Saline tracer methods for tracking leachate leakage-migration and preferential groundwater flow pathways at valley-type landfill sites.

Valley-type landfill sites typically present complex geological conditions that increase groundwater contamination risk. Although hydrological tracer tests are widely recognized as an effective method for evaluating groundwater connectivity, their practical application in such heterogeneous environments is challenging. In this study, a multi-tracer approach using saline compounds (KI, NaBr, LiCl, and (NH4)2MoO4) was developed to characterize leachate migration and preferential groundwater flow pathways at valley-type landfill sites. Five field tracer tests were conducted under natural hydraulic gradients, supplemented by drill core analysis, groundwater flow monitoring, and borehole dilution tests. Analysis of breakthrough curves (BTCs) demonstrated direct hydraulic connectivity between upstream and downstream confined aquifers, with groundwater velocities averaging 4.1 m·h-1 through the fractured network. The high-permeability second fault zone (F2), with an approximate hydraulic conductivity of 8.8 × 10-3 m·s-1, served as a primary pathway for solute transport. Tracer data revealed two distinct contamination patterns: leachate from the storage pond migrated from northwest to southeast, while submembrane water from landfill section Ⅱ travelled from southeast to northwest, both detrimentally affecting downstream groundwater quality. These findings validate the saline tracer method as a practical and reliable approach for characterizing complex groundwater flow and contamination pathways in fractured bedrock environments beneath landfills.