Simulation of Degradation Processes of Volatile Organic Compounds by Using GMS

Gyanashree Bora1, Triptimoni Borah1

 

1 Civil Engineering Department, Assam Engineering College, Guwahati-13, India; E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

 

Abstract

Groundwater is a significant water supply resource. The groundwater table is decreasing in many parts of the world due to excessive overexploitation and unplanned pumping from aquifers and is considered a serious concern. One of the major environmental problems today is hydrocarbon contamination in the subsurface environment. Leakage of underground storage tanks, spills and improper disposal have been considered as major contributors in groundwater contamination. In this study, the pollutants considered are hydrocarbon contaminants (BTEX compounds) leaking from an underground storage tank and our main motive is to simulate the flow and transport processes of BTEX hydrocarbons using MODFLOW and MT3D packages available in the Groundwater Modeling System (GMS). The simulation is performed for a period of five years at intervals of 90 days each using a RT3D model available in GMS. The paper also presents the simulation of the aerobic degradation of BTEX compounds in the aquifer. Oxygen is used to promote natural attenuation process in this study to reduce the concentrations of the organic compounds in the contaminated groundwater.

Keywords: Hydrocarbon contamination, BTEX, MODFLOW, MT3D, RT3D, Oxygen.

Introduction

Volatile organic compounds, mainly benzene, toluene, ethyl benzene and xylene, have been con to be the major contributors to the deterioration of air and water quality. BTEX is an abbreviation used for four related compounds found in coal tar, crude petroleum and a wide range of petroleum products. Once released into the environment, BTEX compounds usually evaporate quickly into the air. BTEX compounds also dissolve in water. These compounds may be found in surface and groundwater at contaminated sites or in close proximity to natural oil, coal and gas deposits. There are different kinds of methods for monoaromatic compounds removal from groundwater, such as physical techniques (electro remediation, air sparging, carbon adsorption and filtration). Among all remediation technologies for treating monoaromatic compounds from contaminated groundwater, bioremediation appears to be an effective and environmentally sound approach. Bioremediation is the process by which organic compounds are broken down into smaller or lesser compounds by living microbial organisms. Aerobic bioremediation takes place in the presence of oxygen and relies on the direct microbial metabolic oxidation of a contaminant. Oxygen can be added directly to the subsurface, or chemical oxidants can be applied and release oxygen as they dissolve or decompose.

Current and recent research on petroleum hydrocarbons as groundwater contaminants is extensive. This reflects the recognition of the pollution threat posed by these compounds (Datta et al. 2011). Fischer, A.J. et al., (1987) have described that organic compounds can be a major pollution problem in groundwater. Futagami et al., (2008) mentioned that because of diverse industrial activities, sites contaminated with man-made chemicals are a world-wide problem. Holliger et al., (1997) described the contaminants in the environment and their bioremediation. Cunningham JA et al., (2001) demonstrated enhancement of in situ anaerobic biodegradation of BTEX compounds at a petroleum-contaminated aquifer.

In this study, the BTEX compounds are considered to be leaking from an underground storage tank. The degradation process of the BTEX compounds are simulated using the most advanced groundwater modeling software GMS. The flow and transport processes of the BTEX compounds in the groundwater aquifer are simulated by using MODFLOW (Modular Finite Difference Flow Model) and MT3DMS (Modular Solute Transport Model) available in GMS. After the flow and transport simulation, aerobic degradation of BTEX compounds are simulated by using RT3D (Reactive Transport in Three Dimensions). The concentrations distributed in the aquifer, without oxygen and with oxygen are observed at the well locations designated as W1, W2, W3, W4, W5, W6 and W7.

 

Methodology

Flow and Transport Equation

A transient, two-dimensional, areal groundwater flow equation for a heterogeneous, anisotropic and fully saturated aquifer given by Bear in 1979 can be written as-

for01                 (1)

Where, S is the storage coefficient; Tij= Kijb is the transmissivity tensor (L2T-1); Kij is the hydraulic conductivity (LT-1); b is the saturated thickness of the aquifer (L); h is the hydraulic head (L); t is the time (T); Q is the pumping rate per unit area (LT-1); W is the recharge flux per unit area (LT1); xi and xj are the Cartesian coordinates.

The two-dimensional solute transport given by Freeze and Cherry in 1979 can be written as-

for02                      (2)

Where, b is the saturated thickness of the aquifer (L); c is the concentration of the dissolved chemical species (ML-3); Dij is the coefficient of hydrodynamic dispersion (second order tensor) (L2T-1); c' is the concentration of the dissolved chemical in a source or sink fluid (ML-3); vi is the seepage velocity in the direction xi (LT-1); ŋ is the effective porosity of the aquifer (dimensionless); Q is the pumping rate per unit area (LT-1); W is the recharge volume flux per unit area (LT-1).

The general macroscopic equations describing the fate and transport of aqueous- and solid-phase species, respectively, in multi-dimensional saturated porous media are written as-

for03                     (3)

Where k=1, 2.....m, m is the total number of aqueous-phase (mobile) species, Ck is the aqueous-phase concentration of the kth species [ML-3], Dij is the hydrodynamic dispersion coefficient [L2T-1], v is the pore velocity [LT-1], φ is the soil porosity, qs is the volumetric flux of water per unit volume of aquifer representing sources and sinks [T-1], Cs is the concentration of source/sink [ML-3], rc represents the rate of all reactions that occur in the aqueous phase [ML3T-1].

 

Overview of the Study Area

The site is a 500 m x 250 m non-uniform section of a confined aquifer with a ground water flow gradient from left to right as shown in Fig 2. There are 50 columns and 25 rows. The east and west boundaries of the aquifer are constant head boundaries and the north and south are no flow boundaries. The orange circular shapes denote the constant head. The red coloured and yellow coloured circular shapes denote the spill location and observation well locations as shown in Fig.1

 

fig01
Figure 1: Study area for BTEX compounds

 

The aquifer has a horizontal hydraulic conductivity (Kxx) of 50 m/day and a horizontal anisotropy of 1. The effective porosity (ŋ) of the aquifer is 0.3. The ratio of horizontal transverse dispersivity to longitudinal dispersivity is 0.3. The level of oxygen is considered as 9 mg/l. The solution scheme used for advection is the Method of Characteristics (MOC). An underground storage tank is leaking fuel hydrocarbon contaminants at 3m3/day at the location. Initially the groundwater was devoid of BTEX compounds. Later at the spill location, the observed concentration of BTEX was 1000mg/L. We will simulate a continuous spill event and compute the resulting hydrocarbon contours and oxygen contours for a duration of five years.

 

Results and Discussions

Head Distribution by Using MODFLOW

The flow processes, which are the head distribution in the entire aquifer, can be observed by using MODFLOW in GMS. Fig.2 shows the head distribution in the entire aquifer during steady pumping from source wells . The steady pumping rate is assumed to be Q = 3m3/d. The figure shows that the flow gradient is increasing from the left and decreasing towards the right side of the aquifer during steady pumping.

 

fig02
Figure 2: Head distribution using MODFLOW

 

BTEX Degradation Using MT3D and RT3D in the Absence of Oxygen

After observing the head distribution by using MODFLOW, the transport process can be simulated by performing a reactive transport simulation using the RT3D model. The simulation is performed for 5 years at an interval of 3 months each.

Fig.3 (a,b) shows the BTEX distribution in the entire aquifer after 90 and 630 days Fig.4 (c,d) shows the distribution of the BTEX compounds in the aquifer after 990 and 1800 days.

 

fig03
(a)                                                                           (b)
Figure 3: BTEX contour after 90 and 630 days.

 

fig04
(c)                                                                                (d)
Figure 4: BTEX contour after 990 and 1800 days

 

Therefore, from the above figures the simulation of the degradation process of BTEX compounds can be observed in a non-uniform aquifer by using MODFLOW, MT3DMS and RT3D. The simulation is performed for 5 years at an interval of 3 months each. From the simulation, it can be observed that starting from 90 days to 1800 days, the BTEX compounds were entirely distributed in the aquifer. The reaction simulated is instantaneous aerobic degradation of hydrocarbons.

 

Aerobic Degradation of BTEX Using MT3D and RT3D in the Presence of Oxygen

BTEX compounds mainly degrade when oxygen is added to them in a subsurface environment. The principal interest, when an aerobic bioremediation system is created, is delivery of oxygen, which is the electron acceptor. Oxygen can be added directly to the subsurface or groundwater. The chemical oxidants can also be applied, which release oxygen as they dissolve or decompose. The end products of aerobic respiration are generally carbon dioxide and water. Aerobic degradation of BTEX compounds after 90 and 630 days are shown in Fig.5 (e,f). Fig.6 (g,h) shows the aerobic degradation of BTEX compounds after 990 and 1800 days.

 

fig05
(e)                                                                        (f)
Figure 5: Aerobic degradation contour of BTEX after 90 and 630 days

 

fig06
(g)                                                                            (h)
Figure 6: Aerobic degradation contour of BTEX after 990 and 1800 days

 

Therefore from the above figures, it can be observed that the BTEX compounds easily degrade when oxygen is added to them. This degradation process is also known as natural attenuation of organic compounds.

 

Time Series Curves by GMS

Time series or breakthrough curves are the curves where the concentrations of the BTEX compounds are plotted with respect to time. Two time series curves are plotted as shown in Fig.7. The blue coloured graph shows the distribution of the BTEX compounds in the aquifer without the presence of oxygen and the red coloured graph shows the aerobic degradation of the BTEX compounds in the entire aquifer. The concentrations distributed in the entire aquifer, without oxygen and with oxygen are observed at the observed well locations. These well locations are designated as W1, W2, W3, W4, W5, W6 and W7 as shown in Fig.1.

From Fig. 7( a, b, c, d, e, f, g), it can be observed that the BTEX distribution in the entire aquifer, due to the spill from an underground tank, gradually increases from 90 days to 1800 days. Initially the aquifer was devoid of oxygen but later when oxygen is applied to the BTEX compounds, aerobic degradation takes place. From the above figures the gradual diminishing of the BTEX compound can be observed over a time period from 90 to 1800 days due to the presence of oxygen and therefore, both the curves show different variations in the entire aquifer.

 

fig07
Figure 7: Time series curves for Well 1, Well 2, Well 3, Well 4, Well 5, Well 6 and Well 7

 

Conclusion

Organic compounds can be a major pollution problem in groundwater or subsurface. Their presence in water can create a hazard to the environment. BTEX are one of the most common sources of soil and groundwater contamination. BTEX are considered one of the major causes of environmental pollution because of widespread occurrences of leakage from underground petroleum storage tanks and spills at petroleum production wells, refineries, etc.

This paper summarizes the distribution of BTEX compounds in the aquifer due to a spill from an underground storage tank. The simulation is performed for a period of 5 years at an interval of 3 months each by using an RT3D model available in the most efficient groundwater modeling system (GMS). The paper also presents the simulation of the aerobic degradation of BTEX compounds in the aquifer., Oxygen clearly promotes a natural attenuation process of reduction of the concentrations of the organic compounds in contaminated groundwater.

 

Acknowledgement

I thank God, the Almighty for his blessings without which nothing would have been possible. I would like to take the opportunity to express my sincere thanks to the entire staff of the Civil Engineering Department and Assam Engineering College, Guwahati, for their kind help and co-operation to carry out this study.

 

References

Bear, J. (1972) Dynamics of Fluids in Porous Media. Dover Publications, Inc. New York.

Holliger, C., Gaspard, S., Glod, G., Heijman, C., Schumacher, W., Schwarzenbach, R.P. & Vazquez F. (1997) Contaminated environments in the subsurface and bioremediation: organic contaminants. FEMS Microbiology Reviews, 20(3-4), 517–523

Cunningham, J.A., Rahme, H., Hopkins, G.D., Lebron, C. & Reinhard M. (2001) Enhanced in situ bioremediation of BTEX-contaminated groundwater by combined injection of nitrate and sulfate. J. of Environ Sci Technol. 35(8), 1663-70

Datta, B.,Chakrabarti,D. & Dhar A. (2011) Identification of unknown groundwater pollution sources using classical optimization with linked simulation, J.of Hydro-environment Research, 5, 25-36

Fischer, A.J., E.A. Rowan & Spalding R.F. (1987) VOCs in Groundwater Influenced by Large Scale Withdrawals. Ground Water 25, 407-413

Futagami, T., Goto, M. & Furukawa K. (2008) Biochemical and genetic bases of dehalorespiration. Chem. Rec, 8(1), 1-12.