Detection of Soil Contamination Using Electrical Resistivity Tomography and Induced Polarization Methods By Tank Model

Abstract


Introduction
Soil contamination is the accumulation and deposition of many hazardous substances, such as harmful substances and hazardous chemicals, affecting the upper part of the soil.The serious impact of these substances creates environmental imbalances that cause problems in crop production.It may also lead to serious health problems for those who consume crops contaminated by hazardous substances (Salloum and Nizam, 2010).The importance of the soil is generally that it represents the important and principal medium of various organisms, from fine plants and organisms to many animals.It is the first element in the production of food necessary for human life (Salloum and Nizam, 2010).
Iraq faces serious environmental problems, ranging from poor water quality to hydrocarbon contamination, soil salinity, and air pollution (Price, 2018).Key issues of soil pollution include oil industry expansion; degradation and acidification of soil, e.g., excess nitrogen fertilizer (NO3-N contamination of soil and groundwater) in agricultural communities; pesticides or agricultural chemicals, over time, added heavy metals to soil; vast quantities of debris and waste, and other hazardous material.Industrial sites and critical infrastructures such as power and water networks store or process toxic chemicals.In general, sewage, waste, and poor drainage are considered to be the most important sources of soil contamination (Price, 2018).
Geophysical surveys are more commonly used to identify and monitor polluted areas than geochemical analysis methods, which require sampling from different sites and depths of the soil (Brewster, 1995;Al-Heety et al., 2022); and addresses environmental and engineering issues (Dahlin, 1996;Pellerin, 2002;Al-Heety et al., 2022).
Resistivity tomography (ERT) uses the Direct Current (DC) technique and is thought to be the most effective method for detecting subsurface resistivity distribution.This distribution may be affected by physical characteristics such as lithology, porosity, the amount of water saturation, and the chemical content of the water (dissolved salts).
Investigations of environmental contaminants are now included in ERT applications.ERT can check for sewage leakage, investigate pipeline or tank leak incidents, identify the size and depth of landfills (Ayolabi et al., 2013;Batayneh, 2005;Godio and Naldi, 2003), locate contaminated soil locations, and analyze the pollutants' lateral and vertical distributions.ERT is used to detect the presence of a non-aqueous phase liquid (NAPL) contaminant plume in the subsoil (Atekwana et al., 2000;Wang et al., 2015).
The induced polarization (IP) method is employed in environmental investigations.(Alegria et al., 2009;Cardarelli and Di Filippo, 2009).Slater and Lesmes (2002) used the IP technique to determine the pollution resulting from the infiltration of polluting liquids in the areas of the waste location, in addition to determining the groundwater in the sandy layers affected by salt infiltration from the mud layers beneath them (Cardarelli and Di Filippo, 2004).The IP method in the time-domain can produce spectral parameters that are equivalent to those from frequency-domain surveys.The IP survey provides additional information for the electrical resistivity survey.Both methods investigate the variation in the medium physical properties and the chemical surface of the particles.So, their use has recently become more popular due to improvements in automatic data collection and the development of 2D and 3D inversion software (Loke and Barker, 1996;Cardarelli and Fishanger, 2008).The study aims to apply a two-dimensional electric method (resistivity and IP) to a synthetic medium (tank model) for different types of soil contamination.It also assesses the electrical resistivity or IP methods to detect a contaminated zone with accuracy and efficiency.

Experimental Tank Facility and ERT & IP Data Acquisition
The successful use of model tanks in geophysical experiments has been reported in the literature (e.g.: Slater et al., 2002;Adeyemi et al., 2006;Fajana et al., 2020;Eluwole et al., 2021).A tank model is an important tool for conducting scientific research under controlled conditions to obtain real results with high accuracy.It allows researchers to test geophysical methods on a wide range of problems in the laboratory that may be difficult to conduct in their natural environments.The experiment was performed in a nonmetallic water channel constructed in the hydraulic laboratory of the Dams and Water Resources Center of the University of Mosul as a tank model (dimensions 50.90.9 m) covered from the inside with a layer of insulation material to prevent the electric current from penetrating to the ground as shown in Fig. 1.The tank is constructed from concrete.The tank can hold about 4 m 3 of material (e.g., over 6 tons of soil).The tank was filled with sandy clay soil and was well compacted to simulate natural agricultural soil.The electrical resistivity and induced polarization measurements of this uncontaminated medium were taken to be adopted as a reference model, with the measurements for the subsequent mediums compared.A number of the most common soil contamination problems have been proposed for testing, as follows: • Salt is present in almost all water (even rain) and accumulates due to evaporation.Plants only use water and very few mineral salts, meaning they become contaminated with salt over time.(Shrivastava and Kumar, 2015).• Liquid oil creates an impenetrable barrier between soil particles and the air, polluting and killing all soil content and living creatures.According to Kavlock et al., (1996), petroleum products from one kind of oil (approximately 4 l of oil) can ruin 1 million gallons of water.The sources of oil contamination are leakage from the ground, human causes, oil exploration operations or accidents during the transportation of oil, and leaks from subsurface oil reservoirs due to cracks.• Sewage water contamination: most of the country lacked sufficient filtration systems for sewage, which led to contamination of the soil and groundwater.Some countries use sewage water to irrigate agricultural lands because of its high nutritional value, which improves plant growth but results in the accumulation of contamination (Ullah et al., 2012).• In cities with high air pollution, nitrogen oxides and sulfur oxides mix with water vapor molecules, and they generate nitric acid and sulfuric acid.Acid rain changes the layers of agricultural soil and dissolves a variety of elements and compounds, which enter the soil cavity (Thomas, 1996).• Fertilizers contain phosphate and nitrate compounds to boost soil fertility and increase production.Excessive use of these fertilizers in proportions that surpass the plant's real needs leads to soil contamination.A large portion of these pesticides remain and contaminate the soil (Savci, 2012).
• Burying waste at shallow depths and exposing it to heat and weathering factors causes the decomposition of a large proportion of its chemical components.With the passage of time and exposure to seasonal rains, these materials penetrate layers and contaminate the soil beneath them (De Donno and Cardarelli, 2017).One by one, contaminated water was added with different contaminants and controlled in quantity and concentration to give a real polluted soil range.(10) liters of each of the following fluids were added: saltwater (NaCl) (salinity concentration greater than 1500 ppm), hydrocarbon (gas, oil), and acid water (distilled water with HCL added at pH < 4).Sewage (from the drain stream of a residential area), contaminated water was added with chemical fertilizer (25 CCS for 10 liters), and finally, about 2 kg of solid waste (plastic bottles and cups, paper) were added.The addition was made by pouring the contaminated liquid onto the surface of the soil and waiting for a few minutes for it to penetrate the soil, Then the measurement is made.The medium soil was replaced with the new soil at each test.
The laboratory ERT and IP data were carried out by using the ABEM Terrameter SAS 4000 Resistivity, an IP meter powered by an external source (12V battery).By using two multi-core cable reels with 42 stainless steel electrodes distributed in a wenner arrangement of electrodes with a space interval of 10cm (Fig. 2).The total profile length was (4.20) m.The penetration depth was 60 cm, which is sufficient for plants to be affected by soil contaminated with it.Among the common arrays, the wenner has the strongest signal strength (Shanshal and Al-Heety, 2020).The choice of the array for the survey was based on obtaining a good signal-to-noise ratio for electrical resistivity measurements in the horizontal and vertical directions.The ERT and IP data processing were implemented in the Res2Dinv X64 software (Geotomo, 2018).A 2D inversion algorithm was used to produce the ERT inverted models (Loke and Barker, 1996).This inversion technique demonstrates a smooth variation in the resistivity distribution and is based on a regularized least-squares optimization (Loke, 2004).Rapid least-squares inversion of apparent resistivity pseudosections by a quasi-Newton method was used for inversion modeling.The latter inversion attempts to minimize the square of the difference (in terms of RMS) between the observed and calculated apparent resistivity values.Res2Dinv calculates chargeability in two phases by performing two DC resistivity forward profiles, one for the DC value and one for the late-time resistivity, according to Oldenburg and Li (1994).The resistivity and essential charge ability are the parameters recovered in the Res2Dinv inversion.To aid in the recognition of horizontal structures, the Res2Dinv inversion was performed with smooth constraints and a vertical to horizontal flatness filter ratio of 0.3 (Fiandaca et al., 2013).

Results
The Electrical resistivity and IP decay measurements were performed for all different contamination mediums, respectively, and the inverted resistivity and chargeability models of each contamination were carried out to evaluate the ability to detect contamination within the soil.The results have described the characteristics of each type of contamination.

Saltwater Contaminant
The natural sandy clay soil (medium uncontaminated) has resistivity values ranging between 1 and 76 Ω.m as shown in Fig. 3a, and the contaminated medium with saltwater shows electrical anomalies with low resistivity values ranging between 10 and 46 Ω.m.The addition of contaminated water results in a decrease in the electrical resistivity from 1-72 Ω.m to 10-40 Ω.m, as shown in Fig. 3b.In this case, the dissolved sodium chloride (NaCl) will increase the concentration of dissolved electrolytes in the natural moisture content of the soil, which leads to an increase in electrical conductivity (decreased resistivity).

Hydrocarbon Contaminant
The natural sandy clay soil (medium uncontaminated) has resistivity values ranging between 1 and 76 Ω.m as shown in Fig. 5a.In Fig. 5b, the model reveals an anomaly of a relatively high electrical resistivity ranging between 100 and 344 reflecting the medium after contamination by hydrocarbons, which consists of a complex mixture of hydrocarbons, other organic components, and some organometallic constituents, including oily substances, which are non-conductive and increase the polluted medium's resistance values.According to Hayley et al. (2007), changes in the fluid electrical resistivity contaminated with hydrocarbons are due to changes in the fluid viscosity.In Fig. 6b, the contaminated zone shows a very low time decay in the model, which is less than 1 msec.Because oily materials don't conduct electricity well, they have low charging capabilities due to the change in physical and chemical properties of the medium.The low anomaly may be an indicator of the contamination's existence.

Sewage Contaminant
The natural sandy clay soil (medium uncontaminated) has resistivity values ranging between 1 and 76 Ω.m as shown in Fig. 7a.In Fig. 7b, the model represents the medium after it has been exposed to sewage contamination, with relatively low resistivity values ranging from 13 to 30 Ω.m.Sewage contains a high percentage of water 99% with the remainder consisting of colloidal, suspended, and dissolved solids.Which is the reason for decreasing values of resistivity or increasing the medium's electrical conductivity.In Fig. 8b, the model shows the medium after contamination, with the contaminated zone having relatively low time decay values of less than 1 msec.This means the chargeability of this medium is very poor due to the change in physical and chemical properties of the medium.

Acid Water Contaminant
The natural sandy clay soil (medium uncontaminated) has resistivity values ranging between 1 and 76 Ω.m as shown in Fig. 9a.In Fig. 9b, the model reflects the medium after contamination.The contaminated zone has relatively low resistivity values ranging from 16 to 30 Ω.m.The low resistance values are due to the medium's being contaminated with acidic water and having a low pH (pH = 5), which enhances the medium's electrical conductivity.In Fig. 10b, the model represents the medium after contamination.We note the contaminated zone with a very low time decay of less than 1msec, which indicates that pollution with acidic water loses the medium's chargeability due to the change in physical and chemical properties of the medium.

Agricultural Fertilizer (Ammonia) Contaminant
The natural sandy clay soil (medium uncontaminated) has resistivity values ranging between 1 and 76 Ω.m as shown in Fig. 11a.In Fig. 11b, the model represents a contaminated medium with ammonia nitrate.The contaminated zone has low resistivity values ranging from 16 to 30 Ω.m.The ammonia molecules are responsible for this decline.It raises the acidity of the soil, which leads to an increase in the medium's electrical conductivity.In Fig. 12b, a significant decrease in the values of the time decay in the contaminated zone of less than 1 msec indicates that pollution with ammonia fertilizer loses the medium's ability to carry an electric charge due to the change in physical and chemical properties of the medium.

Solid Waste Contamination
The natural sandy clay soil (medium uncontaminated) has resistivity values ranging between 1 and 76 Ω.m as shown in Fig. 13a.In Fig. 13b, the model represents a contaminated medium with solid waste.The electrical anomalies represent the contaminated zone, with high resistivity values ranging from 121 to 512 Ω.m.The solid waste contains plastic bodies, which are unconducted materials and have big pores filled with air.The results gave good evidence of the presence of buried waste, its size, and its depth.
In Fig. 14b, a significant decrease in the values of the time decay of less than 1 msec in the contaminated zone represents the medium contaminated with solid waste, which has poor electrical chargeability because it is bad conductive materials.

Discussion
To evaluate the efficiency of the ERT and IP methods in the detection of contamination, a comparison is made between the values resulting from the investigation of the two methods, which are listed in Table 1.When we compare the data in the table, the percentage of the minimum root square of the electrical method models (RMS) for all contaminated media is higher than for the original medium, which may be used as proof of the presence of contamination.In the induced polarization models, the data is of high quality and the inverse models are closer to reality.
The electrical resistivity method produces varied electrical anomalies, as seen in the table above.Electrical resistivity is a function of several soil properties, including the nature of the solid components and the arrangement of voids.It also considers the degree of saturation of water (water content) and the electrical resistance of a liquid (concentration of solute).The resistivity of a water solution is related to the density of electric charges on the surface of the components.These variables affect electrical resistance in a variety of ways and to varying degrees.All tested contamination (except hydrocarbons and waste) decrease the resistivity ranges, but the amount of the decrease varies from one type to another (Samoulian et al., 2005).The resistivity of soil pores is related to the flow of ions in the fluid that fills the pores and changes when dissolved salts are present.This is because the electric current in the soil is primarily electrophilic, meaning it is dependent on ion movement in the pore water.The conductivity of water is determined by its concentration and viscosity (Scollar et al., 1990).Water and salts in soil can be used to make electricity, as the amount of water in the soil determines the conduction paths accessible.The conductivity of the various ions present in the solution is not affected in the same way as the temperature or salinity of the soil.In the same soil, there can be different levels of ionic activity and therefore different potential electrical conductivities, due to changes in concentration and ionic composition in different sections of the soil.This means that different soils can have the same concentration but distinct ionic compositions, resulting in different potential conductivities (Samoulian et al., 2005).
An IP survey can be used to give an insight into the conductivity of the pore space fluid chemistry and/or pore water content of a given material.Similar to a traditional resistivity survey, an IP survey also gives information on the chemical and physical properties of that material (Binely and Kemna, 2005).Electrical conduction within the pore fluid controls the soil's resistivity, but processes at the fluid-grain interface have a significant impact on its conductivity.The resistive and capacitive properties of the soil are measured using an IP survey.As a result, IP surveys can provide important information about lithology and grain-surface chemistry.The electrical method is more significant than the inductive polarization method to measure soil quality, especially when the pore water is mineralized (containing dissolved minerals).When appropriately calibrated and based on an analytical examination, this feature can be used to measure the quality of the soil environment (Loke et al., 2013).Although the effects of regions are simple to recognize, the causes of induced electric polarization also include the polarization of the interfaces of metallic solutions and the effects associated with the coupling of various ion flows.As a result, it is going to be difficult to calculate the polarization factor from electrical measurements (Marshall and Madden, 1959).The electrical resistivity method gives a better indication of all kinds of contamination than the IP method.This is because IP anomalies are mainly caused by structural properties (mainly clay) rather than physical ones, which leads to a stronger relationship between structural properties and IP readings (Slater and Lesmes, 2002).Slater and Lesmes (2002) point out that when contaminants impact the soil, the physical and chemical characteristics change, and the soil's sensitivity to induced polarization declines.

Conclusions
Environmental problems are investigated using geophysical exploration methods.The most effective way of detecting the contaminated zone is by comparing pre-and post-investigations of the medium using the tank model.The electrical resistivity method is better than IP.The presence of contamination can be detected just by the presence of low anomalies in the model.It is due to the low electrical polarization capacity of the medium that affects the physical and chemical methods, except in the case of saltwater contamination, where the two methods are almost identical.Electrical chargeability readings drop to the lowest level when exposed to all types of contamination properties of the soil.
The range of resistance values for contaminating media was extracted separately, where the resistivity value of sandy clay soil contaminated with saltwater ranged from 10 to 46 Ω.m.The resistivity of hydrocarbons-contaminated sandy clay soil ranged from 100 to 344 Ω.m.The resistivity of sandy clay soil contaminated with acidic water (pH 4) ranged from 16 to 30 Ω.m.The resistivity of sandy clay soil contaminated with sewage water was observed in the range of 13 to 30 Ω.m.Ammonia-contaminated sandy clay soil has resistivity values ranging from 16 to 30 Ω.m.
Solid waste contamination sandy clay soil has resistivity values ranging from 121 to 512 Ω.m.In soil contaminated with salt water, the induced polarization method may be considered a successful method to support the resistivity method for investigating the location and depth of the contamination zone.The interpretation of IP data is only possible by distinguishing between the variations in the distribution of chargeability parameters in the medium.Geophysical methods are effective in terms of time, cost, effort, accuracy, and they are considered an environmentally friendly way to determine the zone of contamination in the soil.

Fig. 10 .
Fig. 10.2D Electrical chargeability model for (a) uncontaminated medium and (b) contaminated medium with acid water

Fig. 11 .
Fig. 11.2D Inverse ERT model for (a) uncontaminated medium, and (b) contaminated medium with agricultural fertilizer

Table 1 .
Mediums and corresponding resistivity and Changeability ranges with RMS error