TCE longevity in the vadose zone and loading to the groundwater-The case of episodic NAPL releases from near-surface source

- Multiphase simulations provide insights into TCE-NAPL migration and partitioning.
- Episodic releases of TCE from near-surface source differ than single dose releases.
- NAPL plume is limited to the source zone and the disposal period.
- Adsorption and natural attenuation masked the source pulses.
- Large fraction of TCE volatilized whileTCE loads to the groundwater reduce to <3%.

This paper quantifies, using numerical simulations, the longevity of Trichloroethene (TCE) in the vadose zone and the loading to the groundwater following episodic NAPL releases from a near-surface source. Specifically, the paper exploits the case of episodic leaks from metal plating factory sewer system, and its migration through a sandy unsaturated zone, down to the water table. The numerical investigation was carried out using the T2VOC code of the TOUGH family and the PetraSim2016 graphical user interface (GUI). 3D, multiphase, multicomponent simulaions demonstrated the effects of land-cover conditions (i.e. connection to atmosphere and interception of rain) and vadose zone depth on the distribution, partitioning, and fate of TCE.In all simulated cases, the NAPL plume spread only to a distance of several meters below the source. A general trend of increase TCE(aq) loads to the groundwater appears during the disposal period, followed by a substantial and immediate drop at its termination and a long-term gradual decrease in loads thereafter ('Legacy'). However, each of the simulated cases shows somewhat different characteristics of the time-dependent load function. Ultimately, merely a small fraction (0.08%-3%) of the disposed TCE reached the 5-30 m deep water table during five decades; the cases of the covered (roofed) soils and thicker vadose zone resulted in the lowest loads.