Assessing source zone distribution and persistence at a trichloroethylene contaminated site

Chlorinated solvents are prevalent and persistent contaminants, classified as dense nonaqueous phase liquids (DNAPLs) in both soil and groundwater systems.  They have been identified at numerous sites globally, significantly impacting our environment.  The subsurface distribution of DNAPLs creates a source zone with complex geometry, strongly influenced by the specific characteristics of soil textures and stratification.  Accurately defining the location and extent of DNAPL source zones at contaminated sites poses a considerable challenge due to these complexities.

The aim of this study is to delineate the spatial distribution and persistence of trichloroethylene (TCE) in the subsurface for a long-term period of TCE leakage.  The studied area is situated at a factory in Taiwan.  Employing the T2VOC module within PetraSim 2019 software, we analyzed TCE movement and distribution in the subsurface over a 42-year timeframe which encompasses a 22-year period of TCE leakage and an additional 20-year period preceding remedial activities.  Field data from site investigation and remediation reports were incorporated into the analysis, encompassing information on groundwater table contours, soil layers, lithologies, permeabilities and the historical usage of TCE.  Relevant parameters, such as relative permeability, liquid residual saturation, capillary pressure and fluid saturation relationships, were determined based on literature sources.

Results show that TCE infiltrates to a depth of 13 m, reaching a low permeability zone below the ground surface.  However, the site investigation only extended to a depth of 10 m (the lower bound of a high-permeability zone).  Significant TCE residuals persist in both the upper and lower layers of the high-permeability zone after the 42-year simulation period.  The dissolved phase of TCE follows the groundwater flow, extending up to 80 m downstream with notable concentration levels.  However, if considering volatilization of TCE (resulting in an 80% reduction in leakage) and an 80% reduction in the leakage area, it becomes improbable for TCE to infiltrate to the lower low-permeability layer.  This suggests a potential underestimation of the current assessment of TCE usage.  Moreover, the study underscores the influence of groundwater velocity and TCE residual saturation on the retention and persistence of TCE in the soil layers.  This emphasizes the importance of investigating hydrogeological environment and assessing TCE residuals in various soil textures at such contaminated sites.