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Spatial characterization of leachate plume using electrical resistivity tomography in a landfill composed of old and new cells (Belfort, France)
Engineering Geology 211 (2016) 61–73
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Spatial characterization of leachate plume using electrical resistivity tomography in a landfill composed of old and new cells (Belfort, France) Vincent Bichet a, Elise Grisey b, Lotfi Aleya a,⁎ a b
Laboratoire de Chrono-Environnement, UMR CNRS 6249, Université de Bourgogne, Franche-Comté, France Atelier d'Écologie Urbaine, 9 avenue Philippe Auguste, 75011 Paris, France
a r t i c l e
i n f o
Article history: Received 1 December 2015 Received in revised form 20 June 2016 Accepted 29 June 2016 Available online xxxx Keywords: Landfill Pollution plume Leachates Electrical resistivity tomography
a b s t r a c t Located near Belfort (France), the Etueffont landfill was in operation from 1976 to 2002 for the disposal and storage of domestic waste produced by 47,650 inhabitants. The site is comprised of the original landfill site called the old landfill (OL), in operation from 1976 to 1999, and a newer section known as the new cell (NC) which operated from 1999 to 2002. The objective of this study is to determine, using electrical resistivity tomography (ERT), the extent of the leachate plume from the OL and to monitor the efficiency of the liner of the NC. The entire Etueffont site was crisscrossed with 21 electrical profile lines which were traced in summer between 2009 and 2011 during a period of dry weather. This investigation allowed visualization of the flow and saturation phenomena within the waste without destructive on-site intervention, and also of leachate infiltration into the substratum. The anomalies of low resistivity extending beneath the base of the OL represent a leachate plume that migrates within the substratum. As the water table ceiling is found at nearly the same level as the base of the waste, leachate infiltration is a source of groundwater pollution. A rising gradient in resistivity values is observed at depth indicating progressive vertical dilution in leachate mineralization. Pollution plume extension is limited both at depth and laterally as it is no longer visible a few dozen meters from the storage zone. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The diagnosis and monitoring of polluted sites is currently a major concern. Landfills contain a variable quantity of toxic materials and comprise a risk for human beings and their environment (Bernard et al., 1997; Ben Salem et al., 2014a, 2014b). Former landfills, established at a time when the knowledge of the risks involved in waste storage was nearly non-existent, do not possess the geological or artificial barriers that isolate wastes from the natural environment (Kallis and Buder, 2001; Aleya et al., 2007; Gibbons et al., 2014). Leachate seepage found at the base of landfill sites is a source of soil and groundwater contamination (Grisey et al., 2010; Weber et al., 2011). In addition, not all of these landfills have been inspected; storage zone limits and the type and volume of wastes sometimes remain unknown (Butt et al., 2014). Even in well-monitored landfills, geometry and depth may differ from the information appearing in the plans submitted for inspection of classified sites when the operation permit application is initially filed (Meju, 2006). However, in recent landfills respecting updated rules for technical security (Decree of September 9, 1997, amended) risks have been minimized (Grisey and Aleya, 2016a). Conventional methods for evaluating landfill-related groundwater contamination bring together such topics as reconnaissance drilling, ⁎ Corresponding author. E-mail address: lotfi[email protected] (L. Aleya).
http://dx.doi.org/10.1016/j.enggeo.2016.06.026 0013-7952/© 2016 Elsevier B.V. All rights reserved.
water sampling and chemical analysis (Frid et al., 2008). Though water samples furnish precise information as to the type of contaminants present (Vaudelet et al., 2011), drilling remains costly and invasive (Radulescu et al., 2007) and may sometimes miss the contaminated zone due to poor deployment (Vaudelet et al., 2011). Less costly and non-invasive geophysical prospecting techniques have been developed to study subsoil structures without disturbing them. They are now frequent tools for site investigation and can be used to detect pollution, to describe geological features, to diagnose engineering structures and also for hydrogeological studies (Chambers et al., 2006; Giusti, 2009). Electrical resistivity tomography (ERT), which is based on the measure of the electrical resistivity ρ of subsurface materials, provides information on the properties of the underground environment such as thickness of a layer and its saturation, depth of the substratum, localization and distribution of conducting fluids (leachates), and localization and orientation of fractures and faults (Soupios et al., 2007). Electrical resistivity or inversely, electrical conductivity of wastes, depend on several of the waste mass properties such as porosity, connectivity of pores, water content, the ionic force of leachates and temperature (Imhoff et al., 2007). Thus, ground not saturated with water will be less conductive than if saturated; electrical resistivity diminishes as water mineralization increases. ERT is particularly well adapted to the study of landfills in order to determine their geometry and internal structure (Depountis et al., 2005; Chambers et al., 2006; Meju, 2006; Soupios et al., 2007; Wilkinson et al., 2010), and to detect
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leachate contamination of the substratum (Acworth and Jorstad, 2006; Park et al., 2008; Abdullahi et al., 2010). This technique has some drawbacks that make its features difficult to interpret. One such drawback is that the reduction in resolution as depth increases generates imprecision at deeper levels. In addition, due to the preferential circulation of current in conductive zones, the presence of a non-conductive lining at the bottom of a landfill cell may obstruct flow toward the zone situated below. In a previous investigation via electrical resistivity tomography at the Etueffont site, Belle (2008) brought to light a vertical seepage leachate zone at the base of the OL and a leachate plume migration toward the west, in the direction N325–335. The same author also hypothesized the existence of a leachate leak in the new water-proof cell. For this study, investigation was pursued beyond the limits of the storage zone so as to determine the extent of the leachate plume from the former landfill and to monitor the efficiency of the NC lining. Several profiles have also been traced over the storage zone to verify whether electrical resistivity tomography can appreciate the internal structure of the two cells. 2. Materials and methods 2.1. Study site The Etueffont landfill, located in northeastern France (N 47°43′19″ / E 6°56′57), covers a 2.8 ha surface and was in operation from 1976 to 2002 (Fig. 1A). The site is located in the Permian basin of Giromagny, with the landfill on an eroded horst formed by Devono-Dinantian schists (silt and sandstone). The southeastern part of the horst is bordered by a NE–SW fault, which brings the schists into contact with the
Permian formations (Fig. 1B). The schists dip at a high angle (N75°) and at some places are nearly vertical with an average direction of N75–N80. The NE to SW direction of the groundwater flow was determined by means of a network of piezometers. Two perpendicular fracture families (N40–N70/subvertical and N135–N155/75–80 W) may influence the flow (Fig. 1B). The structural elements of the site show groundwater isopieze curves in a low water period of the water table from 1997 to 2009 and at a mean altitude, indicating that groundwater circulates toward the SW following the stream axis that receives the water from the fill (Fig. 2). During low-water periods the hydraulic gradient averages 7%, indicating a weak permeability of the geological substratum (10−9 b K b 10−6 m s−1). The landfill received municipal solid waste, bulky waste and construction and demolition (C&D) waste. Altogether, approximately 305,000 tons of waste were deposited into two cells, with a waste depth ranging from 6 to 15 m. The first cell, known as the old landfill (OL), was in operation from 1976 to 1998 and was directly established on impermeable schist. The new cell (NC) was in operation from 1998 to 2002 and was equipped with a watertight polyethylene bottom liner, surrounded on either side by a geotextile mat. The Etueffont landfill's mode of operation was non-conventional. Shredded waste from both cells was successively deposited without compaction in 1 m-thick layers after a 2–3-month period of biostabilization between each layer. No alternative daily cover was used and the degradation that did take place was mainly aerobic, which promoted the production of leachate and minimized the levels of methane gas in the landfill (ADEME, 2005). After the landfill closed, the waste was covered by a 0.8 m-thick topsoil layer, without compaction. Considering the low pollution potential of this landfill, an impermeable cover was not required (ADEME, 2005).
Fig. 1. A: Location of the Etueffont landfill with the old landfill (OL) and the new cell (NC), B: Geological map and profile.
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Fig. 2. Structural elements of the site showing groundwater isopieze curves in low water period, at a mean altitude in the water table over 1997–2009.
Leachate that was generated in each cell drained back into a shared leachate collection system, which was then discharged into a natural lagooning treatment system (Khattabi and Aleya, 2007; Ben Salem et al., 2014a, 2014b) (Fig. 2). Inflow was generally adjusted to 59.4 m3 day−1, with a theoretical retention time of approximately 19 days. At the lagooning system's outlet, the treated leachate was released into a brook, the “Gros Près”. The Etueffont site is in the methanogenic phase (Kjeldsen et al., 2002; Claret et al., 2011) as demonstrated by Aleya et al. (2007) and Grisey and Aleya (2016a), illustrating that the site has already achieved advanced stable conditions, i.e. with pH reaching neutral values, methane production and a decreased release of organic and inorganic matter. Leachate and groundwater was chemically characterized for both the old landfill (1989–2010) and the new cell (2000 − 2010), and water electrical conductivity (EC) was measured. The main results were published in Grisey and Aleya (2016b). The EC values of groundwater and leachate cited below were not collected during the geophysical survey but correspond to mean values previously published (Grisey and Aleya, 2016b). 2.2. Geophysical survey The entire Etueffont site was investigated using 2D electrical resistivity tomography (ERT). The area was crisscrossed with 21 electrical profile lines, noted P1 to P21, whose positions are reported in Fig. 3. The Etueffont landfill is a controlled site whose structure is well known. To identify potential contamination beneath the cells, eight profiles (P1 to P8, NE/SW and NW/SE) were traced. Three parallel profile
lines were traced, crossing the storage zone from the NE to the SW, the first along the NW flank (P1), the second within the landfill environment (P2) and the third along the SE flank (P3). Five profile lines oriented NW to SE—P4 and P5 in the OL, P6 at the interface of the two cells, and P7 and P8 in the NC—were traced nearly perpendicularly to the three previous ones. To identify potential contamination surrounding the site, 6 hydraulic profiles (P14 to P19) and 7 lateral profiles were traced (Fig. 3) during a dry period (Table 1) and in sunny weather. The measurements were all taken the day following a light rainy period, which allowed for a better connection between ground and electrodes and favored better propagation of the electrical current. 2.3. Acquisition of data The geoelectrical data were collected via two systems of multi-electrode measurement: the ABEM Terrameter Lund Imaging system (400 V; 100 W; 1 A) and the SYSCAL R1 + Switch-72 (600 V; 200 W; 2.5 A) produced by Iris Instruments. Each prospecting system was equipped with a resistivity meter connected to a cable with multiple outlets connected to stainless steel electrodes planted in the ground at regular intervals (4.5 m for P2 and P4 profiles; 5 m for the others). The resistivity meter automatically exploits, according to a preprogramed sequence, all possible positions of transmission and reception on these electrodes in order to cover the study zone horizontally and vertically. The electrical measurements were taken according to a Wenner-Schlumberger configuration (a hybrid of the Wenner and Schlumberger device) along a rectilinear profile. This device is moderately sensitive to both lateral and depth variations in resistivity and is
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Fig. 3. Site topography with location of electric profiles (black lines), distinguishing those in the storage zone (yellow circles) from those in the surrounding area (white circles). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
a good compromise between the Wenner device (sensitive to horizontal structures) and the dipole-dipole (sensitive to vertical structures) (Loke, 2012). Thus, the Wenner-Schlumberger is particularly well adapted to grounds such as landfills that present both horizontal and vertical structures. The characteristics of each profile are presented in Table 1. Concerning the acquisition set up, the injection time per cycle was 500 ms and the registered value corresponded to a stack of from 3 to 6 measurements with a maximum admitted error of 3%. The minimal tension requested for a valid measurement was 50 mV at the potential dipole. At least 16 depth levels were prospected and, depending on the length of the profiles, from 600 to 925 quadripoles were measured. Finally, topography profiles and electrode locations were measured at a given station to provide geometric correction for apparent resistivity calculation. 2.4. Treatment and modeling of data Acquired data were filtered to eliminate negative values and values with a standard deviation N1% for both measurements at the same point. For each profile b 10% of the values were rejected. The apparent resistivity values calculated from measures of the potential difference between the electrodes M and N furnish an estimation of the geoelectrical parameters of the environment, but do not indicate the resistivity and real form of the subsoil components (Samouëlian et al., 2005). Measurements are represented in the form of a pseudoview of the repartition of apparent resistivity along the profile, where the vertical axis corresponds to a pseudo-depth. After interpretation of the pseudo-view by inversion using the software Res2DInv (Geotomo Software), a view is obtained of the repartition of the subsoil's “real” resistivity in function of depth. The inversion of the data is established following an iterative process through which an attempt is made to minimize the gap between the values of the interpreted and the measured apparent resistivity. The RMS error (root-mean-square error)
quantifies this gap and evaluates the quality of the inversion at each iteration. For the set of profiles, a test of up to 6 iterations was processed to reduce the difference between measured data and the calculated model. It showed that the gain on the RMS error was generally b1% after the fourth iteration (Fig. 4). Under these conditions, it was decided to limit the number of iterations to 4 to avoid distorting real resistivity and to limit model errors (Loke, 2012). The RMS error values were found to be between 9.4 and 37%, usually higher in profiles crossing the OL and NC landfills (15 to 37%; mean at 25.8% – Fig. 4) compared to the profiles located outside of the landfills (9.4 to 22.6%; mean at 15.8%). These high values can be assigned to localized heterogeneities due to materials with a high resistivity contrast (Georgaki et al., 2008). Though expected for the landfills this is more surprising for the profiles calculated on the periphery. However, profiles with RMS errors ranging from 20 to 37% were nevertheless exploited. Finally, the data were interpolated with the software Surfer (Golden Software) in order to obtain a 2D model of repartition of “real” resistivity values in the subsoil. 3. Results and discussion The profiles highlight the presence of contrasting materials. The electrical resistivity values, in Ω m, appear in a common color scale to allow a homogeneous visualization of their distribution in the storage zone and its surroundings. The layers ranging from dark to light blue (b 20 Ω m) correspond to conductive zones, whereas the red to brown zones indicate zones of poor conductivity (N300 Ω m). 3.1. Structure, saturation and leachate circulations in the storage zone Profiles P1, P2 and P3 (NE-SW profiles) are shown in Fig. 5; the NWSE profiles (P4 to P8) are shown in Fig. 6. All these profiles illustrate the storage zone.
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Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 283,5 m; ɑ = 4,5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 283,5 m; ɑ = 4,5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 355 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 265 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 265 m; ɑ = 5 m
Characteristics (length (L); Inter-electrode gap (ɑ))
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07/2011 08/2009 07/2011 08/2009 07/2011 07/2011 04/2011 07/2011 05/2010 07/2010 07/2010 07/2010 04/2011 07/2010 07/2011 04/2011 04/2011 04/2011 04/2011 07/2010 05/2010 P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 P12 P13 P14 P15 P16 P17 P18 P19 P20 P21
Syscal R1+ Switch Lund Syscal R1+ Switch Lund Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch
Date Profile
Table 1 Characteristics of electrical profiles.
System used
Fig. 4. Change of mean RMS error for ERT profiles versus number of iterations.
3.1.1. The old landfill (OL) At the beginning of P2 (0–95 m) (Fig. 5B), poorly conductive ground (ρ N 300 Ω m) corresponds to the existing substratum (schists), according to views from drilling surveys (Belle, 2008) and the observations made while preparing the landfill area. Located from 55 to 88 m from the profile origin (NE), the least resistant layer observed from the surface (ρ ≈ 200 Ω m) is characterized by a bank of clay and shale that lies upon a poorly conductive embankment (ρ N 760 Ω m) which is also observed at the beginning of lines P1 and P3 (Fig. 5A and C). The embankment is in direct contact with an extended zone of conductivity that corresponds to the old landfill (OL) (ρ b 30 Ω m). These electrical resistivity values are in accordance with published values varying from 1.5 to about 20 Ω m (Meju, 2006) and which may reach about 40 Ω m when wastes are not saturated (Chambers et al., 2006). In the heart of the waste mass, several zones of greater conductivity (ρ b 20 Ω m) are visible, reflecting water saturation of wastes and/or the existence of leachate circulation. Based on the landfill's exploitation phases, the real depth of the OL varies from 10 to 15 m and the wastes are in direct contact with the previously scraped and denuded substratum. However, along lines P2 and P3, the conductive zone associated with the waste mass in the landfill extends below the theoretical base of the site along a thickness of 5–15 m. This observation is probably due to a deep leachate migration, the saturated substratum resistivity being reduced under the weight of the mass. In fact, seepage of heavily loaded leachates beneath the base of the landfill generates an intake of ions in the substratum, resulting in a decrease in electrical resistivity in the surrounding environment (Meju, 2006). The waste/substrate interface does not appear clearly along lines P2 and P3, whereas it is well defined along P1. Hydric conditions within the waste mass being variable over time due to the wide variation in rainfall in the area, the flow of P1 leachate may possibly be detected by reiterating the same measurements taken at other moments. An increasing resistivity gradient is observed beneath the landfill, indicating that leachate mineralization decreases with depth (at the base: ρ b 20 Ω m, and from 5 to 15 m beneath the base: ρ N 50 Ω m). In addition to leachate migration in the superficial fringe of the substratum, a deep vertical seepage N 20 m thick is visible along lines P1 and P2, from 75 to 85 m and 188 to 215 m, respectively. The mineralization of infiltrated leachates is heavier along line P2 (ρ = 5–15 Ω m) than along P1 (ρ = 10–20 Ω m). At depth, the dilution of P1 leachates is rapid. This heavy seepage zone may be explained by the presence of the juxtaposed watertight NC whose protective liner forms a hydraulic barrier preventing flow toward the SW and thus causing the water to infiltrate vertically. In addition, the drainage system installed at the foot of the OL, along with the drains below the trench, capture the leachates and halt their progression toward the SW. Line P3 does not indicate this phenomenon since it is situated solely within the OL.
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Fig. 5. P1, P2 and P3 electric profiles traced over the entire storage zone (orientation NE-SW).
Lines P4 and P5 (Fig. 6) confirm the existence of leachate migration beneath the base of the OL. In the transversal part above the OL (profile P4) (Fig. 6A), the infiltration fringe progressively deepens as it moves from the SE to the NW where it attains a thickness of approximately 10 m. The average resistivity of this zone (ρ = 20–50 Ω m) indicates the presence of leachates whose mineralization is weaker than that of the leachates collected downstream (Electrical Conductivity (EC) = 2150–3220 μS cm−1). Their mineralization, however, measured in piezometer PZ5 situated hydraulically upstream (EC = 80–134 μS cm−1), remains higher than in the groundwater which is not influenced by the landfill. The seepage zone observed on line P5 is of greater importance, as much by its thickness (N15 m) as by its leachate mineralization (ρ = 5–12 Ω m) (Fig. 6B). 3.1.2. Interface of the two cells The contact zone between the OL and the NC is covered by a layer of inert waste used to join the landfill's two cells. This material is less conductive than the other wastes and, in function of the specific profile (Fig. 5A, B), is of variable thickness and area. Profile P6 (Fig. 6C), located in the hinge area between the two cells, confirms the presence of a poorly conductive surface layer corresponding to inert waste found on the embankment separating the two cells and of a thickness apparently no N8 m. This first layer covers a conductive and extended layer attributed to leachate seepage in the substratum since, at this place, no shredded
waste lies beneath the layer of inert waste. Profile 6 does not totally intercept the deep vertical seepage zone observed on P1 and P2 (Fig. 5) since the substratum is visible at the base of the profile or therefore confirms the extremity of the leachate plume situated after the vertical infiltration zone, beyond the NW front of the OL mass. 3.1.3. The new cell (NC) Along lines P1 and P2, a conductive surface zone is observed, from 105 to 150 m and from about 215 to 240 m, respectively. It is comprised of the shredded waste in the watertight NC. Zones of greater conductivity are visible within the NC (ρ b 20 Ω m) and show saturation. On the surface, along line P1, a less conductive ground (ρ = 100–220 Ω m) corresponds to materials used for the modeling of the final cover, a layer not visible on line P2, likely due to its shallow depth. At the SW extremity of the new cell, the slightly conductive zone found on the surface (ρ N 200 Ω m) corresponds to bulky waste which shows a higher resistivity than deeper waste; this may be due to the high proportion of nonshredded plastics with a strong insulating power. In addition to the electrical resistivity varying inversely with water content (Grellier et al., 2007), the high resistivity values of bulky waste may indicate a low water content. On line P1, it appears that substratum contamination takes place beneath the new cell whereas it is not visible on line P2. This will be addressed again below. At the base of the cell, on line P2, resistivity
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Fig. 6. P4, P5 (traced over the old landfill: orientation NW-SE), P6 (located at the interface between the old landfill and the new cell), P7 and P8 (located in the new cell) profiles traced over the old landfill (orientation NW-SE).
increases (ρ = 50–100 Ω m), reflecting the efficacy of the base draining layer which progressively evacuates the leachates. The substratum is visible a few meters below the base of the new cell. At the end of the profile line the NC is bordered by a zone of low conductivity (ρ N 760 Ω m) corresponding to a wall of compacted schist. Line P3 does not show the NC since it passes a few meters distant. Lines P7 (Fig. 6D) and P8 (Fig. 6E) were traced perpendicularly to the previous profiles. On the surface, a conductive zone was observed for both profiles, whose length corresponds to that of the new cell. Humidity distribution within the waste was homogeneous along P7 while more heterogeneous along P8. This is closely related to the type of waste buried in these places. In fact, along line P7 the waste is essentially shredded household refuse which allows good leachate circulation, whereas along line P8 a high proportion of bulky waste with a weak
capacity for water retention was discharged among the shredded household refuse that is easily saturated with water. To the SSE (P7) and SE (P8), the conductive zone is about 8 m thick which corresponds to the height of the buried waste. In contrast, a deepening of this zone is observed in the NNW (P7) and NW (P8) directions which may reach a depth of 20 m. Given that the waste was discharged at a nearly uniform height throughout the landfill (6–8 m) and that the waste/substratum interface is defined by an active security barrier at the bottom of the cell (geotextiles, Aleya et al., 2007), this extra thickness is abnormal. Beneath the cell there is a drainage system for its circulating waters, found only in the cell's western part where this supplementary thickness is observed. A failure in the sub-cell water drainage must thus be considered, with seepage of these waters, drained and polluted by leachates, through a depth of several meters.
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Fig. 7. P9 (located to the northeast of the landfill), P10 and P11 (located to the northwest of the landfill), P12 and P13 (traced to the west of the landfill) profiles.
This infiltration may also be present on line P1 which crosses the landfill in the same direction from the NE to the SW (Fig. 5) through the eastern part of the cell. Yet, the absence of the extra thickness in the eastern part, where no drainage network exists, neither for P7 or P8 (under which a drain is located), nor for P2 as illustrated in Fig. 3, appears to indicate that the thickness is closely related to the drainage system and thus supports the hypothesis of a system failure. These interpretations are nevertheless to be considered with caution since the non-conductive geomembrane at the bottom of the cell is a disturbing factor for good circulation in the underlying zone. In the preceding study, Belle (2008) considered the presence of a deep, vertical leachate infiltration well in the center of the NC. Yet the investigation depth along the profile lines of that study does not exceed Fig. 8. Profiles traced in the lagoons (L1–L4) area, to the southwest of the landfill.
8 m, which is to say the maximal theoretical base of the NC's wastes, and does not allow visualization of the ground beneath the new cell. In contrast, along lines P7 and P8 in the present study, the investigation depth does reach the localized substratum underlying the new cell. These two profiles, along with P1 (Fig. 5) underscore the temporary vertical seepage through a thickness of several meters which appears to be generalized under the entire drainage system. Along line P8, the border between the NC and the substratum is better defined. However, two deep and highly conductive zones are observed at depths of 40–50 m and 70–80 m. A topographical map drawn in 2006 shows that the positions of these two zones correspond, respectively, to a shaft of perforated concrete connecting the surface to the base of the cell, and to the multi-collector which gathers the
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leachates from the old landfill, that is to say both the seepage from the NC and the waters circulating beneath it. Two leaks may exist at these places, suggesting a failure of the geomembrane whose installation in sanitary facilities is always difficult. The deficiencies of geomembrane liners placed at the bottom of landfills are frequent, with as many as 17 failures per hectare (Giroud and Bonaparte, 1989). Even if the liners are of excellent quality, one or two leaks per hectare is inevitable (Katsumi et al., 2001). 3.2. Spatial follow up of leachates around the storage zone The preceding profiles confirm the existence of a pollution plume beneath the storage zone. To determine its extent, 13 complementary profiles, noted from P9 to P21 (Figs. 7, 8 and 9) were traced outside of the landfill influence zone: in the northeast (P9), the northwest (P10 and P11), in the west (P12 and P13), the southwest (P14 to P19) and in the southeast (P20 and P21). 3.2.1. Northeastern part Line P9 (Fig. 7A) indicates the presence of discontinuous flows encountered from 0 to 4 m in the SE, and then beyond from 8 m for the first encounter toward the NW. The substratum appears to be fragmented into a vast network of faults. This may produce the discontinuous flows that have been observed. The subvertical schistosity in the Etueffont schist (Belle, 2008) may also be at the origin of these discontinuous flows. Resistivity values ranging from 50 to 300 Ω m correspond to weakly mineralized waters. These results are in accordance with the conductivity values measured in piezometer PZ22 (CE = 32– 150 μS cm−1, piezometric level at 4.5 m), situated a few meters downstream from the profile line and in piezometer PZ5 placed 50 m upstream (CE = 80–134 μS cm−1). Piezometer PZ22 alone is reported by projection in the view because PZ5 was too far. The less conductive ground (ρ N 900 Ω m−1) corresponds to substratum schist. No leachate migration was observed NE of the site. 3.2.2. Northwestern part On line P10 (Fig. 7B), the poorly conductive ground (ρ N 760 Ω m−1) corresponds to dry substratum. A conductive layer is visible underneath, indicating that the substratum is saturated with water which is confirmed by its presence in piezometers PZ7 and PZ70 situated 5 m downstream from the profile. In the NE two concentric zones appear whose resistivity diminishes toward the center, reaching a value in the area of 14 Ω m. These zones correspond to more highly mineralized waters than those found in PZ7 and PZ70 located 5 m from P10. This may reflect the presence or lateral infiltration of diluted leachates. The zones are no longer visible in P11 located 50 m upstream from P10. To the NW the leachate plume is of limited reach. We infer diffusion-related discontinuities (faults, fractures or shear zones) of the substratum as illustrated in profiles 10, 11, 12 and 13 reflecting the secondary fracture network directed N135–N155 (see 2.1), in addition to the main N255–N260 schistosity. 3.2.3. Western part Profile P12 (Fig. 7D), approximately 50 m from the storage zone, presents a weakly conductive substratum (ρ N 760 Ω m), covered on the surface by a layer of more conductive soils no thicker than 7 m. No geological reconnaissance was carried out at this place, but other wells drilled in the sector may lead one to suppose that this conductive horizon corresponds to shear gouge mixed with schist rock or heavily fractured schists. In fact, altered and fractured zones present weaker resistivity than the healthy substratum due to increased porosity and a high shear gouge content (Matsui et al., 2000; Casado et al., 2015). In the eastern part of line P12, a concentric zone more highly mineralized than the groundwater crosses the substratum at a depth of ten meters. It indicates the presence of a leachate plume which generally follows the N270, in approximate agreement with the schistosity
direction (N255–260). The resistivity growing from the center toward the exterior shows a radial dilution and proves that its extent is limited. This contaminated zone therefore corresponds to a diverticulation of the plume which is guided by foliation and the possibly fractured zones of the substratum. The leachate plume is not located on profile P13 (Fig. 7E) and does not appear to extend westward. However, the absence of plume to the west does not mean that leachate progression is interrupted after profile P12. The discontinuities of the substratum may distribute a diluted plume at a depth below the profile. The groundwater piezometry as seen in Fig. 2 indicates that the distribution of contaminant to the NW may extend over a long distance since the local flow indicates a N-S direction in this sector. 3.2.4. Southwestern part The 6 profile lines, P14 to P19, traced downstream from the storage zone at the outlet of each of the lagooning ponds (Fig. 8), highlight the contrast between these very different zones: groundwater circulation (ρ b 300 Ω m) and the non-saturated substratum (ρ N 760 Ω m). Lagooning ponds are generally located along the axis of a thalweg. Their influences have been reported as projections along the profile lines. A zone more conductive than the sector's groundwater is visible in the projected influences of the ponds in all the profiles. This demonstrates a surface migration of leachates beyond the ponds. Their zones of influence present a progressively increasing resistivity from pond 1 to pond 4, ρ = 25–50 Ω m and ρ = 60–110 Ω m, respectively. This gradient is the consequence of leachate treatment in which mineralization decreases from pond to pond, according to measurements of electrical conductivity taken in each pond. Yet, these filtered lagooning waters are passably less mineralized (by a factor of 5–10) than the waters undergoing treatment. Only a slight diffusion of highly diluted leachates through the layer of compacted clay is detected. At depth the groundwater appears to drain following the thalweg axis. Along the P14 profile line (Fig. 8A), situated between the new cell and pond 1, the two small conductive zones in the site's northwestern part probably correspond to slight pollution from the old landfill. This anomaly, underscored on profile line P3 130–140 m distant (Fig. 9A), indicates an east-to-west plume migration. 3.2.5. Southeastern part As seen above for P3 (Fig. 9A), in the southwestern part of the storage zone, leachates from the old landfill infiltrate at the base through a thickness of N10 m, diluting as they seep downwards. Lines P20 and P21 traced in the southeastern part of the site, beyond the influence of the landfill, are presented in Fig. 9B and C. Profile P20 (Fig. 9B), situated 30 m from the landfill's influence zone, shows poorly conductive land on the surface which corresponds to the schist present. A zone of underlying conductivity is visible. The southwestern part of the zone (P21) (Fig. 9C) presents resistivity values near those of the groundwater readings taken nearby in PZ4 (ρzone = 100–250 Ω m; ρwater PZ4 ≈ 190 Ω m). This contamination may be from south of the OL and is not entirely consistent with the piezometry. The southward spread of the contaminants is probably limited by the NE-SW flow of regional groundwater (Fig. 2). In the heart of the northeastern part, the zone includes a more conductive fringe (ρ = 20–50 Ω m), no doubt corresponding to diluted leachates from the old landfill, that spreads horizontally through the schists. At depth, leachates stagnate and do not migrate toward the west-southwest. Within the thirty meters that separate the two profiles, leachate dilution takes place along a horizontal gradient, probably favored by the flow from the water table in the west-east direction as emphasized by Belle (2008). Profile P21, 40 m distant from P20, shows a discontinuous flow of water at varying depths, and a resistivity comparable to that of groundwater as in the SW part of profile 20. The extent of the pollution plume is limited to the SE part of the site since the plume is no longer visible at
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Fig. 9. Profiles traced to the southeast of the landfill.
Fig. 10. Visualization of anomalies detected with ERT. The red lines correspond to the profile sectors exhibiting a leachate plume beneath the waste storage zone. The global extension of the pollution plume is expressed in ochre-yellow. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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a distance of 30 m from the storage zone or is highly diluted in the groundwater. 3.3. Extension of the leachate plume The anomalies of low resistivity observed beneath the base and surrounding the storage zone, representing a leachate plume that migrates within the substratum, are shown in Fig. 10, as deduced from our results. The contamination originates mainly in the OL. Since the water table ceiling is at nearly the same level as the base of the waste mass, the groundwater is polluted through leachate infiltration. However, at depth a rising gradient of resistivity values is observed, indicating a progressive vertical dilution in leachate mineralization. Consequently, plume extension is limited, both at depth and laterally, as no pollution is visible even a few dozen meters distant from the storage zone. Concerning the new cell, its underlying drainage network may have failed. Seepage of concentrated leachates, however, remains moderate (5 m thick). In the NC, too, rapid vertical dilution takes place. 4. Conclusion Twenty-one 2D ERT profiles were obtained from the storage zone of the Etueffont landfill and its periphery, allowing sufficient depth for adequate investigation (from 10 to 40 m) to recognize the substratum under the landfill, while maintaining a correct lateral resolution. Synthesis of the results leads to the following diagnosis: - The landfill's known features, easily identified along the profile lines, were confirmed. The NC, however, created in accordance with more stringent environmental standards than the OL, reveals a failure of its sealing barrier and/or its drainage system. - Flow and saturation phenomena within the waste mass are visualized. - Leachate infiltrations into the substratum are highlighted beneath the storage zone, the main plume originating in the OL. - At depth, a rising gradient of resistivity values is observed, indicating the dilution effect and a progressive vertical leachate mineralization. - Geologically, the ERT profiles revealed a highly fractured substratum traversed by numerous discontinuous groundwater flows. This is a change from the vision obtained through drilling. Any attempt to understand flow phenomena within the schists is complex, but the spatial distribution of the leachate plume appears consistent with the main directions of fractures and the hydraulic gradient. - The plume extension is limited, both at depth and laterally, as no pollution is visible even a few dozen meters distant from the storage zone.
The Etueffont landfill is geographically complex. The realization of 21 profiles required several survey sessions spread over as many days. Water saturation conditions were not completely homogeneous. If we are to understand the migration routes of contaminants and the effects of leachate dilution and mineralization and further study will be required. Indeed, in this context the 2D ERT and the electrical array selected for this study present certain limitations. The high RMS error values calculated for most profiles traced over the storage zone or in the peripheral area introduce only an approximation of the geoelectrical model. Corrections and adjustments may be possible by testing electrical arrays other than the Wenner-Schlumberger and/or by increasing the number of measurements for each quadripole. To reduce any uncertainty, it would be advisable to conduct 3D ERT with multi-channel equipment so as to illustrate the nature of tracer plumes and thus aid in mitigating and controlling contaminating events.
However, 2D ERT has shown itself to be well adapted in determining the extent of the polluting plume and also in characterizing leachate biostabilization (10–50 Ω m). The technique may thus be a useful preliminary step toward the goal of better overall monitoring, if used in association with a network of piezometers.
Acknowledgements This study is part of the Phd research of Elise Grisey. The authors thank J. Mudry, H. Grisey and M. Grapin for their help. The authors gratefully acknowledge the financial support of the ADEME (Agency for the Environment and Energy Management), the SICTOM (Solid Waste Management Service) of Etueffont, Territoire de Belfort, France and the CNRS (National Center for Scientific Research) which made this investigation possible. We also thank the reviewers for their helpful suggestions and corrections.
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Engineering Geology journal homepage: www.elsevier.com/locate/enggeo
Spatial characterization of leachate plume using electrical resistivity tomography in a landfill composed of old and new cells (Belfort, France) Vincent Bichet a, Elise Grisey b, Lotfi Aleya a,⁎ a b
Laboratoire de Chrono-Environnement, UMR CNRS 6249, Université de Bourgogne, Franche-Comté, France Atelier d'Écologie Urbaine, 9 avenue Philippe Auguste, 75011 Paris, France
a r t i c l e
i n f o
Article history: Received 1 December 2015 Received in revised form 20 June 2016 Accepted 29 June 2016 Available online xxxx Keywords: Landfill Pollution plume Leachates Electrical resistivity tomography
a b s t r a c t Located near Belfort (France), the Etueffont landfill was in operation from 1976 to 2002 for the disposal and storage of domestic waste produced by 47,650 inhabitants. The site is comprised of the original landfill site called the old landfill (OL), in operation from 1976 to 1999, and a newer section known as the new cell (NC) which operated from 1999 to 2002. The objective of this study is to determine, using electrical resistivity tomography (ERT), the extent of the leachate plume from the OL and to monitor the efficiency of the liner of the NC. The entire Etueffont site was crisscrossed with 21 electrical profile lines which were traced in summer between 2009 and 2011 during a period of dry weather. This investigation allowed visualization of the flow and saturation phenomena within the waste without destructive on-site intervention, and also of leachate infiltration into the substratum. The anomalies of low resistivity extending beneath the base of the OL represent a leachate plume that migrates within the substratum. As the water table ceiling is found at nearly the same level as the base of the waste, leachate infiltration is a source of groundwater pollution. A rising gradient in resistivity values is observed at depth indicating progressive vertical dilution in leachate mineralization. Pollution plume extension is limited both at depth and laterally as it is no longer visible a few dozen meters from the storage zone. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The diagnosis and monitoring of polluted sites is currently a major concern. Landfills contain a variable quantity of toxic materials and comprise a risk for human beings and their environment (Bernard et al., 1997; Ben Salem et al., 2014a, 2014b). Former landfills, established at a time when the knowledge of the risks involved in waste storage was nearly non-existent, do not possess the geological or artificial barriers that isolate wastes from the natural environment (Kallis and Buder, 2001; Aleya et al., 2007; Gibbons et al., 2014). Leachate seepage found at the base of landfill sites is a source of soil and groundwater contamination (Grisey et al., 2010; Weber et al., 2011). In addition, not all of these landfills have been inspected; storage zone limits and the type and volume of wastes sometimes remain unknown (Butt et al., 2014). Even in well-monitored landfills, geometry and depth may differ from the information appearing in the plans submitted for inspection of classified sites when the operation permit application is initially filed (Meju, 2006). However, in recent landfills respecting updated rules for technical security (Decree of September 9, 1997, amended) risks have been minimized (Grisey and Aleya, 2016a). Conventional methods for evaluating landfill-related groundwater contamination bring together such topics as reconnaissance drilling, ⁎ Corresponding author. E-mail address: lotfi[email protected] (L. Aleya).
http://dx.doi.org/10.1016/j.enggeo.2016.06.026 0013-7952/© 2016 Elsevier B.V. All rights reserved.
water sampling and chemical analysis (Frid et al., 2008). Though water samples furnish precise information as to the type of contaminants present (Vaudelet et al., 2011), drilling remains costly and invasive (Radulescu et al., 2007) and may sometimes miss the contaminated zone due to poor deployment (Vaudelet et al., 2011). Less costly and non-invasive geophysical prospecting techniques have been developed to study subsoil structures without disturbing them. They are now frequent tools for site investigation and can be used to detect pollution, to describe geological features, to diagnose engineering structures and also for hydrogeological studies (Chambers et al., 2006; Giusti, 2009). Electrical resistivity tomography (ERT), which is based on the measure of the electrical resistivity ρ of subsurface materials, provides information on the properties of the underground environment such as thickness of a layer and its saturation, depth of the substratum, localization and distribution of conducting fluids (leachates), and localization and orientation of fractures and faults (Soupios et al., 2007). Electrical resistivity or inversely, electrical conductivity of wastes, depend on several of the waste mass properties such as porosity, connectivity of pores, water content, the ionic force of leachates and temperature (Imhoff et al., 2007). Thus, ground not saturated with water will be less conductive than if saturated; electrical resistivity diminishes as water mineralization increases. ERT is particularly well adapted to the study of landfills in order to determine their geometry and internal structure (Depountis et al., 2005; Chambers et al., 2006; Meju, 2006; Soupios et al., 2007; Wilkinson et al., 2010), and to detect
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leachate contamination of the substratum (Acworth and Jorstad, 2006; Park et al., 2008; Abdullahi et al., 2010). This technique has some drawbacks that make its features difficult to interpret. One such drawback is that the reduction in resolution as depth increases generates imprecision at deeper levels. In addition, due to the preferential circulation of current in conductive zones, the presence of a non-conductive lining at the bottom of a landfill cell may obstruct flow toward the zone situated below. In a previous investigation via electrical resistivity tomography at the Etueffont site, Belle (2008) brought to light a vertical seepage leachate zone at the base of the OL and a leachate plume migration toward the west, in the direction N325–335. The same author also hypothesized the existence of a leachate leak in the new water-proof cell. For this study, investigation was pursued beyond the limits of the storage zone so as to determine the extent of the leachate plume from the former landfill and to monitor the efficiency of the NC lining. Several profiles have also been traced over the storage zone to verify whether electrical resistivity tomography can appreciate the internal structure of the two cells. 2. Materials and methods 2.1. Study site The Etueffont landfill, located in northeastern France (N 47°43′19″ / E 6°56′57), covers a 2.8 ha surface and was in operation from 1976 to 2002 (Fig. 1A). The site is located in the Permian basin of Giromagny, with the landfill on an eroded horst formed by Devono-Dinantian schists (silt and sandstone). The southeastern part of the horst is bordered by a NE–SW fault, which brings the schists into contact with the
Permian formations (Fig. 1B). The schists dip at a high angle (N75°) and at some places are nearly vertical with an average direction of N75–N80. The NE to SW direction of the groundwater flow was determined by means of a network of piezometers. Two perpendicular fracture families (N40–N70/subvertical and N135–N155/75–80 W) may influence the flow (Fig. 1B). The structural elements of the site show groundwater isopieze curves in a low water period of the water table from 1997 to 2009 and at a mean altitude, indicating that groundwater circulates toward the SW following the stream axis that receives the water from the fill (Fig. 2). During low-water periods the hydraulic gradient averages 7%, indicating a weak permeability of the geological substratum (10−9 b K b 10−6 m s−1). The landfill received municipal solid waste, bulky waste and construction and demolition (C&D) waste. Altogether, approximately 305,000 tons of waste were deposited into two cells, with a waste depth ranging from 6 to 15 m. The first cell, known as the old landfill (OL), was in operation from 1976 to 1998 and was directly established on impermeable schist. The new cell (NC) was in operation from 1998 to 2002 and was equipped with a watertight polyethylene bottom liner, surrounded on either side by a geotextile mat. The Etueffont landfill's mode of operation was non-conventional. Shredded waste from both cells was successively deposited without compaction in 1 m-thick layers after a 2–3-month period of biostabilization between each layer. No alternative daily cover was used and the degradation that did take place was mainly aerobic, which promoted the production of leachate and minimized the levels of methane gas in the landfill (ADEME, 2005). After the landfill closed, the waste was covered by a 0.8 m-thick topsoil layer, without compaction. Considering the low pollution potential of this landfill, an impermeable cover was not required (ADEME, 2005).
Fig. 1. A: Location of the Etueffont landfill with the old landfill (OL) and the new cell (NC), B: Geological map and profile.
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Fig. 2. Structural elements of the site showing groundwater isopieze curves in low water period, at a mean altitude in the water table over 1997–2009.
Leachate that was generated in each cell drained back into a shared leachate collection system, which was then discharged into a natural lagooning treatment system (Khattabi and Aleya, 2007; Ben Salem et al., 2014a, 2014b) (Fig. 2). Inflow was generally adjusted to 59.4 m3 day−1, with a theoretical retention time of approximately 19 days. At the lagooning system's outlet, the treated leachate was released into a brook, the “Gros Près”. The Etueffont site is in the methanogenic phase (Kjeldsen et al., 2002; Claret et al., 2011) as demonstrated by Aleya et al. (2007) and Grisey and Aleya (2016a), illustrating that the site has already achieved advanced stable conditions, i.e. with pH reaching neutral values, methane production and a decreased release of organic and inorganic matter. Leachate and groundwater was chemically characterized for both the old landfill (1989–2010) and the new cell (2000 − 2010), and water electrical conductivity (EC) was measured. The main results were published in Grisey and Aleya (2016b). The EC values of groundwater and leachate cited below were not collected during the geophysical survey but correspond to mean values previously published (Grisey and Aleya, 2016b). 2.2. Geophysical survey The entire Etueffont site was investigated using 2D electrical resistivity tomography (ERT). The area was crisscrossed with 21 electrical profile lines, noted P1 to P21, whose positions are reported in Fig. 3. The Etueffont landfill is a controlled site whose structure is well known. To identify potential contamination beneath the cells, eight profiles (P1 to P8, NE/SW and NW/SE) were traced. Three parallel profile
lines were traced, crossing the storage zone from the NE to the SW, the first along the NW flank (P1), the second within the landfill environment (P2) and the third along the SE flank (P3). Five profile lines oriented NW to SE—P4 and P5 in the OL, P6 at the interface of the two cells, and P7 and P8 in the NC—were traced nearly perpendicularly to the three previous ones. To identify potential contamination surrounding the site, 6 hydraulic profiles (P14 to P19) and 7 lateral profiles were traced (Fig. 3) during a dry period (Table 1) and in sunny weather. The measurements were all taken the day following a light rainy period, which allowed for a better connection between ground and electrodes and favored better propagation of the electrical current. 2.3. Acquisition of data The geoelectrical data were collected via two systems of multi-electrode measurement: the ABEM Terrameter Lund Imaging system (400 V; 100 W; 1 A) and the SYSCAL R1 + Switch-72 (600 V; 200 W; 2.5 A) produced by Iris Instruments. Each prospecting system was equipped with a resistivity meter connected to a cable with multiple outlets connected to stainless steel electrodes planted in the ground at regular intervals (4.5 m for P2 and P4 profiles; 5 m for the others). The resistivity meter automatically exploits, according to a preprogramed sequence, all possible positions of transmission and reception on these electrodes in order to cover the study zone horizontally and vertically. The electrical measurements were taken according to a Wenner-Schlumberger configuration (a hybrid of the Wenner and Schlumberger device) along a rectilinear profile. This device is moderately sensitive to both lateral and depth variations in resistivity and is
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Fig. 3. Site topography with location of electric profiles (black lines), distinguishing those in the storage zone (yellow circles) from those in the surrounding area (white circles). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
a good compromise between the Wenner device (sensitive to horizontal structures) and the dipole-dipole (sensitive to vertical structures) (Loke, 2012). Thus, the Wenner-Schlumberger is particularly well adapted to grounds such as landfills that present both horizontal and vertical structures. The characteristics of each profile are presented in Table 1. Concerning the acquisition set up, the injection time per cycle was 500 ms and the registered value corresponded to a stack of from 3 to 6 measurements with a maximum admitted error of 3%. The minimal tension requested for a valid measurement was 50 mV at the potential dipole. At least 16 depth levels were prospected and, depending on the length of the profiles, from 600 to 925 quadripoles were measured. Finally, topography profiles and electrode locations were measured at a given station to provide geometric correction for apparent resistivity calculation. 2.4. Treatment and modeling of data Acquired data were filtered to eliminate negative values and values with a standard deviation N1% for both measurements at the same point. For each profile b 10% of the values were rejected. The apparent resistivity values calculated from measures of the potential difference between the electrodes M and N furnish an estimation of the geoelectrical parameters of the environment, but do not indicate the resistivity and real form of the subsoil components (Samouëlian et al., 2005). Measurements are represented in the form of a pseudoview of the repartition of apparent resistivity along the profile, where the vertical axis corresponds to a pseudo-depth. After interpretation of the pseudo-view by inversion using the software Res2DInv (Geotomo Software), a view is obtained of the repartition of the subsoil's “real” resistivity in function of depth. The inversion of the data is established following an iterative process through which an attempt is made to minimize the gap between the values of the interpreted and the measured apparent resistivity. The RMS error (root-mean-square error)
quantifies this gap and evaluates the quality of the inversion at each iteration. For the set of profiles, a test of up to 6 iterations was processed to reduce the difference between measured data and the calculated model. It showed that the gain on the RMS error was generally b1% after the fourth iteration (Fig. 4). Under these conditions, it was decided to limit the number of iterations to 4 to avoid distorting real resistivity and to limit model errors (Loke, 2012). The RMS error values were found to be between 9.4 and 37%, usually higher in profiles crossing the OL and NC landfills (15 to 37%; mean at 25.8% – Fig. 4) compared to the profiles located outside of the landfills (9.4 to 22.6%; mean at 15.8%). These high values can be assigned to localized heterogeneities due to materials with a high resistivity contrast (Georgaki et al., 2008). Though expected for the landfills this is more surprising for the profiles calculated on the periphery. However, profiles with RMS errors ranging from 20 to 37% were nevertheless exploited. Finally, the data were interpolated with the software Surfer (Golden Software) in order to obtain a 2D model of repartition of “real” resistivity values in the subsoil. 3. Results and discussion The profiles highlight the presence of contrasting materials. The electrical resistivity values, in Ω m, appear in a common color scale to allow a homogeneous visualization of their distribution in the storage zone and its surroundings. The layers ranging from dark to light blue (b 20 Ω m) correspond to conductive zones, whereas the red to brown zones indicate zones of poor conductivity (N300 Ω m). 3.1. Structure, saturation and leachate circulations in the storage zone Profiles P1, P2 and P3 (NE-SW profiles) are shown in Fig. 5; the NWSE profiles (P4 to P8) are shown in Fig. 6. All these profiles illustrate the storage zone.
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Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 283,5 m; ɑ = 4,5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 283,5 m; ɑ = 4,5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 355 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 265 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 175 m; ɑ = 5 m Wenner-Schlumberger; L = 265 m; ɑ = 5 m
Characteristics (length (L); Inter-electrode gap (ɑ))
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07/2011 08/2009 07/2011 08/2009 07/2011 07/2011 04/2011 07/2011 05/2010 07/2010 07/2010 07/2010 04/2011 07/2010 07/2011 04/2011 04/2011 04/2011 04/2011 07/2010 05/2010 P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 P12 P13 P14 P15 P16 P17 P18 P19 P20 P21
Syscal R1+ Switch Lund Syscal R1+ Switch Lund Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch Syscal R1+ Switch
Date Profile
Table 1 Characteristics of electrical profiles.
System used
Fig. 4. Change of mean RMS error for ERT profiles versus number of iterations.
3.1.1. The old landfill (OL) At the beginning of P2 (0–95 m) (Fig. 5B), poorly conductive ground (ρ N 300 Ω m) corresponds to the existing substratum (schists), according to views from drilling surveys (Belle, 2008) and the observations made while preparing the landfill area. Located from 55 to 88 m from the profile origin (NE), the least resistant layer observed from the surface (ρ ≈ 200 Ω m) is characterized by a bank of clay and shale that lies upon a poorly conductive embankment (ρ N 760 Ω m) which is also observed at the beginning of lines P1 and P3 (Fig. 5A and C). The embankment is in direct contact with an extended zone of conductivity that corresponds to the old landfill (OL) (ρ b 30 Ω m). These electrical resistivity values are in accordance with published values varying from 1.5 to about 20 Ω m (Meju, 2006) and which may reach about 40 Ω m when wastes are not saturated (Chambers et al., 2006). In the heart of the waste mass, several zones of greater conductivity (ρ b 20 Ω m) are visible, reflecting water saturation of wastes and/or the existence of leachate circulation. Based on the landfill's exploitation phases, the real depth of the OL varies from 10 to 15 m and the wastes are in direct contact with the previously scraped and denuded substratum. However, along lines P2 and P3, the conductive zone associated with the waste mass in the landfill extends below the theoretical base of the site along a thickness of 5–15 m. This observation is probably due to a deep leachate migration, the saturated substratum resistivity being reduced under the weight of the mass. In fact, seepage of heavily loaded leachates beneath the base of the landfill generates an intake of ions in the substratum, resulting in a decrease in electrical resistivity in the surrounding environment (Meju, 2006). The waste/substrate interface does not appear clearly along lines P2 and P3, whereas it is well defined along P1. Hydric conditions within the waste mass being variable over time due to the wide variation in rainfall in the area, the flow of P1 leachate may possibly be detected by reiterating the same measurements taken at other moments. An increasing resistivity gradient is observed beneath the landfill, indicating that leachate mineralization decreases with depth (at the base: ρ b 20 Ω m, and from 5 to 15 m beneath the base: ρ N 50 Ω m). In addition to leachate migration in the superficial fringe of the substratum, a deep vertical seepage N 20 m thick is visible along lines P1 and P2, from 75 to 85 m and 188 to 215 m, respectively. The mineralization of infiltrated leachates is heavier along line P2 (ρ = 5–15 Ω m) than along P1 (ρ = 10–20 Ω m). At depth, the dilution of P1 leachates is rapid. This heavy seepage zone may be explained by the presence of the juxtaposed watertight NC whose protective liner forms a hydraulic barrier preventing flow toward the SW and thus causing the water to infiltrate vertically. In addition, the drainage system installed at the foot of the OL, along with the drains below the trench, capture the leachates and halt their progression toward the SW. Line P3 does not indicate this phenomenon since it is situated solely within the OL.
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Fig. 5. P1, P2 and P3 electric profiles traced over the entire storage zone (orientation NE-SW).
Lines P4 and P5 (Fig. 6) confirm the existence of leachate migration beneath the base of the OL. In the transversal part above the OL (profile P4) (Fig. 6A), the infiltration fringe progressively deepens as it moves from the SE to the NW where it attains a thickness of approximately 10 m. The average resistivity of this zone (ρ = 20–50 Ω m) indicates the presence of leachates whose mineralization is weaker than that of the leachates collected downstream (Electrical Conductivity (EC) = 2150–3220 μS cm−1). Their mineralization, however, measured in piezometer PZ5 situated hydraulically upstream (EC = 80–134 μS cm−1), remains higher than in the groundwater which is not influenced by the landfill. The seepage zone observed on line P5 is of greater importance, as much by its thickness (N15 m) as by its leachate mineralization (ρ = 5–12 Ω m) (Fig. 6B). 3.1.2. Interface of the two cells The contact zone between the OL and the NC is covered by a layer of inert waste used to join the landfill's two cells. This material is less conductive than the other wastes and, in function of the specific profile (Fig. 5A, B), is of variable thickness and area. Profile P6 (Fig. 6C), located in the hinge area between the two cells, confirms the presence of a poorly conductive surface layer corresponding to inert waste found on the embankment separating the two cells and of a thickness apparently no N8 m. This first layer covers a conductive and extended layer attributed to leachate seepage in the substratum since, at this place, no shredded
waste lies beneath the layer of inert waste. Profile 6 does not totally intercept the deep vertical seepage zone observed on P1 and P2 (Fig. 5) since the substratum is visible at the base of the profile or therefore confirms the extremity of the leachate plume situated after the vertical infiltration zone, beyond the NW front of the OL mass. 3.1.3. The new cell (NC) Along lines P1 and P2, a conductive surface zone is observed, from 105 to 150 m and from about 215 to 240 m, respectively. It is comprised of the shredded waste in the watertight NC. Zones of greater conductivity are visible within the NC (ρ b 20 Ω m) and show saturation. On the surface, along line P1, a less conductive ground (ρ = 100–220 Ω m) corresponds to materials used for the modeling of the final cover, a layer not visible on line P2, likely due to its shallow depth. At the SW extremity of the new cell, the slightly conductive zone found on the surface (ρ N 200 Ω m) corresponds to bulky waste which shows a higher resistivity than deeper waste; this may be due to the high proportion of nonshredded plastics with a strong insulating power. In addition to the electrical resistivity varying inversely with water content (Grellier et al., 2007), the high resistivity values of bulky waste may indicate a low water content. On line P1, it appears that substratum contamination takes place beneath the new cell whereas it is not visible on line P2. This will be addressed again below. At the base of the cell, on line P2, resistivity
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Fig. 6. P4, P5 (traced over the old landfill: orientation NW-SE), P6 (located at the interface between the old landfill and the new cell), P7 and P8 (located in the new cell) profiles traced over the old landfill (orientation NW-SE).
increases (ρ = 50–100 Ω m), reflecting the efficacy of the base draining layer which progressively evacuates the leachates. The substratum is visible a few meters below the base of the new cell. At the end of the profile line the NC is bordered by a zone of low conductivity (ρ N 760 Ω m) corresponding to a wall of compacted schist. Line P3 does not show the NC since it passes a few meters distant. Lines P7 (Fig. 6D) and P8 (Fig. 6E) were traced perpendicularly to the previous profiles. On the surface, a conductive zone was observed for both profiles, whose length corresponds to that of the new cell. Humidity distribution within the waste was homogeneous along P7 while more heterogeneous along P8. This is closely related to the type of waste buried in these places. In fact, along line P7 the waste is essentially shredded household refuse which allows good leachate circulation, whereas along line P8 a high proportion of bulky waste with a weak
capacity for water retention was discharged among the shredded household refuse that is easily saturated with water. To the SSE (P7) and SE (P8), the conductive zone is about 8 m thick which corresponds to the height of the buried waste. In contrast, a deepening of this zone is observed in the NNW (P7) and NW (P8) directions which may reach a depth of 20 m. Given that the waste was discharged at a nearly uniform height throughout the landfill (6–8 m) and that the waste/substratum interface is defined by an active security barrier at the bottom of the cell (geotextiles, Aleya et al., 2007), this extra thickness is abnormal. Beneath the cell there is a drainage system for its circulating waters, found only in the cell's western part where this supplementary thickness is observed. A failure in the sub-cell water drainage must thus be considered, with seepage of these waters, drained and polluted by leachates, through a depth of several meters.
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Fig. 7. P9 (located to the northeast of the landfill), P10 and P11 (located to the northwest of the landfill), P12 and P13 (traced to the west of the landfill) profiles.
This infiltration may also be present on line P1 which crosses the landfill in the same direction from the NE to the SW (Fig. 5) through the eastern part of the cell. Yet, the absence of the extra thickness in the eastern part, where no drainage network exists, neither for P7 or P8 (under which a drain is located), nor for P2 as illustrated in Fig. 3, appears to indicate that the thickness is closely related to the drainage system and thus supports the hypothesis of a system failure. These interpretations are nevertheless to be considered with caution since the non-conductive geomembrane at the bottom of the cell is a disturbing factor for good circulation in the underlying zone. In the preceding study, Belle (2008) considered the presence of a deep, vertical leachate infiltration well in the center of the NC. Yet the investigation depth along the profile lines of that study does not exceed Fig. 8. Profiles traced in the lagoons (L1–L4) area, to the southwest of the landfill.
8 m, which is to say the maximal theoretical base of the NC's wastes, and does not allow visualization of the ground beneath the new cell. In contrast, along lines P7 and P8 in the present study, the investigation depth does reach the localized substratum underlying the new cell. These two profiles, along with P1 (Fig. 5) underscore the temporary vertical seepage through a thickness of several meters which appears to be generalized under the entire drainage system. Along line P8, the border between the NC and the substratum is better defined. However, two deep and highly conductive zones are observed at depths of 40–50 m and 70–80 m. A topographical map drawn in 2006 shows that the positions of these two zones correspond, respectively, to a shaft of perforated concrete connecting the surface to the base of the cell, and to the multi-collector which gathers the
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leachates from the old landfill, that is to say both the seepage from the NC and the waters circulating beneath it. Two leaks may exist at these places, suggesting a failure of the geomembrane whose installation in sanitary facilities is always difficult. The deficiencies of geomembrane liners placed at the bottom of landfills are frequent, with as many as 17 failures per hectare (Giroud and Bonaparte, 1989). Even if the liners are of excellent quality, one or two leaks per hectare is inevitable (Katsumi et al., 2001). 3.2. Spatial follow up of leachates around the storage zone The preceding profiles confirm the existence of a pollution plume beneath the storage zone. To determine its extent, 13 complementary profiles, noted from P9 to P21 (Figs. 7, 8 and 9) were traced outside of the landfill influence zone: in the northeast (P9), the northwest (P10 and P11), in the west (P12 and P13), the southwest (P14 to P19) and in the southeast (P20 and P21). 3.2.1. Northeastern part Line P9 (Fig. 7A) indicates the presence of discontinuous flows encountered from 0 to 4 m in the SE, and then beyond from 8 m for the first encounter toward the NW. The substratum appears to be fragmented into a vast network of faults. This may produce the discontinuous flows that have been observed. The subvertical schistosity in the Etueffont schist (Belle, 2008) may also be at the origin of these discontinuous flows. Resistivity values ranging from 50 to 300 Ω m correspond to weakly mineralized waters. These results are in accordance with the conductivity values measured in piezometer PZ22 (CE = 32– 150 μS cm−1, piezometric level at 4.5 m), situated a few meters downstream from the profile line and in piezometer PZ5 placed 50 m upstream (CE = 80–134 μS cm−1). Piezometer PZ22 alone is reported by projection in the view because PZ5 was too far. The less conductive ground (ρ N 900 Ω m−1) corresponds to substratum schist. No leachate migration was observed NE of the site. 3.2.2. Northwestern part On line P10 (Fig. 7B), the poorly conductive ground (ρ N 760 Ω m−1) corresponds to dry substratum. A conductive layer is visible underneath, indicating that the substratum is saturated with water which is confirmed by its presence in piezometers PZ7 and PZ70 situated 5 m downstream from the profile. In the NE two concentric zones appear whose resistivity diminishes toward the center, reaching a value in the area of 14 Ω m. These zones correspond to more highly mineralized waters than those found in PZ7 and PZ70 located 5 m from P10. This may reflect the presence or lateral infiltration of diluted leachates. The zones are no longer visible in P11 located 50 m upstream from P10. To the NW the leachate plume is of limited reach. We infer diffusion-related discontinuities (faults, fractures or shear zones) of the substratum as illustrated in profiles 10, 11, 12 and 13 reflecting the secondary fracture network directed N135–N155 (see 2.1), in addition to the main N255–N260 schistosity. 3.2.3. Western part Profile P12 (Fig. 7D), approximately 50 m from the storage zone, presents a weakly conductive substratum (ρ N 760 Ω m), covered on the surface by a layer of more conductive soils no thicker than 7 m. No geological reconnaissance was carried out at this place, but other wells drilled in the sector may lead one to suppose that this conductive horizon corresponds to shear gouge mixed with schist rock or heavily fractured schists. In fact, altered and fractured zones present weaker resistivity than the healthy substratum due to increased porosity and a high shear gouge content (Matsui et al., 2000; Casado et al., 2015). In the eastern part of line P12, a concentric zone more highly mineralized than the groundwater crosses the substratum at a depth of ten meters. It indicates the presence of a leachate plume which generally follows the N270, in approximate agreement with the schistosity
direction (N255–260). The resistivity growing from the center toward the exterior shows a radial dilution and proves that its extent is limited. This contaminated zone therefore corresponds to a diverticulation of the plume which is guided by foliation and the possibly fractured zones of the substratum. The leachate plume is not located on profile P13 (Fig. 7E) and does not appear to extend westward. However, the absence of plume to the west does not mean that leachate progression is interrupted after profile P12. The discontinuities of the substratum may distribute a diluted plume at a depth below the profile. The groundwater piezometry as seen in Fig. 2 indicates that the distribution of contaminant to the NW may extend over a long distance since the local flow indicates a N-S direction in this sector. 3.2.4. Southwestern part The 6 profile lines, P14 to P19, traced downstream from the storage zone at the outlet of each of the lagooning ponds (Fig. 8), highlight the contrast between these very different zones: groundwater circulation (ρ b 300 Ω m) and the non-saturated substratum (ρ N 760 Ω m). Lagooning ponds are generally located along the axis of a thalweg. Their influences have been reported as projections along the profile lines. A zone more conductive than the sector's groundwater is visible in the projected influences of the ponds in all the profiles. This demonstrates a surface migration of leachates beyond the ponds. Their zones of influence present a progressively increasing resistivity from pond 1 to pond 4, ρ = 25–50 Ω m and ρ = 60–110 Ω m, respectively. This gradient is the consequence of leachate treatment in which mineralization decreases from pond to pond, according to measurements of electrical conductivity taken in each pond. Yet, these filtered lagooning waters are passably less mineralized (by a factor of 5–10) than the waters undergoing treatment. Only a slight diffusion of highly diluted leachates through the layer of compacted clay is detected. At depth the groundwater appears to drain following the thalweg axis. Along the P14 profile line (Fig. 8A), situated between the new cell and pond 1, the two small conductive zones in the site's northwestern part probably correspond to slight pollution from the old landfill. This anomaly, underscored on profile line P3 130–140 m distant (Fig. 9A), indicates an east-to-west plume migration. 3.2.5. Southeastern part As seen above for P3 (Fig. 9A), in the southwestern part of the storage zone, leachates from the old landfill infiltrate at the base through a thickness of N10 m, diluting as they seep downwards. Lines P20 and P21 traced in the southeastern part of the site, beyond the influence of the landfill, are presented in Fig. 9B and C. Profile P20 (Fig. 9B), situated 30 m from the landfill's influence zone, shows poorly conductive land on the surface which corresponds to the schist present. A zone of underlying conductivity is visible. The southwestern part of the zone (P21) (Fig. 9C) presents resistivity values near those of the groundwater readings taken nearby in PZ4 (ρzone = 100–250 Ω m; ρwater PZ4 ≈ 190 Ω m). This contamination may be from south of the OL and is not entirely consistent with the piezometry. The southward spread of the contaminants is probably limited by the NE-SW flow of regional groundwater (Fig. 2). In the heart of the northeastern part, the zone includes a more conductive fringe (ρ = 20–50 Ω m), no doubt corresponding to diluted leachates from the old landfill, that spreads horizontally through the schists. At depth, leachates stagnate and do not migrate toward the west-southwest. Within the thirty meters that separate the two profiles, leachate dilution takes place along a horizontal gradient, probably favored by the flow from the water table in the west-east direction as emphasized by Belle (2008). Profile P21, 40 m distant from P20, shows a discontinuous flow of water at varying depths, and a resistivity comparable to that of groundwater as in the SW part of profile 20. The extent of the pollution plume is limited to the SE part of the site since the plume is no longer visible at
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Fig. 9. Profiles traced to the southeast of the landfill.
Fig. 10. Visualization of anomalies detected with ERT. The red lines correspond to the profile sectors exhibiting a leachate plume beneath the waste storage zone. The global extension of the pollution plume is expressed in ochre-yellow. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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a distance of 30 m from the storage zone or is highly diluted in the groundwater. 3.3. Extension of the leachate plume The anomalies of low resistivity observed beneath the base and surrounding the storage zone, representing a leachate plume that migrates within the substratum, are shown in Fig. 10, as deduced from our results. The contamination originates mainly in the OL. Since the water table ceiling is at nearly the same level as the base of the waste mass, the groundwater is polluted through leachate infiltration. However, at depth a rising gradient of resistivity values is observed, indicating a progressive vertical dilution in leachate mineralization. Consequently, plume extension is limited, both at depth and laterally, as no pollution is visible even a few dozen meters distant from the storage zone. Concerning the new cell, its underlying drainage network may have failed. Seepage of concentrated leachates, however, remains moderate (5 m thick). In the NC, too, rapid vertical dilution takes place. 4. Conclusion Twenty-one 2D ERT profiles were obtained from the storage zone of the Etueffont landfill and its periphery, allowing sufficient depth for adequate investigation (from 10 to 40 m) to recognize the substratum under the landfill, while maintaining a correct lateral resolution. Synthesis of the results leads to the following diagnosis: - The landfill's known features, easily identified along the profile lines, were confirmed. The NC, however, created in accordance with more stringent environmental standards than the OL, reveals a failure of its sealing barrier and/or its drainage system. - Flow and saturation phenomena within the waste mass are visualized. - Leachate infiltrations into the substratum are highlighted beneath the storage zone, the main plume originating in the OL. - At depth, a rising gradient of resistivity values is observed, indicating the dilution effect and a progressive vertical leachate mineralization. - Geologically, the ERT profiles revealed a highly fractured substratum traversed by numerous discontinuous groundwater flows. This is a change from the vision obtained through drilling. Any attempt to understand flow phenomena within the schists is complex, but the spatial distribution of the leachate plume appears consistent with the main directions of fractures and the hydraulic gradient. - The plume extension is limited, both at depth and laterally, as no pollution is visible even a few dozen meters distant from the storage zone.
The Etueffont landfill is geographically complex. The realization of 21 profiles required several survey sessions spread over as many days. Water saturation conditions were not completely homogeneous. If we are to understand the migration routes of contaminants and the effects of leachate dilution and mineralization and further study will be required. Indeed, in this context the 2D ERT and the electrical array selected for this study present certain limitations. The high RMS error values calculated for most profiles traced over the storage zone or in the peripheral area introduce only an approximation of the geoelectrical model. Corrections and adjustments may be possible by testing electrical arrays other than the Wenner-Schlumberger and/or by increasing the number of measurements for each quadripole. To reduce any uncertainty, it would be advisable to conduct 3D ERT with multi-channel equipment so as to illustrate the nature of tracer plumes and thus aid in mitigating and controlling contaminating events.
However, 2D ERT has shown itself to be well adapted in determining the extent of the polluting plume and also in characterizing leachate biostabilization (10–50 Ω m). The technique may thus be a useful preliminary step toward the goal of better overall monitoring, if used in association with a network of piezometers.
Acknowledgements This study is part of the Phd research of Elise Grisey. The authors thank J. Mudry, H. Grisey and M. Grapin for their help. The authors gratefully acknowledge the financial support of the ADEME (Agency for the Environment and Energy Management), the SICTOM (Solid Waste Management Service) of Etueffont, Territoire de Belfort, France and the CNRS (National Center for Scientific Research) which made this investigation possible. We also thank the reviewers for their helpful suggestions and corrections.
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