Application of Chlorine-36 Technique in Determining the Age of Modern Groundwater in Al-Zulfi Province

 The present study aims to estimate the residence-time of groundwater based on bomb-produced 36Cl. Water samples were collected from some selected wells located around Al-Zulfi province, Riyadh central region, Saudi Arabia. 36Cl/Cl ratios in the water samples are determined by Inductively Coupled Plasma Mass Spectrometry (ICPMS) and Liquid Scintillation Counting (LSC). 36Cl/Cl ratios in the groundwater were estimated to be 1.0–2.0×10−10. The shallow unconfined aquifer of the area is mainly replenished by the water infiltration from the seasonal rains. Estimates of residence time were obtained by comparing the measured bomb-derived 36Cl concentrations in groundwater with the background reference. Dating based on a 36Cl bomb pulse may be more reliable and sensitive for groundwater recharged before 1975, back as far as the mid-1950s. The above 36Cl-background concentration was deduced by determining accurately the background corrected Dye-3 ice core data from the frozen Arctic Data, according to the estimated total 36Cl resources, including bomb-produced 36Cl fallout. Estimated residence time of 7.81×104 y is obtained from extrapolated flow velocity. 36Cl concentration in groundwater does not reflect the input of bomb pulse 36Cl, and it belongs to the era before 1950.