Journal of Environmental Science and Engineering A 10 (2021) 104-115 doi:10.17265/2162-5298/2021.03.002

Using Visual MODFLOW Model to Assess the Efficiency

of Subsurface Barrier Wall for Groundwater Flow

Regulation and Reduction of Saline Intrution

Nguyen Minh Khuyen1, Doan Van Long1, Nguyen Tien Bach1, Tang Huu Dong1, Bui Cong Du1 and Dang Dinh

Phuc2

1. Department of Water Resources Management, Ministry of Natural Resources and Environment, Hanoi 100000, Vietnam

2. Vietnam Association of Hyhrogeology, Hanoi 100000, Vietnam

Abstract: Barrier walls effectively store water, regulate underground flows, improve exploitable reserves and prevent saltwater intrusion. The effectiveness of the underground barrier wall depends not only on the hydrogeological structure, the technical parameters of the wall but also on the layout scheme of the exploitation well system. The results showed that in natural conditions, the ground water level upstream of the barrier wall rose in the presence of a barrier wall. In wells located downstream of high barrier walls, the water level decreased. The amount of underground current flowing into the sea decreased, the annual average value of the whole region decreased was 316 m3/day and night. In presence of a wall, both the water level and the amount of evaporation increased. The average increase in evaporation volume during the calculation period of ten thousand days with walls was 4.114 m3/d. So in presence of a wall, the amount of water that can be exploited increases by the total amount of evaporation plus the decrease in discharge to the sea and is equal to 4,424 m3/d. In the exploitation condition, if the water level in the presence of wall is kept as low as in the absence of wall, the exploitation flow will increase to about 4,400 m3/day and night. From the calculated water level values when there is a wall and without a wall, we can see that if the exploitation flow in presence of a wall and in the absence of wall is the same, the water level drop at the calculated observation wells upstream of the wall will decrease from 0.21 m to 3.97 m. The condition of effective exploitation of the wall depends on the mining scheme. The exploitation scheme is reasonable, the exploitation flow of the wells does not exceed the allowable flow so as not to cause the drying of the aquifer at the location of the well. The upstream area of the wall reflects quite clearly as the Total dissolved solids content in observation wells upstream of the wall at the end of the calculation time is significantly reduced compared to that without the wall, ranging from 69 mg/L to 5,629 mg/L. In the presence of a wall, the water level of observation wells upstream of the wall is higher than that of without a wall from 0.10 m to 0.74 m. Key words: Subsurface barrier wall, store water, regulate underground flows.

1. Introduction

In coastal areas of the Central of Vietnam, there are

many sand dunes from several 10 meters to few

kilometers. They provide the significantly important

water resource for daily life and production of

residents in these areas. In these areas, sand dunes

have been distributed widely and mainly formed by

sand layer in sediments of wind and alluvial origin of

Holocene and Pleistocene ages. According to the

Corresponding author: Dang Dinh Phuc, Ph.D., research

fields: hydrogeology.

geological survey literatures of titanium exploration

and exploitation, the thickness of the sand layers

varies from a few meters to nearly hundred meters.

The water in sand dunes is supplied by rainwater and

drained from the East Sea or low-lands, in which

layers are separated the water into geysers and then

become streams and flow into the East Sea. However,

overexploitation of underground water for production,

especially for aquaculture in sand dunes leads to

salinization. One of the solutions for developing water

resource and preventing saline intrution is to build

subsurface barrier walls [1].

D DAVID PUBLISHING

Using Visual MODFLOW Model to Assess the Efficiency of Subsurface Barrier Wall for Groundwater Flow Regulation and Reduction of Saline Intrution

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2. Material and Methods

We built the assumed model to preliminarily assess

the efficiency of underground barrier wall for water

storage, flow regulation, saline instrution prevention,

and used it to make water resource exploitation and

protection plans. The model used for evaluation is the

Visual MODFLOW model [2]. In the world, the use to

evaluate the effectiveness of subsurface barrier walls

has been conducted by many authors. The model was

set up in Binh Thuan province in which has high

potential for developing tourism as well as agriculture

with many high-value fruits such as dragon fruit,

grape, etc.. Therefore, the demand of domestic

consumption will be high for daily life and

production.

3. Procedure for Selecting the Best Model

The model was set up in coastal area, stretching

along the coast for nearly 6 km and an average width

of nearly 6 km. It was divided into 100 rows and 100

columns consisting of 3 layers (Fig.1, Fig. 2). The first

layer is the surface composed of wind-derived

sand—with the thickness of 0.1 m to 18 m, average

thickness of 4.6 m. The average horizontal and

vertical permeability coefficient after running the

adjustment problem was 3.5 m/d, the average vertical

permeability coefficient was 0.5 m/d. Layer 2 has a

thickness ranging from 0.1 m to 96 m, an average of

7.6 m, is composed of a double layer of sand which

intercropped with clay, silt and gravel originating

from wind and sediment. After running the problem of

adjusting the horizontal permeability coefficient of

layer 2, it is 4 m/day and night, the vertical

permeability coefficient is 0.5 m/d. Layer 3 is the

weathered fracture zone of bedrock with average

thickness of 10 m. After the problem of adjusting the

horizontal permeability coefficient of layer 3 is

determined to be 0.5 m/d, the vertical permeability

coefficient is 0.1 m/d.

The effective gravity water release coefficient of

layer 1 and layer 2 is equal to 0.08 and the elasticity of

these two layers is 0.001. The gravitational water

release coefficient of layer 3 is 0.05, and the elastic

coefficient is equal to 0.001.

In the model, sea surface is considered to be a

defined boundary of water level and has a constant

water level with time and is equal to 0 m.

The amount of seepage supplied was divided into 2

zones. Zone 1 corresponds to a fairly flat terrain in the

eastern half of the model. Zone 2 corresponds to the

hilly terrain in the western part of the model. After the

Fig. 1 Cross section of the mode.

Using Visual MODFLOW Model to Assess the Efficiency of Subsurface Barrier Wall for Groundwater Flow Regulation and Reduction of Saline Intrution

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Fig. 2 The longitudinal profile of the model is parallel to the shoreline.

problem of adjusting the value of permeability supply

of zones 1 and 2 accounts for 23.8% and 11.11%,

respectively, of the monthly rainfall measured at Phan

Thiet station. The average permeation volume for

many years in zone 1 is 249 mm/y, zone 2 is 199

mm/y. The amount of evaporation is 21.6% of

monthly evaporation measured at Phan Thiet

meteorological station, averaging 314 mm/y for many

years (the data of rainfall and evaporation of total

monthly rainfall and evaporation in the period of

2010-2020 were used for calculation). The water level

is initially determined after running the problem in an

unexploited natural state corresponding to the average

value of permeability and evaporation for many years.

The East Sea is taken as the determined concentration

margin and it has a constant TDS over time equal to

20,000 mg/L.

The barrier wall is arranged along the coast a few

tens of meters from the sea and has a length of 1,500

m, a thickness of 1 m, the permeability coefficient is

equal to 0.01 m/day and night. The permeability

coefficient and the wall thickness are constant over the

length of the wall. Retaining walls cut entire layers 1

and 2. In the mode, the retaining wall is simulated by

a weal water-permeability zone with the width of 10

m, the permeability coefficient is equal to 0.1 m and

the permeability resistance is equivalent to that of the

retaining wall, with a thickness of 1 m. The

permeability coefficient is equal to 0.01 m/day and

night.

To evaluate the effectiveness of underground

barrier wall in mining conditions, mining scheme has

been set up including 104 wells, the flow of wells

varies from 50 m3/d to 100 m3/d, total exploitation

flow at the beginning is 9,590 m3/d. Wells in the area

with retaining walls are mainly exploited in layer 2

and layer 1. It arranged by area and concentrated on

the barrier wall area. The model is divided into 3

equilibrium zones. Zone 2 has underground walls

including layers 1 and 2. This is also the area where

mining works are located, zone 3 is layer 3 on the area

with underground barrier wall, and finally zone 1.

4. Applications

The results of the calculation of water level, flow,

saline intrution under natural conditions and the

exploitation state in the presence of underground

barrier wall and without underground barrier wall

Using Visual MODFLOW Model to Assess the Efficiency of Subsurface Barrier Wall for Groundwater Flow Regulation and Reduction of Saline Intrution

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were shown below: (1) natural state without wall and

natural state with wall; (2) walled extraction and wall

less extraction. Fig. 3-6 are show the water level

isometric diagram, isometric TDS, water level

variation graph and TDS according to the exploitation

plan in case of having barrier wall and without barrier

wall in some monitoring wells.

4.1 Natural State

The Tables 1-6 are show the calculation results of

water level and water balance in the natural state with

walls and without walls.

Fig. 3 Diagram of model scope and underground barrier wall location, TDS monitoring well.

Fig. 4 Perpendicular section with underground barrier wall.

Using Visual MODFLOW Model to Assess the Efficiency of Subsurface Barrier Wall for Groundwater Flow Regulation and Reduction of Saline Intrution

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Fig. 5 The longitudinal section of the underground barrier wall.

Fig. 6 Balance zone distribution diagram.

Table 1 Constant head in and out (nature) (unit: m3/day).

Parameter Constant head output speed with wall

Constant head output speed without wall

Difference between with wall and without wall

Max 14,968 15,619 -651

Min 7,885 8,150 -265

Averages 10,193 10,509 -316

Parameter Constant head input speed without wall

Constant head input speed without wall

Difference between with wall and without wall

Max 46.14 46.91 -0.77

Min 0 0 0

Averages 3.46 3.53 -0.07

Using Visual MODFLOW Model to Assess the Efficiency of Subsurface Barrier Wall for Groundwater Flow Regulation and Reduction of Saline Intrution

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Table 2 Evaporation speed value (nature) (unit: mm).

Parameter Evaporation wall speed Evaporation speed without wall Difference between with wall and without wall

Max 6.454 1.289 5.165

Min 3.678 630 3.048

Averages 4.975 861 4.114

Time of a thousand days 5.410 945 4.465

Table 3 Cumulative ET value (nature) (unit: mm).

Time Wall Without wall Difference

Time of a thousand days 49,936,340 47,251,864 2,684,476

Table 4 Distances of water level monitoring obliques to the wall (unit: m).

TGMNQS1 TGMNKT3 TGMNKT2 TGMNKT1 T1MN KT1

MN TT

MN ST

-184 -306 0 +87 +87 -154 -6

+: Downstream wall, -: upstream wall.

Table 5 Water level value (nature) (unit: m).

Parameter Water level with wall

Max 18.59 20.44 9.49 6.06 6.51 11.14 7.60

Min 13.79 16.53 7.48 3.47 4.22 7.95 3.37

Averages 16.98 18.87 8.43 3.95 4.71 9.87 6.47

Parameter Water level without wall

Max 14.78 17.25 9.81 6.95 7.41 8.28 3.70

Min 12.07 14.79 7.47 5.07 5.51 6.72 2.74

Averages 13.01 15.70 8.22 5.68 6.10 7.35 3.10

Parameter Difference between with wall and without wall

Max 3.81 3.19 -0.32 -0.90 -0.90 2.86 3.89

Min 1.72 1.74 0.01 -1.60 -1.29 1.23 0.63

Averages 3.97 3.17 0.21 -1.73 -1.39 2.52 3.37

Table 6 Storage in value in, out (nature) (unit: m3/day).

Parameter Stored value in with wall Stored value in without wall Difference between with wall and without wall

Max 3,346,583 3,351,166 -4.583

Min 0 0 0

Averages 70,415 69,373 1.042

Parameter Stored Value out with wall Stored Value out without wall Difference between with wall and without wall

Max 3,290,319 743,620 2,546,699

Min 0 0 0

Averages 69,881 15,977 53,904

Using Visual MODFLOW Model to Assess the Efficiency of Subsurface Barrier Wall for Groundwater Flow Regulation and Reduction of Saline Intrution

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Fig. 7 Water balance in the presence of walls and isometric and isotropic diagrams of the whole natural state area with without walls.

Fig. 8 Graphs water levels of monitoring wells to calculate natural conditions without walls.

4.2 In Mining Condition

Figs. 9-13 are graph of water level variation and

TDS at some calculated monitoring wells, water level

diagram, TDS in mining condition.

Using

Fig. 9 Wate

Fig. 10 Grap

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ph of TDS var

DFLOW ModelFlow Re

without wall, e

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t wall, exploita

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th a flow of 6,0

ation of 6,000 m

of Subsurfacof Saline Intru

000 m3/d.

m3/d.

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all for Grounddwater 111

Using Visual MODFLOW Model to Assess the Efficiency of Subsurface Barrier Wall for Groundwater Flow Regulation and Reduction of Saline Intrution

112

Fig. 11 Diagram of isotopic layer 2 wall area at the time of 10,000 d with walls.

Fig. 12 TDS diagram of wall area with wall.

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Fig. 13 Graph water level of wells with wall.

Table 7 Comparing the water level at the time after 10,000 d of exploitation (unit: m).

Time of a thousand days Water level of mining wells

Well With wall Without wall Difference between with wall and without wall

TGMNQS1 2.33 1.59 0.74

TGMNKT3 3.38 2.87 0.51

TGMNKT2 1.05 0.58 0.47

TGMNKT1 -0.13 -0.24 0.11

T1MNKT1 1.08 1.17 -0.09

MNTT -0.24 -0.20 -0.04

MNST 0.30 0.20 0.1

Table 8 Compare the amount of salt between walled and unwalled (unit: kg).

Time of a thousand days

With wall Without wall Signal between TDS with wall and without wall

Accumulation Speed Accumulation Speed Speed

463,326,000 46,333 466,000.000 46,566 -233

Table 9 Comparison of evaporation between walled and unwalled (unit: m3/d).

Time of a thousand days

Without wall With wall

Accumulation Averages Accumulation Averages

32,443,000 3,244 32,428,824 3,242

Signal between TDS with wall and without wall

Accumulation Averages

14,176 2

Table 10 Constant head in comparison results between walled and wallless (unit: m3/d).

With wall Without wall Signal between TDS with wall and without wall

Max 250 324 -74

Min 0 0 0

Averages 49 64 -15

Using Visual MODFLOW Model to Assess the Efficiency of Subsurface Barrier Wall for Groundwater Flow Regulation and Reduction of Saline Intrution

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Table 11 Results of comparing storage in between wallless and walled (unit: m3/d).

Parameter Wall Without wall Signal between TDS with wall and without wall

Averages 5,745 5,752 -7

Max 3,181,871 3,259,449 -77,578

Min 0 0 0

Accumulated 57,456,700 57,525,444 -68,744

Table 12 Results of comparing storage out between wallless and walled (unit: m3/d).

Parameter Wall Without wall Signal between TDS with wall and without wall

Averages 5,286 5,282 4

Max 3,152,771 3,259,449 -106,678

Min 0 0 0

Time of a thousand days 52,865,488 52.828,850 36,638

Table 13 TDS comparison results between wallless and walled (unit: mg/L).

Obrevatiion wells Without wall Wall Difference between walled and unwalled

T1ND5 7,368 1,739 5,629

NDTT/B 783 714 69

NDTT/A 672 480 192

NDKTTT/2 907 209 698

NDKTN2/A 672 480 192

NDKTN1/A 1,150 316 834

NDKTB3/A 571 202 369

NDKTB2/A 201 202 -1

NDKTB1/A 201 202 -1

Table 14 Constant head in, out of zone 2 comparison results between wallless and walled (unit: mg/L).

Parameter With wall Without wall Signal between TDS with wall and without wall

Averages 34 51 -17

Max 170 244 -74

Min 0 0 0

Constant head out of zone 2

Averages 418 432 -14

Max 3,129 3,232 -103

Min 228 229 -1

Max/Min 13 14 -1

5. Conclusions

Barrier walls effectively store water, regulate

underground flows, improve exploitable reserves and

prevent saltwater intrusion. The effectiveness of the

underground barrier wall depends not only on the

hydrogeological structure, the technical parameters of

the wall but also on the layout scheme of the

exploitation well system.

The results showed that in natural conditions, the

ground water level upstream of the retaining wall rose

in the presence of a barrier wall. In wells located

downstream of high retaining walls, the water level

decreased.

The amount of underground current flowing into

the sea decreased, the annual average value of the

whole region decreased was 316 m3/day and night. In

presence of a wall, both the water level and the

amount of evaporation increased. The average increase

in evaporation volume during the calculation period of

ten-thousand days with walls was 4,114 m3/d. So in

presence of a wall, the amount of water that can be

Using Visual MODFLOW Model to Assess the Efficiency of Subsurface Barrier Wall for Groundwater Flow Regulation and Reduction of Saline Intrution

115

exploited increases by the total amount of evaporation

plus the decrease in discharge to the sea and is equal

to 4,424 m3/d. In the exploitation condition, if the

water level in the presence of wall is kept as low as in

the absence of wall, the exploitation flow will increase

to about 4,400 m3/day and night.

From the calculated water level values when there

is a wall and without a wall, we can see that if the

exploitation flow in presence of a wall and in the

absence of wall is the same, the water level drop at the

calculated observation wells upstream of the wall will

decrease from 0.21 m to 3.97 m.

The condition of effective exploitation of the wall

depends on the mining scheme. The exploitation

scheme is reasonable, the exploitation flow of the

wells does not exceed the allowable flow so as not to

cause the drying of the aquifer at the location of the

well. The upstream area of the wall reflects quite

clearly as the TDS content in observation wells

upstream of the wall at the end of the calculation time

is significantly reduced compared to that without the

wall, ranging from 69 mg/L to 5,629 mg/L. In the

presence of a wall, the water level of observation

wells upstream of the wall is higher than that without

a wall from 0.10 m to 0.74 m.

References

[1] Nguyen, V. D. 1996. Water Resources in the North

Central and South Central Plains. General Department of

Geology and Minerals of Viet Nam.

[2] Waterloo Hydrogeologic. 2021. “Hydrogeologic Software Solutions.” https://www.waterloohydrogeologic.com/.