Feasibility of reconstructing atmospheric chloride deposition rates based on tritium tracer in the unsaturated zone

The scarcity of chloride (Cl) deposition rate data is a serious impediment to assess the variability of atmospheric Cl deposition, develop Cl tracer techniques and improve the understanding of hydrological process. In this study, we use collocated tritium tracer data in the unsaturated zone (USZ) to attempt the reconstruction of Cl deposition rates. Sixteen data sets of atmospheric Cl deposition and soil profile observations from different locations were analyzed, with an aim to evaluate the feasibility of reconstructing Cl deposition rate based on tritium information in the deep USZ. Reconstructed models for Cl deposition rates include the inverse Cl mass balance method (ICMB) and the inverse Cl accumulation method (ICA). The two methods were basically consistent with the field observations. But the Cl deposition rate reconstruction by the ICA method (R2 = 0.96, RMSE*= 0.29, and MEC = 0.92) is more accurate than the ICMB method (R2 = 0.78, RMSE*= 0.48, and MEC = 0.82) under natural conditions. The ICMB method resulted in a larger uncertainty of reconstructions due to uncertainties from the tritium peak method and soil water chloride concentration. In contrast, the bias from the ICA method reconstruction results from the anthropogenic input of chloride sources in the USZ. This study offers valuable and complementary tools for determining Cl deposition rates and can aid in promoting the development of global Cl deposition databases or networks, enhancing their accuracy and providing essential information for relevant research and applications.