Environmental Monitoring and Assessment (2005) 109: 147–160 DOI: 10.1007/s10661-005-5845-5
c Springer 2005
A NOVEL TECHNIQUE FOR MONITORING CONTAMINANT TRANSPORT THROUGH SOILS P. R. KUMAR1,∗ and D. N. SINGH2 1
Department of Civil Engineering, Faculty of Technology, Alemaya University, DireDawa, Ethiopia; 2 Department of Civil Engineering, Indian Institute of Technology Bombay, Powai, Mumbai, India (∗ author for correspondence, e-mail: rajeev
[email protected])
(Received 30 March 2004; accepted 1 November 2004)
Abstract. An experimental setup which is capable of simulation as well as monitoring contaminant transport through soil mass has been developed. Efficiency of the experimental setup has been demonstrated by comparing the results obtained from the argentometric method. Reynolds number (Re ) and the Peclet number (Pe ) have been found to be less than unity. This indicates that flow of the solute through soil mass is laminar and the dominant contaminant transport mechanism is diffusion. Keywords: clays, contaminant transport, diffusion, monitoring, Peclet number, Reynolds number, silty soil
1. Introduction Due to excessive usage of pesticides and fertilizers in various agricultural activities, and the indiscriminate disposal of hazardous wastes, such as radionuclide wastes, contamination of the soil is becoming a matter of great concern to mankind. These chemicals interact with water and contaminate the ground water regime as well as the subsurface soil layers. Such a situation calls for continuous monitoring of the chemicals present in the soils, and their migration through it, subsequently. This necessitates development of a technique that can be used to study and monitor the interaction and migration of various chemicals in the soil–water system. Various geo-chemical processes control the interaction and migration of these chemicals when they come in contact with the soil–water system (Folkes, 1982; Mackay et al., 1985; Rowe, 1988). Many researchers have employed mathematical models or numerical methods (Freeze and Cherry, 1979; Pinder, 1984; Rowe and Booker, 1985; Shackelford, 1995) to study migration of contaminants through the soil mass. Input parameters for these models are obtained by conducting laboratory column tests (Crooks and Quigley, 1984; De Smedt and Wierenga, 1984; Gillham et al., 1984; Rowe et al., 1985; Shackelford and Daniel, 1991; Badv and Rowe, 1996) wherein the soil and pore water samples are collected from different locations along the length of the soil sample (column) to determine the concentration of the contaminant present. Using these data, the concentration profiles for a contaminant can be developed as a function of time for a given soil (Crooks and Quigley, 1984;
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Acar et al., 1985; Shackelford, 1988; Acar and Haider, 1990). However, this is a cumbersome method and suffers from several disadvantages such as changes in the pore structure of the soil column due to sampling and chemical analysis of the collected samples is quite tedious. This calls for development of a technique that is efficient in detecting the presence of contaminants in the soil mass, without sampling the soil and the pore water. Efforts were made to develop a soil contaminant detector (SCD) for determining concentration of contaminants in the soil mass. NaCl solution, which is a non-reactive contaminant, has been used to demonstrate efficiency of the SCD in detecting presence and migration of Cl− through locally available silt and clay samples. Validity of the technique has been demonstrated with the help of argentometric method (APHA, 1989). As several factors such as the effect of compaction energy, degree of saturation and effect of clay content play important roles, their influence on the migration of contaminants through the soil mass has been studied in detail.
2. Details of the Technique The technique developed for the purpose is a non-destructive testing methodology and deals with development of an instrument, the soil contaminant detector (SCD), and is depicted in Figure 1. This consists of an electrode array, a signal conditioning circuit, a data acquisition module and a computer for processing the obtained data. The presence of contaminant(s) in the soil mass can be detected by sending a low frequency sinusoidal signal into it and recording the differential output voltage, OV, in mV, across each pair of the electrodes. To avoid changes in the water content, soil structure, and pore-fluid chemistry i.e., polarization of the pore-fluid, an alternating
Figure 1. Basic schematic diagram for the soil contaminant detector, SCD.
A NOVEL TECHNIQUE FOR MONITORING CONTAMINANT TRANSPORT
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current has been used as suggested by earlier researchers (Yong and Hoppe, 1989; Abu-Hassanein et al., 1996).
3. Experimental Investigations 3.1. M ATERIAL
PROPERTIES
Locally available silty soil, denoted as S, and commercially available white clay, denoted as C, were used in the present study. By mixing 60% white clay and 40% silty soil, a soil mix CS, was created. Minerals present in soils S and C were identified with the help of XRD (X-ray diffraction) analysis, using an XRD-spectrometer (RIGAKU, Japan). The presence of anorthite, albite, illite, montmorillonite, and microcline in soils S, and halloysite, muscovite, and illite in soil C has been confirmed (JCPDS, 1994). The specific gravity of the soil samples has been determined as per ASTM D854-92. The specific surface area of the soil samples has been determined as per ASTM C-204-84, using a Blaine’s apparatus and Portland cement as the standard material. For obtaining the gradational characteristics of the soil samples, dry sieving along with the hydrometer analysis has been conducted (ASTM D-422-63). The standard Proctor compaction characteristics of the three soil samples were obtained in accordance with the guidelines provided by ASTM D-698-91. Table I presents details of the soil characteristics. 3.2. C ALIBRATION
OF THE SOIL CONTAMINANT DETECTOR ( SCD )
Calibration of the SCD has been done with the help of these soils corresponding to their virgin (mixed with distilled water) and contaminated (mixed with NaCl solution of different concentrations) states. For calibration of the SCD, six soil samples (S1, S2, S3, C1, C2, C3, CS1, CS2 and CS3), as depicted in Table II have been selected. These soil samples correspond to different compaction states (i.e., degree of saturation, Sr , dry density, γdry , molding moisture content, w and saturated permeability, ksat ). A 1 N NaCl solution was prepared by dissolving 58.44 g NaCl in 1 L of distilled water. Similarly, solutions with different concentration of NaCl were prepared. However, solutions of higher concentration (>1 N) were not prepared for the present study due to the fact that at higher concentration the effect of van Der Waal forces (electrostatic attraction based upon closeness of atoms) on the electrical conductivity of the NaCl solution cannot be measured accurately (Worthington et al., 1990; USEPA, 1990; Butler and Knight, 1995). An appropriate amount of oven-dried soil is mixed with different weight percentages of NaCl solutions, of different concentrations (=0.000001–1 N), and is
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TABLE I Properties of soils Soil Property Physical characteristics Specific gravity Specific surface area (cm2 /g) Particle size distribution Clay (%) (
