撒哈拉地下水盐度上升

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（绿洲） farms in the arid（干旱的）, sub-Saharan Draa Basin. But a new study by American and Moroccan scientists finds that far from alleviating water woes for the six farm oases in the basin, the inflow of imported water has exacerbated（加重，恶化） problems by dramatically increasing the natural saltiness of their groundwater.

Researchers from Duke University in Durham, N.C., 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（岩层） to dissolve and leach into local groundwater supplies," said 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."

The team of Duke and Ibn Zohr scientists was able to know this by identifying the distinctive geochemical and isotopic signatures of different elements in the water, such as oxygen, strontium（锶） 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," explained Nathaniel Warner, a Ph.D. student at Duke's Nicholas School who led the study. "We can also track the source of low-saline water flowing into a system."