But this new
research shows that when saltwater and fresh water meet,
they mix in complex ways, depending on the texture of
the sand along the coastline. In some cases, a zone of
mixed, or brackish, water can extend 50 percent further
inland underground than it does above ground.
Like saltwater, brackish water is not safe to drink
because it causes dehydration. Water that contains less
than 250 milligrams of salt per liter is considered
fresh water and safe to drink.
Motomu Ibaraki, associate professor of earth sciences
at Ohio State, led the study. Graduate student Jun
Mizuno presented the results Tuesday, October 30, 2007,
at the Geological Society of America meeting in Denver.
“Most people are probably aware of the damage that
rising sea levels can do above ground, but not
underground, which is where the fresh water is,”
Ibaraki said. “Climate change is already diminishing
fresh water resources, with changes in precipitation
patterns and the melting of glaciers. With this work,
we are pointing out another way that climate change
can potentially reduce available drinking water. The
coastlines that are vulnerable include some of the
most densely populated regions of the world.”
In the
United States, lands along the East Coast and the Gulf
of Mexico -- especially Florida and Louisiana -- are
most likely to be flooded as sea levels rise.
Vulnerable areas worldwide include Southeast Asia, the
Middle East, and northern Europe.
“Almost 40 percent of the world population lives in
coastal areas, less than 60 kilometers from the
shoreline,” Mizuno said. “These regions may face loss
of freshwater resources more than we originally
thought.”
Scientists have used the IPCC reports to draw maps
of how the world's coastlines will change as waters
rise, and they have produced some of the most striking
images of the potential consequences of climate
change.
Ibaraki said that he would like to create similar
maps that show how the water supply could be affected.
That's not an easy task, since scientists don't
know exactly where all of the world's fresh water is
located, or how much is there. Nor do they know the
details of the subterranean structure in many places.
One finding of this study is that saltwater will
penetrate further into areas that have a complex
underground structure.
Typically, coastlines are made of different sandy
layers that have built up over time, Ibaraki
explained. Some layers may contain coarse sand and
others fine sand. Fine sand tends to block more water,
while coarse sand lets more flow through.
The researchers simulated coastlines made entirely
of coarse or fine sand, and different textures in
between. They also simulated more realistic, layered
underground structures.
The simulation showed that, the more layers a
coastline has, the more the saltwater and fresh water
mix. The mixing causes convection -- similar to the
currents that stir water in the open sea. Between the
incoming saltwater and the inland fresh water, a pool
of brackish water forms.
Further sea level rise increases the mixing even
more.
Depending on how these two factors interact,
underground brackish water can extend 10 to 50 percent
further inland than the saltwater on the surface.
According to the United States Geological Survey,
about half the country gets its drinking water from
groundwater. Fresh water is also used nationwide for
irrigating crops.
“In order to obtain cheap water for everybody, we
need to use groundwater, river water, or lake water,”
Ibaraki said. “But all those waters are disappearing
due to several factors --including an increase in
demand and climate change.”
One way to create more fresh water is to desalinate
saltwater, but that's expensive to do, he said.
“To desalinate, we need energy, so our water
problem would become an energy problem in the future.”
