Aug 28, 2013 - The plant is located on Hymus Boulevard in the industrial park of Pointe Claire across the street from residential area and various schools in ...
CSCE 2014 13th International Environmental Specialty Conference - 13e Conférence internationale spécialisée sur l’environnement 2014 de la SCGC
Halifax, NS May 28 to 31, 2014 / 28 au 31 mai 2014
ANALYSIS OF PCBs FATE AND TRANSPORT MECHANISM THROUGH CASE STUDY OF TRANSFORMER OIL LEAKAGE IN MONTREAL 1
Zunaira Asif and Dr. Zhi Chen
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PhD student, Environmental Engineering, BCEE, Faculty of Engineering & Computer Sciences Concordia University, Montreal, Canada 2 Associate Professor, Faculty of Engineering & Computer Sciences Concordia University, Montreal, Canada
Abstract: Transformer oil leakage having large quantity of PCBs is one of the most deplorable incidents resulting in huge negative impacts on the environment. In this paper a multimedia environmental model and the hydrocarbon spill screening model (HSSM) are applied to the case study adapted from a real case of PCBs- transformer oil spill in Montreal. The oil migration is examined through these two methods in three dimensions including site specific data, soil characteristics and hydro-geological properties. It aims to investigate the fate, transport and transformation rates of PCB in the soil, the unsaturated and the saturated zones. The proposed modeling concept help to define the processes to be used in characterization of soil and subsurface environment as a receptor of transformer oil spill. This study includes the simulation techniques for assessing the effect of PCBs on groundwater and soil. Results from multimedia environmental model and HSSM model verify with each other and both show that maximum concentrations of PCBs are found in soil as compared to groundwater in context of transformer oil spill at site. Key words: Hydrocarbon spill screening Model, Multimedia environmental model, CHEMCAN model, PCBs, Transformer Oil spill, Fate and transport.
1. INTRODUCTION Polychlorinated biphenyls (PCBs) have been previously used as a coolant fluids for example in electric motors, capacitors, and electric transformers. Due to PCBs' environmental toxicity and consideration as a persistent organic pollutant, its usage was banned in 1979 by the United States Congress (William 1994) and announced by the Stockholm in convention held in 2001 (Porta and Zumeta 2002). According to the U.S. Environmental Protection Agency (EPA), PCBs have been known to cause cancer in animals as well as in human beings. PCBs are corrosive for the skin, eyes and respiratory tract. It can directly affect nervous and immune system. It can also cause many of the diseases related to skin and consider as carcinogenic compound (Lauby-Secretan et al. 2013). In this research paper, assessment of PCB-Transformer oil leakage through the subsurface soil is studied based on the principle of multiphase flow. PCB-Transformer oil is categorized as non aqueous phase liquid (NAPL) (Environmental Agency 2003) .There are many environmental multimedia models that considered the transport mechanism and partitioning behaviour of PCB in vadose zone as well as in
ENV-43-1
groundwater (Russell and Jon 2004). However, there are very few screening models that deal with PCBs and NAPLs together in subsurface soil media. Screening models offer minimum data requirement and provide an alternative approach to study physical behaviour of Non Aqueous Phase Liquids (NAPLs) releases through simplifications. An interpretation of hydrological data based on certain assumptions including homogenous soil conditions and uniform aquifer is used in screening models to get the analytical solutions. The Hydrocarbon Spill Screening Model (HSSM) is one of the screening models which simulate subsurface leakages of oil with the intent of assessing migration of potential contaminant in the groundwater (Weaver et al. 1994 and Charbeneau et al. 1995). The HSSM consists of three modules, the Kinematic Oily Pollutant Transport (KOPT) that treats transport of NAPL through the vadose zone to capillary fringe, OILENS module simulates the formation and spreading of an oil lens in the capillary fringe; and Transient Source Gaussian Plume (TSGPLUME) which simulates transport of soluble constituents of the LNAPL in the aquifer to receptor locations. This model addresses the questions that how far an oil release might go into the soil and how soon it might join the ground water (Weaver et al. 1994). There are numerous factors influencing the migration, transformation and degradation of NAPL when it is released into the environment. In order to deal with these factors several environmental multimedia models exist which attempt to study and predict the partitioning behaviour of contaminant in the environment. They include CEMC Level III (Mackay 2001), CALTOX (McKone and Enoch 2002), MEPAS model can be used for sites that release radionuclide or toxic chemicals from different sources in landfill and ponds (Strenge and Smith 2006).Other representatives models are HWIR (USEPA 1999), MULTIMEDIA 2.0 (Solhotra et al.1995),3MRA (Babendreier and Castleton 2005) and the one studied in this paper is known as CHEMCAN which is developed by Mackay and his colleague (Mackay et al.1991).CHEMCAN is one of the notable example of regional scale models, parameterized for 24 regions of the Canada based on the concept of fugacity. The primary objective of this research paper is to study three dimensional multi-phase flow of PCBtransformer oil through vadose zone to ground water using HSSM model. Particularly formation and spreading of oil lens in the capillary fringes is also studied. This paper also focused on mass transfer of PCBs from NAPL lens to ground water. Additionally comparison is made between HSSM and CHEMCAN model in order to check validation of analyses.
2. METHOD AND MATERIAL
2.1. Hydrocarbon Spill Screening Model (HSSM) The model is intended to address the problem of NAPL flow and migration from the sub surface to a water table aquifer. The importance of the model is to determine the NAPL lens size and the mass flux of contaminants into the ground water. These key factors define conditions of the source and must be based upon multiphase flow phenomena in the unsaturated zone. The first two modules of HSSM i.e. KOPT and OILENS address the vadose zone flow of the NAPL and its migration. These two modules are combined into one computer code. HSSM-KO, which provides a time-variable source condition for the aquifer model has separate computer code (Weaver et al.1994). A chemical pollutant dissolved in both the NAPL and water phase is tracked by KOPT and OILENS. Once the chemical pollutant reaches the water table, it contaminates the aquifer by contact and by dissolution from the NAPL oil lens. Thus, the third part of the model is transported pollutant through the NAPL to aquifer. Notably, the mass flux from OILENS is time dependent, so that the aquifer model must be capable of simulating by varying a time with conditions of the source. In keeping with the level of approximation used in KOPT and OILENS, one suitable choice is the Transient Source Gaussian Plume (TSGPLUME) model, which uses different numerical techniques than KOPT and OILENS; so it is not
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incorporated within HSSM-KO, but rather it is implemented in the computer code HSSM-T (Charbeneau et al.1995).
2.2. CHEMCAN Model CHEMCAN is a multimedia model based on fugacity approach designed to estimate the distribution of single chemical pollutant in multiple media. It was developed for 24 different regions of Canada. It permits temporary and permanent additions or changes of environmental properties to a simulation. It estimates average concentration of contaminant in air, soil, sediments and water etc. It is intended to help in human exposure assessment. In fact, this model can be used to check the validation of modelling results obtained through other screening models for case study of Canada region (Webster et al. 2004).
2.3. Case Study The Reliance Power Equipment was storing Polychlorinated biphenyls (PCBs) contaminated materials at their facility since 1998. The plant is located on Hymus Boulevard in the industrial park of Pointe Claire across the street from residential area and various schools in neighbourhood. In Canada the import and th manufacturing of PCB was made illegal in 1977. On March 26 , 2013 the leak of 800 to 1,200 litres of oil containing large concentrations of PCBs had spilled in Reliance Company’s warehouse. This incident would result from the opening of the valve of an outside reservoir containing some oil and oily water (Mddefp Report 2013 and CTV Montreal News 2013) The analysis of samples taken from the site has confirmed that the oil poured out in 86, Hymus Blvd. contain PCB in concentration of 430 mg/kg. Two days later, PCB laced oil was found in St-Louis Lake, the results of its sample have concentration of 445 mg/kg (Mddefp Report and Press Release 2013). It’s important to know site characteristics as they play key role in migration of pollutant through subsurface soil to groundwater. Table 1 describes some of the important characteristics of the incident site and all other inputs parameters required for respective models (IRDA 2013).
Table 1: Input Parameters parameters
Values
Parameters
Values
NAPL density (g/cm )
0.885
Porosity θ
0.33
NAPL dynamic viscosity (cp)
2.0
Hydraulic conductivity (m/d)
.22
NAPL flux (m/d)
.452
Depth to water table (m)
10.0
Bulk density of unsaturated Zone
1012
Aquifer saturated thickness (m)
15
3.8
PCB in NAPL (ppm)
473
NAPL/water partition coefficient
251
Average Infiltration rate (m/d)
.365x10
NAPL surface tension (dyne/cm) 0 @20 C Transverse dispersivity (m)
25
Longitudinal dispersivity (m)
10.00
1.00
Vertical dispersivity (m)
.100
3
3
(kg/m ) Soil-water partitioning coefficient (L/kg ) Kd
3. RESULT ANALYSIS
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3.1. HSSM Modeling The saturation profile (Figure 1) represents the simulated distribution of transformer oil in the vadose zone. The cross-hatched region on the left represents the assumed uniform water saturation. The profile times are listed on the lower right of the Figure 1. It could be assumed that the PCB reaches the water table after 200 days. It is clearly observed that oil reached at the depth of 10 m of vadose zone after 200 days. Rather than accepting the results of one simulation, several times simulations should be run in order to get some feel for the effects of parameter variability. If the hydraulic conductivity was twice greater than the average value of 0.22 m/d, the transformer oil would flow deeper into the subsurface earlier than 200 days.
Figure 1: NAPL to water saturation profile (Source: Direct output of HSSM) The lens profile graph (Figure 2- 4) depicts the configuration of the lens at the selected times. The vertical axis illustrates the configuration of the lens in the vicinity of the water table .The spread of lens in radius (m) of water table is indicated by the horizontal axis. The horizontal axis starts from the source out to some distance beyond the edge of the lens. The vertical line from the top to the lens indicates contamination due to PCB in the vadose zone due to the transformer oil leakage. The shape of lens shows the configuration of the actively spreading NAPL. PCB-transformer oil contaminated only 0.3 m of water table vertically from 75 to 200 days. Whereas, oil lens spread horizontally and increased in radius from 2 m to 5 m in 200 days.
Figure 2: Oil Lens profile after 75 days (Source: Direct output of HSSM)
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Figure 3: Oil Lens profile after 100 days (Source: Direct output of HSSM)
Figure 4: Oil Lens profile after 200 days (Source: Direct output of HSSM) The oil lens i.e. NAPL spread laterally in radius as a function of time as shown in Figure 5.The lens radius increases rapidly as the oil enters the lens. Later on the lens tends toward limiting the radius. The Figure 5 shows that mass flux of contaminant i.e. PCB into the aquifer as a function of time. This mass flux is used as the input boundary condition to HSSM-T. As the oil lens formed, the mass flux to the aquifer -4 increased rapidly to the value of 4.5 x10 kg/d, due to the increasing radius of the oil lens. If the spill is one time, as in this case study, the mass flux to the NAPL declines because of leaching of the PCB into the aquifer. Typically, the mass flux shows a tailing effect after 1 year.
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Figure 5: Radius History by NAPL and PCB (Source: Direct output of HSSM) The oil lens contaminant mass balance Figure 6 illustrates the mass of PCB contained within the oil lens as a function of time in years. Figure 7 plots the cumulative mass of contaminant i.e. PCB which has been dissolved into the ground water from the oil lens. As the PCB mass contained within the lens declines, the proportionality of cumulative amount of PCB dissolved in groundwater increases.
Figure 6: PCB mass flux rate as function of time (Source: Direct output of HSSM)
Figure 7: Contaminant mass in oil lens (Source: Direct output of HSSM)
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3.2. CHEMCAN Modelling PCB was transferred from NAPL to water; soil and air phase in the subsurface soil. According to the Table 2 CHEMCAN multimedia modelling results show that 92.7 % of PCB sorbed into the soil while rest of PCBs become the part of ground water. Advection and diffusion were two main transport mechanism process or the dominant processes in migration of PCB from NAPL to water media and soil media in subsurface environment. The total residence time of contaminant was 1700 days. The value of fugacity -08 -10 for soil was 3.43 x 10 Pa (Pascal) and for ground water was 2.52 x 10 Pa.
Table 2: Output of CHEMCAN model Compartment
Z Value (mol/m³.Pa)
Fugacity (Pa)
Amount* (%)
Concentrations -3 (g.m )
Ground Water
0.268
4.98E-10
4.21
1.27E-05
Soil
1.60E+02
3.43E-08
92.7
1.03E-03
*Rest of percentage is divided among air voids, water present in vadose zone
3.3. Comparison of HSSM and CHEMCAN model The computed results from HSSM model were carefully analyzed with respect to physical and dynamics of pollutant i.e. PCB in subsurface environment. Then the simulated results from HSSM model were compared to those calculated from the similar case study by using regional multimedia model CHEMCAN to verify the performance of model. -3
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According to HSSM, concentration of PCB in soil was 4.67 x 10 g.m after 1788 days (approximately -3 -3 more than 4 years).Whereas by using CHEMCAN multimedia model PCB in soil was 1.03 x 10 g.m for the same time scale (see Table 3). Both models predicted almost the same value of PCBs in soil. Most of the time migration of pollutant depends upon the porosity and permeability of the soil. -3
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The concentration of PCBs in groundwater after a year using HSSM model was 1.61 x10 g.m .This concentration was gradually increased with time as mass of contaminant in oil lens decreased. Similarly, -5 -3 PCB concentration using CHEMCAN was 1.27 x 10 g.m .This concentration was less as compared to HSSM model as in CHEMCAN NAPL phase is not keep into considerations. The average PCB dissolution -3 -1 -1 -1 flux rate was 0.1779 x 10 kg.d and NAPL flux rate was 0.375 x10 kg.d using HSSM model. Whereas, -4 -1 PCB flux rate to ground water using CHEMCAN model was 5.3x10 kg.d (see Table 3).
Table 3: Comparison of HSSM and CHEMCAN model Advection flux Average Predicted PCB -2 -1 -3 (g.m .d ) concentration* (g.m )
Diffusion Flux -2 -1 (g.m .d )
Environment
HSSM
CHEMCAN
HSSM
CHEMCAN
HSSM
CHEMCAN
Soil
4.67E-03
1.03E-03
1.78E-03
2.33 E-03
3.65E-01
1.43E-01
1.80E-04
6.45E-03
Groundwater 1.61E-03 1.27E-05 8.60E-03 4.8E-02 * Concentrations are based on average of first year of incident.
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The advection and diffusion flux for soil obtained by CHEMCAN model were 2.33 x 10 g.m .d and -2 -1 0.1425 g.m .d respectively. Whereas, advection and diffusion flux for soil obtained by HSSM model -3 -2 -1 -2 -1 were 1.78 x 10 g.m .d and 0.365 g.m .d respectively. In addition to this, advection and diffusion flux -2 -1 -3 -2 -1 for ground water obtained by CHEMCAN model were .048 g.m .d and 6.45x10 g.m .d respectively. -3 -2 -1 While, advection and diffusion flux for ground water obtained by HSSM model were 8.6 x10 g.m .d and
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-4
-2
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1.80x10 g.m .d respectively. Hence, diffusion was the dominant transport mechanism in soil media reported by both the two models (see Table 3).
4.
DISCUSSION
The HSSM model uses semi-analytical solutions of the transport equations which include many of the important physico-chemical processes. Based on this approach, it can incorporate three dimensional effects easily as compared to CHEMCAN model. According to HSSM, concentration of PCB in the soil was more than 92 %. One of the reasons is PCBs stick strongly to soil and do not readily break down in soil. They may stay in the soil for months or years; generally, the more chlorine atoms that the PCBs contain, the more slowly they break down (Abriola et al.1985b).In this case study soil is composed of clay and well sorted gravels that impede flow of transformer oil due to its low hydraulic conductivity i.e. 0.22 -1 m.d .As ability to transmit NAPL through vadose zone to groundwater depends upon hydraulic conductivity (Awobajo 1981). PCBs are often blended with carrier fluids such as mineral oil and chlorobenzenes. In this case study, the carrier fluid was transformer oil which is classified as Light NAPLs (LNAPLs) as it has a density less than water (Daubermann 2002). Depending upon the type of carrier fluid, the density of PCB oils encountered -3 in range from 1,100 to 1,500 (kg.m ) while viscosity is from 10 to 50 cp (Environmental Agency 2003). The relative high density and viscosity indicates that PCBs as a part of dense NAPLs (DNAPLs) may be still migrating at the site where they were introduced through subsurface in the past decade. It means density and viscosity along with other site specific factors are important for timescale of migration of NAPLs. Moreover, HSSM is designed for LNAPLs. It is not suitable for denser than water NAPLs (DNAPLs) as the NAPL is assumed to float over the water table. The vadose zone module of HSSM could, however, be used for a DNAPL, as the qualitative behaviour of that module is not affected by density of fluid (Weaver 1994). CHEMCAN is one of the multimedia models which are based on regional characteristics and offers great simplicity and minimum data requirements for input of parameters (Kawamoto et al.2001). As compared to HSSM this model does not account for partitioning behaviour of PCB in NAPL phase. Although this model is suitable for large scale but spreading of contaminant plume in radius with respect to time is not considered in CHEMCAN model. According to CHEMCAN, advection and diffusion were primary mechanism for migration of transformer oil which is also true for all NAPLs (Russell and Jon 2004). However, there are certain limitations in HSSM model as it does not include all processes which may be important, and because it is based on semi-analytical solutions, it does not account directly for heterogeneity of the soil. In addition, a 3 D multiphase numerical models should be further study to address limitations of HSSM model especially heterogeneity of soil as these models are employed to stimulate the migration of NAPLs (Borden 2000).
5. CONCLUSIONS The study illustrates that HSSM model is one of the screening models which has advantage to assess the PCB constituent dissolved in both NAPL and water phase. The predicted concentrations of PCB in subsurface soil as well as in ground water were obtained by using HSSM model. The oil lens profiles as a function of time were also generated that can assist in remediation technologies by considering where the contaminants reside in the subsurface soil. Particularly more oil was trapped in vadose zone and less was available to migrate to aquifer depending upon porosity of soil. This reflected the occurrence of PCB concentration in soil was 92.7 % as compared to other media of environment. CHEMCAN model was also studied to evaluate the partitioning behaviour of oil laced with PCBs in subsurface soil environment. A comparison showed that the CHEMCAN model responds to analysis similar to HSSM model. The studies of both the models conducted in this paper indicated that dominant transport mechanism in subsurface soil was diffusion in vadose zone. However, both the models have their own limitations when applied for NAPL.
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