Mapping of contaminant plumes with geoelectrical methods. A case study in urban context
Highlights
► Urban context. ► 3 contaminated zones located: organic phase detection, 2 mineralized. ► SIP contaminant nature.
Introduction
During the past two decades, the diagnosis and monitoring of polluted sites have become more important. European regulations are more and more restrictive concerning contamination. Two French ministerial circulars in April 1996 initiated initial diagnosis and evaluation of simplified risk at about 1300 industrial activity sites in France. The European Parliament and of the Council defined a thematic strategy for soil protection (SEC (2006) 620, 2006). Article 10 of this proposal provides rules for the identification of sites containing hazardous substances from human activity.
The preliminary assessment required information about a contaminated site is the geology and the hydrogeology of the site. Afterwards, it is necessary to identify and locate the potentially contaminated area, to detect waste disposal towards a sustainable remediation. The environmental assessment of urban sites is difficult because they are contaminated with various pollutants, and because there is a large physical and chemical heterogeneity. This heterogeneity comes from the landfilling of various solid wastes and remolded soil (endogenous or exogenous) from which they were constituted over time. It is very common to find the following contaminants in urban landfills: metals (e.g. Cd, Cr, Cu, Pb, and Zn), solvents, and polycyclic aromatic hydrocarbons (PAH) (Boudreault et al., 2010). The task becomes very tricky because large debris may also be found (i.e., coal, concrete, bricks, metal scraps), as well as large buried structure (foundation walls or underground tanks).
The traditional techniques to evaluate soil contamination are drilling associated to sampling. The water quality relies on the chemical analysis of groundwater samples collected from monitoring wells. Although the water samples provide a good and generally accurate inventory of contamination, monitoring wells are invasive, and they can even be misplaced and may not intercept the contaminated zone.
Taking into account the high heterogeneity of the study sites, the drilling and sampling techniques are often insufficient to offer a continuous and consistent image of the subsoil contamination, whereas the accuracy of the results is essential for the estimation of management and remediation costs.
For this reason the integration of geophysical investigation at large scale with accurate geochemical analysis of samples of the subsoil and groundwater is recommended in order to quantitatively estimate the extent and the level of the contamination.
The geophysical strategy has to be defined, as a function of geology, contaminants, and limitations inherent to the study site.
Since urban landfills show distinct properties (electrical resistivity, permittivity) from and surrounding medium, geophysical methods may be employed. These landfills contain most of the time various pollutants, some of them are electrically resistive (concrete debris, fresh hydrocarbons) and others are electrically conductive (metal scraps, alterated hydrocarbons, ions). That's why ground penetrating radar (GPR), electromagnetic induction at low frequency (EM), and electric resistivity tomography including induced polarization, should be considered.
GPR is very useful to detect buried structures or underground tanks, as proven by its massive use in environmental companies. This method has also been used to characterize the geometry of the soil and define the heterogeneity (Asprion and Aigner, 1999, Beres and Haeni, 1991, Beres et al., 1999, Huggenberger et al., 1994, Kowalsky et al., 2004, Neal, 2004), and hydrocarbon contamination (Atekwana and Atekwana, 2010, Bermejo et al., 1997, Bradford, 2007, Cassidy, 2007, Cassidy, 2008, Daniels et al., 1995).
Electrical Resistivity Impedance and EM studies have been used to delineate hydrocarbon pollution (Benson et al., 1997, Halihan et al., 2005, Kaufmann and Deceuster, 2007). Some studies show that hydrocarbon plumes may be defined either as areas of high resistivity (Benson, 1991, Benson et al., 1997), since hydrocarbons typically have higher resistivity than interstitial water (Asquith and Gibson, 1982), or as low resistivity zones (Atekwana et al., 2000, Benson, 1992, Benson et al., 1991, Sauck et al., 1998) due to biodegradation of hydrocarbons that may increase the amount of total dissolved solids (TDS) in the interstitial water. The same techniques had been successfully used to locate groundwater contamination (Frohlich et al., 2008, Guérin et al., 2004, Meju, 2000, Noguera et al., 2002, Soupios et al., 2007).
The IP method shows a increasing potential for environmental studies since the 80s (Olhoeft, 1985, Vanhala et al., 1992) with the advent of very sensitive devices. Induced polarization is a phenomenon that takes place at the interface between an electrolyte and a mineral grain. It is a current induced effect observed as a delay of the voltage response of the ground. This effect may be measured in the time domain, or in the frequency domain; in the first case the measured parameter is the chargeability, and in the second one it is an amplitude and a phase-lag. The surfaces of most solids usually possess a net charge by adsorption of essentially fixed ions. The net surface charge on the solid is a property measured by the ion exchange capacity, and it attracts charges of opposite polarity to form a diffuse layer next to the fixed layer. This double layer presents capacitive impedance to current passage across the solid interface. Thus, ionic conduction paths via the pore fluid electrolyte primarily control current conduction in resistivity; whereas the IP effect, is linked to the buildup of excess charge. Therefore, the surface properties of solid mineral grains and adsorption are very important to the IP effect.
Several IP studies have been done for organic contaminants reporting sometimes an increase of the IP phase lag (using spectral induced polarization, SIP)/chargeability (using temporal induced polarization, TIP) (Atekwana and Atekwana, 2010, Börner et al., 1993, Kemna et al., 2004, Olhoeft, 1986a, Olhoeft, 1986b, Olhoeft, 1986c, Schmutz et al., 2010, Sogade et al., 2006), but also sometimes decrease of these parameters (Börner et al., 1993, Li et al., 2001, Vanhala, 1997, Vanhala and Soininen, 1995).
The different signal over the same type of contaminant has been explained in Revil et al. (accepted): the increase of phase shift is linked to non wetting oil (undegraded oil), and the decrease to wetting oil (biodegraded oil). The second case corresponds to the formation of polar components at the oil/water interface that increases the cation exchange capacity.
One of the difficulties to carry out SIP is related to the EM coupling effects, increasing with frequency and with array length. One of the solutions is to analyze the low frequency spectra (< 1 Hz), that are consistent with the frequency range needed for Stern layer (double layer) effect analysis, and most of the time, with the EM coupling effects associated with the array length (AB/2 = 20 m at the maximum to reach about 4–5 m deep).
The objective of our study is to define the horizontal and vertical extension of hydrocarbon and solvent contaminants (between 0 and 5 m deep), taking into account all the limitations induced by the urban situation. This study will be done with multi-method measurement strategy that involves geoelectrical techniques with a detailed geochemical campaign.
The steps are the following, going from the fastest technique to the most time consuming:
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EM induction mapping with EM31 device (Geonics Ltd.) that will allow some anomalies
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Electrical resistivity tomography implemented thanks to EM31 mapping, geochemical analysis, or visual inspection
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Temporal induced polarization (for chargeability) profiles implemented thanks to electrical profiles
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Spectral induced polarization (for amplitude and phase shift) soundings implemented on anomalies detected both on electrical and chargeability profiles.
Literature concerning geophysical characterization of urban landfills is scarce (e.g. Boudreault et al., 2010, Holt et al., 1998, Mundell and Byer, 2004), so the novelty of this paper is based on the fact that (i) the crossvalidation of EM induction, electrical resistivity, temporal induced polarization and spectral induced polarization may differentiate 3 kinds of contaminants (one organic phase, and 2 different mineralized pollutions) with the suited methodology, and (ii) only spectral induced polarization soundings (located on resistivity and chargeability anomalies) may differentiate contaminants that have the same electrical resistivity and chargeability.
Section snippets
Geological setting and site description
Our area of investigation is in a river valley of the Paris Basin (France). On the whole, the valley setup consists of alternating layers of clay, sandstone and limestone of variable thickness, as shown by a geological column noted down close to the site (Fig. 1). This column is representative of the formations present on the site. The first 2 m layer is mainly composed of silty-sand soil and clay (old site coverage), clean sand with a fine grain diameter well sorted, stony ground and debris of
Soil and water sampling
There are more than 15 piezometers, some of them were unusable during our campaigns because they were not maintained (blocked) or inaccessible. Piezometers used to sample groundwater, localized on the map (Fig. 2), are built in the alluvial aquifer except piezometer P1 which is in the chalk aquifer and piezometer P16, which is both in alluvial aquifer and chalk aquifer. Before sampling, the piezometers were purged. Conductivity and pH measurement on the thickness of piezometers showed that the
Soil and water data analysis
The major anions (chloride, sulfate and nitrate) and cations (sodium, potassium, magnesium and calcium) were analyzed by ion chromatography. The chlorinated ethenes, which are the major chlorinated solvent contaminants, were analyzed in head space by gas chromatography. The chlorinated ethenes are PCE, TCE, dichloroethyloene (DCE) and vinyl chlorinated (VC). TOC were analyzed with a ThermoAppolo HiperToc. The contents of metals (As, Cd, Cr, Cu, Ni, Pb, Zn, Hg, Ti, Mn and Fe) and PAHs, and the
Soil analysis
Major results of soil analysis concern very shallow samples extracted between 0 m and 1 m deep, and are reported in Table 2. Naphthalene represents about 90% of PAHs and it is present at 1 m deep on S1 and S2 and at 0.4 m deep on S3. Metal concentrations decrease with depth. Hydrocarbon concentrations are lower between 0.25 and 0.6 m deep and they are more important at 1 m deep. Long chains (between 16C to 40C) are more represented in surface (> 97%) than deeper (44%). At 1 m deep, S2 is more
Synthesis of results
The aim of this study is to show how a non-destructive geoelectrical methodology associated to a few borehole information permits to successfully characterize a complex problem as diagnosis of urban contamination. Effectively, with few geochemical data we may distinguish 3 different contaminated zones (one organic phase and two high mineralized zones) in an urban context.
About 5 h of data acquisition was enough to obtain the electrical conductivity map with the slingram method EM31. A large part
Acknowledgments
We thank “Région Aquitaine” and the Agency of Environment and Energy Management “ADEME” for funding.
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