**1. Introduction**

 One of the greatest worldwide concerns is related to the increase and destination of waste volume produced by world's population. Waste disposal imposes a serious risk of contamination of groundwater by the migration of contaminants from the waste site to the local environment. To protect the environment from this type of contamination, waste disposal sites must follow regulation standards.

 Modern landfill facilities are specially engineered for the disposal of solid waste. They operate to ensure protection of the environment from groundwater contamination and landfill gas produced by residue degradation. Municipal solid waste (MSW) landfills are areas properly prepared to receive household waste, as well as other types of nonhazardous wastes. Ideally, MSW landfill projects should ensure that landfills: are built in suitable geological areas away from faults, wetlands, flood plains or other restricted areas; include flexible membranes (i.e., geomembrane) overlaying compacted clay soil lining the bottom and sides; collect leachate (formed when rain water filters through wastes) for treatment and disposal; and operate according with standard practices (compacting and covering waste frequently with soil) [1].

However, inadequate waste disposal activities were a frequent routine in many developing countries until recent years. The direct disposal of residues on the ground, without any selection of waste types and without any protection to avoid soil and groundwater contamination, had been a common practice for decades. Inactivated waste disposal sites can continue to contaminate the groundwater, especially when they are located in hydrological vulnerable areas.

Contamination plumes are formed when leachate reaches the local water table and contaminating the groundwater. **Figure 1** illustrates the formation of a contamination plume. Contaminants are diluted into groundwater and are carried through hydrodynamic dispersion along with the groundwater flow.

Leachate typically presents high concentrations of total dissolved solids (TDS), ammonia, organic carbon, chloride, and iron, among other organic and inorganic contaminants [2]. Even though the exact chemical composition of the leachates produced by MSW disposal sites, they can be associated with high electrical resistivity values. This makes geophysical electrical methods ideal to detect contamination plumes generated by waste disposal sites.

The advantages of employing noninvasive geophysical methods over direct sampling are cost and time consuming of analysis. Geophysical methods can therefore optimize investigation campaigns, maximizing the investigated area and minimizing drilling needs. Another important advantage is that geophysical data are real time measurements of the investigated system. However, due to the inherent ambiguity of geophysical methods, it is often necessary to use direct measurements to validate interpretation.

 According with Sharma [3], geophysics can assist waste disposal problem by: locating geological features of interest (i.e., faults and contacts); locating aquifers and hydraulic active features for contamination plume detection; and detecting the waste volume and searching for areas appropriate for waste disposal. Electrical methods have been widely used as a tool for environmental investigations. Its application in contaminated site investigations consists in detecting and mapping the affected area and providing information about groundwater flow and saturated zone and bedrock depth. When detecting the impacted area, electrical imaging

**Figure 1.**  *Contamination plume formed by waste disposal site leachate.* 

*Resistivity and Induced Polarization Application for Urban Waste Disposal Site Studies DOI: http://dx.doi.org/10.5772/intechopen.81225* 

techniques can not only map but also infer contaminant immobilization and long term monitoring.

The direct current (DC) resistivity method is frequently conducted simultaneously to time domain induced polarization (TDIP). In this measurement setup, electrical current is applied as a reversal square wave. When the current injection is "on," the observed voltage, *Vc* [mV], is measured. When the current injection is "off," the voltage decay curve, *V(t)*, is registered during a period of time [*t1*,*t2*], from with chargeability, m [dimensionless], can be computed:

$$dm\_1 = \frac{1}{V\_\varepsilon} \int\_{t\_1}^{t\_2} V(t)dt\tag{1}$$

 In the frequency domain, resistivity and induced polarization (IP) methods consist in injecting an alternating current and measuring the amplitude and phase lag between applied current and measured potential, from which in-phase and ′ ′′ ′ quadrature components of resistivity, ρ<sup>∗</sup> [ohm.m], can be calculated: ρ and ρ . ρ represents ohmic conduction (energy loss), whereas ρ′′ represents media polarization (energy storage). While m is associated with the intensity of the polarization effect, normalized chargeability, *mn* <sup>=</sup>*m*/ρ [dimensionless], is considered a direct estimate of polarization, analogous to ρ′′ [4].

Distinct mechanisms can generate the polarization response of the media. The IP phenomena are observed when metallic bodies and metallic dispersed particles are present in the subsurface, resulting from differences in ionic mobility in the metallic particles and ions in the pore fluid (electrode polarization). Another source of polarization is ion selective zones formed by clay particles and/or pore throats (membrane polarization). Charge motion along the electrical double layer (EDL) formed at the mineral surface also contributes to polarization (electrochemical polarization) [5, 6].

Resistivity is traditionally applied in waste disposal sites and contamination studies. However, resistivity does not separate different zones in these sites, and low resistivity zones are associated with the whole affected zone, both by wastes and leachate. Johansson et al. [7] and Leroux et al. [8] argue that this limitation can be suppressed by taking into account the normalized chargeability. Despite this seems to be efficient in environments poor in clay content which is not the case of Brazil [9]. According to Slater and Lesmes [4], normalized chargeability is highly influenced by clay content.

 Several examples in the resistivity/IP literature report excellent applications of these methods for waste disposal site studies. Bernstone et al. [10] conducted a pre-remediation investigation over a MSW disposal site and identified preferential paths for the leachate. Gazoty et al. [11] obtained a 3D shape of the waste body of the former landfill in Denmark. Maurya et al. [12] investigated the migration of leachate from a landfill in Denmark by 2D and large-scale 3D electrical resistivity tomography. 2D profiles showed variations along the groundwater flow and the plume extension across the flow directions. The 3D model revealed low resistivity variation patterns corresponding to differences in the ionic strength of the landfill leachate.

In this chapter, we will present case studies conducted over municipal waste disposal sites in Brazil. We intend to show the ability of resistivity and IP method in providing useful and fundamental information for investigations of waste disposal and its impact on the environment.

## **2. MSW disposal site in Ribeirão Preto, SP, Brazil**

 Ribeirão Preto is a growing population city in Stet of São Paulo, and the water supply is almost completely provided by groundwater water. The quality of the

**Figure 2.**  *Local map with survey lines.* 

 groundwater in this region is therefore of critical importance. In this MSW disposal site that operated from 1974 to 1990, wastes were disposed inside two trenches of approximately 15-m deep. **Figure 2** shows the location of the trenches, geophysical lines and soundings, and groundwater wells. The area consists of a surface water divider, and the bedrock is Botucatu Formation sandstone, beneath unconsolidated material composed by sand residual soils and clayey material from Serra Geral basalts [13]. Decomposed basalt is observed at north and south from the trenches, between sandstones and colluvium. In the center, the trench base is in direct contact with the sandstones. The hydrogeological scenario is composed by a deeper aquifer (more than 30 m) within Botucatu sandstone and a shallow aquifer (~ 10 m) sustained by clayey materials originated from basalt alteration. Monitoring wells confirm contamination of groundwater and provide the groundwater flow direction as from southwest to northeast. **Figure 3** presents a geological session of the area, based on the geological wells and geophysical data. The suspended aquifer is assumed to form in the north portion of the trenches, being contaminated by leachate, and the main aquifer that is contaminated since the wastes were directly disposed above the sandstone.

 In this site, resistivity and time domain induced polarization profile lines were carried out with dipole-dipole array (10 m of spacing and six investigation levels) using Syscal Pro (Iris Instruments). Metallic electrodes were used for current injection, and nonpolarizing electrodes (Cu/CuSO4) for potential measurement. Current was injected in cycles of 2 s, and the IP measurements were recorded with 160 ms delay after current shut off. Data were inverted with the software RES2Dinv [14] generating 2D models that allowed a detailed analysis of the relationships between the natural materials and the trenches filled of waste.

**Figure 4** presents the resistivity and chargeability models of line C4. The resistivity session clearly shows the two trenches filled with wastes and leachate marked by low resistivity values (<15 ohm m). Although the horizontal limits of the trenches

*Resistivity and Induced Polarization Application for Urban Waste Disposal Site Studies DOI: http://dx.doi.org/10.5772/intechopen.81225* 

**Figure 3.**  *Profile A–A'.* 

**Figure 4.** 

 are well marked by resistivity, the bottoms of the trenches were not detected. We interpret these results as an indication that the permeable sandstones directly below the wastes as being filled by contaminated water, giving low resistivities. The chargeability session on the other hand, successfully detects the wastes bodies, marked by high chargeability values (>20 mV/V). The sandstone groundwater contaminated by highly saline leachate produces a low chargeability zone (<10 mV/V), identifying the trench base. The same geophysical fingerprint is observed for line C3 (**Figure 5**). The trenches are marked by low resistivity and high chargeability values, while the infiltration zone at the trench bases presents low resistivity and chargeability values. The high chargeability observed in the trenches is explained by the presence of metallic and polarizable material that composes the wastes, whereas the low chargeability signature of the contaminated groundwater is attributed to the decrease of ionic mobility due to increase of solution concentration [12].

Line C1 (**Figure 6**) is located outside and downstream from the trenches. The resistivity session detects the upper portion of the contaminated aquifer, identified by resistivity values lower than 50 ohm m. A polarization anomaly is observed in the position of 135 m along the survey line, produced by the metallic coating of part of the groundwater well P2. The contamination plume is not as well defined by

*Resistivity and chargeability sessions of line C4.* 

#### **Figure 5.**

*Resistivity and chargeability sessions of line C3.* 

#### **Figure 6.**

*Resistivity and chargeability sessions of line C1, outside and downstream from the trenches.* 

chargeability as it is by resistivity, but overall this region shows low chargeability values (8 mV/V). The behavior of chargeability against salinity (and clay content) does follow a linear trend, and its interpretation is not always straightforward. Lithological variations might also be affecting chargeability, competing with the salinity effect.

 **Table 1** presents chemical analysis of groundwater wells. High TDS values explain the observed low resistivity values inside the trenches. P2 TDS values confirm groundwater contamination. Well P19 (60 m deep) also present high TDS concentration, confirming the contamination of the deeper aquifer.


*Resistivity and Induced Polarization Application for Urban Waste Disposal Site Studies DOI: http://dx.doi.org/10.5772/intechopen.81225* 

#### **Table 1.**

*Important water parameters of monitoring well analysis.* 

**Figure 7.**  *3D resistivity model of the site, showing the source area (dashed lines) and contaminant plume flowing NE.* 

**Figure 7** shows a 3D resistivity model obtained by the interpolation of all geophysical data. The obtained image shows the contamination source and a contamination zone flowing toward NE to the shallower aquifer.
