April 8, 2013
Salt Levels In Draa Basin Groundwater A Detriment To Farming
Brett Smith for redOrbit.com — Your Universe Online
Arid but habitable regions around the world depend on limited water resources for drinking and crop irrigation. Any disruption or contamination of the water supply could be a major problem for a local population.
The Draa Basin in Sub-Saharan Africa is one such region, as it has relied on runoff from Morocco's High Atlas Mountains. It has also been dammed and directed toward oases hundreds of miles to the south.
A group of American and Moroccan researchers has found that, far from sustaining life, the transported water has been dramatically increasing the natural saltiness of the local groundwater — to the point of being a detriment.
According to a report in the journal Applied Geochemistry, salt levels of water samples collected at the basin measured as high as 12,000 milligrams per liter — much saltier than the 1,000 to 2,000 milligrams per liter that most crops can accept.
"The flow of imported surface water onto farm fields has caused natural salts in the desert soil and underlying rock strata to dissolve and leach into local groundwater supplies," explained co-author Avner Vengosh, professor of geochemistry and water quality at Duke University's Nicholas School of the Environment. "Over time, the buildup of dissolved salt levels has become irreversible."
In the study, the research team investigated the salinity information of a water sample based on the identification of unique geochemical and isotopic signatures of various elements in the water; such as oxygen, strontium and boron.
"Once we get a water sample's fingerprint, we can compare it to the fingerprints of other samples and track the nature of the salinity source," said lead author Nathaniel Warner, a graduate student at the Nicholas School. "We can also track the source of low-saline water flowing into a system."
Because climate change models forecast lower precipitation levels in the Southern Mediterranean and Northern Africa regions in coming decades, the basin´s local groundwater may be the only source of water remaining for many communities.
"Protecting this vital resource, and helping governments in desert areas worldwide find new, untapped sources of it, is the wiser approach in the long run," Vengosh said. "The forensic tracing technologies we used in this study can help do that."
By using the isotopic fingerprints, the researchers found an overlooked source of low-saline water that connects to the Draa Basin from the nearby Anti-Atlas Jabel Saghro Mountains. The fresh water allows for the dilution of local, high-salt groundwater aquifers and adds to the potential for the future of farming at three of the basin´s oases. The dissolved salt levels in these oases were found to be between 450 and 4,225 milligrams per liter. According to the scientists, these are much more sustainable levels, particularly for growing date palms, which are a major commercial crop in the basin and fairly salt-tolerant.
"Prior to our study, people didn't think this was a major water input into the Draa system," Vengosh said. "We now know it is -- and that it deserves to be protected as such."