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

Abstract

 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.