Fig. 9: Groundwater "ages" in the Syrian desert range up to 40,000 years. More negative values of 2H indicate moister past climates with a very wet period about 10,000 to 20,000 years ago. More positive values, also found in the Sahara desert, indicate peristent aridity in the region. Isotope methods determined that the age of major groundwater reserves beneath the Sahara is greater than 10,000 years.