A TDR SYSTEM FOR SUBSURFACE POLLUTANTS DETECTION (II) : APPLICATION AND ANALYSIS

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1 A TDR SYSTEM FOR SUBSURFACE POLLUTANTS DETECTION (II) : APPLICATION AND ANALYSIS Abdel-Mohsen O. Mohamed, R. A. Said 2 and N. AlShawawreh 2 Civil Engineering Department, UAE University, P.O. Box: 7555, Al Ain, United Arab Emirates; Mohamed.a@uaeu.ac.ae 2 Electrical Engineering Department, UAE University, P.O. Box: 7555, Al Ain, United Arab Emirates; rasaid@uaeu.ac.ae Paper Submitted to: Symposium TDR2: Innovative Applications of TDR Technology, Infrastructure Technology Institute, Northwestern University, Evanston, Illinois, September 5-7, 2. March 2

2 A TDR SYSTEM FOR SUBSURFACE POLLUTANTS DETECTION (II) : APPLICATION AND ANALYSIS Abdel-Mohsen O. Mohamed, R. A. Said 2 and N. AlShawawreh 2 Civil Engineering Department, UAE University, P.O. Box: 7555, Al Ain, United Arab Emirates; Mohamed.a@uaeu.ac.ae 2 Electrical Engineering Department, UAE University, P.O. Box: 7555, Al Ain, United Arab Emirates; rasaid@uaeu.ac.ae ABSTRACT The proposed work utilizes a TDR based system for the in-situ detection of subsurface pollutants. Natural soil samples from Al Ain, UAE and calcium solutions were used in this investigation. Three conjugate anions of chloride, sulfate and carbonates were used for the preparation of the calcium solutions. All calcium solutions were maintained at a fixed concentration of ppm. Specimens with various moisture contents were prepared and used in the testing program. The TDR system was modeled using transmission line theory and the associated electrical parameters were optimized. Based on the optimized parameters (resistance and capacitance), calibration curves for different conjugate anions and soil moistures were obtained. INTRODUCTION Monitoring techniques relay mainly upon driven wells, lysimeters, and leachate under-drains. Wells are the most common means of monitoring the groundwater contamination. This technique tends to be expensive and time consuming to implement. Timely detection of contaminant plume is obviously dependent on the initial layout and the number of monitoring wells. Unfortunately, wells can sample only a small volume of the aquifer. If samples collected from wells are not representative of the area or conditions for which they are intended, misleading and erroneous conclusions may result. It should be noted that by the time a pollutant becomes detected in a monitoring well, a substantial volume of the surrounding soil and groundwater has already been polluted. In addition, the risk of drilling wells and exploratory holes in unknown hazardous waste sites can be substantial. As the number of holes needed to define a problem area increases, the possibility of puncturing buried containers is increased hence, toxic fumes and liquids may be released. Also, explosions and fire may occur in extreme cases. The limitations associated with present monitoring techniques underscore the need for an alternate approach. Undeniable, early detection and characterization of subsurface pollutants can minimize its negative impact. Therefore, there is an urgent need for the development of a field diagnostic technique, which allows a rapid determination of the extent of pollutants present in subsurface soils. The developed method should assist in locating a leak in the impounding boundary so that a corrective action can be taken to alleviate the problem. It should also be adaptable to a wide range of chemicals, as opposed to being ion specific. Currently there are about 6, substances classified as hazardous by the various environmental agencies.

3 Recently, several researchers have discussed the large potentials of the use of electrical detection techniques for measuring tracer movements in laboratory as well as in field conditions (Mohamed et al., 2; Mallants et al., 994; Ward et al., 994; Wraith et al., 993; Vanclooster et al., 993; Kachanoski et al., 992). These electrical detection methods operate by analyzing the characteristics of a fast rising electric potential signal as it reflects at the probing end of a transmission line (coaxial cable) immersed in a sample under investigation. The characteristics of the reflecting signal are highly influenced by the electrical properties of the specimen, which are dependent on the concentration of the constituents. A common application of the electrical detection principle is the time-domain-reflectometry (TDR) that is widely used as a concentration detection technique. TDR enables the measurement of total resident concentration from both mobile and immobile water regions for a sample volume with a well-defined geometry and allows data collection of both moisture content and salt concentration at a high spatial and temporal resolution. Automation of TDR has made it more powerful for use in the laboratory as well as in field application at remote sites (Heimovaara & Bouten, 99). Application of TDR in the area of contaminant hydrology has been reported (Vanclooster et al., 995; Mallants et al., 994; Ward et al., 994; Vanclooster et al., 993; Kachanoski et al., 992; Elrick et al., 992). The success or failure of TDR technique to accurately predict pollutant concentration depends on the appropriateness of the calibration procedure used. Application of electrical polarization in the area of pollutant detection in subsurface has yet to be developed due to the nature of the flow and the characteristics (signature) of each pollutant. This paper discusses the application of the developed TDR system and analysis of the experimental results. The design and modeling aspects of the developed TDR system are reported previously by Mohamed et al. (2) and Said et al. (2).. Experimental Setup MATERIALS AND METHODS Figure shows the components of the proposed measuring system that consist of pulse generator, oscilloscope, co-axial cables, monitoring probe, and a computer. The scope meter is used to acquire the system response through a measurement point on the transmission line. The measuring probe is connected to a pulse generator and to a scope meter via transmission lines (co-axial cables). The selected co-axial cable had line capacitance of pf/m. The probe was connected to the scope meter using 3.6-meter long co-axial cable. The generated pulse has the following characteristics 8.3 MHz, 4V, and rise and fall times around 3 nano seconds. Oscillosco Oscilloscope Pulse signal V g Transmission Line Z Soil Probe Pulse Probe Figure : A schematic model of the system. Figure 2: The Measurement system (with the probe appears enlarged)

4 During measurement, an electrical pulse is generated periodically by the pulse generator and launched toward the probe. The pulse signal propagating toward the probe appears on the scope meter as it passes a measuring point located at a known distance from the generator on the transmission line. As the pulse reaches the probe end it reflects back on the transmission line with pulse characteristics dependent on the properties of the media surrounding the probe. The reflected pulse signal propagates back toward the generator and thus appears on the scope meter after a delay time with respect to the time of appearance of the original pulse. To characterize the system, a test was conducted by leaving the probe in an open air. In this case a major reflected wave appeared. 2. Soil Material Soil samples obtained from Al-Jimi district in Al-Ain municipality of UAE were used in this investigation. The selected testing procedures were carried out following procedures described by ASTM standards. Measurements of specific gravity and consistency limits were performed according to ASTM D854, test method for specific gravity of soils, and ASTM D422, method for particle-size analysis of soils. Determination of compaction parameters and permeability coefficients (at maximum dry density) were carried out following ASTM D698, test methods for moisture-density relations of soils and soil-aggregate mixtures, and ASTM D2434, test method for permeability of granular soils, respectively. ph and conductivity measurements of : soil-water extract were conducted according to ASTM D293, test method for ph of water, and ASTM D25, test methods for electrical conductivity and resistivity of water, respectively. A summary of these results is shown in Table. Table : Selected properties and composition of tested soil. Soil Properties Geotechnical Specific gravity = 2.68 Consistency limits Liquid limit & Plastic limit = NP Soil Gradation 35% Gravel; 43 %Sand ; 8 % Silt; 4 % Clay Soil Texture Silty Sand Compaction Max. dry density =.79 Mg/m 3 & Opt. water content = 2% Hydraulic conductivity = 7.85E-6 m/sec Pore Fluid Analysis ph = 8. Conductivity (S) =.8E-3 Ion Concentrations (ppm) Na + 4 K Mg Ca CaCO HCO 3 4 Cl - 46 Mineralogical composition Major: Quartz, Calcite, Plagioclase Minor: Dolomite, Feldspar, Kaolinite 3. Sample Preparation Solutions of CaCl, CaCO 3 2-, and CaSO 4 2- were prepared with specified concentration of ppm. Soil samples were mixed with different amounts of moisture containing the above calcium solutions. The moisture content in the tested specimens varies as 3, 6, 9, 2, 5 and 8% by weight. To ensure homogeneity of the moistures, samples were prepared in advance of testing for about 8- hours. Soil specimens were then placed in the testing column at a specified density that allows one to push the testing probe into the soil material without any major disturbance. Then, the probe was connected to the scope meter as shown in Fig.. 4. Experimental measurements The setup shown in Fig. 2 was adjusted, so that the scope meter can take 32 reading at a time and count for the average. Special computer software (FlukeView) was used to acquire the data measured for each sample. The voltage profile at the measuring point, shown in Fig., was captured and recorded. Six profiles corresponding to moisture contents of 3, 6, 9, 2, 5, and 8% by weight were captures. Focusing on the reflected part of the signal, the measured voltage profiles were compared for CaCl, CaCO 3 2-, and CaSO 4 2- concentration of ppm as shown in Figs. 3, 4, and 5.

5 .2 3%.2 3% 6% 6%.8.4 9% 2% 5%.8.4 9% 2% 5% 8% 8% Figure 3: Reflected signal for CaCl 2 with ppm Figure 4: Reflected signal for Ca CO 3 with ppm concentration and various moisture contents. concentration and various moisture contents % 6% 9% 2% 5% 8% Figure 5: Reflected signal for with Ca SO 4 2- with ppm concentration and various moisture contents. SIMULATION ANALYSIS In order to characterize the properties of the soil medium, it is convenient to represent the system by electrical circuit model for simulation purposes as discussed by Said et al (2). A simulation was performed to match the real case profiles and extract the electrical parameters representing the type and concentration of the soil medium around the probe. The simulation results will provide a proper mechanism for distinguishing between different salt species and/or concentrations. Electrical circuit simulation software (Circuit-Maker) was used to simulate the system model. The co-axial transmission lines in the simulation diagram were considered as lossless transmission lines. This approximation is valid since the length of the coaxial cable is not too long. The probe combined with the soil medium can be expressed electrically by RC circuit (Resistance and a Capacitance). Experiments have shown that the metal probe by it self has no significant effect on the response. In this case, the RC circuit model can best represent the medium properties (Mohamed et al, 2). The

6 system parameters (pulse frequency, rise time, fall time, pulse width and pulse amplitude) are used in the simulation to match the real pulse and calibrate the system. Another RC circuit is used to simulate the model of the scope meter. The coaxial cable is modeled as a lossless transmission line with Z = 5Ω, C = pf/m, and L G = m. L L Measured Simulated Figure 6.a: CaCl 2 Case 2 Measured Simulated Figure 6.b: CaCO 3 Case 2 Measured Simulated Figure 6.c: CaSO 4 Case Figure 6: Measured and simulated voltage profiles for CaCl 2, CaCO 3 and CaSO 4 for 9% soil moisture.

7 To simulate the system with calcium moisture profiles of ppm concentrations, another transmission line model is used with Z = 5Ω, C= pf/m, and L P = 3. 6m. The simulation strategy was based on varying resistance and capacitance parameters until both the real and simulated cases were matched. The simulation was performed for calcium concentrations of ppm with various conjugate anions of chloride, carbonates, and sulfates. Each tested sample was simulated and compared with the real one. Some of these results are shown in Fig. 6. The figure indicates a very good matching between measured and simulated results. 25 Capacitance (pf) NaCl ppm oo NaCl 5ppm ppm ppm -- CaCl 2 ppm ** CaCO 3 ppm CaSO 4 ppm % soil moisture Figure 7: The capacitance variations with moisture content and calcium chloride, calcium carbonate, and calcium sulfate concentrations. Resistance (O ) NaCl ppm oo NaCl 5ppm -- CaCl 2 ppm ** CaCO 3 ppm CaSO 4 ppm % soil moisture Figure 8: The resistance variations with moisture content and calcium chloride, calcium carbonate, and calcium sulfate concentrations. The variations of the optimized parameters (resistance and capacitance) with moisture content and calcium chloride, calcium carbonate, and calcium sulfate concentrations are shown in Figs. 7 and 8, respectively. In addition, for comparing the results with previously published data, sodium chloride concentrations of and 5 ppm (Mohamed et al., 2), are shown in the same figures. From these figures, one can highlight the effect of soil solution concentration, moisture content, and conjugate anions on the variations of resistance and capacitance.

8 . Concentration Effect As shown in Fig. 7, for the same moisture content, as NaCl concentration increases, the soil medium capacitance decreases. Also, it is known from the diffuse ion layer theory that as pore fluid concentration increases, soil water potential decreases (Mohamed and Antia, 998). On the other hand, as soil concentration increases, soil structure tends to be flocculated leading to a decrease in soil water potential. This in turn leads one to conclude that as concentration increases, both capacitance and soil water potential are decreased. With respect to soil medium resistance, the calculated results shown in Fig. 8 indicate that as NaCl concentration increases, the resistance decreases. This is attributed to the fact that as soil pore fluid concentration increases, electrical conductivity increases hence, decreasing soil medium resistance. Therefore, for the same moisture content, as concentration increases, both capacitance and resistance are decreased. 2. Moisture Content Effect The results shown in Fig. 7 indicate that for the same concentration, as moisture content increases, the capacitance increases. This phenomenon could be attributed to the fact that as moisture content increases, the diffuse ion layer will be fully developed and the soil water potential will increase hence, increasing the capacitance on the soil medium. Furthermore, the results shown in Fig. 8 indicate that for the same concentration, as moisture content increases, soil medium resistance decreases. Once again this could be explained via the fully expanded diffused ion layer as demonstrated previously. Therefore, as moisture content increases, soil medium capacitance increases while, soil medium resistance decreases. 3. Conjugate Anion Effect In order to highlight the effect of conjugate anions one has to keep in mind the following phenomena: () As concentration increases, both soil water potential and capacitance are decreased; (2) As concentration increases, soil tends to have a flocculated structure hence, the resulting soil water potential and capacitance are decreased; and (3) If the negative charge on a soil colloid surface remains constant, anions of higher charge are repelled than anions of lower charge (i.e., CO 3 2- > SO 4 2- > Cl - ). Therefore, inter-particle repulsion energies would be less for Cl - than CO 3 2- ion association resulting in the formation of flocculated structures in the case of Cl - and dispersed structure in the case of CO 3 2- ions (Mohamed and Antia, 998). Therefore, from items, 2, and 3, it can be concluded that soil medium capacitance is higher for dispersed structure than for flocculated structures. And with respect to conjugate anions, the order of increasing capacitance could be expressed as SO 4 2- > CO 3 2- > Cl - as shown in Fig. 9. In view of soil medium resistance changes with conjugate anions, the results shown in Fig. 8 indicate that the order of decreasing resistance is SO 4 2- < CO 3 2- < Cl -. This once again could be attributed to the formation of flocculated structures in the case of SO 4 2- and CO 3 2 anions while dispersed structure in the case of Cl - anion. Noting that at mid-plane, pore fluid concentration in dispersed structure is larger that that of flocculated structure hence, both soil water potential and soil medium resistance are less for dispersed structure than that for flocculated structure. Therefore, soil medium resistance for both SO 4 2- and CO 3 2 anions is less than that of Cl - anion. CONCLUSION In this study, the soil system performance due to an externally applied electrical pulse is quantified via two electrical parameters that are capacitance and resistance. These parameters can easily be obtained for various ionic species and set of databases can be developed. This in turn will enable one to detect various pollutants in subsurface soils and determine their approximate concentrations. Knowing that the frequency used in this developed method is about 8.3 MHz and the load response depends on the pulse frequency used, it is convenient to study the frequency spectrum of the soil medium response to create another data base that can indicate what is the moisture content for a specific response based on spectrum analysis. Sweeping the frequency is another successful method to link the soil medium response with its actual moisture content in the soil sample, which is currently under investigation by the authors.

9 ACKNOWLEDGEMENT The authors greatly appreciate the financial support provided by the Scientific Research Council of the United Arab Emirates University. REFERENCES Elrick, E.E., Kachanoski, R.G., Pringle, E.A., and Ward, A Parameter estimation of field scale transport models based on time domain reflectometry measurements. Soil Sci. Soc. Am. J., 56, pp Heimovaara, T.J., and Bouten, W. 99. A computer-controlled 36-channel time-domain reflectometry system for monitoring soil water contents. Water Resour. Res., 26, pp Kachanoski, R.G., Pringle, E., and Ward, A Field measurement of solute travel times using time domain reflectometry, Soil Sci. Soc. Am. J., 56, pp Mallants, D., Vanclooster, M., Meddahi, M., and Feyen, J Estimating solute transport parameters on undisturbed soil columns using time domain reflectometry. J. Cont. Hydrol., 7, pp.9-9. Mohamed, A.M.O., Said, R.A., and AlShawawreh, N. 2. Development of an electrical polarization technique for subsurface pollutants detection. In: Geoengineering in Arid Lands, Mohamed & AlHosani (eds.), Balkema, Rotterdam, pp Mohamed, A.M.O., and Antia, H.E. 998.Geoenvironmental Engineering. Elsevier, Science Publishers, Amsterdam, 77p. Said, R.A., AlShawawreh, N, and Mohamed, A.M.O. 2. A TDR system for subsurface pollutants detection: I. design and modeling. Proceedings of the Symposium TDR2: Innovative Applications of TDR Technology, Infrastructure Technology Institute, Northwestern University, Evanston, Illinois, September 5-7, 2. Vanclooster, M., Mallants, D., Vanderborght, I., Diels, I., Van Orshoven, J., and Feyen, J Monitoring solute transport in a multi-layered sandy lysimeter using time domain reflectometry, Soil Sci. Soc. Am. J., 57. Vanclooster, M., Mallants, D., Diels, J., and Feyen, J Determining local-scale solute transport parameters using time domain reflectometry, J. Hydrolo., 48, Ward, A.L., Kachanoski, R.G., and Elrick, D.E Laboratory measurements of solute transport using time domain relectometry, Soil Sci. Soc. Am. J. 56, pp. -. Wraith, J.M., Comfort, S.D., Woodbury, B.L., and Inskeep, W.P A simplified waveform analysis approach for monitoring solute transport using time-domain reflectometry, Soil Sci. Soc. Am. J., 57, pp