Water table fluctuations affect dichloromethane biodegradation in lab-scale aquifers contaminated with organohalides

Dichloromethane (DCM) is a toxic industrial solvent frequently detected in multi-contaminated aquifers. It can be degraded biotically or abiotically, and under oxic or anoxic conditions. The extent and pathways of DCM degradation in aquifers may thus depend on water table fluctuations and microbial responses to hydrochemical variations. Here, we examined the effect of water table fluctuations on DCM biodegradation in two laboratory aquifers fed with O-2-depleted DCM-spiked groundwater from a well-characterized former industrial site. Hydrochemistry, stable isotopes of DCM (delta C-13 and delta Cl-37), and bacterial community composition were examined to determine DCM mass removal and degradation pathways under steady-state (static water table) and transient (fluctuating water table) conditions. DCM mass removal was more pronounced under transient (95%) than under steady-state conditions (42%). C and Cl isotopic fractionation values were larger under steady-state (epsilon(C)(bulk) = -23.6 +/- 3.2 parts per thousand, and epsilon(C)(bulk) = -8.7 +/- 1.6 parts per thousand) than under transient conditions (epsilon(C)(bulk) = -11.8 +/- 2.0 parts per thousand, and epsilon(C)(bulk) = -3.1 +/- 0.6 parts per thousand). Dual C-Cl isotope analysis suggested the prevalence of distinct anaerobic DCM degradation pathways, with Lambda(C/Cl) values of 1.92 +/- 0.30 and 3.58 +/- 0.42 under steady-state and transient conditions, respectively. Water table fluctuations caused changes in redox conditions and oxygen levels, resulting in a higher relative abundance of Desulfosporosinus (Peptococcaceae family). Taken together, our results show that water table fluctuations enhanced DCM biodegradation, and correlated with bacterial taxa associated with anaerobic DCM degradation. Our integrative approach allows to evaluate anaerobic DCM degradation under dynamic hydrogeological conditions, and may help improving bioremediation strategies at DCM contaminated sites.

Automated summary

The study investigated the impact of water table fluctuations on DCM biodegradation in aquifers, showing that fluctuations enhanced DCM degradation and were associated with bacterial taxa involved in anaerobic DCM degradation.