Climate change threatens drinking water, as rising sea penetrates coastal aquifers

As sea levels rise, coastal communities could lose up to 50 percent more of their fresh water supplies than previously thought, according to a new study from Ohio State University।

Hydrologists here have simulated how saltwater will intrude into fresh water aquifers, given the sea level rise predicted by the Intergovernmental Panel on Climate Change (IPCC). The IPCC has concluded that within the next 100 years, sea level could rise as much as 23 inches, flooding coasts worldwide.
Scientists previously assumed that, as saltwater moved inland, it would penetrate underground only as far as it did above ground.
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 October 30, 2007, at the Geological Society of America meeting in Denver.
“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.”
“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.”
Adapted from materials provided by Ohio State University.

Seawater Intrusion Is The First Cause Of Contamination Of Coastal Aquifers

Seawater intrusion is often the consequence of freshwater aquifers overexploitation। This is a very common and serious phenomenon all over the Mediterranean basin, as well as in other areas with similar weather conditions and population.

In Spain, the most severely affected areas by seawater intrusion are the Mediterranean and South-Atlantic coastlines. Given that Spain is located on a peninsula, seawater intrusion is currently one of the main causes of groundwater pollution. In fact, “about 60% of Spanish coastal aquifers are contaminated by seawater intrusion, a generalised phenomenon in 20% of cases”, points out Prof. José Benavente Herrera, a researcher from the Water Institute of the University of Granada, Spain, and senior lecturer at the department of Geodynamics.
According to Prof। Benavente, “freshwater contaminated by a 5% of seawater can no longer be used for common purposes, such as human use, agriculture or farming. That is the reason why salinisation of coastal aquifers – mainly a consequence of an uncontrolled or deficient management – is such a serious phenomenon”. In Southern Spain, “seawater intrusion is contaminating some of the most important aquifer systems in economic terms”, both on the Mediterranean and the Atlantic coastlines. In the world, the most affected areas include Mexico, the North of the Pacific and Atlantic coastlines, Chile, Peru and Australia.

Solutions: prevention and control
A good knowledge of aquifers (subsoil) enables scientists to determine the ‘critical discharge’, i.e. the extent to what aquifers can support water catchments without seawater intrusion taking place. Experts in hydrogeology acknowledge that such is a complex question, but they can currently give advice on prevention and control of situations caused by human activity.
Prof. Benavente states that solutions to prevent salinisation should start by studying every aquifer individually. Therefore, reducing freshwater pumping should be followed by other measures, such as analysing the aquifer’s situation before building reservoirs upstream, as this will account for a serious minimisation of its natural recharge and, possibly, for salinisation if the return flow is not guaranteed. In fact, “ironic as it may be, building up an artificial reservoir could render useless the natural groundwater reservoir downstream.”
Also, says Benavente, in very localised seawater intrusion areas, reducing pumping or extracting water from either smaller or greater depths become indispensable measures. According to Benavente, artificial recharge of aquifers is another efficient measure to prevent salinisation, as it stops seawater intrusion and increases freshwater levels. In this sense, for instance, clean water obtained from urban sewage purification can be used for irrigation of crops and golf fields as well as to create a hydraulic barrier against seawater intrusion.
Some regions in the world – including Spain – are already implementing these measures। Prof. Benavente highlights Los Angeles, USA, and river Llobregat delta, Spain, where sewage water injection as mentioned above has proven to be useful to solve salinisation problems.

Quick summary
- About 60 per cent of coastal aquifers are contaminated by seawater intrusion, a generalised phenomenon in 20 per cent of cases.
- Experts have found viable solutions implying not so much investment, but rather an ‘intelligent’ management of water resources.
- Scientists from France, Italy, Morocco, the Palestinian Authority, Spain, Switzerland and Tunisia have taken part in an international project to address sustainable water management in Mediterranean coastal aquifers.
Note: This story has been adapted from a news release issued by Universidad de Granada.