DUKE (US) — Efforts to divert water from mountains in Morocco to irrigate oases farms have dramatically increased the natural saltiness of groundwater.
For more than 40 years, snowmelt and runoff from Morocco’s High Atlas Mountains has been dammed and redirected hundreds of kilometers to the south to irrigate oases farms in the arid, sub-Saharan Draa Basin.
Researchers from Duke University and Ibn Zohr University in Agadir, Morocco, measured dissolved salt levels as high as 12,000 milligrams per liter at some locations—far above the 1,000 to 2,000 milligrams per liter most crops can tolerate.
Dissolved salt levels in the groundwater of the three southernmost farm oases are now so high they endanger the long-term sustainability of date palm farming there.
“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,” says Avner Vengosh, professor of geochemistry and water quality at Duke’s Nicholas School of the Environment. “Over time, the buildup of dissolved salt levels has become irreversible.”
Researchers sampled surface water being diverted to irrigate six oases in Morocco and performed isotopic “fingerprinting” to identify sources of rising salinity. (Credit: Nathaniel Warner/Duke University)
The scientists were able to know this by identifying the distinctive geochemical and isotopic signatures of different elements in the water, such as oxygen, strontium, and boron. Elements in low-saline water have different stable isotope signatures, or fingerprints, than those in high-saline water.
“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,” explains Nathaniel Warner, a PhD student at Duke who led the study. “We can also track the source of low-saline water flowing into a system.”
The practice of importing freshwater to irrigate crops is widespread throughout much of the world’s arid regions, Vengosh notes. Governments have invested billions of dollars to construct reservoirs, dams, pipelines, canals and other infrastructure to bring the vital resource from areas where it is plentiful to where it is scarce.
Future climate change models predict significant reductions in precipitation in the Southern Mediterranean and Northern Africa regions in coming decades. Snowmelt and runoff will diminish.
Local groundwater may be the best—perhaps 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 says. “The forensic tracing technologies we used in this study can help do that.”
Warner notes that by using the isotopic fingerprinting technologies, the researchers discovered a previously overlooked low-saline water source that flows naturally into the Draa Basin from the adjacent Anti-Atlas Jabel Saghro Mountains.
The natural flow of freshwater from this source dilutes the saltiness of nearby groundwater aquifers and improves prospects for the future of farming at the basin’s three northernmost oases.
Dissolved salt levels in these oases’ groundwater are between 450 and 4,225 milligrams per liter—a more sustainable level, especially for growing date palms, which are the primary commercial crop in the basin and relatively salt-tolerant.
“Prior to our study, people didn’t think this was a major water input into the Draa system,” Vengosh says. “We now know it is—and that it deserves to be protected as such.”
The study appears online in the journal Applied Geochemistry.
NATO’s Science for Peace Program funded the project.
Source: Duke University