Compound Specific Isotopic Analysis (CSIA) of landfill leachate DOC components

Abstract

In an attempt to better characterize the biogeochemical evolution of dissolved organic carbon (DOC) in landfill leachates, and to interpret the origin of DOC in groundwater, we have developed a new analytical technique for the compound specific isotope ( 13C) analysis (CSIA) of DOC. This is a new operational system that measures chromatographically separated DOC compounds with a total inorganic/organic carbon analyzer (TCA) interfaced with an isotope ratio mass spectrometer (IRMS), and it represents a significant contribution to analytical technology. Leachate samples were collected from the Trail Road Landfill (TRL) site located about 25 km to the west of the city of Ottawa, Canada. Measurements of Eh, pH, electrical conductivity, dissolved oxygen, total dissolved solids, and temperature were completed at the field site. High performance liquid chromatography (HPLC) was used to separate DOC components into fractions for separate analysis on TCA. The TCA is operated in-line with a Thermo-Finnigan Delta Plus continuous-flow isotope ratio mass spectrometer (CF-IRMS) that oxidises organic carbon to CO 2 for measurement of both concentration, by infrared absorption, and δ 13C. DOC fraction collection was based on the detection of discrete peaks of individual compounds, allowing identification of key peaks, such as acetate, with recoveries of up to 100%. The difference in δ 13C values for leachate acetate (−10.7‰ to −16.9‰ VPDB) and the bulk DOC (−24.7‰ VPDB) can be used to distinguish landfill leachate derived DOC and identify biogeochemical reactions. The enrichment of δ 13C in the acetate suggests that this biologically derived compound has become a substrate for secondary biogeochemical reaction, likely methanogenesis.