Fate and degradation of methoxychlor in a contaminated aquifer: Insights from dual carbon-chlorine isotope analysis and isomeric fraction

The combined use of isomeric fraction (IF) and multi-element compound-specific isotope analysis (ME-CSIA) was evaluated for the first time to assess the fate and degradation of methoxychlor in the environment. The concentration and carbon and chlorine isotope composition of methoxychlor and its transformation products were monitored in water and solid phases of a fractured aquifer. The results from the interception trenches water samples demonstrated that induced alkaline conditions promoted alkaline hydrolysis. Natural attenuation of methoxychlor isomers was evidenced by carbon and chlorine isotopic fractionation. The field C-Cl isotope slope (Λ_C/Cl = 0.42 ± 0.06; R² = 0.98) was statistically indistinguishable (p > 0.05) from that obtained in a previous experiment (0.44 ± 0.14), confirming the occurrence of reductive dechlorination of methoxychlor isomers. P,p'-methoxychlor δ¹³C values in groundwater samples revealed variations linked to rainfall patterns. The extent of p,p'-methoxychlor biodegradation was calculated to be greater than 89 % across the monitoring period. The combined use of CSIA and IF evidenced that alkaline hydrolysis and reductive dechlorination did not exhibit isomeric selectivity. Differences in IF values between slurry and water samples, as well as between upstream and downstream wells, suggested variations in the environmental behaviour of the p,p' and o,p'-isomers, likely due to differing water solubilities. Overall, ME-CSIA proved to be a valuable tool for identifying, quantifying, and tracing methoxychlor degradation in this aquifer. Additionally, IF provided insights into the distinct environmental behaviour of the p,p'- and o,p'-isomers. These tools offer crucial information, valuable for decision-makers in developing remediation strategies for methoxychlor-contaminated sites.