Isotopic and chemical composition of precipitation in Riyadh, Saudi Arabia

• We establish a Local Meteoric Water Line for Riyadh: δ2H = 5.22(± 0.38) · δ18O + 14.8(± 0.9) ‰.
• Moisture source-related isotopic fingerprints are masked by several isotope effects.
• 3H concentrations (2.8 to 6.4 TU) are close to the natural background level.
• Solutes in Riyadh rain mainly originate from local limestone outcrops and sabkhas.
• Sabkha impact causes a relatively high precipitation-weighted mean Cl−concentration (9.5 mg L− 1).

Only limited data on the isotopic and chemical composition of Riyadh rain are currently available. In this study, we complement these data by analyzing integral samples covering 28 precipitation events between 2009 and 2013. Results of stable isotope analyses are used to establish a Local Meteoric Water Line: δ2H = 5.22(± 0.38) · δ18O + 14.8(± 0.9) ‰. Moisture source-related isotopic fingerprints are masked by the continental effect, the altitude effect, sub-cloud evaporation, and moisture recycling. The study of one event for intra-storm variability revealed strong isotopic depletion due to rainout and Rayleigh distillation processes, thus highlighting the general need for integral samples. Tritium analyses of grab samples from 12 events yielded concentrations between 2.8 and 6.4 tritium units (TU), which are close to the natural background of a few TU.Major ion concentrations and ratios indicate that solutes are predominantly derived from atmospheric dust originating from limestone outcrops and sabkha deposits. The latter play a role with respect to the elevated Cl−and Na+ contents, but are probably also responsible for the SO42 −and a part of the Ca2 + found in Riyadh rain. Observed intra- and inter-storm variabilities of major ion levels necessitate the collection of integral samples and the calculation of precipitation-weighted means, respectively.The obtained isotopic signatures and the precipitation-weighted mean Cl−concentration (9.5 mg L− 1) may be useful in groundwater assessments, e.g., for the identification of modern recharge and quantification thereof by means of the Chloride Mass Balance method.