Geoscience Reference
In-Depth Information
and it has been only recently that field measurements of groundwater dis-
charge have shown the importance of subsurface flow on water and nutrient
budgets in coast and estuaries. Groundwater discharge has shown to be a
source of nitrogen, typically as nitrate, in shallow sediments of lakes and
coastlines. For example, upland aquifers contribute to direct groundwater
discharge greater than 20% of the freshwater and 75% of the nitrogen that
enters the Great South Bay, New York.
2
Excess nitrogen in groundwater
derived from sewage and fertilizer has drastically affected water quality in
other estuaries and coastal lagoons as well.
3
Ecosystem of the world's coastlines are receiving extraordinary amounts
of nutrients as a consequences of human activities such as fertilizers, indus-
trial emissions to the atmosphere, and disposal of waste water in water-
sheds adjoining coastal waters. The loadings of nitrogen and phosphorous to
coastal aquatic environments even exceed those to fertilized agro-ecosystem.
Increased nutrient loading from anthropogenic sources is pervasive and
function of shallow coastal ecosystems during coming decades.
Although increased nutrient loading by precipitation has been docu-
mented, most research has focused on deeper estuaries in which flow from
rivers and streams dominates water budgets and contributes the major of
nutrients. Rivers and direct precipitation, however, are not the sole source
of freshwater — borne nutrients to coastal environments. Even in places
without rivers, salinity is often depleted in coastal waters due to ground-
water input. Groundwater flow is especially important where underlying
coastal sediments are coarse, unconsolidated sands of glacial or marine ori-
gin. In such situations flow of groundwater may be the major source of
nutrients to coastal waters.
In unconsolidated sediments, groundwater moves through the watershed
shoreward in paths that have downward vertical as well as horizontal vector.
Downward flow is caused by additional water infiltrating along the path of
the water. Freshwater eventually moves close enough to shore to meet the
denser saltwater that saturates interstitial space in sediments beneath the
sea. The presence of seawater in the pore space acts together with lower
head pressures in the near shore zone compared with offshore to deflect
the path of fresh groundwater sharply upward.
1
As a result, most of the
groundwater flow occurs very near the shore.
During the last decade, it has become apparent that groundwater flow
and transport of nutrients into shallow coastal water are far more significant
and widespread than had been realized. The importance of groundwater is
not so much because of the magnitude of flow rates, but rather because
