Effect of water table fluctuations on dichloromethane degradation in laboratory aquifers
Dichloromethane (DCM) is a toxic industrial solvent frequently detected in multi-contaminated aquifers, and degrades under both oxic and anoxic conditions. Anaerobic DCM degradation is preferred in oxygen-depleted aquifers where water table fluctuations may affect the extent and pathways of DCM due to microbial responses to hydrochemical variations. In this study, we examined the effect of water table fluctuations on DCM biodegradation in two lab-scale aquifers fed with anoxic DCM-spiked groundwater from a well-characterized contaminated site. Hydrochemical, stable isotopes (i.e., d13C and d37Cl), and microbial composition was examined to determine DCM mass removal and degradation pathways under steady-state and transient conditions. C and Cl isotope fractionation values ("e" _"bulk" ^"C"  and "e" _"bulk" ^"Cl" ) of -23.6 ± 3.2‰ and -8.7 ± 1.6‰, respectively, were larger under steady-state than transient conditions ("e" _"bulk" ^"C"  and "e" _"bulk" ^"Cl"  values of -11.8 ± 2.0‰ and -3.1 ± 0.6‰, respectively). Dual C-Cl isotope analysis indicated distinct anaerobic DCM degradation pathways under steady and transient conditions with ?C/Cl values of 1.92 ± 0.30 and 3.58 ± 0.42, respectively. Water table fluctuations caused changes in redox condition but did not play a major role on bacterial diversity. Bacterial composition differed mainly according to depth and location within the aquifer flow, however, a major shift of Desulfosporosinus (Peptococcaceae family) was observed under transient conditions. Altogether, our results highlight that water table fluctuations enhanced DCM biodegradation and induced distinct anaerobic DCM degradation pathways compared to steady conditions. Our integrated approached provided insights into in situ anaerobic DCM degradation under environmental dynamic conditions, which may help to improve bioremediation strategies at DCM contaminated sites.