DEFINITION OF SALTWATER INTRUSION

Encroachment of seawater into drinking

and irrigation wells is an increasing problem for many

coastal communities around the world. Porous soils and

rocks beneath the groundwater table in terrestrial environments

are generally saturated with freshwater, whereas

porous sediment and rock beneath the oceans are saturated

with salt water. In coastal environments there must be a

boundary between the fresh groundwater and the salty

groundwater. In some cases this is a vertical boundary,

whereas in other cases the boundary is inclined, with the

denser salt water lying beneath the lighter freshwater. In areas

where there is complex or layered stratigraphy the boundary

may be complex, consisting of many lenses.

In normal equilibrium situations the boundary between

the fresh and salty water remains rather stationary. In times

of drought the boundary may move landward or upward,

and in times of excessive precipitation the boundary may

move seaward and downward.

Many coastal communities have been highly developed,

with many residential neighborhoods, cities, and agricultural

users obtaining their water from groundwater wells. When

these wells pump more water out of coastal aquifers than is

replenished by new rainfall and other inputs to the aquifer

the freshwater lens resting over the saltwater lens is depleted.

This causes the salt water to move in to the empty pore

spaces to take the place of the freshwater. Eventually as

pumping continues the freshwater lens becomes so depleted

that the wells begin to draw salt water out of the aquifer, and

the well becomes effectively useless. This is called saltwater

intrusion or encroachment. In cases of severe drought the

process may be natural, but in most cases seawater intrusion

is caused by overpumping of coastal aquifers, aided by

drought conditions.

Many places in the United States have suffered from seawater

intrusion. For instance, many east coast communities

have lost use of their wells and had to convert to water piped in

from distant reservoirs for domestic use. In a more complicated

scenario, western Long Island, New York, experienced severe

seawater intrusion into its coastal aquifers because of intense

overpumping of its aquifers in the late 1800s and early 1900s.

Used water that was once returned to the aquifer by septic systems

began to be dumped directly into the sea when sewers

were installed in the 1950s, with the result being that the water

table dropped more than 20 feet over a period of 20 years.

This drop was accompanied by additional seawater intrusion.

The water table began to recover in the 1970s when much of

the area converted to using water pumped in from reservoirs in

the Catskill Mountains to the north of New York City.

See also GROUNDWATER.

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