Analysis of the remediation of coal tar-contaminated groundwater using ex situ remediation

Citation: Marschalko, M.; Kempa, T.; Popielarczyk, D.; Cernik, M.; Vicherkova, M.; Vicherek, P.; Niemiec, D. Analysis of the Remediation of Coal Tar-Contaminated Groundwater Using Ex Situ Remediation. Water 2022,14, 2182. https://doi.org/ 10.3390/w14142182 Academic Editors: Jiangshan Li, Xiao Yang and Fei Wang Received: 16 June 2022 Accepted: 6 July 2022 Published: 10 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). water Article Analysis of the Remediation of Coal Tar-Contaminated Groundwater Using Ex Situ Remediation Marian Marschalko 1, Tomas Kempa 2, Dariusz Popielarczyk 3,* , Miroslav Cernik 4, Michaela Vicherkova 1, Petr Vicherek 1and Dominik Niemiec 1 1 Department of Geological Engineering, Faculty of Mining and Geology, VSB-Technical University of Ostrava, 17 listopadu 15, 708 33 Ostrava, Czech Republic; [email protected] (M.M.); [email protected] (M.V.); petr.vicher[email protected] (P.V.); [email protected] (D.N.) 2Tomas Kempa Hydrogeology, Ivana Sekaniny 1802/11, 708 00 Ostrava, Czech Republic; [email protected] 3Department of Geodesy, Faculty of Geoengineering, University of Warmia and Mazury in Olsztyn, Oczapowskiego 2, 10-719 Olsztyn, Poland 4Institute of New Technologies and Applied Informatics, Faculty of Mechatronics, Technical University of Liberec, Studentska 1402/2, 461 17 Liberec, Czech Republic; miroslav[email protected] *Correspondence: dariusz.popielar[email protected] Abstract: The article describes the remediation of contaminated groundwater during the ex situ remediation of coal tar contamination following the closure of a coking plant in the Moravian– Silesian Region (Czech Republic). The aim of the article is to point out the advantages of ex situ soil remediation via the excavation of the contaminated geological environment combined with thermal desorption, a method of removing contaminants both from soil and groundwater. Its advantage is the absolute qualitative and quantitative control over the contaminated soil with the possibility of precise segmentation into contaminated and non-contaminated soils. Next, all contaminated groundwater may be pumped off upon the construction of sealing walls to control groundwater flows. To excavate the soil, it is necessary to reduce the contaminated groundwater level inside the sealing walls and thus create conditions for the extraction of contaminated soils using standard machinery. In detail, the article describes the removal of the contaminated groundwater and compares the quality of the pumped and inflowing water before and after the remediation. The locality is characteristic of a high horizontal and vertical grain-size heterogeneity of gravel-sand, which led to a varying filtration coefficient affecting the capacities of pumped groundwater quantity during the remediation. At the start of the remediation process, the contaminant levels exceeded the limits by the Czech Environmental Inspectorate several times. The post-remediation monitoring showed that all the contaminant levels were below the limit. Surprisingly, the overall groundwater contamination amounted to 232.86 t of contaminants as non-aqueous phase liquids, and 6872.9 kg of dissolved contaminants. As much as 12,200 t of contaminants were removed from the soil. Keywords: groundwater decontamination; coal tar-contamination; ex situ remediation; remediation dugout; contaminated soil excavation; alluvial sediments; Moravian–Silesian Region; Czech Republic 1. Introduction The motivation behind the study was an analysis of remediation of groundwater contaminated by coal tar when applying ex situ dugout decontamination. The aim is to indicate the advantages of ex situ remediation of a locality contaminated by coal tar (Figure 1). An application of in situ remediation leads to the formation of a gap between the water-lowering wells in the lower part of the contaminated permeable environment, where water does not flow toward the wells (Figure 1). This gap is characteristic of a cone of depression with an angle dependent on the friction slope, permeability of the rock (soil) environment, filtration coefficient, and distance between the wells. However, when the soil Water 2022,14, 2182. https://doi.org/10.3390/w14142182 https://www.mdpi.com/journal/water Water 2022,14, 2182 2 of 16 is extracted and the decontamination occurs ex situ, e.g., by means of thermal desorption, as in the case of the locality of interest, this problem is eliminated. Water 2022, 14, x FOR PEER REVIEW 3 of 16 concerned contaminants as follows: benzo(a)pyrene 12.5 μg/L, benzene 250 μg/L, naphthalene 6300 μg/L, phenol 25,000 μg/L (25 mg/l), and non-polar extractable substances 50,000 μg/L (50 mg/l) [15]. Figure 1. Scheme of the research aim of the study. 2. Methods 2.1. Study Area The industrial complex, where the environmental strain was located, was established in connection with the start of the black coal mine in 1842. From 1858 there was a coking plant, a power plant, an iron and steel production facility, and a heavy machinery industry. The complex was closed down in 1990. The combination of all of the industrial activities, predominantly due to the operation of the coking plant, led to one of the largest environmental strains in the Czech Republic. Figure 2a is an aerial photo of the locality of interest in 1955, when the coking plant was in full operation. It also gives the lines of foundations that were dug out during the remediation process. The spread of the contamination by coal tar is shown in Figure 2b. The next photo shows the area where the remediation dugout was refilled with decontaminated soil (Figure 2c) and Figure 2d is a photo of a newly built shopping center. Figure 1. Scheme of the research aim of the study. When dealing with coal tar remediation, it is important to distinguish between ex situ remediation [ 1 – 4 ], and in situ remediation methods applied directly in the soil massif [ 5 – 8 ]. An important specific condition for the application of the ex situ remediation method of removing coal tar from alluvial sediments is the extraction of all the soil and its ex situ decontamination using thermal desorption. At the same time, it is important to drain the whole remediation dugout and treat the contaminated groundwater in a decontamination station. If in situ remediation was implemented using wells to pump away the contaminated water, an overlap of cones of depression could occur. Groundwater occurring in the overlap of cones of depression at the bottom of the dugout cannot be pumped or treated. This means that using ex situ remediation, when all the soil is extracted and the impermeable bedrock is exposed, no contaminated groundwater remains in the dugout. The thermal desorption mentioned is an ex situ remediation method used for cleaning soil materials. It has the advantage of short site remediation time and, above all, high contaminant removal efficiency. The principle of this method is to heat the contaminants present in the soil. This heating is carried out in a so-called rotary desorber, which is resistant to high temperatures. A distinction is made between direct and indirect thermal desorption, whereby in direct desorption, specific soil grains are heated by burners, and in indirect desorption, the desorber shell is heated, which indirectly transfers heat to the Water 2022,14, 2182 3 of 16 contaminated soil grains. The heating transfers the contaminants to the gaseous state. After cooling, the contaminants are concentrated in the liquid condensate, which is disposed of as hazardous waste. The main goal of the study was to focus on ex situ remediation of groundwater contaminated by coal tar. It was implemented by means of a case study of one of the biggest environmental strains in the Czech Republic (Moravian–Silesian Region, east part of the Czech Republic). This is mainly to highlight the main advantages of using ex situ remediation through this mentioned case study, since ex situ remediation refers to the pumping and subsequent treatment of water away from the site of pollution. The case study had the following specific conditions (Figure 1). The first specific condition was contamination by coal tar. Contamination by any chemical brings along specific limiting conditions that affect the type of remediation and many other aspects. The same applies to coal tar contamination [ 9 – 11 ]. In the locality of interest, the environmental strain was caused by the long operation of a coking plant, lack of technological discipline, industrial accidents, and changing technologies in time that had their drawbacks. The second specific condition is the geological conditions in the locality. There are alluvial sediments of partially permeable fine-grained soils of an average 4 m thickness and coal tar-contaminated permeable gravel sand of an average 4 m thickness. The geological structure of the wider area of interest was also described [ 12 – 14 ]. The third specific condition is the hydrogeological conditions with an unconfined groundwater body in the depth of approximately 4 m. The unconfined groundwater body is predominantly bound onto permeable gravel-sand. The fourth specific condition is the applied ex situ remediation method, which means that all the soil was remediated outside its original location. The main decontamination method was a low-temperature thermal desorption based on the principle of heating the contaminated materials over the temperature (560 ◦ in this case) at which the pollutants volatilize. The fifth specific condition is the technology of ex situ remediation executed using a remediation dugout. As much as 1.5 million tons of contaminated soils were extracted all the way to the impermeable clayey Miocene bedrock. After the decontamination, the soil was returned to the dugout, while the original grain-size composition was preserved. The sixth specific condition was the occurrence of the different contaminants that predominantly corresponded to coal tar contamination. The Czech Environmental Inspectorate determined the limits for the concerned contaminants as follows: benzo(a)pyrene 12.5 µ g/L, benzene 250 µ g/L, naphthalene 6300 µ g/L, phenol 25,000 µ g/L (25 mg/L), and non-polar extractable substances 50,000 µg/L (50 mg/L) [15]. 2. Methods 2.1. Study Area The industrial complex, where the environmental strain was located, was established in connection with the start of the black coal mine in 1842. From 1858 there was a coking plant, a power plant, an iron and steel production facility, and a heavy machinery industry. The complex was closed down in 1990. The combination of all of the industrial activities, predominantly due to the operation of the coking plant, led to one of the largest environmental strains in the Czech Republic. Figure 2a is an aerial photo of the locality of interest in 1955, when the coking plant was in full operation. It also gives the lines of foundations that were dug out during the remediation process. The spread of the contamination by coal tar is shown in Figure 2b. The next photo shows the area where the remediation dugout was refilled with decontaminated soil (Figure 2c) and Figure 2d is a photo of a newly built shopping center. Water 2022,14, 2182 4 of 16 Water 2022, 14, x FOR PEER REVIEW 4 of 16 Figure 2. Locality of interest: (a) Aerial photo of 1955 marking the lines of foundations which were dug out during the remediation process, (b) spread of the contamination by coal tar, (c) remediation dugout refilled with decontaminated soil, (d) current photo of a newly built shopping center. 2.2. Groundwater Remediation The aim of the article was to point out the advantages of ex situ remediation achieved by the excavation of the contaminated geological environment combined with thermal desorption. It is a method of removing contaminants both from soil and groundwater. Its advantage is the absolute qualitative and quantitative control over the contaminated soil with the possibility of precise segmentation into contaminated and non-contaminated soils. Next, all contaminated groundwater may be pumped off having constructed sealing walls to control the groundwater flows. To excavate the soil, it was necessary to reduce the contaminated groundwater level inside the sealing walls and thus create conditions for the extraction of contaminated soils using standard machinery. When in situ remediation and water-lowering wells are applied, the overlap of cones of depression forms at the level of the aquifer. This means that contaminated residues remain at the bottom as they do not flow toward the wells. The section does not aim to describe all the technological details of the remediation process, but only those related to groundwater remediation. At the start, cutoff walls and diaphragm walls were installed into the impermeable Miocene clay bedrock (Figure 3a). Figure 2. Locality of interest: ( a ) Aerial photo of 1955 marking the lines of foundations which were dug out during the remediation process, ( b ) spread of the contamination by coal tar, ( c ) remediation dugout refilled with decontaminated soil, (d) current photo of a newly built shopping center. 2.2. Groundwater Remediation The aim of the article was to point out the advantages of ex situ remediation achieved by the excavation of the contaminated geological environment combined with thermal desorption. It is a method of removing contaminants both from soil and groundwater. Its advantage is the absolute qualitative and quantitative control over the contaminated soil with the possibility of precise segmentation into contaminated and non-contaminated soils. Next, all contaminated groundwater may be pumped off having constructed sealing walls to control the groundwater flows. To excavate the soil, it was necessary to reduce the contaminated groundwater level inside the sealing walls and thus create conditions for the extraction of contaminated soils using standard machinery. When in situ remediation and water-lowering wells are applied, the overlap of cones of depression forms at the level of the aquifer. This means that contaminated residues remain at the bottom as they do not flow toward the wells. The section does not aim to describe all the technological details of the remediation process, but only those related to groundwater remediation. At the start, cutoff walls and Water 2022,14, 2182 5 of 16 diaphragm walls were installed into the impermeable Miocene clay bedrock (Figure 3a). Their purpose is the geotechnical and hydraulic insulation of the contaminated space. This made it possible to gradually lower the groundwater level and thus create conditions for the subsequent extraction of the contaminated soil and refill when remediated. Next, groundwater was pumped off from the dugout in combination with gutters (Figure 3b). The water-lowering wells were gradually shortened and reconnected. When drained, the different branches or their parts were replaced with pumping from the gutters at the bottom of the dugout (Figure 3c). Water pumping was terminated after the contaminants were extracted and the lower soil layer refilled. Monitoring wells were gradually constructed in parts refilled with the decontaminated soil. Water 2022, 14, x FOR PEER REVIEW 6 of 16 Samples were regularly drawn at the aquifer level. The aim of the monitoring was the verification of the groundwater quality and quantity, and whether the contaminant levels were below the limits set by the Czech Environmental Inspectorate. Figure 3. Principle of ex situ remediation in the context of groundwater conditions. Photodocumentation of the site of interest during remediation is shown in Figure 4. Figure 4. Photodocumentation of the locality of interest in the course of remediation (a) decontamination station, (b) inflow into the stripping unit via gravity separators, (c) detail of a gutter to trap contaminated groundwater that cannot be pumped by water-lowering wells (a view from the west), (d) panorama of the locality taken from the east. The contaminants (benzo(a)pyrene, benzene, naphthalene, non-polar extractable substances, phenol) were determined in certified laboratories according to the applicable Czech standards and according to the limits set by the Czech Environmental Inspectorate, Figure 3. Principle of ex situ remediation in the context of groundwater conditions. The groundwater was decontaminated (Figure 3d) using a decontamination station with a constant flow of 10–15 L/s. The treatment combined 4 subsequent methods, namely gravitational separation, aeration, filtration, and activated carbon adsorption. The first applied method was gravitational separation, during which substances of different densities than water and of a supersaturated solution concentration were separated as nonaqueous phase liquids. Among others, this method separated the following contaminants: benzo(a)pyrene, benzene, and non-polar extractable substances. At the same time, mechanical impurities and partially iron hydroxides were separated via sedimentation. The second was aeration, which caused the oxidation of bivalent iron ions and their conversion into ferric hydroxides. A displacement of volatile contaminant components also occurred, which passed from water into the air to be cleaned via the sorption filters and activated carbon. The third method was filtration with the use of two parallel sand pressure filters to remove the remaining mechanical impurities and iron hydroxides. The fourth water treatment method was activated carbon adsorption (6 sorption filters) to remove the remaining concentrations of contaminants (non-volatile matter in non-polar extractable substances, benzene, benzo(a)pyrene, and phenols). The water cleaned to the required limits was discharged as wastewater into the Ostravice River. Water needed to be pumped away before the remediation dugout and earthwork could be executed (Figure 3e). This means that each soil level to be extracted had to be dewatered. Moreover, the bottom of the dugout was drained by extra gutters. The Water 2022,14, 2182 6 of 16 excavated soil was predominantly transported for thermal desorption. Thermal desorption was applied to decontaminate 92.5% of the soil and 0.1% of the soil was decontaminated using biodegradation. The remaining percentages of the soil were transported to 3 types of disposal sites, namely a waste disposal site (5.1%), a combined site for hazardous waste and a single-type waste disposal site (2.0%), and a combined waste disposal site (0.3%). The remediation dugout was refilled (Figure 3f) with the decontaminated soil in slanted beds of 0.5 m thickness so that the soil could be compacted using a vibrating roller. The soil was refilled to respect the original grain-size structure of the soil environment. This means that the bottom of the dugout, consisting of gray-blue Miocene clay, was filled with gravel and complemented with rock material of analogous grain-size (particularly spoil). This layer was followed by two layers of fine-grained soils, each 2 m thick. The beds had an inclination of 3.33 ◦ so that rainwater could run off naturally. The ground morphology was almost analogous to the original, including the fact that the new anthropogenic geological environment had to have analogous hydrogeological conditions with an unconfined groundwater body. In the next phase, the cutoff walls were removed (Figure 3g). This phase was important to gradually create hydrogeological conditions analogous to those before remediation. The diaphragm walls remained to protect the nearby buildings. Having removed the cutoff walls, the natural groundwater flow could be restored in the refilled dugout and the groundwater level could be adjusted in line with the existing unconfined groundwater body (Figure 3h). This took 2 years based on the heterogeneity of the alluvial sediments and in line with the distribution of filtration coefficient in the existing soils. Having terminated the remediation earthwork, post-remediation monitoring was implemented at the site of the dugout to observe the quality of groundwater and its depth (Figure 3i). For this purpose, 12 monitoring wells were drilled evenly across the dugout. Samples were regularly drawn at the aquifer level. The aim of the monitoring was the verification of the groundwater quality and quantity, and whether the contaminant levels were below the limits set by the Czech Environmental Inspectorate. Photodocumentation of the site of interest during remediation is shown in Figure 4. Water 2022, 14, x FOR PEER REVIEW 6 of 16 Samples were regularly drawn at the aquifer level. The aim of the monitoring was the verification of the groundwater quality and quantity, and whether the contaminant levels were below the limits set by the Czech Environmental Inspectorate. Figure 3. Principle of ex situ remediation in the context of groundwater conditions. Photodocumentation of the site of interest during remediation is shown in Figure 4. Figure 4. Photodocumentation of the locality of interest in the course of remediation (a) decontamination station, (b) inflow into the stripping unit via gravity separators, (c) detail of a gutter to trap contaminated groundwater that cannot be pumped by water-lowering wells (a view from the west), (d) panorama of the locality taken from the east. The contaminants (benzo(a)pyrene, benzene, naphthalene, non-polar extractable substances, phenol) were determined in certified laboratories according to the applicable Czech standards and according to the limits set by the Czech Environmental Inspectorate, Figure 4. Photodocumentation of the locality of interest in the course of remediation ( a ) decontamination station, ( b ) inflow into the stripping unit via gravity separators, ( c ) detail of a gutter to trap contaminated groundwater that cannot be pumped by water-lowering wells (a view from the west), (d) panorama of the locality taken from the east. Water 2022,14, 2182 7 of 16 The contaminants (benzo(a)pyrene, benzene, naphthalene, non-polar extractable substances, phenol) were determined in certified laboratories according to the applicable Czech standards and according to the limits set by the Czech Environmental Inspectorate, with the values of input (before remediation), output (after remediation) and their difference (what was removed by remediation). 3. Results Changes in the Hydrogeological Conditions during Remediation Changes in the hydrogeological conditions will be explained using 4 model maps of groundwater-table contours and 4 diagrammatic sections that document the principal hydrogeological changes during the remediation process. The gradual extraction and refilling of soil is explained in two time frames (Figure 5a,b), where parts of the contaminated locality (left-north) are extracted gradually all the way to the permeable gravel and later refilled with decontaminated soil. A gap is formed to separate the contaminated section of the locality (right-south) all the way to the impermeable Miocene bedrock, where water is drained by means of two gutters. The course of groundwater level may be observed in the section as it lowers towards the gutters. This procedure was applied in the direction north–south. Thus, the last section to be decontaminated was in the south. The two diagrammatic sections represent the whole process behind the remediation dugout, while the bottom geometry changes when progressing from the north to the south. Water 2022, 14, x FOR PEER REVIEW 7 of 16 with the values of input (before remediation), output (after remediation) and their difference (what was removed by remediation). 3. Results Changes in the Hydrogeological Conditions during Remediation Changes in the hydrogeological conditions will be explained using 4 model maps of groundwater-table contours and 4 diagrammatic sections that document the principal hydrogeological changes during the remediation process. The gradual extraction and refilling of soil is explained in two time frames (Figure 5a,b), where parts of the contaminated locality (left-north) are extracted gradually all the way to the permeable gravel and later refilled with decontaminated soil. A gap is formed to separate the contaminated section of the locality (right-south) all the way to the impermeable Miocene bedrock, where water is drained by means of two gutters. The course of groundwater level may be observed in the section as it lowers towards the gutters. This procedure was applied in the direction north–south. Thus, the last section to be decontaminated was in the south. The two diagrammatic sections represent the whole process behind the remediation dugout, while the bottom geometry changes when progressing from the north to the south. Figure 5. Two time diagrammatic sections of the gradual extraction and refilling documenting the work sequence in the dugout (a) earlier in time, (b) later in time. The following section describes the basic changes in the hydrogeological conditions that occurred during the remediation works. The first situation represents the time (Figure 6a or Figure 7a) when the soil in the locality was extracted and refilled with the decontaminated soil (92.5%, or 11,290 t) using thermal desorption. It is still possible to see the hydraulically closed system of the locality due to the cutoff and diaphragm walls. As for hydrogeology, the groundwater level inside the closed system is influenced only by Figure 5. Two time diagrammatic sections of the gradual extraction and refilling documenting the work sequence in the dugout (a) earlier in time, (b) later in time. The following section describes the basic changes in the hydrogeological conditions that occurred during the remediation works. The first situation represents the time (Figures 6aor7a) when the soil in the locality was extracted and refilled with the decontaminated soil (92.5%, or 11,290 t) using thermal desorption. It is still possible to see Water 2022,14, 2182 8 of 16 the hydraulically closed system of the locality due to the cutoff and diaphragm walls. As for hydrogeology, the groundwater level inside the closed system is influenced only by precipitation; no water flows in from the surroundings. Outside the closed system, there is an unconfined groundwater body characteristic of water flow from SW to NE. Water 2022, 14, x FOR PEER REVIEW 9 of 16 Figure 6. Map of groundwater-table contours in the area of interest: (a) start of flooding, (b) situation after the partial removal of cutoff walls in the west, (c) situation after 2 years–groundwater leveled off before the 106-meter-long underground piling wall was opened in the east, (d) steady state. Figure 6. Map of groundwater-table contours in the area of interest: ( a ) start of flooding, ( b ) situation after the partial removal of cutoff walls in the west, ( c ) situation after 2 years–groundwater leveled off before the 106-meter-long underground piling wall was opened in the east, (d) steady state. Water 2022,14, 2182 9 of 16 Water 2022, 14, x FOR PEER REVIEW 10 of 16 Figure 7. Diagrammatic sections of the area of interest: (a) section A-A’ of the start of flooding, (b) section B-B´ having partially removed the cutoff walls in the west, (c) section C-C’-situation 2 years later when the groundwater leveled off and before the 106-meter-long underground piling wall was opened in the east, (d) section D-D’-steady state. 4. Discussion Evaluation of Groundwater Contamination The groundwater in the locality was contaminated by coal tar, both in its dissolved form and as non-aqueous phase liquids. It had the character of “wash oil”. The spread of coal tar contamination depends on the geological structure and its permeability. It means that in the environment of alluvial sediments, the spread of contamination is influenced by the specific conditions of alluvial sediments [16,17]. Coal tar as dense, non-aqueous phase liquids (DNAPL) was identified in the amount of 232.96 t (as a mixture of organic compounds). It was only analyzed for characteristics vital for its disposal, such as sulfur content, viscosity, and high heat value. In the dissolved form, the contaminants were identified, and their values were observed in the course of the remediation work and compared with the limits set by the Czech Environmental Inspectorate. DNAPL was predominantly drawn from the wells. When the level of DNAPL could not be pumped directly, it was pumped along with water and separated using gravitational separation in the decontamination station. DNAPL, possibly with tar-in-water emulsion, and remaining contaminated water were pumped into containers to settle for several days into contaminants and water. Having been separated into two phases, the Figure 7. Diagrammatic sections of the area of interest: ( a ) section A-A’ of the start of flooding, ( b ) section B-B ´ having partially removed the cutoff walls in the west, ( c ) section C-C’-situation 2 years later when the groundwater leveled off and before the 106-meter-long underground piling wall was opened in the east, (d) section D-D’-steady state. The second situation represents the time Figure 6b or Figure 7b, when the closed system was partially opened as the cutoff walls were removed in the west. This was done to hydrogeologically accommodate and restore the remediated locality, i.e., to re-flood it with groundwater and restore the natural groundwater flows in the form of an unconfined groundwater body. The cutoff wall was kept in the east, where remediation still continued in the heap of the Žofie’s foundry using in situ water pumping and treatment, including vapor. The course of the groundwater-table contours is characteristic of gradual flooding of the remediated locality and leveling-off the groundwater level in the dugout after the soil refill. The third situation represents the time (Figures 6c or 7c) characteristic of the locality approximately 2 years after the removal of the cutoff walls in the west (106-meter-long underground piling wall), when the groundwater level had leveled off with the surroundings. The steeper hydraulic gradient in the west and east (Figure 6c) is given by higher soil permeability of the original conditions when compared to the newly refilled and compacted remediated soil of a lower hydraulic gradient. The fourth situation represents the time (Figures 6dor7d) upon the opening of the cutoff walls in the east, 3 years after the soil refill (one year later than the previous situation). The groundwater level inside and outside the walls documents the fact that the original groundwater flows were fully restored . 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