The impact of water saturation on the infiltration behaviour of elemental mercury DNAPL in heterogeneous porous media

Industrial use has led to the presence of liquid elemental mercury (Hg-0) worldwide in the subsurface as Dense NonAqueous Phase Liquid (DNAPL), resulting in long lasting sources of contamination, which cause harmful effects on human health and detrimental consequences on ecosystems. However, to date, insight into the infiltration behaviour of elemental mercury DNAPL is lacking. In this study, a two-stage flow container experiment of elemental mercury DNAPL infiltration into a variably water saturated stratified sand is described. During the first stage of the experiment, 16.3 ml of liquid Hg-0 infiltrated and distributed into an initially partially water saturated system. Afterwards, during the second stage of the experiment, consisting of the simulation of a "rain event" to assess whether the elemental mercury already infiltrated could be mobilized due to local increases in water saturation, a significant additional infiltration of 4.7 ml of liquid mercury occurred from the remaining DNAPL source. The experiment showed that, under conditions similar to those found in the field, Hg-0 DNAPL infiltration is likely to occur via fingers and is strongly controlled by porous medium structure and water saturation. Heterogeneities within the porous medium likely determined preferential pathways for liquid Hg-0 infiltration and distribution, as also suggested by dual gamma ray measurements. Overall, this study highlights that the infiltration behaviour of mercury DNAPL is strongly impacted by water saturation. In the field, this may result in a preferential infiltration of Hg-0 DNAPL in wetter areas or in its mobilization due to wetting during a rain event, as indicated by this study, or a groundwater table rise. This should be considered when assessing the likely distribution pathways of historic mercury DNAPL contamination as well as the remediation efforts.