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ANNALS OF GEOPHYSICS, VOL. 50, N. 3, June 2007
Key words landfill hydrogeological modeling salinity resistivity geophysical forward and in-verse modeling
Characterizing and monitoring the under-ground conditions of landfills and the locationof subsurface contaminants has always been achallenge. Conventional environmental moni-
toring used to determine the spread and fate ofgroundwater contamination is performed by theexpensive and labour intensive task of drillingclosely spaced boreholes for point sampling(Granato and Smith, 1999). More advanced,non-invasive geophysical techniques, less cost-ly and more environmentally friendly, were de-veloped as tools for interpreting the under-ground structures without disturbing them.Contaminant plumes usually have a sufficientlyhigh contrast in physical properties against thehost media due to an increase in dissolved saltsin the groundwater and a resulting decrease inpore water resistivity; therefore, they may bedetected by geo-electrical techniques.
Early studies have shown that the electricalresistivity method is one of the most efficient
Time-lapse electrical resistivity anomaliesdue to contaminant transport
Monica Radulescu (1), Chifu Valerian (2) and Jianwen Yang (3)(1) Conestoga Rovers and Associates, Waterloo, Ontario, Canada
(2) Avalanche Mobile Development, Bucharest, Romania(3) Department of Earth and Environmental Sciences, University of Windsor, Windsor, Ontario, Canada
AbstractThe extent of landfill leachate can be delineated by geo-electrical imaging as a response to the varying electri-cal resistivity in the contaminated area. This research was based on a combination of hydrogeological numeri-cal simulation followed by geophysical forward and inversion modeling performed to evaluate the migration ofa contaminant plume from a landfill. As a first step, groundwater flow and contaminant transport was simulatedusing the finite elements numerical modeling software FEFLOW. The extent of the contaminant plume was ac-quired through a hydrogeological model depicting the distributions of leachate concentration in the system.Next, based on the empirical relationship between the concentration and electrical conductivity of the leachatein the porous media, the corresponding geo-electrical structure was derived from the hydrogeological model. Fi-nally, forward and inversion computations of geo-electrical anomalies were performed using the finite differencenumerical modeling software DCIP2D/DCIP3D. The image obtained by geophysical inversion of the electric da-ta was expected to be consistent with the initial hydrogeological model, as described by the distribution ofleachate concentration. Numerical case studies were conducted for various geological conditions, hydraulic pa-rameters and electrode arrays, from which conclusions were drawn regarding the suitability of the methodologyto assess simple to more complex geo-electrical models. Thus, optimal mapping and monitoring configurationswere determined.
Mailing address: Dr. Monica Radulescu, ConestogaRovers and Associates, Waterloo, Ontario, N2V 1C2, Cana-da; e-mail: email@example.com
Monica Radulescu, Chifu Valerian and Jianwen Yang
geophysical tools in detecting, delineating andmonitoring groundwater contamination (Ben-son et al, 1983; Greenhouse and Slaine, 1983).More recently, Khair and Skokan (1998) inves-tigated the early detection of salt water intru-sion through systematic observation of electri-cal resistivity in selected positions with fixedelectrode arrays. Geo-electrical techniqueswere also used in the detection of gasoline andsaline leaks (Bevc and Morrison, 1991; Rami-rez et al., 1996a), characterization of hazardouswaste disposal sites (Ogilvy et al., 1999; Sim-mons et al., 2002), and the mapping and moni-toring of contaminant plumes in groundwatersystems (Ramirez, 1996b,c; Tekzan, 1999; Nau-det et al., 2003). Electrical resistivity tomogra-phy was used to determine the spreading patternof a saline tracer into groundwater (Singha andGorelick, 2005).
However, field geo-electrical experimentsare very expensive. In addition, characterizinglandfills and location of subsurface contami-nants is problematical as the extent and locationof contaminant plumes are largely unknownduring real site investigation. In order to mini-mize site characterization costs, it is advanta-geous to investigate theoretical responses ofplanned experiments before field programs areinitiated. In addition, geophysical forward stud-ies followed by inversion are essential to the fi-nal interpretation of observed geophysical data.
Electrical conductivity governing the elec-trical field is the key physical parameter used ingeo-electrical exploration. As the dissolvedcontaminant travels with groundwater flow, ad-vection and hydrodynamic dispersion cause thespreading of the contaminant plumes in direc-tions transverse and longitudinal to the flowpath. Consequently, the electrical conductivityof the targets varies not only with space but al-so with time. Thus it is essential to include thetime-dependent feature of electrical conductivi-ty in the forward and inversion studies per-formed during geo-electrical exploration.
The present research used the hydrogeolog-ical modeling of a groundwater contaminantflow path and the resulting distribution of resis-tivity/conductivity as a basis for the geophysi-cal modeling of the geo-electrical structure. Im-ages of the resistivity/conductivity distribution
acquired through forward geophysical compu-tations followed by inversion were expected topredict the initial distribution of concentrationsin the hydrogeological model. The presentedmodels and scenarios were designed to concep-tualize a diverse range of hydrogeological envi-ronments, from simple to more complex, withvarious types of deposits and shapes of theplume.
The methodology used in the paper hasbeen developed by Yang (2005) for the theoret-ical investigation of geo-electrical responses as-sociated with hydrothermal fluid circulation ina mid ocean ridge environment. The study pres-ents the hypothetical, highly conceptualizedscenario of a landfill, designed to assess thesuitability of geo-electrical methods in describ-ing the spreading of leachate plumes into thegeologic environment. Several hypotheticalscenarios of the leachate spreading into differ-ent geological structures were investigated.
The hydrogeological model simulated ground-water flow and contaminant transport by usingFEFLOW 5.1, a 3D finite element code developedby WASY (WASY, 2005) capable of performingnumerical modeling of density-dependent fluidflow and mass transport based on the hydrogeo-logical components included in the conceptualmodel.
Once the spatial-temporal pattern of contam-ination was obtained by hydrogeological model-ing as a distribution of concentrations, the con-centration data was converted into resistivities/conductivities, depicting the geo-electrical mod-el. The conversion was based on the empiricalArchies law, describing the relation betweenconcentration of contaminant and resistivity ofgroundwater.
The resulting electrical anomalies werecomputed by geophysical electrical modeling,using a finite difference software DCIP2D/DCIP3D (2004), with different electrode con-figurations.
Electrical resistivity profiles and pseudosec-tions corresponding to various electrode config-urations were obtained by forward computation
Time-lapse electrical resistivity anomalies due to contaminant transport around landfills
of the geo-electrical model. The direct conver-sion of salinity data acquired over a finely dis-cretized mesh into electrical response offeredthe possibility of simulating the use of verylarge numbers of locations for the electrodes.Based on the results obtained at this stage, theconfigurations of electrodes which describedmost accurately the transient and space depend-ent image of the geo-electrical structure associ-ated with the contaminant were selected. Geo-physical inversion of the electrical data wasthen performed in order to recover the initialgeo-electrical models. The inverted geo-electri-cal models were compared with the electricalmodels obtained previously by hydrogeologicalsimulation.
Further analysis was conducted to evaluatethe most suitable electrode configurations ableto describe the spatial and temporal evolution ofthe contaminating plume originating from thelandfill and consequently, the viability of theproposed methodology.
3. Hydrogeological modeling
Flow modeling is based on Darcys law inporous media. As a density-dependent transportphenomena is simulated, FEFLOW uses the ex-tended Oberbeck Boussinesq approximation(Nield and Bejan, 1999). The solution of thegoverning equations is achieved using an im-plicit adaptive time stepping scheme. FEFLOWperforms calculations in discrete time steps, im-posed by the stability criteria required duringthe numerical simulation. Numerical stabiliza-tion is achieved with Petrov-Galerkin least-square upwinding, an alternative numericalscheme used to solve advective dominant flowand transport (Nguyen and Reynen, 1984).
A two-dimensional transient ground-waterflow model was developed, describing variousgeological settings, according to different hy-draulic conditions. The hydrogeological modelsare displayed in fig. 1a-c. The hydrostratigraph-ical conceptualization of the models was cho-sen to depict gradually more complex geologi-cal and hydraulic settings in order to assess theviability of the proposed methodology on awide range of conditions.
Triangle, a specialized code for creating two-dimensional finite element meshes (Shewchuk,1996, 2002), was used to build Delaunay trian-gulations during mesh generati