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Characterizing Landfill Extent, Composition, and Biogeochemical Activity using Electrical Resistivity Tomography and Induced Polarization under Varying Geomembrane Coverage
Abstract
Landfill monitoring is essential for sustainable waste management and environmental protection. Geophysical methods can provide quasi-continuous spatial and temporal insights into subsurface physical properties and processes in a non-intrusive manner. The effectiveness of monitoring landfill extent, composition, and degradation under varying geomembrane coverage was evaluated using electrical resistivity tomography (ERT) and induced polarization (IP) methods. Synthetic electrical models for landfill with different geomembrane damage degrees were inverted to assess data reliability. The current conduction channels into the geomembrane during the electrical survey were quantified. Reliable electrical data was obtained when the inverted conduction channel ratio of the geomembrane (representing damage to the geomembrane) was 51.6% or higher. This criterion was validated in a landfill experiencing aeration and anaerobic treatments. ERT and IP data captured construction and domestic waste distribution and identified the landfill boundary. The chargeability of domestic waste proved sensitive to microbial degradation activity, corroborated by characteristic ammonium and nitrate ions and a linear relation between chargeability and subsurface temperature. Temperature variations between the aerobic and anaerobic reaction zones (>20°C and = 12°C) were observed to correlate with high chargeability values (>80.4 mV/V), signifying the presence of biogeochemically active zones. IP excels in characterizing geomembrane-covered landfill boundaries and discerning biogeochemical activity, thereby enhancing landfill monitoring and waste management strategies.
... borehole or well data), since they derive from measurements not directly sensing hydrogeological or contaminant properties (Day-Lewis et al., 2017). Recently, the combined use of different geoelectrical methods, such as electrical resistivity tomography (ERT) and induced polarization (IP), has been growing for leachate detection (e.g., Xia et al., 2023) and MSW landfill characterization (e.g., De Donno and Cardarelli, 2017b;Flores-Orozco et al., 2020;Ma et al., 2024). In fact, low resistivity values can be mainly correlated with an increase on bulk (electrolytic) conduction, due to the leachate produced within the landfill. ...
... Furthermore, M and MN are linked to surface polarization that is directly proportional to the biogeochemical activity that can be highly variable in the waste mass (Flores-Orozco et al., 2020). Conversely, DC resistivity is mainly related to the ion concentration and water content (Ma et al., 2024) that are not expected to significantly vary in the saturated area. Looking at the distribution of points belonging to the different profiles in parameter space (Fig. 6a), we note how the peaks in the point cloud (filled circles in Fig. 6a) clearly display different ranges of the geophysical parameters. ...
... Landfilling needs large open space and can cause many adverse effects on human health and the environment, including emitting landfill gases (Bruce et al., 2017;Karimi et al., 2020), generating leachate (Pan et al., 2019), causing unpleasant odor and visual concerns (Tansel and Inanloo, 2019;Panzone et al., 2021), and releasing pollutants that can cause soil and water contamination and threaten agricultural activities (Wdowczyk and Szymańska-Pulikowska, 2021;Gu et al., 2022). As such, a well-designed landfill plan supports landfill maintenance and routine monitoring is crucial for environmental sustainability and protection (Pasternak et al., 2023;Ma et al., 2024). ...
The evolution of landfill footprint design and the associated environmental impacts have been mostly ignored in literature. This study examines landfill footprint geometrical shape using four different shape factors, including compactness (CS), rectangularity (RS), stretch (SS), and ease of access (ES) on 99 active landfills in UK. ES had the highest mean value of 0.69, while SS had the lowest mean value of 0.24. RS and CS showed similar mean values of 0.55 and 0.53. The footprints are categorized based on the landfill's age into “young”, “intermediate” and “old” groups to assess the evolution of landfill design. Younger landfills exhibit the lowest compactness (median CS = 0.52), followed by intermediate (median CS = 0.55) and older sites (median CS = 0.62). In terms of RS, all the sites showed median values around 0.55. The young landfills have the lowest SS with a median of 0.20, while the old and intermediate groups had medians of 0.23 and 0.24 respectively. The variabilities of sites' land surface temperature were also assessed. This study underscores the role of landfill footprint design on landfill operation and introduces new aspects of design strategies to mitigate environmental risks.
... Carlo et al. used ERT to examine the bottom of the landfill, identifying a lowimpedance element well above the original landfill depth [25]. Ma et al. combined ERT and induced polarization (IP) method to evaluate the effectiveness of monitoring the range, composition, and degradation of landfills under different geomembrane covers [26]. These ERT methods are mainly performed along 2D profiles. ...
At present, the mainstream technology for leachate detection in landfills is electrical resistivity tomography (ERT), known for its efficiency and non-destructive nature. However, the conventional ERT data interpretation primarily uses inversion based on structured grids, which cannot accurately simulate the complex and thin impermeable layers of landfills, leading to unreliable results. To address this issue, we propose a novel ERT observation system and a new three-dimensional (3D) inversion technology. In our observation system, all measuring electrodes are placed around the landfill at once, and only a limited number of transmitting sources are needed to sequentially inject current, which effectively reduces the time for data acquisition. For 3D inversions we employ unstructured tetrahedral grid for fine discretization of structures at various scales. The node-based finite-element method is used for high-precision forward and adjoint forward calculations, while the gradient filtering method in combination with L-BFGS algorithm is used to update the inversion model. Numerical experiments indicate that the proposed method can mitigate the influence of thin impermeable landfill layers, provide more accurate imaging results compared to the conventional methods. Additionally, we also test the effects of water-bearing layers, faults, and near-surface interferences on the leakage inversion results. The results show that the proposed method can achieve reliable high-resolution imaging under various conditions, making it an effective technique for landfill leachate detection.
... ERT is widely used in site investigation and is employed across various disciplines. It has been employed successfully to investigate pipeline or tank leakage (Li et al. 2021;McClymont et al. 2017;Wang et al. 2015), identify aquifer structure (Das et al. 2021;Yeh et al. 2015), monitor the remediation process (Ma et al. 2024;Mao et al. 2016;Xia et al. 2023), delineate the scope of a landslide (Falae et al. 2019;Imani et al. 2021) and evaluate rock mass quality (David et al. 2009;Hasan et al. 2023), etc. ...
The diagnosis and monitoring of contaminated sites have become progressively critical. The effectiveness of electrical resistivity tomography (ERT) has been demonstrated in the assessment of contaminated sites, and its resolution can be significantly improved by array optimization. Gradient array involves the deployment of multiple electrodes in an array configuration to measure the variation in electrical potential across the subsurface, whereas there are some improved arrays which can often provide more accurate measurements. This study investigates the conventional gradient and improved Compare Resolution protocol to identify an abandoned quarry pit where pharmaceutical wastes were dumped. The acquired results were subsequently validated with borehole sampling. The results obtained from the gradient protocol revealed the presence of low-resistivity area. Nevertheless, it is difficult to define the range of the contamination plume area. The results from the optimized array illustrated that the maximum lateral contamination range in the vertical direction appears at a depth of 9 m. A portion of the contaminated area shown in the profile is confined by bedrock. Considering the high cost, only four locations are chosen for drilling to collect samples. The findings revealed that low resistivity corresponds to contamination of inorganic substances. The resistivity and drilling results further clarify that the site has a three-layer structure. The spatial variability of the contaminated site has been ascertained by the high-resolution ERT, and further identify four contaminant sources. This study proves the effectiveness of the ERT method to avoid the drawbacks of costly drilling in bedrock and accomplishes an efficient assessment of contamination.
Landfills have become the prevalent waste disposal method due to their convenience and cost effectiveness. However, landfills face numerous challenges including drainage and treatment of leachate, as well as pollution of surface water or groundwater. These issues pose significant health risks, while also detrimentally affecting natural ecosystems and agricultural resources. Note that the degradation process can persist for decades after closure. To comprehend its environmental impacts, it is crucial to investigate waste distribution and biogeochemical activity. Conventional approaches for landfill characterization typically involve direct techniques, such as chemical analyses of waste and water samples, whose effectiveness is limited by the number and spatial distribution of sampling points. In order to overcome these limitations, geophysical methods such as electrical resistivity tomography and induced polarization provide valuable information to enhance the characterization of landfill sites. In general, these methods provide quasi-continuous spatial and temporal information on subsurface properties and processes in a noninvasive manner and for relatively low cost. In “Characterizing landfill extent, composition, and biogeochemical activity using electrical resistivity tomography and induced polarization under varying geomembrane coverage,” Ma et al. highlight the main advantages of this methodology in portraying landfills, especially with geomembrane cover, and outline practical indicators for effective measurements. Additionally, the authors evaluate the effectiveness of electrical resistivity tomography and induced polarization techniques in monitoring the degradation of landfill waste under real-world conditions.
In recent times, 3D electrical resistivity and induced polarization tomographies are being used more frequently. However, it is often not possible to have regular grids of electrodes due to irregular topography, difficulty accessing urbanized or industrialized places, and other environmental and health problems. In these cases, the use of unconventional arrays is necessary, arranging the electrodes around the inaccessible area according to one or more open or closed polygonal traces. In this work, three different perimeter arrangements of electrodes are considered, and, for each, three different electrode array configurations are tested by calculating their apparent resistivity and solving the inverse problem on a three-dimensional model with resistive and conductive blocks. The comparison of the results showed that the dataset that produces the most realistic inverse model consists of electrodes arranged in concentric squares and the use of the Full Range Gradient (FRG) Array. This combination was evaluated in the field on a waste landfill, in which electrical resistivity and induced polarization tomographies were carried out, exploiting the access paths to the various sectors of the landfill to arrange the electrodes on approximately concentric polygons. The 3D models of electrical resistivity and induced polarization allowed the detection of zones of high concentration of leachate, defining their extension, and monitoring the functioning of the waterproofing membrane at the bottom of the landfill. The results proved that when it is not possible to arrange a regular grid of electrodes, the use of perimeter disposals of electrode joined to the FRG array provide a sufficiently homogeneous resolution below the area to be investigated.
Spectral Induced Polarization (SIP) has been suggested as a non‐invasive monitoring proxy for microbial processes. Under natural conditions, however, multiple and often coupled polarization processes co‐occur, impeding the interpretation of SIP signals. In this study, we analyze the sensitivity of SIP to microbially‐driven reactions under quasi‐natural conditions. We conducted flow‐through experiments in columns equipped with SIP electrodes and filled with natural calcareous, organic‐carbon‐rich aquifer sediment, in which heterotrophic denitrification was bio‐stimulated. Our results show that, even in the presence of parallel polarization processes in a natural sediment under field‐relevant geochemical conditions, SIP is sufficiently sensitive to microbially‐driven changes in electrical charge storage. Denitrification yielded an increase in imaginary conductivity of up to 3.1 μScm−1 {\upmu }\mathrm{S}\,{\mathrm{c}\mathrm{m}}^{-1} (+140%) and the formation of a distinct peak between 1 and 10 Hz, that matched the timing of expected microbial activity predicted by a reactive transport model fitted to solute concentrations. A Cole‐Cole decomposition allowed separating the polarization contribution of microbial activity from that of cation exchange, thereby helping to locate microbial hotspots without the need for (bio)geochemical data to constrain the Cole‐Cole parameters. Our approach opens new avenues for the application of SIP as a rapid method to monitor a system's reactivity in situ. While in preceding studies the SIP signals of microbial activity in natural sediments were influenced by mineral precipitation/dissolution reactions, the imaginary conductivity changes measured in the biostimulation experiments presented here were dominated by changes in the polarization of the bacterial cells rather than a reaction‐induced alteration of the abiotic matrix.
Landfill leachates are high-strength complex mixtures containing dissolved organic matter, ammonia, heavy metals, and sulfur species, among others. The problem of leachate treatment has subsisted for some time, but an efficient and cost-effective universal solution capable of ensuring environmental resources protection has not been found. Aerobic granular sludge (AGS) has been considered a promising technology for biological wastewater treatment in recent years. Granules’ layered structure, with an aerobic outer layer and an anaerobic/anoxic core, enables the presence of diverse microbial populations without the need for support media, allowing simultaneous removal of different pollutants in a single unit. Besides, its strong and compact arrangement provides higher tolerance to toxic pollutants and the ability to withstand large load fluctuations. Furthermore, its good that settling properties allow high biomass retention and better sludge separation. Nevertheless, AGS-related research has focused on carbon-nitrogen-phosphorus removal, mainly from sanitary sewage. This review aims to summarize and analyze the main findings and problems reported in the literature regarding AGS application to landfill leachate treatment and identify the knowledge gaps for future applications.
The signal level and shape of induced polarization responses are significantly affected by the current pulse duration and waveform. If not accounted for, this data dependency on the current will propagate trough the inversion to results rendering unquantifiable subsurface models. While this problem has been addressed in full-response induced polarization modelling, questions remain as to how to accurately retrieve quantitative induced polarization inversion models from the types of apparent integral chargeability data often used in data interpretation. Although several methodologies have been proposed for handling and inverting apparent resistivity and integral chargeability, these cannot compensate for the data dependency on the current waveform and pulse duration. This paper presents a novel inversion method for such data. The method considers current waveform and receiver transfer functions for retrieving quantitative IP models unbiased by transmitter waveform. The method uses the constant phase angle model, expressed in terms of the medium resistivity and phase. Specifically, four field data sets for the same profile but with different 100 per cent duty cycle pulse durations (4, 2, 1 and 0.5 s) serve as examples of data sets giving models dependant on current waveform when inverted with standard approaches. The novel inversion method presented here gives quantifiable models independent on the current waveform and pulse duration. These results resemble models retrieved with existing, full-response induced polarization inversions. The results still contain some degree of uncertainty in relation to underlying assumptions and parametrizations. Managing this source of uncertainty is considered in terms of full-response induced polarization inversions with constant phase angle and maximum phase angle inversions.
For 3D monitoring of leachates in a closed landfill, 3D electrical resistivity tomography is done by a 21 × 21 square lattice in an area of 1600 m². 56 samples were provided from 6 boreholes to measure the physical properties. In this study, one specific electrode array was used in laboratory measurements and also in field operations to improve comparison between measurements in laboratory and field operation with each other. After correction of temperature and compression effects on the electrical resistivity data in laboratory, a strong correlation was developed between electrical resistivity and volumetric water content of five borehole samples. The validity of the obtained mathematical relation was confirmed by using it in volumetric water content estimation of the sixth borehole. 3D monitoring of volumetric water content was done via measured electrical resistivity values in the landfill. The results indicated that uniform changes have occurred in those layers with depth from 7 to 10 m in comparison with lower depth layers. This is due to biodegradation and compression of wastes in deeper points. The results of this study are indicative of high efficiency of electrical resistivity tomography in 3D monitoring of leachate distribution in landfill.
Natural attenuation is very often the remediation method of necessity, rather than choice, for beach environments impacted by offshore exploration/drilling accidents. Robust methods that can be efficiently utilized in difficult to access and ecologically sensitive areas are needed for the long‐term monitoring of such degradation processes. A prime candidate for such a monitoring tool is the spectral induced polarization (SIP) method, a geophysical technique successfully used for characterization and monitoring of hydrocarbon degradation in freshwater environments. In this laboratory experiment the SIP method successfully monitored the natural degradation of beach sediments impacted by the Deepwater Horizon oil spill. Using the SIP, we were able to differentiate between biotic (e.g., microbial driven) and abiotic (e.g., dilution) degradation processes and infer degradation rates. To our knowledge this is the first effort to use the SIP method as a monitoring aid in high salinity environments.
Contaminant mass discharge is a commonly applied tool to evaluate the environmental impact of contaminated sites on water resources. At large contaminated sites with heterogeneous sources, such as landfills, the number of wells available is often not sufficient, leading to a high uncertainty of mass discharge estimates. In this study, we tackle the uncertainty of the contaminant mass discharge due to low sampling densities by interpolating limited water-sample data with the support of surface direct current resistivity and induced polarization geophysical data. The method relies on finding a conceptual link between the bulk conductivity imaged from geophysics and the contaminant concentrations. We investigate the link between (1) imaged bulk and electrical water conductivity, (2) water conductivity and conservative ionic species, (3) water conductivity and redox-sensitive species, (4) water conductivity and semipersistent organic species, and (5) water conductivity and biodegradable organic compounds. The method successfully identify similarities between the distribution of the bulk conductivity and chloride and pharmaceutical compounds in a landfill leachate plume and between the bulk conductivity data and benzene and chlorinated ethenes for a contaminant plume from a former pharmaceutical factory. Contaminant concentrations were interpolated through regression kriging, using geophysical data as the dependent variable. The distribution of concentration determined with the novel method showed a lower mean relative estimation error than the traditional method of kriging only contaminant concentration data. At large sites, the method can improve contaminant mass discharge estimates, especially if surface geophysical measurements are integrated in the site investigation at an early stage.
Efficient non-invasive techniques are desired for monitoring the remediation process of contaminated soils. We applied the direct current resistivity technique to image conductivity changes in sandbox experiments where two sandy and clayey soils were initially contaminated with diesel hydrocarbon. The experiments were conducted over a 230 day period. The removal of hydrocarbon was enhanced by a bioelectrochemical system (BES) and the electrical potentials of the BES reactors were also monitored during the course of the experiment. We found that the variation in electrical conductivity shown in the tomograms correlate well with diesel removal from the sandy soil, but this is not the case with the clayey soil. The clayey soil is characterized by a larger specific surface area and therefore a larger surface conductivity. In sandy soil, the removal of the diesel and products from degradation leads to an increase in electrical conductivity during the first 69 days. This is expected since diesel is electrically insulating. For both soils, the activity of BES reactors is moderately imaged by the inverted conductivity tomogram of the reactor. An increase in current production by electrochemically active bacteria activity corresponds to an increase in conductivity of the reactor.
The secondary voltage associated with time-domain induced olarization data of disseminated metallic particles (such as pyrite nd magnetite) in a porous material can be treated as a transient lf-potential problem. This self-potential field is associated with e generation of a secondary-source current density. This source urrent density is proportional to the gradient of the chemical po-ntials of the p-and n-charge carriers in the metallic particles or nic charge carriers in the pore water including in the electrical ouble layer coating the surface of the metallic grains. This new ay to treat the secondary voltages offers two advantages with spect to the classical approach. The first is a gain in terms of quisition time. Indeed, the target can be illuminated with a w primary current sources, all the other electrodes being used multaneously to record the secondary voltage distribution. The second advantage is with respect to the inversion of the obtained data. Indeed, the secondary (source) current is linearly related to the secondary voltage. Therefore, the inverse problemof inverting the secondary voltages is linear with respect to the source current density, and the inversion can be done in a single iteration. Sev- eral iterations are, however, required to compact the source cur- rent density distribution, still obtaining a tomogram much faster than inverting the apparent chargeability data using the classical Gauss-Newton approach. We have performed a sandbox experi- ment with pyrite grains locallymixed to sand at a specific location in the sandbox to demonstrate these new concepts. A method ini- tially developed for self-potential tomography is applied to the inversion of the secondary voltages in terms of source current distribution. The final result compares favorably with the classical inversion of the time-domain induced polarization data in terms of chargeability, but it is much faster to perform.
Spectral induced polarization measurements are affected by temperature variations due to a variety of temperature-dependent parameters that control the complex electrical conductivity. Most important is the influence of the ion mobility, which increases with increasing temperature. It is responsible for the increase of the conductivity of the fluid in the pores with temperature and influences the electrical double layer on the mineral surface. This work is based on laboratory measurements of 13 sandstone samples from different sources with different geological and petrophysical characteristics. We measured the complex impedance in a frequency range from 0.01 to 100 Hz and a temperature range from 0 to 40 °C. The main observation is a decrease of the characteristic time (defined by the inverse of the frequency, at which the phase shift is maximum) with increasing temperature. The strength of this decrease differs from one sample to another. The temperature dependence of the phase shift magnitude cannot easily be generalized, as it depends on the particular sample. The experimental findings suggest that neglecting the influence of temperature on complex conductivity may lead to significant errors when estimating hydraulic conductivity from relaxation time. We also simulate the temperature dependence with a theoretical model of membrane polarization and review some of the model properties, with an emphasis on the temperature dependence of the parameters. The model reproduces several features characterizing the measured data, including the temperature dependence of the characteristic times. Computed tomography and microscope images of the pore structure of three samples also allow us to associate differences in the geometrical parameters used in the modelling with pore scale parameters of the actual samples.
An interpretation approach for analysis of electrical resistivity and chargeability inverse models generated at two upgraded semi-aerobic landfill sites was developed. The inverse models produced from a 2D inversion program (RES2DINV) were used to set up a grid of electrical resistivity and chargeability imaging at the landfill sites. The investigations confirmed a high chargeability unit (>70 ms) depicting waste deposits and a saturated clayey layer, while the leachate plume showed low resistivity (<10 Ωm) and weak chargeability zone (<20 ms). Moreover, the IP responses of a diffused leachate plume downstream of a separate semi-aerobic landfill site in a similar geological setting were evaluated, obtaining an analogous result. The ion-dominated plume that exhibited a weak chargeability promoted membrane polarization rather than electrode polarization in its IP responses. However, the high concentration of ions in the diffused leachate restrained the ionic polarization, diminishing the IP effects in host sediments. The interpretation of the resistivity and chargeability profiles at the characterization sites allowed delineating the different zones of the subsurface strata. In particular, comparison of the leachate zones at the two locations revealed that the active landfill with a lesser proportion of waste deposits contained a greater accumulation of leachate than the closed site. The results in this study support the effectiveness of contaminant plume monitoring through the joint application of resistivity and IP methods. These non-invasive, rapid and cost-effective, geophysical techniques could lead to a promising reconnaissance tool for remediation or reclamation of solid waste disposal sites.
Keywords
Resistivity IP Chargeability Leachate Waste deposit Landfill
In floodplain environments, a naturally reduced zone (NRZ) is considered to be a common biogeochemical hotspot, having distinct microbial and geochemical characteristics. Although important for understanding their role in mediating floodplain biogeochemical processes, mapping the subsurface distribution of NRZs over the dimensions of a floodplain is challenging, as conventional wellbore data are typically spatially limited and the distribution of NRZs is heterogeneous. In this study, we present an innovative methodology for the probabilistic mapping of NRZs within a three-dimensional (3D) subsurface domain using induced polarization imaging, which is a non-invasive geophysical technique. Measurements consist of surface geophysical surveys and drilling-recovered sediments at the U.S. Department of Energy field site near Rifle, CO (USA). Inversion of surface time-domain induced polarization (TDIP) data yielded 3D images of the complex electrical resistivity, in terms of magnitude and phase, which are associated with mineral precipitation and other lithological properties. By extracting the TDIP data values co-located with wellbore lithological logs, we found that the NRZs have a different distribution of resistivity and polarization from the other aquifer sediments. To estimate the spatial distribution of NRZs, we developed a Bayesian hierarchical model to integrate the geophysical and wellbore data. In addition, the resistivity images were used to estimate hydrostratigraphic interfaces under the floodplain. Validation results showed that the integration of electrical imaging and wellbore data using an Bayesian hierarchical model was capable of mapping spatially heterogeneous interfaces and NRZ distributions thereby providing a minimally invasive means to parameterize a hydro-biogeochemical model of the floodplain. This article is protected by copyright. All rights reserved.
Measuring induced polarization in the time domain with relatively compact multi‐channel multi‐electrode systems is attractive because of the simplicity of the procedure and thus its efficiency in the field. However the use of this technique is sometimes discouraged by the bad quality of the measurements in cases of high electrode contact resistances that can render data interpretation infeasible or at least unreliable. It is proposed that capacitive coupling in the multi‐core electrode cables has a significant role in creating this problem.
In such cases separation of current and potential circuits by using separate multi‐conductor cable spreads can yield significant improvement in data quality. The procedure is relatively simple and can be implemented with common resistivity and time‐domain IP equipment.
We show here three field examples from Southern Sweden, all measured as 2D electrical imaging sections. The first one is an example where the use of a single cable spread is sufficient thanks to moderate electrode contact resistance and high signal levels. The following two examples are from sites where induced polarization measurements could not yield consistent results using only a single multi‐conductor cable spread. Useful results were subsequently obtained by using separate cable spreads.
The first example is a 280 m long line measured over an old covered municipal waste deposit where the waste body stands out as a zone of high chargeability. The second example is a 120 m line measured on a sandy glaciofluvial structure that is host to an aquifer of regional importance. The improvement led to discrimination between materials of different grain sizes, with potential bearing for understanding the aquifer. The third example is a 300–400 m line measured across an esker lying on clay till. The improvement led to a clear visualization of the esker and to the identification of a possible fault in the underlying gneissic bedrock.
In all cases pseudosections and examples of chargeability decay curves are shown and discussed as tools for assessing data quality. Inversion results are shown together with background geological information and it is concluded that they are in good agreement.
Field-based time domain (TD) induced polarization (IP) surveys are
usually modelled by taking into account only the integral chargeability,
thus disregarding spectral content. Furthermore, the effect of the
transmitted waveform is commonly neglected, biasing inversion results.
Given these limitations of conventional approaches, a new 2-D inversion
algorithm has been developed using the full voltage decay of the IP
response, together with an accurate description of the transmitter
waveform and receiver transfer function. This allows reconstruction of
the spectral information contained in the TD decay series.
The inversion algorithm is based around a 2-D complex conductivity
kernel that is computed over a range of frequencies and converted to the
TD through a fast Hankel transform. Two key points in the implementation
ensure that computation times are minimized. First, the speed of the
Jacobian computation, time transformed from frequency domain through the
same transformation adopted for the forward response is optimized.
Secondly, the reduction of the number of frequencies where the forward
response and Jacobian are calculated: cubic splines are used to
interpolate the responses to the frequency sampling necessary in the
fast Hankel transform. These features, together with parallel
computation, ensure inversion times comparable with those of direct
current algorithms.
The algorithm has been developed in a laterally constrained inversion
scheme, and handles both smooth and layered inversions; the latter being
helpful in sedimentary environments, where quasi-layered models often
represent the actual geology more accurately than smooth
minimum-structure models. In the layered inversion approach, a general
method to derive the thickness derivative from the complex conductivity
Jacobian is also proposed.
One synthetic example of layered inversion and one field example of
smooth inversion show the capability of the algorithm and illustrates a
complete uncertainty analysis of the model parameters.
With this new algorithm, in situ TD IP measurements give access to the
spectral content of the polarization processes, opening up new
applications in environmental and hydrogeophysical investigations.
A direct current (DC) resistivity and time domain induced polarization (TDIP) survey was undertaken at a decommissioned landfill site situated in Hørløkke, Denmark, for the purpose of mapping the waste deposits and to discriminate important geological units that control the hydrology of the surrounding area. It is known that both waste deposits and clay have clear signatures in TDIP data, making it possible to enhance the resolution of geological structures compared to DC surveys alone.
Four DC/TDIP profiles were carried out crossing the landfill, and another seven profiles in the surroundings provide a sufficiently dense coverage of the entire area. The whole dataset was inverted using a 1-D laterally constrained inversion scheme, recently implemented for TDIP data, in order to use the entire decay curves for reconstructing the electrical parameters of the soil in terms of the Cole-Cole polarization model.
Results show that it is possible to resolve both the geometry of the buried waste body and key geological structures. In particular, it was possible to find a silt/clay lens at depth that correlates with the flow direction of the pollution plume spreading out from the landfill and to map a shallow sandy layer rich in clay that likely has a strong influence on the hydrology of the site. This interpretation of the geophysical findings was constrained by borehole data, in terms of geology and gamma ray logging. The results of this study are important for the impact of the resolved geological units on the hydrology of the area, making it possible to construct more realistic scenarios of the variation of the pollution plume as a function of the climate change.
Two different equations, both of which are often called ‘the Cole–Cole equation’, are widely
used to fit experimental Spectral Induced Polarization data. The data are compared on the
basis of fitting model parameters: the chargeability, the time constant and the exponent. The
difference between the above two equations (the Cole–Cole equation proposed by the Cole
brothers and Pelton’s equation) is manifested in one of the fitting parameters, the time constant.
The Cole–Cole time constant is an inverse of the peak angular frequency of the imaginary
conductivity, while Pelton’s time constant depends on the chargeability and exponent values.
The difference between the time constant values corresponding to the above two equations
grows with the increase of the chargeability value, and with the decrease of the Cole–Cole
exponent value. This issue must be taken into consideration when comparing the experimental
data sets for high polarizability media presented in terms of the Cole–Cole parameters.
The basic idea of the Aarhus Workbench was to develop a single and integrated software platform for handling a number of different data types. The Workbench uses an open-source client server database to manage data and settings. The benefits of using a databases compared to flat ASCII column files should not be underestimated. Firstly, user-handled input/output is nearly eliminated, thus minimizing the chance of human errors. Secondly, data are stored in a well described and documented format which is well suited for both exchange and storage of data. The Workbench allows the user to present the output of inversions as point themes or as color contoured thematic maps, such as mean resistivity slices, depth to a conductor etc. Models can also be shown on sections which are linked both to the GIS and to displays of data and forward data. The sections can contain numerous layers representing different data types. Over the years HGG has developed stable processing and inversion algorithms for airborne and ground-based EM data. The inversion is known as Laterally Constrained inversion (LCI) for quasi 2-D modeling and Spatial Constrained inversion (SCI) for quasi 3-D inversion. The Workbench implements a user friendly interface to these algorithms enabling non- geophysicists to carry out inversion of complicated airborne data sets without having in- depth knowledge about how the algorithm actually works. Just as important is an extensive system for evaluation of inversion results with plots of results like resistivity models, flight height, pitch, roll, residuals etc. THE PLATFORM The Aarhus Workbench is best described as a platform gathering a number of different data handling methods and visualization modules under the same roof. This is illustrated in Figure 1. In common are the GIS and and the data management modules. Both are used by the other modules, importer, data processing, inversion and thematic maps. The importer modules transforms instrument specific data formats to the database format.. Most of these files are proprietary ASCII files which document to different degrees the measurement process and the data. Currently, importer modules are developed for ground-based transient electromagnetic (TEM), Pulled Array Continuous Electric Sounding (PACES) (Sørensen et al., 2005), multi electrode resistivity and induced polarization (DC/IP), ground conductivity meters (GCM), SkyTEM and helicopter frequency domain EM (HEM). Finally, if data are inverted using other inversion algorithms like EMFlow, the models can be imported and visualized in the Workbench.
Hybrid anaerobic–aerobic biological systems are an environmentally sustainable way of recovering bioenergy during the treatment of high–strength wastewaters and landfill leachate. This study provides a critical review of three major categories of anaerobic–aerobic processes such as conventional wetland, high–rate and integrated bioreactor systems applied for treatment of wastewaters and leachate. A comparative assessment of treatment mechanisms, critical operating parameters, bioreactor configurations, process control strategies, efficacies, and microbial dynamics of anaerobic–aerobic systems is provided. The review also explores the influence of wastewater composition on treatment performance, ammonium nitrogen removal efficacy, impact of mixing leachate, energy consumption, coupled bioenergy production and economic aspects of anaerobic–aerobic systems. Furthermore, the operational challenges, prospective modifications, and key future research directions are discussed. This review will provide in–depth understanding to develop sustainable engineering applications of anaerobic–aerobic processes for effective co–treatment of wastewaters and leachate.
Light non-aqueous phase liquids (LNAPLs) are commonly used in industrial processes, and they are well known for their potential to contaminate groundwater and their toxic effects on ecosystems. The adequate delineation of contaminant plume distribution is critical for the effective remediation of contaminated sites and aquifers. Electrical resistivity tomography (ERT) surveys on LNAPL contaminated soils have shown great potential in this regard. In this study in China, six ERT profiles were conducted at a former perfumery plant with a benzene and ethylbenzene spill history to evaluate whether ERT could be used to delineate the distribution of the LNAPL plume beneath the plant. Based on the survey results, the electrical resistivity was consistent with borehole sampling results, where high resistivity corresponded to increased LNAPL concentration. A linear relationship was built between resistivity and contaminant concentration, with a threshold value of 18 Ω·m used to identify contaminated areas. It was possible to construct a detailed three-dimensional characterization of the LNAPL distribution. In addition, four local sites were excavated to verify the results of the ERT profiles. The contamination sources were further categorized into four types that were considered useful for the selection of remediation strategies. In conclusion, ERT was an effective non-invasive technique for delineating LNAPL plume distribution at a high resolution.
Historic landfills that were constructed to standards that would not meet current regulations represent environmental and human-health risks. The adequate characterization of legacy landfills is paramount to manage the risks they pose effectively. In this study, the non-invasive geophysical technique of electrical resistivity tomography has been used to characterize two sites that have been identified as posing risks to the local environment. The sites are significantly different in the type of waste present, moisture content, type of bedrock and shape and distribution of the waste; although they have in common that neither site is lined.
The results of the resistivity imaging have allowed defining the boundaries of the landfill and the depth of the waste. Different types of waste and their distribution have been identified. Additionally, a relationship between the resistivity distribution and the release of residual gas from the landfill has been described; where the presence of saturated clay in the waste prevents the release of said gasses. This is be associated with locations with higher resistivity and lower clay content. That relationship is not observed when the clay is dry.
When interpreting the results, the heterogeneous nature of the waste can lead to misinterpretations due to resistivity overlap between the bedrock and the waste materials. Therefore, comparison of the resistivity models with direct information like borehole logs can significantly improve the reliability of the interpretations. However, the resistivity survey should predate the installation of bores to identify the most suitable locations for them.
Although sanitary landfilling currently effectively treats municipal solid waste (MSW) semi-aerobic bioreactor landfills (SABLs) are designed to accelerate biological processes and shorten the landfill stabilization phase. In this study, the effects of SABLs with the joint treatment of leachate recirculation and preaeration on landfill stabilization were determined by four landfill reactors, one anaerobically as a control, and three semi-aerobically with different leachate recirculation treatments (reactor I, II and III: 300 mL leachate, 600 mL leachate, and 600 mL aerated leachate per week, respectively). Results showed that reactor III had the highest removal rates of chemical oxygen demand (COD) and ammonia-nitrogen (NH4+-N), reaching 97% and 88%, respectively. Degradation of the organic substances could be described using an exponential attenuation model; the rates of COD, NH4+-N and biochemical oxygen demand (BOD5) in the SABLs increased from 0.019, 0.018, and 0.035 to 0.029, 0.025, and 0.053 day⁻¹, respectively, when leachate recirculation quantity was increased and preaeration was applied. Finally, classification index evaluation revealed that reactor III had the lowest I value of 312, indicating that the combination of increasing leachate recirculation quantity and preaeration could positively affect the MSW stabilization phase. This finding provides experimental evidence for improving landfill management for accelerating MSW stabilization in SABLs.
Tire derived aggregate (TDA) is currently being used as a cost-effective substitute for gravel in landfill leachate collection systems. TDA is composed of tires that have been shredded into pieces. However, the particles often contain a small portion of high risk protruding wires that may puncture a geomembrane placed below the TDA. The chance that these wires puncture is a function of the number of wires present in a sample, how the particles land, and the efficiency of the protection layer to prevent punctures. Using heavier nonwoven geotextile protection layers with larger size TDA and thicker geomembranes, the number of punctures may be expected to be fewer than 20 per hectare for the materials tested in this study. A second component to long term geomembrane performance is the presence of localised zones of high strain resulting from the point loading on the geomembrane. The strain resulting from TDA was found to be less than the strain which occurred from gravel using the same protection layers. It is concluded that, for different reasons, gravel and TDA both require effective protection layers of soil or heavy nonwoven geotextile to ensure long-term performance.
In this study, we investigate the applicability of the Induced Polarization (IP) imaging method to discriminate between biogeochemically active and inactive areas of a landfill. The elevated amount of degradable organic carbon in landfills results in the development of biogeochemical hot-spots associated with high rates of microbial activity and the generation of landfill gas and leachate as metabolic products. Our results demonstrate that the electrical conductivity is mainly sensitive to the increase in the fluid conductivity associated to leachate production and migration. Whereas images of the polarization effect, expressed in terms of the imaginary component (σ″) or the phase of the complex conductivity (ϕ), reveal the potential to characterize variations in the architecture and biogeochemical activity of the landfill. Correspondingly, biogeochemically active zones (leachable TOC contents above 1500 mg/kg dry waste) are related to high polarization values (σ″ > 10 mS/m, ϕ > 40 mrads), whereas low leachable TOC contents (<300 mg/kg dry waste) in the inactive areas are characterized by low polarization values (σ″ < 1 mS/m, 10 < ϕ < 25). Additionally, landfill sections corresponding to construction and demolition waste (CDW), associated to negligible TOC content, exhibit the lowest polarization response (σ″ < 0.1 mS/m, ϕ < 15). We prove that IP imaging is a well-suited method for landfill investigations that permits an improved characterization of landfill geometry, variation in waste composition, and the discrimination between biogeochemically active and inactive zones.
The Jigongshan mountain tunnel (Shenzhen, China) was designed to cross underneath the Xiaping Municipal Solid Waste (MSW) Landfill with the shortest vertical distance of 36 m. To take suitable measures to make the tunnel leachate-proof, the geological conditions and the leachate distribution in and under the landfill were detected and evaluated in this study. Firstly, geological and geophysical methods, including electrical resistivity tomography (ERT), transient electromagnetic method (TEM), and geological drilling and sample analysis, were employed on the ground and in the underground to perform a multi-pronged investigation. Meanwhile, the leakage levels were established to evaluate the extent of the leakage as well as its effects on the tunnel. Furthermore, the corresponding measures to achieve a leachate-proof tunnel were also provided. The test results indicated that the leachate was well confined by the liner system in the landfill. Although fracture zones containing water were detected under the landfill, which may preferentially act as pathways for the leachate flow, no pollution index of the underground water exceeded the allowable value, and no leachate leakage had occurred yet. Eventually, an integrated strategy comprised of a multi-pronged investigation, leakage assessment, and engineering countermeasures was developed to ensure the safety of the tunnel construction and operation. This study offers a valuable reference for similar investigative projects concerning underground construction near MSW landfills worldwide.
The assessment of landfill in-situ aeration eludes standardization as its application highly depends on varying local conditions. The prevailing work tries to assess the procedure performance by using typically available data.
In the here presented case study the aeration pipes were applied horizontally. To evaluate the air-distribution and its effect on the landfill solid body, two monitoring fields with 10 × 10 m were created. From there in total 60 solid and 336 gaseous samples were taken over five years. As the material from the landfill was rather old and characterized by comparatively low reactivity, “new” material from a mechanical biological treatment (MBT) plant was introduced in the landfill. Additionally online gas data from eight technically separated landfill sections were analyzed during in-situ aeration.
In total, about 46 Mm3 (0.27 m3/kg waste) air was introduced into the landfill body. The eight sections showed differences in reactivity (overall C-discharge was 8 g C kg−1 dry weight, ranging from 4.5 to 11). With solid sampling we could not show a significant decrease in landfill TOC but for the introduced MBT-material. Ammonium in solid samples was significantly reduced (to 14.7% initial) and NO3 significantly increased (2.1% initial). The reduction of the initial TOC (4.58%) was on average 11%.
The application of horizontal landfill aeration led to a widespread air-distribution in a rather shallow landfill. Monitoring fields allowed for a screening of the impact of the measures on the solid body with reduced sampling costs.
Remediation of soils and groundwater in a municipal solid wastes (MSW) landfill site emerges as a global challenge to the living environment on earth with significant market potential. Unlike contaminants in an industry or agricultural site, contaminants from MSW landfills are diverse, primarily consisting of chemical oxygen demand (COD), inorganic matter (ammonia-nitrogen, nitrate-nitrogen, total phosphorus) and heavy metals. This renders new challenges to remediation contaminants of different characters altogether. A status quo of existing technologies, including permeable reactive barriers, electrokinetic remediation, microbial remediation, and injection of either solubilizing agents or micro or nanobubbles were thoroughly reviewed, with an emphasis on removal efficiency based on existing projects at lab, pilot or field scales. A design chart tailored for the remediation of a landfill contaminated site was developed, verified by a few case studies, which supplement the chart. Future trends of technical innovation (such as multi-layer permeable reactive barriers (PRBs)) and challenges (such as flow pattern) were identified.
Landfill leachate, generated from the wastes in a landfill, is a type of wastewater with high concentrations of ammonia and organics, causing a serious environmental pollution. Because of its complex and changing characteristics, it is difficult to remove nitrogen from landfill leachate economically and effectively. Hence, nitrogen removal is a significant research priority of landfill leachate treatment in recent years. Biological processes are known to be effective in nitrogen removal. In this work, the biological nitrogen removal treatments were divided into the following processes: conventional nitrification-denitrification process, nitritation-denitritation process, endogenous denitritation process, and anaerobic ammonium oxidation (Anammox) process. This manuscript summarized the theories and applications of these approaches in detail, and concluded that appropriate processes should be selected in accordance with different characteristics of landfill leachate, in order to effectively remove nitrogen from all stages of landfill leachate and reduce disposal costs. Finally, perspective on the challenges and opportunities of biological nitrogen removal from landfill leachate was also presented.
The induced polarization phenomenon, both in time domain and frequency domain, is often parameterised using the empirical Cole-Cole model. To improve the resolution of model parameters and to decrease the parameter correlations in the inversion process of induced polarization data, we suggest here three re-parameterisations of the Cole-Cole model, namely the maximum phase angle Cole-Cole model, the maximum imaginary conductivity Cole-Cole model, and the minimum imaginary resistivity Cole-Cole model. The maximum phase angle Cole-Cole model uses the maximum phase φmax and the inverse of the phase peak frequency, τφ, instead of the intrinsic charge-ability m0 and the time constant adopted in the classic Cole-Cole model. The maximum imaginary conductivity Cole-Cole model uses the maximum imaginary conductivity σmax″ instead of m0 and the time constant τσ of the Cole-Cole model in its conductivity form. The minimum imaginary resistivity Cole-Cole model uses the minimum imaginary resistivity ρmin″ instead of m0 and the time constant τρ of the Cole-Cole model in its resistivity form. The effects of the three re-parameterisations have been tested on synthetic timedomain and frequency-domain data using a Markov chain Monte Carlo inversion method, which allows for easy quantification of parameter uncertainty, and on field data using 2D gradient-based inversion. In comparison with the classic Cole-Cole model, it was found that for all the three re-parameterisations, the model parameters are less correlated with each other and, consequently, better resolved for both time-domain and frequency-domain data. The increase in model resolution is particularly significant for models that are poorly resolved using the classic Cole-Cole parameterisation, for instance, for low values of the frequency exponent or with low signal-to-noise ratio. In general, this leads to a significantly deeper depth of investigation for the φmax, σmax″, and ρmin″ parameters, when compared with the classic m0 parameter, which is shown with a field example. We believe that the use of reparameterisations for inverting field data will contribute to narrow the gap between induced polarization theory, laboratory findings, and field applications.
We have investigated the influence of temperature and salinity upon the spectral induced polarization of 10 samples including rocks with their mineralization (galena, chalcopyrite) plus sand mixed with semiconductors such as magnetite grains, graphite, and pyrite cubes of two different sizes. Measurements are made in a temperature-controlled bath with a high-precision impedance meter and using NaCl solutions. We cover the temperature range 5°C-50°C and the frequency range 10⁻² Hz to 45 kHz. For one large pyrite cube, we also investigated six salinities from 0.1 to 10 Sm⁻¹ (at 25°C, NaCl) and three salinities for graphite. The spectra are fitted with a Cole-Cole complex parametric conductivity model for which we provide a physical meaning to the four Cole-Cole parameters. As expected, the Cole-Cole exponent and the chargeability are independent of the temperature and salinity. The instantaneous and steady state (direct current [DC]) conductivities depend on the salinity and temperature. This temperature dependence can be fitted with an Arrhenius law (combining the Stokes-Einstein and Vogel-Fulcher-Tammann equations) with an activation energy in the range of 15±1 kJ Mol⁻¹. This activation energy is the same as for the bulk pore-water conductivity demonstrating the control by the background electrolyte of these quantities, as expected. The instantaneous and DC conductivities depend on the salinity in a predictable way. The Cole-Cole relaxation time decreases with the temperature and decreases with the salinity. This behavior can be modeled with an Arrhenius law with an apparent activation energy of 7±3 kJ mol⁻¹. A finite-element model is used further to analyze the mechanisms of polarization, and it can reproduce the temperature and salinity dependencies observed in the laboratory.
As it is well-known, the characterization plan of an old landfill site is the first stage of the project for the treatment and reclamation of contaminated lands. It is a preliminary in-situ study, with collection of data related to pollution phenomena, and is aimed at defining the physical properties and the geometry of fill materials as well as the possible migration paths of pollutants to the surrounding environmental targets (subsoil and groundwater). To properly evaluate the extent and potential for subsoil contamination, waste volume and possible leachate emissions from the landfill have to be assessed. In such perspective, the integrated use of geophysical methods is an important tool as it allows a detailed 3D representation of the whole system, i.e. waste body and hosting environment (surrounding rocks). This paper presents a very accurate physical and structural characterization of an old landfill and encasing rocks obtained by an integrated analysis of data coming from a multi-methodological geophysical exploration. Moreover, drillings were carried out for waste sampling and characterization of the landfill body, as well as for calibration of the geophysical modeling.
Anaerobic digestion of organics is one of the most used solution to gain renewable energy from waste and the final product, the digestate, still rich in putrescible components and nutrients, is mainly considered for reutilization (in land use) as a bio-fertilizer or a compost after its treatment. Alternative approaches are recommended in situations where conventional digestate management practices are not suitable. Aim of this study was to develop an alternative option to use digestate to enhance nitrified leachate treatment through a digestate layer in a landfill bioreactor. Two identical landfill columns (Ra and Rb) filled with the same solid digestate were set and nitrified leachate was used as influent. Ra ceased after 75 day's operation to get solid samples and calculate the C/N mass balance while Rb was operated for 132 days. Every two or three days, effluent from the columns were discarded and the columns were refilled with nitrified leachate (average N-NO3-concentration = 1,438 mg-N/L). N-NO3- removal efficiency of 94.7% and N-NO3- removal capacity of 19.2 mg N-NO3-/gTS-digestate were achieved after 75 days operation in Ra. Prolonging the operation to 132 days in Rb, N-NO3- removal efficiency and N-NO3- removal capacity were 72.5% and 33.1 mg N-NO3-/gTS-digestate, respectively. The experimental analysis of the process suggested that 85.4% of nitrate removal could be attributed to denitrification while the contribution percentage of adsorption was 14.6%. These results suggest that those solid digestates not for agricultural or land use, could be used in landfill bioreactors to remove the nitrogen from old landfill leachate.
Electrical transport properties of saturated porous media, such as soils, rocks and fractured networks, typically composed of a non-conductive solid matrix and a conductive brine in the pore space, have numerous applications in reservoir engineering and petrophysics. One of the widely used electrical conductivity models is the empirical Archie's law that has a practical application in well-log interpretation of reservoir rocks. The Archie equation does not take into account the contributions of clay minerals, isolated porosity, heterogeneity in grains and pores and their distributions, as well as anisotropy. In the literature, either some modifications were presented to apply Archie's law to tight and clay-rich reservoirs or more modern models were developed to describe electrical conductivity in such reservoirs. In the former, a number of empirically derived parameters were proposed, which typically vary from one reservoir to another. In the latter, theoretical improvements by including detailed characteristics of pore space morphology led to developing more complex electrical conductivity models. Such models enabled us to address the electrical properties in a wider range of potential reservoir rocks through theoretical parameters related to key reservoir-defining petrophysical properties. This paper presents a review of the electrical conductivity models developed using fractal, percolation and effective medium theories. Key results obtained by comparing experiential and theoretical models with experiments/simulations, as well as advantages and drawbacks of each model are analyzed. Approaches to obtaining more reasonable electrical conductivity models are discussed. Experiments suggest more complex relationships between electrical conductivity and porosity than experiential models, particularly in low-porosity formations. However, the available theoretical models combined with simulations do provide insight to how microscale physics affects macroscale electrical conductivity in porous media.
The Handbook of Environment and Waste Management, Volume 2, Land and Groundwater Pollution Control, is a comprehensive compilation of topics that are at the forefront of many of the technical advances and practices in solid waste management and groundwater pollution control. These include biosolids management, landfill for solid waste disposal, landfill liners, beneficial reuse of waste products, municipal solid waste recovery and recycling and groundwater remediation. Internationally recognized authorities in the field of environment and waste management contribute chapters in their areas of expertise. This handbook is an essential source of reference for professionals and researchers in the areas of solid waste management and groundwater pollution control, and as a text for advanced undergraduate and graduate courses in these fields. © 2014 by World Scientific Publishing Co. Pte. Ltd. All rights reserved.
The purpose of this study was to track the long-term trends of contaminants distribution in the old landfill of Singapore through monitored natural attenuation and to explore the main parameters that rule such transition. Contaminants distribution, including dissolved organic matter (DOM), inorganic species, heavy metals, and organic compounds, was investigated via monitoring wells in the years 1997, 2004 and 2011. The data revealed that the distribution of contaminants possessed selective attention of spots associated with leachate movement. The hydrogeology of the landfill governed the fate and transportation of contaminants. More specifically, strong statistical correlations were identified between DOM and certain constituents in the leachate, suggesting enhanced mobilization potential. However, the leachate composition exhibited limited correspondence to the nearby solid waste, indicating the minor effect induced by the partitioning coefficient. The presence of sulphate unveiled air intrusion, suggesting increased stability of the landfill, where enhanced biodegradation occurred at earlier period responsible for higher BOD removal. Afterwards other parameters continued to facilitate the compounds removal resulting in overall low concentrations of the contaminants.
Resistivity and induced polarization (IP) measurements on soil contaminated with non-aqueous phase liquids (NAPLs) show a great variety in results in previous research. Several laboratory studies have suggested that the presence of NAPLs in soil samples generally decrease the magnitude of the IP-effect, while others have indicated the opposite. A number of conceptual models have been proposed suggesting that NAPLs can alter the pore space in different ways, e.g. by coating the grain surfaces and thus inhibiting grain polarization, or by changing the pore throat size and thus affecting the membrane polarization mechanism. The main aim of this paper is to review previously published conceptual models and to introduce some new concepts of possible residual NAPL configurations in the pore space. Time domain induced polarization measurements were performed at a NAPL contaminated field site, and the data were inverted using the Constant Phase Angle (CPA) model and the Cole-Cole model respectively. No significant phase anomalies were observed in the source area of the contamination when the CPA inverted profiles were compared with soil sampling results of free-phase contaminant concentrations. However, relatively strong phase and normalized phase anomalies appeared next to the source area, where residual free-phase presence could be expected according to the chemical data. We conclude that depending on the NAPL configuration, different spectral IP responses can be expected. In previous research, the NAPL configurations in different samples or field sites are often unknown, and this may to some extent explain why different results have been achieved by different authors. In our field case, we believe that the NAPL forms a more or less continuous phase in the pore space of the source zone leading to an absence of IP anomalies. The increase in phase and normalized phase angle observed next to the source zone is interpreted as a degradation zone. The ongoing biodegradation may have led to a fractionation of the continuous NAPL in the outer part of the original source zone, leading to residual presence of isolated NAPL droplets in the soil pores. With such NAPL configurations, an increased polarization can be expected according to the electrochemical- and membrane polarization mechanisms. More research is needed to confirm the effects of different NAPL configuration on spectral IP parameters.
Landfill leachate samples (n=11) were collected from five USA municipal solid waste (MSW) landfills and analyzed for ten trace organic pollutants that are commonly detected in surface and municipal wastewater effluents (viz., carbamazepine, DEET, fluoxetine, gemfibrozil, PFOA, PFOS, primidone, sucralose, sulfamethoxazole and trimethoprim). Carbamazepine, DEET, PFOA and primidone were detected in all leachate samples analyzed and gemfibrozil was detected in samples from four of the five-landfill sites. The contaminants found in the highest concentrations were DEET (6900-143000ngL(-1)) and sucralose (<10-621000ngL(-1)). Several compounds were not detected (fluoxetine) or detected infrequently (sulfamethoxazole, trimethoprim and PFOS). Using the average mass of DEET in leachate amongst the five landfills and scaling the mass release from the five test landfills to the USA population of landfills, an order of magnitude estimate is that over 10000kg DEETyr(-1) may be released in leachate. Some pharmaceuticals have similar annual mean discharges to one another, with the estimated annual discharge of carbamazepine, gemfibrozil, primidone equating to 53, 151 and 128kgyear(-1). To the authors knowledge, this is the first time that primidone has been included in a landfill leachate study. While the estimates developed in this study are order of magnitude, the values do suggest the need for further research to better quantify the amount of chemicals sent to wastewater treatment facilities with landfill leachate, potential impacts on treatment processes and the significance of landfill leachate as a source of surface water contamination.
Copyright © 2015. Published by Elsevier Ltd.
This study uses time-domain induced polarization data for the delineation and characterization of the former landfill site at Eskelund, Denmark. With optimized acquisition parameters combined with a new inversion algorithm, we use the full content of the decay curve and retrieve spectral information from time-domain IP data. Thirteen IP/DC profiles were collected in the area, supplemented by el-log drilling for accurate correlation between the geophysics and the lithology. The data were inverted using a laterally constrained ID inversion considering the full decay curves to retrieve the four Cole-Cole parameters. For all profiles, the results reveal a highly chargeable unit that shows a very good agreement to the findings from 15 boreholes covering the area, where the extent of the waste deposits was measured. The thickness and depth of surface measurements were furthermore validated by el-log measurements giving in situ values, for which the Cole-Cole parameters were computed. The 3D shape of the waste body was pinpointed and well-defined. The inversion of the IP data also shows a strong correlation with the initial stage of the waste dump and its composition combining an aerial map with acquired results.
Municipal solid waste landfills pose a threat on environment and human health, especially old landfills which lack facilities for collection and treatment of landfill gas and leachate. Consequently, missing information about emission flows prevent site-specific environmental risk assessments. To overcome this gap, the combination of waste sampling and analysis with statistical modeling is one option for estimating present and future emission potentials. Optimizing the tradeoff between investigation costs and reliable results requires knowledge about both: the number of samples to be taken and variables to be analyzed.
This article aims to identify the optimized number of waste samples and variables in order to predict a larger set of variables. Therefore, we introduce a multivariate linear regression model and tested the applicability by usage of two case studies. Landfill A was used to set up and calibrate the model based on 50 waste samples and twelve variables. The calibrated model was applied to Landfill B including 36 waste samples and twelve variables with four predictor variables.
The case study results are twofold: first, the reliable and accurate prediction of the twelve variables can be achieved with the knowledge of four predictor variables (Loi, EC, pH and Cl). For the second Landfill B, only ten full measurements would be needed for a reliable prediction of most response variables. The four predictor variables would exhibit comparably low analytical costs in comparison to the full set of measurements. This cost reduction could be used to increase the number of samples yielding an improved understanding of the spatial waste heterogeneity in landfills.
Concluding, the future application of the developed model potentially improves the reliability of predicted emission potentials. The model could become a standard screening tool for old landfills if its applicability and reliability would be tested in additional case studies.
Estimation of the municipal solid waste settlements and the contribution of each of the components are essential in the estimation of the volume of the waste that can be accommodated in a landfill and increase the post-usage of the landfill. This article describes an experimental methodology for estimating and separating primary settlement, settlement owing to creep and biodegradation-induced settlement. The primary settlement and secondary settlement have been estimated and separated based on 100% pore pressure dissipation time and the coefficient of consolidation. Mechanical creep and biodegradation settlements were estimated and separated based on the observed time required for landfill gas production. The results of a series of laboratory triaxial tests, creep tests and anaerobic reactor cell setups were conducted to describe the components of settlement. All the tests were conducted on municipal solid waste (compost reject) samples. It was observed that biodegradation accounted to more than 40% of the total settlement, whereas mechanical creep contributed more than 20% towards the total settlement. The essential model parameters, such as the compression ratio (Cc’), rate of mechanical creep (c), coefficient of mechanical creep (b), rate of biodegradation (d) and the total strain owing to biodegradation (EDG), are useful parameters in the estimation of total settlements as well as components of settlement in landfill.
In spectral-induced polarization (SIP) studies of sites contaminated by organic hydrocarbons, it was shown that biodegradation by-products in general, and organic acids in particular, significantly alter the SIP signature of the subsurface. Still a systematic study that considers the effect of organic acid on the physicochemical and electrical (SIP) properties of the soil is missing. The goal of this work is to relate between the effect of organic acid on the physicochemical properties of the soil, and the soil electrical properties. To do so, we measured the temporal changes of the soil chemical (ion content) and electrical (low-frequency SIP) properties in response to influx of organic acid at different concentrations, gradually altering the soil pH. Our results show that organic acid reduces the soil pH, enhances mineral weathering and consequently reduces both the in-phase and quadrature conductivity. At the pH range where mineral weathering is most significant (pH 6-4.5) a negative linear relation between the soil pH and the soil formation factor was found, suggesting that mineral weathering changes the pore space geometry and hence affecting the in-phase electrical conductivity. In addition, we attribute the reduction in the quadrature conductivity to an exchange process between the natural cation adsorbed on the mineral surface and hydronium, and to changes in the width of the pore bottleneck that results from the mineral weathering. Overall, our results allow a better understanding of the SIP signature of soil undergoing acidification process in general and as biodegradation process in particular.
A three-stage sequencing batch reactor (SBR), comprising pretreating SBR (SBRpre), nitritation SBR (SBRni), and anaerobic ammonium oxidation (Anammox) SBR (SBRana), was developed for the nitrogen removal from mature landfill leachate. The concentrations of ammonia and chemical oxygen demand (COD) in the leachate were 2000 ± 100 and 2200 ± 200 mg/L, respectively. About 100 mg/L of organic substance was removed from SBRpre to reduce the negative effect on the Anammox process under real-time control. After acclimation for 40 days, the nitrite to nitrogen oxide ratio (NO2-/NOx) in SBRni was above 0.95. The nitrogen removal efficiency reached 90% in SBRana, and nitrogen load rate and nitrogen removal rate were 0.81 and 0.76 kg N/(m3 d), respectively. The continuous filling process was used to avoid the nitrite inhibition on the Anammox activity. The quantitative PCR analysis of Anammox indicated the average Anammox gene ratio increased from 0.23% to 4.77% after 220 days operation.
The spectral induced polarization (SIP) signature of soil contaminated
with organic pollutant was studied. Using a flow column experiment, the
effect of crystal violet (CV, a polar organic pollutant) on the temporal
change of the SIP response over a broad frequency range (1 mHz to 45
KHz) was determined. Complimentary measurements were used to determine
the effect of CV on the chemical composition of both the pore water and
the solid surface. In addition, analysis of the experimental results was
carried out by using both chemical complexation and induced polarization
models. Our results shows that adsorption of CV to the mineral surface
resulted in release of inorganic ions to the soil solution, increasing
the solution electrical conductivity and therefore also the real part of
the complex conductivity. Despite the increase in the real part of the
complex conductivity, the imaginary part of the complex conductivity
decreased with increasing concentration of adsorbed CV. Using the Revil
induced polarization model, we were able to show that the contribution
of the adsorbed CV to the polarization of the soil is negligible, and
that the main process affecting the polarization is the decrease in the
density of the inorganic surface species. The results of this study can
be used to better interpret SIP signature of soils contaminated by
organic compounds.
The long-term service life of HDPE geomembrane liners depends upon the rate of generation of holes in the liner and the acceptability of leachate or gas leakage at a particular site. The development of holes with time until the landfilled waste no longer poses an environmental hazard is important in assessing the long-term performance of landfills. A thorough review of physical damage, material degradation processes and the development of holes by stress cracking has been undertaken. This paper summarises the findings of the research project, which forms the basis on which guidance and assumptions on medium to long-term landfill liner performance can be formulated in the UK. A conceptual model of hole generation in six stages throughout the service life of an HDPE liner is presented. Electrical leak location surveys are seen to be effective means of identifying holes caused by physical damage during liner installation and waste disposal, and permitting their repair. Degradation of the HDPE liner is controlled by the liner exposure conditions, the activation energy of the antioxidant depletion process and the oxidative resistance of the material. Where the liner is subjected to long-term stresses, stress cracking will lead to the development to holes, and the rate of cracking will increase once oxidation of the liner commences. Methods to minimise and retard the development of holes are discussed.
Electrical resistivitymethods arewidely used for environmental applications, and they are particularly useful for the characterization and monitoring of sites where the presence of contamination requires a thorough understanding of the location and movement of water, that can act as a carrier of solutes. One such application is landfill studies, where the strong electrical contrasts between waste, leachate and surrounding formations make electrical methods a nearly ideal tool for investigation. In spite of the advantages, however, electrical investigation of landfills poses also challenges, both logistical and interpretational. This paper presents the results of a study conducted on a dismissed landfill, close to the city of Corigliano d'Otranto, in the Apulia region (Southern Italy). The landfill is located
in an abandoned quarry, that was subsequently re-utilized about thirty years ago as a site for urban waste disposal.
The waste was thought to be more than 20 m thick, and the landfill bottom was expected to be confined with an HDPE (high-density poli-ethylene) liner. During the digging operations performed to build a nearby new landfill, leachate was found, triggering an in-depth investigation including also non-invasivemethods. The principal goal was to verify whether the leachate is indeed confined, and to what extent, by the HDPE liner.We performed
both surface electrical resistivity tomography (ERT) and mise-à-la-masse (MALM) surveys, facing the severe challenges posed by the rugged terrain of the abandoned quarry complex. A conductive body, probably associated with leachate,was found as deep as 40 mbelowthe current landfill surface i.e. at a depth much larger than the expected 20 mthickness of waste. Given the logistical difficulties that limit the geometry of acquisition, we utilized synthetic
forward modeling in order to confirm/dismiss interpretational hypotheses emerging from the ERT and MALM results.
This integration between measurements and modeling helped narrow the alternative interpretations and strengthened the confidence in results, confirming the effectiveness of non-invasive methods in landfill investigation and the importance of modeling in the interpretation of geophysical results.
In electrolyte-saturated sands, the reversible storage of electrical charges is responsible for a phase lag between the current (injected and retrieved by two current electrodes) and the electrical field recorded by two voltage electrodes. This phenomenon is called 'spectral induced polarization' in geophysics and can potentially be used to monitor salt tracer tests in shallow aquifers to infer their permeability and dispersivity tensors. We demonstrate analytically that the polarization of the inner part of the electrical triple layer coating the surface of the grains (named the Stern layer in electrochemistry) is consistent with available data. We also perform new experiments using silica sands saturated by NaCl and CaCl2 pore water solutions. The salinity dependence of quadrature conductivity can be modelled using an analytical solution of the triple layer model, which offers a simple way to interpret laboratory and field data. This analytical solution depends on the total site density of the mineral surface, the pH value and the sorption coefficient of the cation in the Stern layer. This model shows that both the specific surface conductivity of the Stern layer and the quadrature conductivity of the porous material depend on the conductivity of the pore water. The quadrature conductivity is becoming independent of the salinity above 1 S m-1. The parameters entering the analytical model are consistent with independent estimates from titration data and zeta potential measurements, which are two classical methods to characterize the electrical triple layer at the pore water mineral interface.
The possibility to estimate permeability from the electrical spectral induced polarization (SIP) response might be the most important benefit offered by SIP measurements. It can thus be deduced that, in the future, SIP measurements will be carried out more frequently at the field scale or in a well-logging context to estimate permeability. In the shallow subsurface, however, the temperature generally exhibits seasonal variability, and in the deeper subsurface, it usually increases with depth. Hence, knowledge about the dependence of the SIP response on temperature is necessary in order to avoid possible misinterpretation of datasets impacted by thermal effects. In our study, we present a semiempirical framework to describe the temperature dependence of the SIP response. We briefly introduce the SIP response and its relation to permeability in terms of an electrochemical polarization mechanism and combine this formulation with relationships for the dependence of ionic mobility on temperature. We compare the predictions of our formulation with the experimental data from SIP measurements performed on sandstone at temperatures from 0°C to 80°C. The measured SIP response was transformed into a relaxation time distribution, using the empirical Cole-Cole model and a regularized Debye decomposition procedure. The SIP response was found to be in good agreement with the theoretical model. The temperature dependence of both direct current conductivity and relaxation time is controlled mainly by the dependence of ionic mobility on temperature, and the shape of the relaxation time distribution of the investigated sandstone is almost independent of temperature. The temperature effect on the SIP response can therefore be easily corrected.
Electrical Resistivity Tomography (ERT) has been used to map the 3D spatial distribution of waste and leachate concentrations within a closed and unconfined landfill. Borehole sampling and a 2D ERT survey down-gradient of the landfill boundary had failed to detect a pollution plume. Accordingly, a 3D survey was undertaken to determine the pattern of leachate drainage within the waste so that a more refined contaminant transport model could be developed. A full 3D survey was undertaken by sub-dividing the landfill into a number of discrete rectangular blocks and acquiring data on multiple parallel lines. The line data were merged into a single x-y matrix file and then inverted using a 3D finite element algorithm. The results are presented as 3D volumetric tomograms to show the inferred waste distribution and leachate flow-paths. The resistivity models indicate that leachate has accumulated at several discrete localities within the landfill. The controlling mechanism appears to be the depth and geometry of the original Chalk quarries used for waste disposal. The deepest leachate infiltration of the underlying Chalk bedrock invariably occurs below these drainage sinks. A narrow leachate plume is indicated in one part of the landfill, migrating down-gradient to the north-west. The inferred leachate concentrations in the Chalk were subsequently confirmed by drilling and hydrochemical sampling which showed elevated solute levels coincident with distinct resistivity lows of < 15 ohm ‐ m .
Electrical resistivity imaging surveys are used to monitor variations in pore fluid chemistry and saturation as well as time-lapse changes. Temperature variations in the near surface can produce larger magnitude changes in electrical conductivity than changes due to slow moving solute plumes or spatial variations in chemistry and soil moisture. Relationships between temperature and electrical conductivity based on previous studies conducted over 25-200°C do not explain 0-25°C laboratory data. A modification to the temperature dependence within a petrophysical model is proposed that may allow general application over this temperature range. An empirical linear approximation of 1.8 to 2.2 percent change in bulk electrical conductivity per degree C is consistent with low temperature electrical conductivity studies and the predictions of the petrophysical model used. This relationship can be used to account for the effect of temperature variations within individual images or time-lapse difference images.