Tracking abiotic transformation of 1,1,1-trichloroethane to 1,1-dichloroethylene in contaminated groundwater on a national scale

1,1,1-trichloroethane (TCA) is a chlorinated aliphatic compound that was increasingly applied as a replacement for trichloroethylene. TCA can simultaneously follow two abiotic degradation processes: hydrolysis and dehydrochlorination, where the latter lead to the formation of a relatively recalcitrant product 1,1-dichloroethylene (DCE). The abiotic processes are relatively rapid and can be assessed based on the ratio between TCA and DCE. Using national data collected in Israel since the year 2000, this work aimed to examine the abiotic degradation extent of TCA nationally and whether it is affected by the aquifer type and unsaturated zone thickness. We have also examined temporal shifts in TCA and DCE concentrations and tested whether they follow expected trends. Our results show that the abiotic degradation of TCA is significant on a national scale, with higher DCE over TCA concentrations in ≈ 89 % of the wells. Furthermore, in ≈ 85 % of the data points, TCA over DCE concentrations indicate that TCA underwent three or more half-lives. Comparing the different lithologies of contaminated groundwater, higher concentrations are observable in karst relative to unconsolidated aquifers. Nevertheless, the ratio between the two and correspondingly the degradation rates are not affected by lithology. Finally, temporal shifts in TCA and DCE concentrations, as well as the ratio between the two, are different than expected in an idealized closed system. Complexities such as lithological heterogeneity, multiple sources, transport parameters, and more must be considered in interpreting these trends when quantification of the degradation rate is attempted.