Mixture effects on the tracing significance of carbon and hydrogen isotopes for identifying leaking points at landfills: Insights from theoretical analysis and field investigation.

Quantitative technologies for identifying leaking points at landfills remain a pervasive challenge. This study aimed to elucidate the mixture effects on tracing significance of carbon and hydrogen isotopes in leachate plumes caused by leachate leakage. Theoretical analysis based on isotopic mass balance, along with field investigation, were conducted. The apparent isotope enrichment factors of C-difference in dissolved carbon (εC) and H (εH) were selected as the primary isotopic indicators. Consequently, the mixing process between leachate under highly anaerobic landfill conditions (LHAL) and groundwater was found to significantly control the hyperbolic evolution path of (εC, εH) points in leachate plumes. Key factors influencing this mixing process, such as εC of LHAL, size and shape of the leak hole, and the Darcy flow velocity of groundwater (v), were revealed. Subsequently, the (εC, εH) tracing method was improved. Specifically, in a zoned εC versus εH plot, the lower-right boundary for LHAL was determined by a deviation (d) of 10 ‰ from the fitted line based on LHAL and background groundwater points, and the upper-left boundary for leachate that significantly interacted with background groundwater or precipitation (LBGP) was d=15‰. For landfill sites with v≤1 × 10 m/s, the distance deviation between a single LBGP point outside landfill and the leak hole was generally less than 20 m, and the tracing accuracy can be enhanced by increasing the leak-indicating points.