Quantifying dual recharge mechanisms in deep unsaturated zone of Chinese Loess Plateau using stable isotopes

Apportioning recharge to piston flow and preferential flow is critical for groundwater resource management and hydrologic connectivity evaluation, but the occurrence of the two flows in thick unsaturated zones is still poorly understood. The main objective of this study was to quantify the relative contributions of these two flows in the deep unsaturated zone of the Loess Plateau, China (CLP). The Bayesian mixing model (MixSIAR) was integrated with the line conditioned excess (lc-excess) of stable isotopes (delta H-2 and delta O-18) in precipitation, soil water (8-25 m), and groundwater to determinate the contributions. Piston flow contributed 62 +/- 8% to the total recharge while preferential flow did only 38 +/- 8%, indicating piston flow was the dominant groundwater recharge mechanism. On comparison with results from similar study sites in the CLP, the primary groundwater recharge mechanism differed among sites and may depend on spatial scales or geomorphology. The information is critical for understanding hydrological process and evaluating groundwater quantity and quality in the highlands of the CLP and other regions with similar hydrogeological conditions.