Chlorine isotopes as tracers of solute origin and age of groundwaters from the Eastern Desert of Egypt

Abstract

Chlorine-36 abundances and stable chlorine isotope ratios (37Cl/35Cl) were analyzed to decipher sources of chloride and to estimate subsurface residence times of groundwaters from the Nubian Sandstone Aquifer System (NSAS), alluvial aquifers, and fractured basement aquifers in the Eastern Desert of Egypt. Chlorine-36 isotopic abundances (36Cl/Cl) ranged from 5 × 10−15 to 343 × 10−15. The range in δ37Cl values was −0.61 to +0.90‰. Groundwater solute compositions were dominated by Na, Ca, Cl, and SO4 with total dissolved solids ranging from 445 to 13,700 mg/L. Chemical and isotopic data indicate that dry fallout of marine aerosols and evaporated vadose-zone residues from precipitation that recharged the aquifers may have been the main sources of groundwater solutes. Apparent groundwater 36Cl model ages range from 0 (+/−40) to 1300 (+300/−180) kyr. Groundwater from the fractured basement aquifers in the southwestern part of the study area had the youngest apparent ages, while that from deep wells tapping the Nubian aquifer had the oldest apparent ages. Shallow alluvial aquifers having anomalously old apparent 36Cl model ages indicate mixing of old Nubian aquifer water with modern recharge in alluvial aquifers. Stable isotope ratios of hydrogen and oxygen support this inferred mixing relationship and allowed quantitative estimation of old/young water mixing fractions.