Assessing recharge transit times in multilayered soil profiles influenced by preferential flow

The fate and transport of water and contaminants through soils or the unsaturated zone remains an active field of research due to its implications for environmental and public health. Agricultural soils in particular pose a high risk for contaminants (e.g., fertilizers, pesticides) leaching to groundwater due to repeated chemical inputs and farming practices such as irrigation or intentional flooding for groundwater recharge. Transit time approaches are commonly used to assess the vulnerability of groundwater to contamination. However, assessing transit times in multilayered deep soil profiles influenced by preferential flow remains a major challenge. This study quantifies recharge transit times using well-established synthetic tracer methods available in HYDRUS-1D, including i) particle tracking, ii) the peak displacement method, and iii) a convolution integral-based transit time distribution (TTD) analysis in a 70 m vadose zone, comparing three soil profiles. Our results reveal that particle tracking consistently yields the shortest transit times because it represents piston-flow advection with no dispersion or tailing. The peak displacement method results in intermediate estimates as it reflects advective–dispersive transport but ignores the BTC tailing. The convolution integral-based TTD analysis produces the longest ages since it reconstructs the full transit time distribution, including slow matrix flow and long-tail contributions. We also examined how soil texture combinations influence water ages at the groundwater table. By exploring 3600 combinations of soil textures in the deep vadose zone, we found that coarser layers can either accelerate or delay recharge depending on their position and degree of saturation, highlighting the complex role of stratification. The mutual information analysis identified key controlling layers that govern water age variability at the groundwater table. These findings underscore the importance of selecting appropriate modeling approaches and accounting for site-specific soil structure and saturation dynamics when evaluating recharge effectiveness and contaminant transport risk in deep vadose zones.