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Abstract

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 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 Hg0 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, Hg0 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 Hg0 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 Hg0 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.

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... Therefore, it causes a slower infiltration rate. This is relevant with [18] and [19] that the infiltration rate is influenced by soil saturation, which can occur due to the changes in groundwater levels which is getting shallower. Abdullahi and Garba [20] explain that the maximum infiltration estimated during October-December correlates with a decrease in groundwater levels from June-August. ...
... Less potential areas are likely to be distributed in a small part of the research areas which are usually located around a reservoir or spring. This is due to the influence of soil moisture which causes a slow infiltration rate as explained by [19,28]. ...
Article
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Liquid elemental mercury occurrence in the subsurface as DNAPL is reported worldwide in proximity of several industrial facilities, such as chlor-alkali plants. Insight into Hg0 DNAPL infiltration behaviour is lacking and, to date, there are no experimental observations of its infiltration and distribution in water saturated porous media, except for capillary pressure-saturation column experiments. To better understand the processes governing elemental mercury DNAPL flow behaviour, a series of flow container experiments were performed using mercury DNAPL (in sands and glass beads) and PCE (in sands). While liquid Hg0 was not able to infiltrate in the sand filled container due to an overall lower permeability of the sample and a defect of the setup, in the glass beads experiment mercury DNAPL infiltration occurred. Dual gamma ray measurements showed that, in glass beads, liquid Hg0 preferentially migrated towards higher porosity zones. As for PCE, infiltration and distribution of Hg0 DNAPL are strongly affected by the heterogeneities within the porous formation. However, compared to other DNAPLs, liquid Hg0 shows a strong attenuation potential of gamma rays. Finally, numerical simulations of the glass beads experiment showed an overall good agreement with the experimental results, highlighting that, among the factors influencing the prediction of liquid Hg0 migration in water saturated porous media, the most critical are (i) the knowledge of the inflow rate, (ii) the reliable estimation of the porous formation permeability, and (iii) the accurate representation of the correlation between retention properties and intrinsic permeability.
Thesis
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This thesis focuses on the characterization of the flow behaviour of elemental mercury (Hg0) DNAPL (Dense NonAqueous Phase Liquid) in porous media. In the subsurface, Hg0 DNAPL can act as a long lasting source of contamination, causing detrimental consequences for the human health and the environment. Therefore, insight into the flow behaviour of elemental mercury in porous media is needed and is critical in assessing the control of contaminant spreading as well as remediation approaches. However, the scientific literature on Hg0 DNAPL is still very limited and, to date, it remains unclear to what extent the validity of the classical constitutive relations, used to describe DNAPLs flow behaviour, as well as the scaling theory, holds for elemental mercury. These issues become crucial in partially water saturated porous media, where liquid Hg0 is likely to behave as a nonwetting phase with respect to both air and water. To address these knowledge gaps, experimental and numerical analysis were performed. In particular, the properties affecting the constitutive relations governing liquid Hg0 infiltration behaviour were explored first, using capillary pressure-saturation, Pc(S), experiments in different granular porous media, and in two- and three-phase fluid systems. Then, the infiltration and (re)distribution behaviour of Hg0 DNAPL was studied in variably water saturated stratified porous media with flow container experiments and dual gamma ray measurements of porosity and fluid saturations. Experimental results indicated that elemental mercury infiltration is strongly controlled by the porous medium water content and can be triggered by its changes due to, for example, rain events. Finally, a new theoretical formulation of elemental mercury retention properties in variably water saturated porous media was proposed and, to assess to what extent numerical modelling can predict elemental mercury migration in porous media, the flow container experiments were simulated using GDAn, the code developed by the Author.
Article
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Mercury is a contaminant of global concern. The use of elemental mercury in various (former) industrial processes, such as chlorine production at chlor-alkali plants, is known to have resulted in soil and groundwater contaminations worldwide. However, the subsurface transport behaviour of elemental mercury as an immiscible dense non-aqueous phase liquid (DNAPL) in porous media has received minimal attention to date. Even though, such insight would aid in the remediation effort of mercury contaminated sites. Therefore, in this study a detailed field characterization of elemental mercury DNAPL distribution with depth was performed together with two-phase flow modelling, using STOMP. This is to evaluate the dynamics of mercury DNAPL migration and the controls on its distribution in saturated porous media. Using a CPT-probe mounted with a digital camera, in-situ mercury DNAPL depth distribution was obtained at a former chlor-alkali-plant, down to 9m below ground surface. Images revealing the presence of silvery mercury DNAPL droplets were used to quantify its distribution, characteristics and saturation, using an image analysis method. These field-observations with depth were compared with results from a one-dimensional two-phase flow model simulation for the same transect. Considering the limitations of this approach, simulations reasonably reflected the variability and range of the mercury DNAPL distribution. To further explore the impact of mercury's physical properties in comparison with more common DNAPLs, the migration of mercury and PCE DNAPL in several typical hydrological scenarios was simulated. Comparison of the simulations suggest that mercury's higher density is the overall controlling factor in controlling its penetration in saturated porous media, despite its higher resistance to flow due to its higher viscosity. Based on these results the hazard of spilled mercury DNAPL to cause deep contamination of groundwater systems seems larger than for any other DNAPL.
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Mercury contamination of soils and vegetation close to an abandoned Hg-fulminate production plant was investigated. Maximum concentrations of Hg (>6.5 g kg−1 soil) were found in the soils located in the area where the wastewater produced during the washing procedures carried out at the production plant used to be discharged. A few meters away from the discharge area, Hg concentrations decreased to levels ranging between 1 and 5 g kg−1, whereas about 0.5 ha of the surrounding soil to the NE (following the dominant surface flow direction) contained between 0.1 and 1 g kg−1. Mercury contamination of soils was attributed (in addition to spills from Hg containers) to (i) Hg volatilization with subsequent condensation in cooler areas of the production plant and in the surrounding forest stands, and (ii) movement of water either by lateral subsurface flow through the contaminated soils or by heavy runoff to surface waters.
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Mercury emissions from chlor-alkali plants have been past and present sources of soil contamination with Hg. Here we calculate net mercury (Hg) deposition to soils in the vicinity (100-1000-m downwind) of three-chlor alkali plants. Calculations were based on spatial distribution patterns of Hg concentrations in soils, which were extrapolated by kriging. Moreover, we investigated to what extent Hg deposition depends on the elevation of receptors and canopy throughfall. Mercury concentrations in soil exceed background values up to a factor of 56 and show enrichment factors between 2 and 5.8 calculated from the median Hg concentration. Net deposition rates range between 2356 and 8952 microg m(-2) year(-1), which is up to 224-fold the background values. Net deposition of Hg to soils at the three sites varies between 1.2 and 2.4% of total emitted Hg. Highest deposition rates were found at sites with extended elevated or forested areas. Here, Hg concentrations in soils increased by a factor of up to 7.3 in elevated (+180 m) forest areas compared to non-elevated grassland soils.
Article
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Article
Understanding of flow and entrapment of non-aqueous-phase liquids (NAPLs) in aquifers contaminated with organic chemicals is important in the effective design of recovery and remediation schemes. Soil heterogeneities play a significant role in the physical behavior of these chemicals. An experimental facility consisting of a large soil tank (lysimeter) and a dual-gamma spectroscopy system for fluid saturation measurements was developed to simulate and monitor plume migration in water-table aquifers after chemical spills. Experimental techniques and results form a preliminary set of experiments conducted in unsaturated and saturated soils under homogeneous and heterogeneous conditions are presented. the effects of the layered homogeneities were pronounced in modifying the migration pattern and velocity of the plume. Pockets of coarse sand placed across the path of the plume resulted in the soil acting as a light NAPL trap. A fine-sand pocket acted as a barrier. Qualitative and quantitative data generated in the type of experiments presented in this paper can be used to validate multiphase flow models.
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This study investigates the influence of two factors—geological heterogeneity and variability in water infiltration—on non-aqueous phase liquid (NAPL) migration in the unsaturated zone. NAPL migration under three-phase flow conditions resulting from a ground surface spill is modeled for multiple heterogeneous realizations of a porous medium with various water infiltration scenarios. Increased water infiltration before the spill has two counteracting effects: NAPL relative permeability (k rn) increases with increasing water saturation (S w) for a given NAPL saturation, while higher S w in the soil near the NAPL source zone leads to less NAPL mass infiltration. It is found that the former effect is overwhelmed by the latter effect, the net effect being that with longer infiltration durations before the spill, both the infiltrated NAPL mass and the depth of the front migration decrease. Simulation results also show strong effect of the medium heterogeneity. Results suggest that total infiltrated mass, front depth and plume spread increase with an increasing standard deviation of log-permeability. Also variability in modeling results among realizations is largely impacted by the log-permeability standard deviation. Spatial correlation in permeability also strongly influences NAPL infiltration. An increase in the isotropic correlation length from 0.75 to 1.5 m leads to a decrease in total infiltrated mass, plume migration depth as well as vertical spread. Lateral spread in this case is not shown to be affected by the correlation length.
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'Viscous fingering' generally refers to the onset and evolution of instabilities that occur in the displacement of fluids in porous materials. In most but not all cases, the mechanism of the instability is intimately linked to viscosity variations between phases or within a single phase containing a solute - hence the term ' viscous fingering. ' Shown in the article are three examples of the complex and intriguing patterns that evolve as a result of this instability. In the article author provides a detailed discussion of the mechanisms that govern these flows, which he refers to as shielding, spreading, and splitting, that will enable us to at least qualitatively understand these fascinating patterns.
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Mercury occurs naturally in the environment and can be found in elemental (metallic), inorganic, and organic forms. Modern uses for mercury include chemical manufacturing, thermometers, and lighting (mercury vapor and fluorescent lamps). The chemical and allied products industry group is responsible for the largest quantity of mercury used in the United States. Mercury, particularly the organic methylmercury form, is a potent neurotoxin capable of impairing neurological development in fetuses and young children and of damaging the central nervous system of adults. Mercury regulations span multiple federal and state environmental statutes, as well as multiple agency jurisdictions. In August 2007, the U.S. Environmental Protection Agency's (US EPA's) Office of Superfund Remediation and Technology Innovation (OSRTI) published a report titled “Treatment Technologies for Mercury in Soil, Waste, and Water.“ The report identifies eight treatment technologies and 57 projects, 50 of which provide performance data. This information can help managers at sites with mercury-contaminated media and generators of mercury-contaminated waste and wastewater to identify proven and effective mercury treatment technologies; screen technologies based on application-specific goals, characteristics, and costs; and apply experiences from sites with similar treatment challenges. This article provides a synopsis of the US EPA report, which is available at http://clu-in.org/542R07003. © 2007 Wiley Periodicals, Inc.†
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There have been many studies of mercury geochemistry in the environment and its bioconcentration/bioaccumulation through the aquatic food chain. However, there is a dearth of information regarding the bioaccessibility of mercury in human receptors exposed primarily by soil ingestion. This paper reviews the current state of knowledge of mercury bioaccessibility and speciation in soils, and the utility of speciation methods to estimate mercury bioaccessibility. We conclude that additional research is necessary to determine: (1) whether analytical measurements can adequately determine the bioaccessibility of mercury in sediments and soils; (2) the accuracy of in vitro analyses in assessing mercury bioaccessibility; (3) the ability of mercury to cross tissue membranes of the mouth, esophagus, stomach, and the small and large intestines; (4) the speciation and distribution of mercury in biological fluids; and (5) mercury bioavailability using an in vivo animal model relevant to human gastrointestinal tract conditions.
Article
The theory governing the stability of immiscible displacements in porous media is reviewed with the specific needs of contaminant hydrogeologists in mind. Criteria to determine whether or not displacements occurring in homogeneous porous media will bring about fingered fluid distributions are first addressed. These criteria are a function of the density, viscosity, and surface tension of the fluids involved. Immiscible displacements occurring in natural porous media are next reviewed with an emphasis on porous media heterogeneity. It is concluded that natural porous media contain heterogeneities at a fine enough scale such that no distinction need be made between stable and unstable displacements. The limitations associated with the use of the conventional multiphase flow equations in simulating immiscible fingering are examined. The use of random walk models and percolation theory to describe immiscible fingering is also reviewed.
Article
Soil samples collected in the surroundings of a chlor-alkali plant in the Netherlands were characterised by synchrotron-based techniques and conventional analytical procedures, in order to evaluate the environmental impact of Hg emissions and other heavy metals present in these locations. Analysis of total metal content by inductively coupled plasma-optical spectroscopy (ICP-OES) revealed a heterogeneous contamination of Hg, with concentrations ranging from 4.3 to 1150 μg g−1. In addition, significant concentrations of Cu, Ni, Pb, Zn, Mn and principally Fe were also identified within the studied samples. Direct determination of mercury species by X-ray absorption near edge spectroscopy (XANES) showed inorganic Hg compounds to prevail in all soils, being Cinnabar (HgSred) and Corderoite (Hg3S2Cl2) the main species. Nevertheless, more soluble mercury compounds, such as HgO and HgSO4, have been also identified in significant proportion (from 6 to 20% of total mercury content), indicating a potential risk of mercury mobilisation. On the other hand, the application of sequential extraction schemes (SES) revealed large portions of weakly available Hg extracted in the residual fraction, while Hg associated to the exchangeable phase amounts as much as 19% of total Hg, thus, supporting the results obtained by XANES.Finally, synchrotron-based micro X-ray fluorescence (μ-XRF) was applied to identify qualitative trends on elemental associations in sample particles through a systematic mapping of its surface. In this concern, results show a well-defined correlation between Hg and Cu/Ni in the analysed particles. On the other hand, an absence of correlation between Hg and several other elements (Fe, Ti, Ca, Zn, Mn and S) was also observed. These effects have been attributed to chemical and physical interactions of mercury species on both enriched particles and sample matrices.
Article
An experimental study of the migration of denser-than-water nonaqueous-phase organic contaminants through heterogeneous porous media was carried out. The purpose of the study was to observe the flow and record the migration of the contaminant to gain a fundamental insight into the way aquifer heterogeneities influence the movement and subsequent distribution of immiscible contaminants after a spill. In addition, the experiments were designed to gather quantitative data to validate multiphase flow models. The experimental results demonstrate the importance of layering in the soil in determining the flow and entrapment behavior of dense nonaqueous-phase contaminants. The findings are of use for model conceptualization and developing field characterization strategies in aquifer remediation.
Article
Between 1950 and 1963 approximately 11 million kilograms of mercury (Hg) were used at the Oak Ridge Y-12 National Security Complex (Y-12 NSC) for lithium isotope separation processes. About 3% of the Hg was lost to the air, soil and rock under facilities, and East Fork Poplar Creek (EFPC) which originates in the plant site. Smaller amounts of Hg were used at other Oak Ridge facilities with similar results. Although the primary Hg discharges from Y-12 NSC stopped in 1963, small amounts of Hg continue to be released into the creek from point sources and diffuse contaminated soil and groundwater sources within Y-12 NSC. Mercury concentration in EFPC has decreased 85% from ∼2000 ng/L in the 1980s. In general, methylmercury concentrations in water and in fish have not declined in response to improvements in water quality and exhibit trends of increasing concentration in some cases.
Article
Elemental mercury is an immiscible liquid with high density and high interfacial tension with water. Its movement in the saturated subsurface region is therefore considered as a case of two phase flow involving mercury and water and is expected to be governed by gravity, viscous, hydrodynamic and capillary forces. This paper investigates the migration and capillary entrapment of mercury in the subsurface based on controlled laboratory capillary pressure-saturation experiments. In the first place, entrapment of mercury was observed in homogeneous porous media. Residual mercury saturation and van Genuchten's parameters for mercury entrapment were generated. These data will provide vital inputs for mercury migration and entrapment models. Secondly, the dependency of residual saturation on fluid properties was brought out in this work by comparing the experimental results of mercury-water system and DNAPL-water systems. Capillary forces were large enough in mercury-water systems to counteract the high gravity forces and caused the entrapment of mercury. Large density differences between mercury and water lead to a high Bond number and thus a low residual mercury saturation was obtained which corroborates with existing DNAPL theories. However, the inverse relationship between residual saturation and capillary number established for NAPL-water systems cannot be compared with mercury-water systems. Moreover, the critical capillary numbers and Bond numbers to mobilize DNAPLs may not be applicable to mercury since mercury has a low capillary number and high Bond number. This work has enabled the understanding of the process of migration and entrapment of mercury and provided useful inputs for two phase flow models specific to mercury-water systems. It has also highlighted the influence of fluid properties on entrapment and mobilization particularly for highly dense, viscous fluid which also possesses high interfacial tension with water.
Article
Previous site-specific investigations have found that mercury concentrations in water, sediments, and biota of the Brazilian Amazon are elevated above global averages, and that these concentrations are a direct result of widespread mercury amalgamation mining operations conducted by non-organized prospectors. In order to assess the regional impacts of Hg contamination from these non-organized gold mining activities, water, sediments, and fish were systematically collected in 1997 along a 900-km reach of the Madeira River. The sampling program extended from the Amazon River upstream to Porto Velho, the site of historic and ongoing mercury amalgamation mining. Mercury concentrations were found to be elevated above global averages in all sampled media. However, the geochemical data suggest that the high mercury levels are due largely to natural sources and natural biogeochemical processes, and that the impacts of anthropogenically released mercury from mine sites is relatively localized.
Article
Since pre-industrial times, anthropogenic emissions of Hg have at least doubled global atmospheric Hg deposition rates. In order to minimize environmental and human health effects, efforts have been made to reduce Hg emissions from industries and power plants, while less attention has been paid to Hg mining. This paper is a compilation of available data on primary Hg production and associated emissions with regional and annual resolution since colonial times. Globally, approximately one million tons of metallic Hg has been extracted from cinnabar and other ores during the past five centuries, half already before 1925. Roughly half has been used for mining of gold and silver, but the annual Hg production peaked during a short period of recent industrial uses. Comparison with total historic Hg deposition from global anthropogenic emissions (0.1-0.2 Mtons) suggests that only a few percent of all mined Hg have escaped to the atmosphere thus far. While production of primary Hg has changed dramatically over time and among mines, the global production has always been dominant in the region of the mercuriferous belt between the western Mediterranean and central Asia, but appears to be shifting to the east. Roughly half of the registered Hg has been extracted in Europe, where Spanish mines alone have contributed one third of the world's mined Hg. Approximately one fourth has been mined in the Americas, and most of the remaining registered Hg in Asia. However, the Asian figures may be largely underestimated. Presently, the dominant Hg mines are in Almadén in Spain (236 t of Hg produced in 2000), Khaydarkan in Kyrgyzstan (550 t), Algeria (estimated 240 t) and China (ca. 200 t). Mercury by-production from mining of other metals (e.g. copper, zinc, gold, silver) in 2000 includes 48 t from Peru, 45 t from Finland and at least 15 t from the USA. Since 1970, the recorded production of primary Hg has been reduced by almost an order of magnitude to approximately 2000 t in the year 2000. Mining is thus still of similar magnitude as all current anthropogenic Hg emissions to the atmosphere, and mined Hg may account for more than one third of these emissions. Also before use, mercury is emitted from Hg mines locally during the mining and refining processes and from mining waste. Global direct emissions to the atmosphere amount to 10-30 t per year currently (up to 10 at Almadén alone), and probably exceed 10000 t historically. Termination of Hg mining will reduce associated local emissions to the atmosphere and biosphere. Since several economically viable Hg-free alternatives exist for practically all applications of Hg, the production and use of Hg can be further reduced and all primary production of Hg other than by-production terminated.
Article
Substrate-electron acceptor combinations and specific metabolic inhibitors were applied to anoxic saltmarsh sediment spiked with mercuric ions (Hg) in an effort to identify, by a direct approach, the microorganisms responsible for the synthesis of hazardous monomethylmercury. 2-Bromoethane sulfonate (30 mM), a specific inhibitor of methanogens, increased monomethylmercury synthesis, whereas sodium molybdate (20 mM), a specific inhibitor of sulfate reducers, decreased Hg methylation by more than 95%. Anaerobic enrichment and isolation procedures yielded a Desulfovibrio desulfuricans culture that vigorously methylated Hg in culture solution and also in samples of presterilized sediment. The Hg methylation activity of sulfate reducers is fully expressed only when sulfate is limiting and fermentable organic substrates are available. To date, sulfate reducers have not been suspected of Hg methylation. Identification of these bacteria as the principal methylators of Hg in anoxic sediments raises questions about the environmental relevance of previous pure culture-based methylation work.
Article
Since the 19th century, mercury(II)chloride (HgCl(2)) has been used on wood impregnation sites to prevent wooden poles from decay, leaving behind a legacy of highly contaminated soil/aquifer systems. Little is known about species transformation and mobility of HgCl(2) in contaminated soils and groundwater. At such a site the behaviour of HgCl(2) in soils and groundwater was investigated to assist in risk assessment and remediation. The soil is low in organic carbon and contains up to 11,000 mg Hg/kg. Mercury (Hg) concentrations in groundwater decrease from 230 to 0.5 microg/l within a distance of 1.3 km. Hg species transformations in soil and aqueous samples were analysed by means of solid-phase Hg pyrolysis and CV-AAS. In aqueous samples, Hg species were distinguished between ionic/reactive Hg and complex-bound Hg. Potential mobility of Hg in soils was studied through batch experiments. Most Hg in the soil is matrix-bound HgCl(2), whereas in the aquifer secondary formation to Hg(0) could be observed. Aqueous Hg speciation in groundwater and soil solutions shows that an average of 84% of soluble Hg exists as easily reducible, inorganic Hg species (mostly HgCl(2)). The proportion of complex-bound Hg increases with distance due to the transformation of inorganic HgCl(2). The frequent occurrence of Hg(0) in the aquifer suggests the formation and degassing of Hg(0), which is, in addition to dilution, an important process, lowering Hg concentrations in the groundwater. High percentage of mobile Hg (3-26%) and low seepage fluxes will result in continuous Hg release over centuries requiring long-term groundwater remediation. Results of soluble Hg speciation suggest that filtering materials should be adapted to ionic Hg species, e.g. specific resins or amalgamating metal alloys.
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Detail of the Hg 0 (re)distribution front in proximity of the left side wall at the end of stage 1 (top) and stage 2 (bottom)
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Fig. 12. Detail of the Hg 0 (re)distribution front in proximity of the left side wall at the end of stage 1 (top) and stage 2 (bottom). 271-311.
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Soil and groundwater contamination: nonaqueous phase liquids, principles and observations
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