Chemical and strontium isotope characterization of rainwater at an urban site in Loess Plateau, Northwest China

Major ion concentrations and strontium isotopic ratios (87Sr/86Sr) were measured in rainwater samples collected at the urban site of Lanzhou, a city located on the Loess Plateau in the arid and semi-arid areas of northwest China. The rainwater samples possessed alkaline pH, at a reference level of 5.6, with a range of 6.82 to 8.28 and a volume-weighted mean (VWM) pH value of 7.70. The alkaline character of rainwater in Lanzhou is due to the result of neutralization caused by the alkaline soil dusts which contain large amount of CaCO3. It was observed that Ca2+ was the most abundant cation with a VWM value of 886 µeq l− 1 (115–2184 µeq l− 1), accounting for 87.8% of the total cations. Without considering HCO3−, SO42− and NO3− were dominant among the anions, accounting for 64.2% and 23.0%, respectively, of the total measured anions. Using Na as an indicator of marine origin and Al for terrestrial inputs, the proportions of sea salt and non-sea-salt elements were estimated from elemental ratios. The precipitation in this region has typical continental characteristics. The Sr concentrations varied from 0.004 to 0.885 µmol l− 1, and strontium isotopic ratios (87Sr/86Sr) lay in the range of 0.71025–0.71302, with an average of 0.71143. The 87Sr/86Sr ratios of Lanzhou rainwater are higher than that of seawater, which reflects contributions from the radiogenic Sr sources of the aerosols. The most suitable candidate for the source would be the soil dust originating from local and distant loess and desert areas. The 87Sr/86Sr ratios were used to characterize different sources of base cations in rainwater, suggesting that the samples could be interpreted in terms of combinations of at least three components: soil dust derived from the Loess Plateau and desert areas in northwest China (with 87Sr/86Sr ~ 0.7130), seawater (with 87Sr/86Sr ~ 0.70917), and anthropogenic inputs (with 87Sr/86Sr ~ 0.7103). The high 87Sr/86Sr ratio and Ca and Sr content in the rainwater from Lanzhou can be attributed to the dissolution of calcium carbonate in soil dust.