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Measured Hg 0 inflow rate and level in the reservoir over time-Stage 1. 

Measured Hg 0 inflow rate and level in the reservoir over time-Stage 1. 

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Industrial use has led to the presence of liquid elemental mercury (Hg0) 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 elemen...

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... the first stage of the flow container experiment, liquid Hg 0 infiltration and distribution occurred in the partially water saturated stratified sand under an observed DNAPL head of 7.76 cm. Infiltration of Hg 0 took place directly at the bottom of the trench, with no evidence of lateral spreading until the MS2 layer was encountered. A volume of 16.3 ml of elemental mercury infiltrated within the system. Infiltration stopped after 50.9 h (3054 min), and the Hg 0 level in the reservoir reached the final value of 5 cm. The Hg 0 inflow rate did not show a clear trend over time (Fig. 2), other than highest infiltration rates oc- curring in the first 2 ...
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... the 3 cm of DNAPL head below the bottom of the trench (Fig. 3), the total liquid Hg 0 head on top of the MS2 layer was at maximum 10.5 cm, according to the mercury level in the reservoir (Fig. 2). However, this head was not sufficient to allow liquid Hg 0 to penetrate MS2. This is in keeping with the higher entry heads found under stable flow behaviour for Hg 0 in this sand (Table 3; D'Aniello et al., 2018a), in both fully (12.51 cm) and partially water saturated conditions (15.74 cm). No infiltration via fingers occurred in MS2 as its narrower particle size distribution and lower particles sizes than MS1 (Table 1) likely prevented mercury from invading even a single pore of this layer. Therefore, elemental mercury found easier pathways to continue its infiltration into the upper MS1 layer, thus distributing on . D'Aniello et al. Journal of Contaminant Hydrology xxx (xxxx) xxx-xxx top of the MS2 layer. Once mercury reached the top of MS2, it created a pool of about 1 cm height. Elemental mercury distribution over MS2 was not sym- metric (Figs. 6 and 7), and the infiltration front was initially faster on the left side because of the higher porosity, hence higher intrinsic permeability, found (Fig. 4), since water saturation was practically the same in the first 6-7 cm on both sides of the lateral spreading zone above MS2 (Fig. 5). This trend reversed after 30 min (Fig. 7) because of the lower water saturation found at the left side (Fig. 5) as a result of the higher porosity (Fig. 4). The larger the pores, the lower the entry head, so air easily filled this part of the system, thus locally dampening the infiltration, as Hg 0 is nonwetting with respect to air and is required to overcome higher entry heads due to its presence (Table 3; D' Aniello et al., 2018a). Then, at about 42 min, the infiltration temporarily ceased on the right side, even though the water saturation was nearly the same at the Hg 0 front locations, and the higher porosity on the left side al- lowed liquid mercury to infiltrate easier and faster through this part of the system (Fig. 7). Therefore, the effect induced by the heterogeneity of the porous medium structure likely prevailed in determining a pre- ferential pathway for liquid Hg 0 infiltration and distribution. The in- spection of Fig. 8 further suggests this, thus showing a tendency of li- quid Hg 0 to migrate towards the left side of the container, where porosity was the highest (0.392) above the MS2 layer, as well as the Hg 0 saturation (0.615), and the height of the DNAPL pool increased to 2 ...
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... mercury reached the top of the MS2 layer in about 9 min, when a volume of 0.79 ml of liquid Hg 0 had infiltrated, based on the measured inflow rate (Fig. 2). The dual gamma ray measurements (Figs. 4 and 8) performed at the end of the first stage of the experiment indicated that 0.69 ml of liquid Hg 0 infiltrated in the first 3 cm beneath the trench. The comparison of the volume obtained by integration of the dual gamma ray measurements with that calculated based on the measured inflow rate suggests that, in the first 3 cm below the DNAPL reservoir, most of the elemental mercury infiltrated in the first minutes of the experiment, and that no significant changes were observed at later times. Therefore, once mercury found a pathway from the DNAPL source in the trench down to the bottom of the upper MS1 layer, all the additional infiltrated Hg 0 probably migrated through this pathway be- fore distributing on top of the MS2 ...
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... stage 2, the infiltration of elemental mercury mostly oc- curred in the first 40 min following the end of the rain event (Fig. 9). In this time interval, the Hg 0 inflow rate reached its maximum value (0.592 · 10 −1 ml/min; Fig. 9), of about half of the maximum rate measured during stage 1 (0.116 ml/min; Fig. 2) but of about one order of magnitude higher than the average inflow rate of stage 1 (0.536 · 10 −2 ml/min; Fig. 9). Both inflow rate and DNAPL level in the trench showed a stepwise pattern over time (Fig. 9), with large time intervals where no (or negligible) liquid Hg 0 infiltration occurred. This may be explained based on the time required for water to redistribute within the system. As water slowly imbibed the pores, a displacement of air occurred, allowing further elemental mercury ...