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.

Radionuclide transport in fractured chalk under abrupt changes in salinity.

Internationally, it has been agreed that geologic repositories for spent fuel and radioactive waste are considered the internationally agreed upon solution for intermediate and long-term disposal. In countries where traditional nuclear waste repository host rocks (e.g., clay, salt, granite) are not available, other low permeability lithologies must be studied. Here, chalk is considered to determine its viability for disposal. Despite chalk's low bulk permeability, it may contain fracture networks that can facilitate radionuclide transport. In arid areas, groundwater salinity may change seasonally due to the mixing between brackish groundwater and fresh meteoric water. Such salinity changes may impact the radionuclides' mobility. In this study, radioactive U(VI) and radionuclide simulant tracers (Sr, Ce and Re) were injected into a naturally fractured chalk core. The mobility of tracers was investigated under abrupt salinity variations. Two solutions were used: a low ionic strength (IS) artificial rainwater (ARW; IS ∼0.002) and a high IS artificial groundwater (AGW; IS ∼0.2). During the experiments, the tracers were added to ARW, then the carrier was changed to AGW, and vice versa. Ce was mobile only in colloidal form, while Re was transported as a conservative tracer. Both Re and Ce demonstrated no change in mobility due to salinity changes. In contrast, U and Sr showed increased mobility when AGW was introduced and decreased mobility when ARW was introduced into the core. These experimental results, supported by reactive transport modeling, suggest that saline groundwater solutions promote U and Sr release via ion-exchange and enhance their migration in fractured chalk. The study emphasizes the impact of salinity variations near spent fuel repositories and their possible impact on radionuclide mobility.