Numerical Simulation of the Effect about Groundwater Level Fluctuation on the Concentration of BTEX Dissolved into Source Zone

Abstract

The water level fluctuation of groundwater will affect the BTEX dissolution in the fuel leakage source zone. In order to study the effect, a leakage test of gasoline was performed in the sand-tank model in the laboratory, and the concentrations of BTEX along with water level were monitored over a long period. Combined with VISUAL MODFLOW software, RT3D module was used to simulate the concentrations of BTEX, and mass flux method was used to evaluate the effects of water level fluctuation on the BTEX dissolution. The results indicate that water level fluctuation can significantly increase the concentration of BTEX dissolved in the leakage source zone. The dissolved amount of BTEX can reach up to 2.4 times under the water level fluctuation condition. The method of numerical simulation combined with mass flux calculation can be used to evaluate the effect of water level fluctuation on BTEX dissolution.

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Numerical Simulation of the Effect about Groundwater Level Fluctuation
on the Concentration of BTEX Dissolved into Source Zone
To cite this article: Liqun Sun et al 2018 IOP Conf. Ser.: Earth Environ. Sci. 111 012017
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2017 International Conference on Advanced Environmental Engineering (ICAEE2017) IOP Publishing
IOP Conf. Series: Earth and Environmental Science 111 (2018) 012017 doi :10.1088/1755-1315/111/1/012017
Numerical Simulation of the Effect about Groundwater Level
Fluctuation on the Concentration of BTEX Dissolved into
Source Zone
Liqun Sun, Yudao Chen *, Lingzhi Jiang and Yaping Cheng
College of Environmental Science and Engineering, Guilin University of Technology,
Guilin 541004, China
Project (No. 41172229) supported by the National Natural Science Foundation
E-mail: cyd0056@vip.sina.com; 1106316393@qq.com
Abstract: The water level fluctuation of groundwater will affect the BTEX dissolution in the
fuel leakage source zone. In order to study the effect, a leakage test of gasoline was performed
in the sand-tank model in the laboratory, and the concentrations of BTEX along with water
level were monitored over a long period. Combined with VISUAL MODFLOW software,
RT3D module was used to simulate the concentrations of BTEX, and mass flux method was
used to evaluate the effects of water level fluctuation on the BTEX dissolution. The results
indicate that water level fluctuation can significantly increase the concentration of BTEX
dissolved in the leakage source zone. The dissolved amount of BTEX can reach up to 2.4 times
under the water level fluctuation condition. The method of numerical simulation combined
with mass flux calculation can be used to evaluate the effect of water level fluctuation on
BTEX dissolution.
1. Introduction
Monoaromatic hydrocarbons mainly including benzene, toluene, ethyl-benzene, and xylenes (referred
to as BTEX) have several characteristics about solubility and mobility, which make it easy to penetrate
through the soil vadose zone and stay in the aquifer. Additionally, it has some disadvantages
about toxic, deformed, carcinogenic and hard to degrade. The water quality deterioration caused by oil
leakage is a severe problem for humanity at present.
Monoaromatic hydrocarbons have a state of natural attenuation in the aquifer. The fluctuation of
groundwater level will obviously improve the state, changing the migration of nitrate, dissolved
oxygen and oxidation-reduction potential and so on [1]. The seasonal change can lead to the repeated
eluviation at the smear zone. It also causes LNAPL (light non-aqueous phase liquid) to make vertical
motion, and then influences the BTEX concentration [2]. At the same time, the fluctuation of water
level around the smear zone may improve the biodegradation, but the increase amount of BTEX
dissolved in groundwater is much more than consumed by degradation. Therefore, the amount of
dissolved BTEX will increase significantly [3]. The trend of LNAPL pollution caused by water level

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2017 International Conference on Advanced Environmental Engineering (ICAEE2017) IOP Publishing
IOP Conf. Series: Earth and Environmental Science 111 (2018) 012017 doi :10.1088/1755-1315/111/1/012017
fluctuation has been studied by some scholars, and they have made the simulation about natural
attenuation by giving the adsorption conditions or electron acceptor for a certain organic pollutants or
total petroleum hydrocarbon [4, 5]. There are few reports about the amount of BTEX dissolving in the
source area using numerical simulation methods.
In order to determine the dissolved amount of BTEX caused by water level fluctuation,
MODFLOW with its sub-module RT3D (multi-component interaction model of the migration) were
used to simulate the natural attenuation process and the concentration changes of BTEX under the
water level fluctuation condition. Combined with the effective dissolved amount of 63 mg/L of BTEX
in gasoline with the steady water level, the BTEX dissolution was simulated under the condition of
water level fluctuation and used to calculate BTEX mass flux. This work about BTEX dissolution
under the condition of water level fluctuation can provide a reference for the actual contaminated site.
2. Materials and Methods
2.1. Experimental Situation
The design of aquifer model used in this experiment is based on the generalized shallow sandy
aquifers in the field. The dimension of the aquifer model is 5.8 m long, 2.9 wide and 1.30 high. Two
narrow slots of 0.2 m wide is located on both east and west sides, respectively, and a 0.2 m
impermeable wall divides the aquifer model into the north and south parts. The aquifer is 0.9 m thick,
made of sand with 0.05-0.25 cm particle size. Groundwater pumped from nearby shallow aquifer was
injected as supply water. The model has 40 sampling holes including A-E and 8 water level
observation holes including W1-W8. The specific model structure is shown in Figure 1.
Figure 1. The plane figure of the aquifer model.
Before the experiment, the model was pumped and injected systematically to estimate
hydrogeologic parameters by using fresh shallow groundwater. The water level was controlled at the
height of 50 cm above the bottom of the model. After that, 3 L traditional gasoline was injected into
the north part and equivalent of 10% ethanol gasoline was injected into the south part at the same time.
The average flow rate of injection was about 500 mL/h, and the injection points was designed at the 45
cm height of the source wells of the model
After the experiment began, water level was relatively stable before the first 77 days. Water level
occurred fluctuations in some extent, as shown in Table 1. During the period of experiment, BTEX
concentrations were monitored A-E holes per month, detected by using an Agilent 6890 N gas
chromatograph, and BTEX attenuation was paid attention. Significantly, The BTEX concentrations
W11 W12
W10
W9
W-Slot
sampling well (A~E) screened well source well
pumping-injecting well water slot
Sources-1
Sources-2
0.0
0.5
1.0
1.5
2.0
2.5
(m)
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
(m)
A1A2A3A4
B1B2B3B4
C1C2C3C4
D1D2D3D4
E1E2E3E4
W1W2W3
A5A6A7A8
B5B6B7B8
C5C6C7C8
D5D6D7D8
E5E6E7E8
W5W6W7
W8
W4
West East
section
the 2th section
the 6th section

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IOP Conf. Series: Earth and Environmental Science 111 (2018) 012017 doi :10.1088/1755-1315/111/1/012017
increased, correspondingly to each rise of the water level.
Table 1. The fluctuation of water level
2.2. Mass Flux
Mass flux refers to the total mass of a certain solute in a unit of time through a section perpendicular to
the flow direction. In this paper, the in-site cross-section method and time-moment method were used
to estimate the mass flux of BTEX concentration. The formulas are shown in the reference [6].
2.3. Numerical Simulation
The initial water level of the flow model was 0.5 m and the initial BTEX concentration was no
detectable. Boundaries of the east and west sides belong to the first type of boundary condition, and
other boundaries belong to zero flow condition. The top has evaporation boundaries. And set up the
well boundary at east-west flume head observation well location and pollutant injection hole. The
water flow model was corrected by parameter inversion, adjustment of hydraulic conductivity, water
supply and water storage coefficient.
3. Results and Discussion
3.1. The BTEX Concentration Changed with Water Level Fluctuation in the Source Area
In the water quality model, a water injection well was set up at the pollution source, simulating the
release of pollutants by setting a continuous boundary of point source injection. The pollutant flow
rate was 500 ml/h. Since the model has electron acceptor, the initial concentration of each component
is 4.5 mg/L dissolved oxygen, 16 mg/L sulfate, 3.8 mg/L nitrate. Shallow groundwater contains a large
number of electron acceptors, so the influent sink boundary conditions was set up for the concentration
of continuous injection on the boundary. In the shallow groundwater, the dissolved oxygen was 7.5
mg/L, nitrate was 3.5 mg/L and sulfate was 16 mg/L, and they were the average values of multiple
measurements. The time step was calculated as 10 d [7]. By adjusting the effective solubility of BTEX
within a reasonable range and fitting with the observation concentration data which changes caused by
water level fluctuations to adjust parameters repeatedly. The model parameters are shown in Table 2.
Because the injection hole is located at a depth of 45 cm and the simulated water flow is similar to
the one-dimensional flow. The central line at the water level of 45 cm is most impacted by the water
level fluctuation. Therefore, the change of BTEX concentration is the most significant near the source
region or within the source region affected by water level fluctuation, and it is more likely to be due to
other factors at the downstream pore position. The model simulation results of pollution source area
are shown in Figure 2.
Period(day)
Water level changes
Water level range (cm)
Average water level (cm)
-76th
Stable
48.1-51.4
49.8
77th-78th
Rise
50.4-64.3
57.4
78th-128th
Fall
64.3-50.8
56.1
129th-147th
Rise
50.8-63.4
57.0
148th-167th
Stable
63.4-64.1
61.3
168th-173th
Fall
64.1-53.1
54.1
174th-237th
Stable
53.1-49.3
50.9
238th-285th
Rise
49.3-61.5
57.4

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IOP Conf. Series: Earth and Environmental Science 111 (2018) 012017 doi :10.1088/1755-1315/111/1/012017
Table 2. Parameters that have been corrected
77th day 106th day 128th day 146th day
163th day 173th day 237th day 285th day
Figure 2. The BTEX concentration changed along with the water level fluctuation at north
source zone.
The water level fluctuated 3 times in all. During the period with a stable water level before the 76th
day, the concentration of BTEX in the source area first increased and then decreased. The
concentration becomes higher when pollutants flow through the hole at this stage, and then decreased
gradually with the natural attenuation effect. After a sudden increase in water level on day 77, the
BTEX concentration increased significantly, and after the rise of the water level slowed down, the
concentration decreased gradually. After a slow rise in water level at 129 days, the contaminant
concentration increased. The water level gradually decreased from 148 days to 167 days after being
stable for 20 days, and by day 182, it dropped to the initial level. At this point BTEX concentration
also decreased gradually. At the end of the experiment, when the concentration of pollutants was very
low and after the water level had risen, the BTEX concentration increased.
3.2. The Mass Flux of Source Area Fluctuates with Water Level Fluctuation in Downstream Section
With the flux analysis can be seen that the difference of mass flux is obvious before and after the three
water level fluctuations. Before the first water level fluctuation, the mean mass flux on both sides was
Model parameters
Value, unit
Source
Horizontal hydraulic conductivity
Vertical hydraulic conductivity
41 m/d
4.1 m/d
measured
[8]
Porosity
0.3
measured
Groundwater velocity
0.207 m/d
measured
Bulk density
1700 kg/m3
[9]
Specific yield
0.2
[8]
Specific storage
1.0×10-5 (1/m)
Amount of evaporation
0.1 mm/d
Dispersion (N side model):
Longitudinal
Transverse horizontal
Transverse vertical
0.025 m
0.075m
0.003 m
[10]
parameter calibration
parameter calibration
Dispersion (S side model):
Longitudinal
Transverse horizontal
Transverse vertical
0.025 m
0.012m
0.006 m
[10]
parameter calibration
parameter calibration
BTEX transport velocity
0.019 m/d
calculated
First-order reaction rate
0.0002(1/day)
[11]
Partition coefficient (Kd)
7.73×10-4 m3/kg
[12]
BTEX effective solubility (gasoline)
44-112 mg/L
[13]

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IOP Conf. Series: Earth and Environmental Science 111 (2018) 012017 doi :10.1088/1755-1315/111/1/012017
35.5 mg/d on day 71. After the fluctuation and on day 103, the mean mass flux on both sides was 78.1
mg/d. Before the second fluctuation, the mean mass flux on both sides was 75.1 mg/d on day 128.
After the fluctuation and on day 163, the mean mass flux on both sides was 110.7 mg/d. Before the
third water level fluctuation, the mean mass flux on both sides was 12.7 mg/d on day 237. After the
water level fluctuation and on day 285, the mean mass flux on both sides was 50.4 mg/d. The
observations, calculated results of mass flux are shown in Table 3. The mass flux results of
observations are derived directly from the concentration observations into the relevant calculation
formula, and the mass flux results of calculated is calculated by the value which is simulated by the
MODFLOW software. Comparing the results showed that the simulation accuracy has improved.
Table 3. The mass flux changed along with the times.
In summary, water level fluctuation will promote the dissolution of BTEX. After gasoline polluted
the 45 cm layer of the aquifer, it will move with the flow of water. Most of the pollutants floating in
the surface of groundwater are absorbed in the soil and rocks. Coupled with its own volatility,
diffusion and dispersion, the concentration of BTEX in the smear zone increased greatly. Therefore, a
sudden increase in water level will dissolve the pollutants in the water body, resulting in a substantial
increase in the concentration of pollutants in the water body. When the water level drops, BTEX
concentration will be reduced due to the water away from the source area. In real subsurface aquifers,
away from the source area, the concentration of pollutants increases as the water level rises, due to the
effect of dilution, which is also illustrated in the literature [14].
3.3. The BTEX Concentration Simulation Dissolved in Source Zone
Water level fluctuations will have an effect of elution for LNAPL, and it will accelerate the dissolution
of monocyclic aromatic hydrocarbons significantly. So the amount of dissolved pollutants increased
sharply [6]. According to the literature, the effective solubility of BTEX is generally 63 mg/L.
Table 4. The concentration gradient of contamination in source zone.
After concentration correction, it was concluded that the continuous injection flow of the
corresponding injection hole was 0.0036 m3/d. The value we set can accurate simulate of BTEX
dissolution relatively without water level fluctuation. Furthermore, through the calibration fitting with
experimental results, the effective dissolved concentration of BTEX under every fluctuating condition
was obtained with the stable injection flow. Finally, the model simulated the release concentration of
BTEX pollutants caused by water fluctuation in the laboratory aquifer model by calibration parameters.
And the concentration is the value of the point source boundary. The specific data are shown in Table 4,
and it can provide data for the real reference to contaminated sites.
Time(d)
71
103
106
118
128
142
163
173
237
285
Mass
flux
(mg/d)
The 2th
section
calculated
26.3
54.9
73.4
71.0
61.1
55.0
98.9
79.7
5.5
53.8
observations
30.3
-
84.1
62.8
-
51.1
-
70.8
3.8
-
The 6th
section
calculated
39.7
101.3
146.3
112.4
92.0
93.4
122.4
111.9
19.9
47.0
observations
40.7
-
148.5
113.5
-
96.5
-
123.3
15.9
-
Period(d)
0-77
77-78
78-128
128-148
148-167
167-173
173-237
237-285
North side (mg/L)
74.8
129.9
118.1
133.9
124.0
122.1
21.7
63
South side (mg/L)
63.0
126.0
144.2
145.7
149.6
129.9
25.6
68.9

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IOP Conf. Series: Earth and Environmental Science 111 (2018) 012017 doi :10.1088/1755-1315/111/1/012017
4. Conclusions
Fluctuation of water level will seriously affect the quality of groundwater in the source area. A rise in
the water level will cause a lot more BTEX to dissolve into the groundwater sharply, resulting in a
high BTEX concentration in the source area. A drop in the water level will alleviate the pollution in
the source area. The method of numerical simulation combined with mass flux calculation can be used
to evaluate the effect of water level fluctuation on BTEX dissolution. The BTEX dissolution under the
condition of water level fluctuation can provide a reference for the actual contaminated site.
5. Acknowledgments
We gratefully acknowledge the National Natural Science Foundation of China (Project No. 41172229)
and Guilin University of technology for their financial and technical support, and thank Li Liu-yue and
Huang Jun-yu their assistance during fieldwork. We would like to thank the anonymous reviewers for
their helpful comments and suggestions.
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