Abstract
The present study aims at estimating the residence-time of groundwater based on bomb-produced Cl-36. Water samples were collected from some selected wells located around Al-Zulfi province, Riyadh central region, Saudi Arabia. (36) The Cl/Cl ratios in the water samples were determined by Inductively Coupled Plasma Mass Spectrometry (ICPMS) and Liquid Scintillation Counting(LSC). (36) The Cl/Cl ratios in the groundwater were estimated to be 1.0-2.0x10(-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 bombderived (36) Cl concentrations in groundwater with the background reference. Dating based on a Cl-36 bomb pulse may be more reliable and sensitive for groundwater recharged before 1975, back as far as the mid-1950s. The above Cl-36-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 Cl-36 resources, including bomb-produced Cl-36 fallout. An estimated residence time of 7.81x10(4) y was obtained from extrapolated flow velocity. It was noticed that Cl-36 concentration in groundwater does not reflect the input of bomb pulse Cl-36, it belongs to the era before 1950.