharge in the
coastal zone via multiple methods. Science of the Total
Environment 367: 498--543.
Burnett WC, Bokuniewicz H, Huettel M, Moore WS, and
Taniguchi M (2003) Groundwater and pore water inputs
to the coastal zone. Biogeochemistry 66: 3--33.
Cable JE, Burnett WC, Chanton JP, and Weatherly GL
(1996) Estimating groundwater discharge into the
northeastern Gulf of Mexico using radon-222. Earth
and Planetary Science Letters 144: 591--604.
Charette MA and Sholkovitz ER (2002) Oxidative
precipitation of groundwater-derived ferrous iron in the
subterranean estuary of a coastal bay. Geophysical
Research Letters 29: 1444.
Charette MA and Sholkovitz ER (2006) Trace element
cycling in a subterranean estuary. Part 2: Geochemistry
of the pore water. Geochimica et Cosmochimica Acta
70: 811--826.
Heath R (1998) Basic ground-water hydrology. US
Geological Survey Water Supply Paper 2220. Washington,
DC: USGS.
Kohout F (1960) Cyclic flow of salt water in the Biscayne
aquifer of southeastern Florida. Journal of Geophysical
Research 65: 2133--2141.
Lee DR (1977) A device for measuring seepage in lakes and
estuaries. Limnology and Oceanography 22: 140--147.
Li L, Barry DA, Stagnitti F, and Parlange J-Y (1999)
Submarine groundwater discharge and associated
chemical input to a coastal sea. Water Resources
Research 35: 3253--3259.
Michael HA, Mulligan AE, and Harvey CF (2005)
Seasonal water exchange between aquifers and the
coastal ocean. Nature 436: 1145--1148 (doi:10.1038/
nature03935).
Miller DC and Ullman WJ (2004) Ecological consequences
of ground water discharge to Delaware Bay, United
States. Ground Water 42: 959--970.
Moore WS (1996) Large groundwater inputs to coastal
waters revealed by Ra-226 enrichments. Nature 380:
612--614.
GROUNDWATER FLOW TO THE COASTAL OCEAN 473