Groundwater lost to rising sea levels could be greater than thought

Rising sea levels could swallow up to 40 percent more potable groundwater than previously thought because of tricks of topography, a new study has found.

Many current predictions about the impact of global warming look at how much land would be lost to rising sea levels.

But researchers at Ohio State University have found that in many coastal regions sea water will leach into the water table and contaminate groundwater well beyond the shoreline.

The degree to which groundwater is contaminated depends on shoreline structure: sandy beaches allow for much greater subsurface mixing than solid cliffs.

"The complex structure of the soil can enhance mixing between salt water and fresh water and that area can extend more than the distance that the coastal line recedes," said hydrology professor Motomu Ibaraki, who designed the study.

"In most studies, people say if the coastline recedes 100 meters then freshwater recedes 100 meters. Well, our study shows that it's going to be extended (up to 40 percent) more by the mixing process."

Ibaraki and a graduate student built a computer simulation to study how different coastal soil structures would affect subsurface mixing of salt water and fresh water.

The next step is to take the model and apply it to specific geographical locations to determine how much freshwater would be lost as sea levels rise.

The UN's Intergovernmental Panel on Climate Change predicts that global mean sea levels will rise by 14 to 44 centimeters (5.5-17 inches) by 2100 as a result of global warming. The impact would be far greater in low-lying coastal areas.

Since it takes relatively small amounts of salt water to render fresh water undrinkable, even nominal increases in sea levels can have dramatic effects on fresh water resources, Ibaraki said.

"The amount of water we have on the earth is constant. However, the amount of fresh water we can use is decreasing," Ibaraki said in a telephone interview.

"Only two percent of the earth's water is fresh water and most of it is contained in glaciers. We are losing glaciers but we don't know how much and because we have more demand for water, groundwater is also diminishing."...