[C. a. Brebbia, A. G. Kungolos] Water Resources Ma(Bookos.org)

Proceedings of the 19th IAHR-APD Congress 2014, Hanoi, Vietnam

ISBN xxx-xxxx-xx-x

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Water Quality Assessment in Coastal Areas of Lower Mekong Delta, South Vietnam

TRAN DANG AN(1), TRUONG THU HANG(1), TRIEU ANH NGOC(1), VO LE PHU(2)

(1) Water Resources University, 2nd Base, Ho Chi Minh City, Vietnam [email protected]

(2) Ho Chi Minh City University of Technology-VNU,Vietnam, 268 Ly Thuong Kiet, HCM City, Vietnam

ABSTRACT

Water quality is first consideration in choosing water resources for many purposes, especially in drinking and irrigation. In the Mekong Delta, the quality of water resources is seriously affected by many complicated factors such as wastewater, arsenic contamination and seawater intrusion. This paper aims to assess water quality, servicing for domestic and irrigation purposes in coastal areas of Soc Trang province, the Mekong Delta, Vietnam. The water quality index (WQI) and variable irrigation water quality indexes (IWQIs) such as sodium adsorption ratio ( SAR), Kelly's ratio (KR), residual sodium carbonate (RSC), solute sodium percentage (SSP) and permeability index (PI) were deployed to evaluate water quality regarding domestic and agricultural water demands. Water samples were taken by seasonal characteristics and tidal regime in 2013 and 2014. Main parameters include Temperature (T0C), Electrical Conductivity(EC), pH, Dissolved Oxygen(DO), Total Dissolved Solid (TDS), Total Hardness (TH), calcium (Ca2+), magnesium (Mg2+), sodium (Na+), bicarbonate (HCO3-), chloride (Cl-), sulfate (SO42-) and nitrate (NO3-) were analyzed for the calculation of WQI and IWQIs. The results show that, almost groundwater samples at shallow aquifers, some of the deeper aquifers and river water samples in dry season were seriously affected by seawater intrusion and nitrate contamination. This means that these water resources cannot use for both drinking and agricultural purposes. Meanwhile, most of groundwater samples at middle and lower Pleistocene aquifers and surface water in rainy season seem to be good for both drinking and irrigation. However, groundwater source in Soc Trang province has been excessively extracted without suitable regulation and appropriate management. This situation may threat to groundwater source because it can be easily contaminated by seawater intrusion and wastewater. In order to extract, use and manage effectively fresh water source in coastal areas in the Mekong delta, emergent management practices should be considered.

Keywords: Water quality assessment, Water quality index (WQI), Irrigation water quality indexes (IWQIs), Soc Trang, Mekong Delta

1. INTRODUCTION

Water plays an extremely important role to human's life and ecosystems on the planet. And ensuring water quality for drinking and irrigation is a big challenge in the context of water resources deterioration due to negative effects of anthropogenic activities and climate change (UN-Water, 2010). Monitoring and evaluating water quality are needed to understand water resources status and consider relevant solution for sustainable use and effective management. Practically, however, monitoring system provides us a large and complex water quality data while the reviewers who would access these data are variable, including managers, policy makers and publics (Saffran et al., 2001). Therefore, the overall water quality index that comprises main water parameters to assess water quality for drinking and irrigation purposes have been developed and globally applied as well-known water quality index (WQI), (GEMS, 2007; Tyagi et al., 2013) and irrigation water quality indexes (IWQIs) such as sodium adsorption ratio (SAR), Kelly's ratio (KR), residual sodium carbonate (RSC), solute sodium percentage (SSP) and permeability index (PI). Although, these water quality indexes have been widely applied in many countries in the world, the application of these water quality indexes in assessing water resources quality in Vietnam is still limitation. Therefore, this paper intends to make clear understanding on water quality in coastal areas of the Mekong Delta.

Mekong Delta is the most important delta in Vietnam where contributing more than 60% of agricultural

production of the country and accounting for more than 95% of rice production exporting over the world (Anh, 2010). In the Mekong Delta, surface water and groundwater contribute significant benefits to socio-economic development (Ghassemi.F and Brennan.D, 2000). However, water quality, especially surface water has been increasingly deteriorated due to human activities (Toan et al., 2013), seawater intrusion (Nguyen et al., 2012). Consequently, groundwater source has been excessively extracted and seriously threaten by seawater intrusion directly from East Sea, saline diffusion (IUCN, 2011; Wagner et al., 2010) and arsenic contamination (Laura E. Erbana et al., 2013). These affect not only on groundwater uses at present but also for future sustainability of water use in the Mekong Delta, especially in coastal regions.

In Soc Trang province, the coastal region of Mekong Delta, surface water is strongly affected by seawater. Thus, groundwater has become a preliminary fresh water source for domestic, industrial and agricultural activities. However, because of locating in coastal areas, groundwater source in Soc Trang province is highly vulnerable than other parts of the Mekong Delta due to the effects of over groundwater extraction, human wastes and waste water, seawater intrusion, acid sulfate soils dissolution. Therefore, the assessment of both surface water and groundwater quality is a vital concern for sustainable use and effective management of water resources.

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2. STYDY AREA

This study was carried out in Soc Trang province, a coastal region of the Mekong Delta, Vietnam. The province is located in the lower reach of the Mekong River. It covers an area of 2311.76 km2, approximately 0.7% and 5.9 % areas of Vietnam and Mekong Delta, respectively with average population of 1,292,796 people (VGSO, 2009)

Figure 1. Location of Soc Trang province, Southern Vietnam

Soc Trang province falls in strong tropical monsoon activity region with two different seasons; the dry and the rainy seasons. In the wet season, the province is affected by the Southwest Monsoon, which brings more than 85% amount of rainfall annually, high temperatures and high humidity. Meanwhile, the Northeast Monsoon dominates in the dry season from November until April, receiving dry air with hardly any precipitation (15% of the annual rainfall), lower temperature and lower humidity. The average annual air temperature in Soc Trang province is 26.8°C with over 28°C in the warmest months (April and May) and the lowest mean monthly temperature in January is 24.6°C; average annual relative humidity of 84%; and annual precipitation of 1,772 mm with lack of rain from January to April annually (MIST,2013). Because topography is relatively low, around 0.5 to 2.5 m above mean sea levels, coupled with facing directly with East Sea and the Hau river (Bassac River). Soc Trang province is frequently influenced by inflow from upstream, flooding and river-marine dynamic interactions such as tidal regime, sea wave, sediment transportation and deposition as well as riverbank erosion. These factors can affect significantly on water quality in the province.

3. MATERIALS AND METHODS

The field investigation and sampling of river, canal and groundwater (Fig.1), were performed in dry and rainy seasons between 2013 and 2014. Totally, 142 water samples were stored in 100ml bottles, including 101 groundwater and 41 surface water samples. Groundwater samples were collected from private tube-wells at seven aquifers, including Holocene (qh),upper Pleistocene (qh3), middle Pleistocene (qp23), lower Pleistocene (qp1), middle Pliocene (n22), lower Pliocene (n21) and Miocene (n13) in Soc Trang province (as shown in Figure 2) with the depth of wells ranges from 4.5 m to 480m under land surface. Surface water samples were taken along the Hau river and its tributaries at Soc Trang province. The pH, Dissolved Oxygen (DO), Electrical Conductivity (EC), Total dissolved solids (TDS) were on-site measured using portable meters. Meanwhile, main ion compositions including major cations (Na+, K+, Ca2+, Mg2+) and major anions (Cl-, SO42-, NO3-, HCO3-) were analyzed at University of Tsukuba, Japan.

The results of hydrochemistry as shown in Table 1, were used to calculated WQI and irrigation water quality indexes( SAR, KR, RSC, SSP, PI) in order to understand acceptable water quality for drinking and irrigation purposes.

4. RESULTS AND DISCUSSION

4.1 Hydrochemistry

The hydrochemistry of surface and groundwater was

performed in Tables 1 and 2 by the descriptive statistic

method. The box and Whisker plot for different water

quality parameters are given in the Figures 2 and 3, and

classification of water types and general groundwater

evolution were clearly described in Figure 4. Generally,

pH of river and canal water samples was slight alkaline,

whilst groundwater is from slight acidic to alkaline in

nature. The electrical conductivity (EC) values of both

surface and groundwater varied widely.

0.1

1

10

100

1000

10000

100000

TDS Na K Ca Mg CL NO3 S04 HCO3-

Main water parameters of Groundwater

Conce

ntration(m

g/L

) Max

Median

75 %

25%

Min

Legend

Figure 2. Box and Whisker Plot of groundwater for different

water quality parameters

1

10

100

1000

10000

100000


Conce

ntration(m

g/L

)

Main water parameters of River water in Dry season

0.1

1

10

100

1000

10000


Conce

ntration(m

g/L

)

Main water parameters of River water in Rainy season

Max

Median

75 %

25%

Min

Legend

Figure 3. Box and Whisker Plot of surface water in the dry and rainy seasons for different water quality parameters

G1: Mg-Na-Ca-HCO3G2: Na-ClG3: Na-HCO3-Cl

Mg-Ca

Gypsum-anhydriteDissolution

Ion exchangesSeawater intrusion

Calcite saturation

Desalination

G1

G2

G3

Figure 4. Durov diagram for classification water type

The EC values of river and canal water ranges from 118 to

24,300 µS/cm with average value of 9,465.24 µS/cm,

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whereas that of groundwater varies from 116.70 µS/cm to

21,200 µS/cm with average of 1,678.32 µS/cm. As shown

in Tables 1 and 2, calcium (Ca2+), magnesium (Mg2+) and

sodium (Na+) ions were major cations, while anions were

dominated by bicarbonate (HCO3-) and chloride (Cl-). The

possible sources of sodium, calcium and magnesium

concentration in these areas are due to the calcite and

dolomite dissolution, meanwhile high chloride

concentration may be driven from dissolution rock salts

and seawater intrusion. Fig.4 showed the different

characteristics of water resources in this area. Almost

shallow groundwater and surface water samples were

seriously affected by seawater intrusion, characterized by

Na-Cl type. Groundwater samples at middle and lower

Pleistocene and Miocene aquifers were dominated by Mg-

Ca-HCO3 and Na-HCO3-Cl types, respectively. It is also

obviously evidenced that groundwater in this study area

represents variable hydrogeological processes. Most of

shallow groundwater shows ion exchanges processes,

particularly influenced by seawater intrusion, while some

of deeper aquifers (Pliocene and Miocene) were affected

by paleo-saline and calcite rocks dissolution and tends to

calcite saturation. Groundwater at middle and lower

Pleistocene aquifers reveal two main tendencies, on the

one hand these samples were affected by dolomite

dissolution, representing an increase of calcite (Ca2+) and

magnesium (Mg2+) ions concentration, some of them have

been experienced by gypsum and anhydrite dissolution,

denoting an increase of sulfate (SO42-) constituent.

4.2 Water quality for drinking

Water quality index (WQI) was widely used as a very

useful tool to evaluate water quality servicing for drinking

purpose (Jagadeeswari and Ramesh, 2012; Tyagi et al.,

2013). WQI is defined as a rating reflecting the composite

influence of different water parameters (Sahu and

Sikdar, 2008). Calculating WQI based upon perspective

of suitability of water sources for human consumption

via three steps. In the first step, each of the chemical

parameter was assigned a weight (wi), based on its

perceived effects on human health. The highest weight

of five was assigned to parameters, which have the

major effects on water quality. In this area, nitrate

concentration (NO3-) was assigned the highest weight

because of its importance to water quality assessment.

The second step involves in computing the relative

weight of each parameter via using the equation Eq. [1]

as below:

=∑

i

i

Wi W

W [1]

where,

∑ iw is the sum of the weights of all parameters.

In this research, ∑ iw were 31. Table 3 shows the wi,

Wi and WHO standard for each chemical composition in this study. In the third step, a quality rating scale (qi) was computed for each parameter using the equation Eq. [2], given below:

= × 100i

i

Ci Sq [2]

where,

Ci and Si, respectively present the concentration and the WHO standard for each parameter, in mg/L. The water quality subindex, SIi of each parameter was calculated by using equation Eq. [3], given below:

= ×i iiSI W q [3]

Eventually, WQI is calculated by summing ten sub-indexes (SIi) of each water sample, as described in Eq. [4] below:

=∑ iWQI SI [4]

The WQI values of surface water and groundwater in this study were presented in Table 4, showing different tendencies. The WQI values of groundwater samples vary largely, ranging from 22.27 to 2319.25 with coefficient of variation of 217.04%, indicating the different water quality between aquifers. The WQI values of surface water demonstrate distinctly changes between rainy and dry seasons. In the dry season, despite of seawater intrusion effects, the WQI values of surface water were higher than that of values in the rainy season with average of 534.93 and 88.77, respectively. Groundwater samples at Holocene (qh) and upper Pleistocene (qh3) aquifers show very poor water quality and cannot use for drinking purpose and approximately 30%-40% groundwater samples at deeper aquifers (Pliocene, Miocene) can use for that purpose. Most of groundwater samples at middle and lower Pleistocene aquifers present good quality and are favorite aquifers for extracting to meet the demand of fresh water in coastal areas of Soc Trang province. Surface water shows very different quality between dry and rainy seasons. In the dry season, 95.24% of surface water samples cannot use for human consumption due to seawater intrusion, however in the rainy season 75% of surface water samples can be used for drinking purpose, excepting some areas along the coast despite of effect of salinity. Notably, although surface

water can be used in the rainy season, it should be carefully considered for human use because of being contaminated by wastes and wastewater from human activities via runoffs.

Table 1 Descriptive statistics of groundwater hydrochemistry in Soc Trang province in 2013 and 2014 Parameter Min Max Mean SD CV(%)

Temp 25.00 40.30 29.70 3.01 10.13 Depth 4.50 490.00 139.90 106.73 76.29 pH 6.52 8.68 7.25 0.36 4.96 DO 0.59 5.13 1.92 0.71 36.92 EC 116.70 21200.0 1678.32 3493.89 208.18 TDS 75.86 14204.0 1120.3 2342.1 209.07 Na+ 16.88 8535.80 437.82 1340.30 306.13 K+ 1.95 278.81 17.30 34.17 197.50 Ca2+ 1.05 970.48 66.42 157.81 237.61 Mg2+ 3.15 1290.06 69.17 168.36 243.39 Cl- 2.85 16970.45 727.18 2762.54 379.90 NO3- 0.11 264.18 18.52 52.54 283.68 SO42- 0.02 2511.29 148.09 351.77 237.53 HCO3- 37.83 779.23 353.45 130.03 36.79 TH 15.61 7712.98 451.03 1059.84 234.98 WQI 22.27 2319.25 163.55 354.97 217.04 SAR 0.92 73.72 7.30 12.53 171.69 KR 0.25 58.55 3.35 8.72 260.09 RSC 0.14 146.30 6.73 20.57 305.37 SSP 20.17 98.32 48.34 21.08 43.61 PI 15.02 95.14 39.05 10.40 26.63 MS 0.00 5.00 3.88 1.61 41.59 Notes: EC: Electrical Conductivity ( µS/cm), Tem. (0C), ions concentration, DO: Dissolved Oxygen (mg/L); TDS: Total Dissolved Solid (mg/L), RSC (meq/L), remain parameters (no unit): TH: total hardness; SAR: sodium ad sorption ratio; KR: Kelly’s ratio; RSC: residual sodium carbonate; SSP: solute sodium percentage; PI: permeability index.

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4.3 Water quality for irrigation

Despite of salt intrusion, surface water quality in Soc

Trang province changes significantly between the dry and

rainy seasons, especially in coastal areas. Meanwhile,

groundwater has been intensively exploited for both

domestic and irrigation purposes. Therefore, assessing

water quality of both surface and groundwater, provides

useful information for balancing between usage and

extraction water resources in coastal areas. Similar to

other coastal areas of the Mekong Delta, the province's

groundwater plays an important role to agricultural

activities such as onion, legume, sugarcane cultivation

household consumption. It is estimated that more than

85% of water for agricultural water in coastal areas such

as Tran De, Vinh Chau some parts of Cu Lao Dung and

Long Phu districts, depends on groundwater resource.

River and canal water can use for irrigation in other parts

of Soc Trang province where are not affected by seawater

intrusion . In order to evaluate water quality for irrigation

purposes, variable indicators including sodium

adsorption ratio (SAR), Kelly's ratio (KR), residual sodium

carbonate (RSC), solute sodium percentage (SSP) and

permeability index (PI) were calculated. The sodium

absorption ratio (SAR) for surface and groundwater was

estimated as the given equation Eq [5] below:

+

+ ++=

2 2

2

Ca Mg

NaSAR [5]

where,

Na+, Ca2+ and Mg2+ are solute concentrations of sodium, calcite and magnesium in meq/L.

The water samples having SAR values less than 10 are

categorized excellent water quality, from 10 to 18 as good,

from 18 to 26 as fair and above 26 are unsuitable for

irrigation use (USDA, 1954). The Kelley's ratio was

calculated by using the following expression.

+

+ ++= 2 2

Na

Ca MgKR [6]

where,


The Kelley's ratio of unity or less than one indicates good

water quality for irrigation, while above one is unsuitable

for agricultural purpose because of alkaline hazards

(Karanth, 1987 ). The residual sodium carbonate (RCS) is

determined by using the equation (Eq.[7]) below.

− − + ++ − += 2 2 23 3 ) ( )(RCS HCO CO Ca Mg [7]

where, the concentration of ions are expressed in meq/L.

Table 2 - Descriptive statistics of surface water hydrochemistry in the rainy and dry seasons, 2013 -2014

Parameter Dry Season

Rainy Season

Min Max Mean SD CV(%)

Min Max Mean SD CV(%)

Temp 25.90 30.10 28.44 1.02 3.59

27.80 33.30 29.36 1.02 3.49

pH 7.36 8.04 7.81 0.22 2.86

6.57 7.75 7.39 0.29 3.97

DO 3.64 6.65 5.27 0.88 16.62

1.53 5.37 3.99 0.98 24.66

EC 118.00 24300.00 9465.24 7062.63 74.62

133.20 13320.00 1372.22 3024.73 220.43

TDS 138.16 13329.39 5694.91 3812.86 66.95

89.24 8924.40 919.38 2026.57 220.43

Na+ 13.40 4877.02 1673.11 1418.07 84.76

5.98 2632.92 233.12 598.54 256.75

K+ -1.67 255.06 77.02 66.82 86.76

1.58 98.98 10.37 22.20 214.07

Ca2+ 9.36 174.34 70.45 49.18 69.81

13.22 111.39 23.94 22.01 91.94

Mg2+ 0.66 625.91 209.41 180.65 86.27

3.48 334.46 32.54 75.36 231.58

Cl- 14.50 9354.58 3103.31 2646.25 85.27

2.44 4642.72 396.87 1058.67 266.75

NO3- 0.06 234.40 107.18 93.23 86.99

0.49 23.43 3.14 5.30 168.53

SO42- 20.50 1677.79 624.50 502.58 80.48

5.94 536.06 53.13 120.22 226.25

HCO3- 67.10 137.00 90.72 15.21 16.77

71.39 136.07 84.06 17.15 20.41

TH 47.31 3014.58 1038.89 864.31 83.20

50.14 1656.44 193.94 365.30 188.36

WQI 31.53 1415.58 534.93 380.82 71.19

24.02 743.99 88.77 164.37 185.15

SAR 0.62 38.71 20.12 10.82 53.77

0.37 28.19 4.30 6.87 159.91

KR 0.33 4.07 3.25 0.83 25.50

0.26 3.47 1.29 1.15 89.14

RSC 0.27 58.42 19.24 17.03 88.51

0.02 30.90 2.58 7.01 271.17

SSP 24.89 80.28 74.46 11.77 15.81

20.60 77.63 46.38 21.40 46.13

PI 18.39 57.81 23.69 11.39 48.10

18.83 91.60 54.68 23.59 43.14

MS 0.00 5.00 1.05 1.24 118.75

0.00 5.00 3.70 1.78 48.11 Notes: EC: Electrical Conductivity ( µS/cm), Tem. (0C), ions concentration, DO: Dissolved Oxygen (mg/L); TDS: Total Dissolved Solid (mg/L), RSC (meq/L), remain parameters (no unit): TH: total hardness; SAR: sodium ad sorption ratio; KR: Kelly’s ratio; RSC: residual sodium carbonate; SSP: solute sodium percentage; PI: permeability index.

Table 3 The weight and relative weight of each of the chemical parameters used for the WQI determination

Physico-chemical

parameters

WHO

Standard wi W

pH 7.5 4 0.1290

Total Dissolved Solid (TDS) 1000 4 0.1290

Total Hardness (TH) 300 2 0.0645

Bicarbonate (HCO3-) 300 3 0.0968

Calcium (Ca2+) 75 2 0.0645

Magnesium (Mg2+) 30 2 0.0645

Sodium (Na+) 200 2 0.0645

Chloride (Cl-) 250 3 0.0968

Nitrate (NO3-) 50 5 0.1613

Sulfate (SO42-) 250 4 0.1290

Sum 31 1.0

Notes: Ions concentration , TDS, and TH (mg/L);

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According to USDA (1954), RCS less than 1.25 meq/L, the

water source is safe for irrigation, RCS ranges from 1.25 to

2.5 meq/L, indicating quite suitability, while RCS exceeds

2.5 meq/L, the water is generally unsuitable for irrigation.

Since 1955, Wilcox proposed classification scheme for

rating water irrigation on the basis of soluble sodium

percentage (SSP). The SSP was calculated by using the

following formulate:

+

++ +×

++= 2 2

100

Na

Na

Ca MgSSP [8]

where,


The values of SSP less than 50 reveal good quality of

water for irrigation whilst higher values (above 50) are

unsuitable for agriculture (USDA, 1954). The permeability

index is computed by using equation Eq. [9] below:

+ −

++ ++

++×= 3

2 2

\100

HCO

Na

Na

Ca MgPI [9]

where, solute concentrations are expressed in meq/L.

The PI values more than 75 illustrate excellent quality of

water for irrigation, while the PI values are between 25

and 75, indicating good quality of water. However, the PI

values are less than 25, reflecting unsuitable water source

for irrigation purpose.

The results of water quality assessment of both surface

and groundwater for irrigation purpose are presented in

Table 4. It is clearly demonstrated that water quality for

irrigation practices in this area varies widely depending

on seasons and types of aquifers. About 60% of surface

water samples in the wet weather from June to December

can be used for irrigation, excepting some areas close to

the coast, while in the dry season just only 4.76% of

surface water in northwestern parts of Ke Sach, Cu Lao

Dung Island and Long Phu districts can be met irrigation.

This shows strong effects of seasonal seawater intrusion in

the coastal river system. Unlikely, groundwater quality

depends significantly on features of aquifers. Most

groundwater samples of Holocene (qh), upper Pleistocene

(qh3), some of middle and lower Pliocene (n22. n21) and

Miocene (n13) aquifers cannot use for irrigation, while

almost groundwater samples of middle and lower

Pleistocene (qp23, qp1) aquifers are good quality for both

drinking and irrigation purposes, spelling the reason for

these aquifers have been intensively pumped for those

purposes, resulting in rapid groundwater drawdown of

the large scale of the province.

Furthermore, Wilcox diagram was deployed to classify

water quality for irrigation. It can seen clearly that almost

groundwater at middle and lower Pleistocene (qp23, qp1)

aquifers and surface water in the rainy season fall in the

categories C1-S1, C2-S1 and C3-S1, while almost

groundwater of Holocene (qh), upper Pleistocene (qh3)

and surface water in the dry season were characterized by

C4-S3 and C4-S4. Remained groundwater samples of

Pliocene (n22, n21) and Miocene (n13) aquifers were

ranged between C3-S3 and C3-S4 groups.

Figure 5. Wilcox diagram of water classification for irrigation

4.4 Groundwater pollution

Figure 6 shows nitrate concentration of river and groundwater in the dry season, Feb., 2014. The majority of groundwater and surface water samples were contaminated by nitrate constituent and exceeding desirable values of nitrate concentration, 10 mg/L for drinking water in comparison with WHO and US drinking water standards. More seriously, high nitrate concentration presents not only at shallow aquifers (qh, qh3) where groundwater was intensive extracted for shrimp farming but also at deep groundwater around Soc Trang City. Groundwater samples from shallow and some of deep aquifers are high nitrate concentration, ranging from 32.86 mg/L(G3) to 256. 31 mg/L (Q59801Z ). This can be explained by influence of wastewater and fertilizers from agricultural practices in the province, while nitrate sources of groundwater samples at deep aquifer surrounding Soc Trang city may be caused by geologic conditions (Boyce et al., 1976) and wastewater.

12.38

256.31 240.75264.18

113.10

32.86

5.01 4.515.44 5.90

7.53

128.73125.67

257.29

11.40 11.17 11.98

98.24

37.36

445.49

10

1.00

10.00

100.00

1000.00

Natrite concentration of water samples, dry season2014

Q59801T Q59801Z M1 M2 M3 G3 N1

G5 G6 G7 Q598020M1 Q598030 Q59804T Q59804Z

G1 G2 G4 B1 S1-TV SW STD

Shrimp farming

Soc Trang City

Figure 6. Nitrate concentration of groundwater and surface water in dry season, Feb.,2014

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5. CONCLUSIONS

In coastal areas of Soc Trang province, the quality of surface water and groundwater is seasonal variation and influenced by geological conditions. Groundwater in Soc Trang province is mainly characterized by Mg-Na-Ca--HCO3 type at middle and lower Pleistocene aquifers, while deeper aquifers are dominated by Na-HCO3-Cl type. Groundwater of Holocene and upper Pleistocene aquifers were grouped into heavy brackish water (Na-Cl), indicating effects of seawater intrusion. Surface water in this area was strongly affected by seawater intrusion in the dry season and some parts close to the coast suffer from salinity in the rainy season, containing high salt concentration. More notably, some groundwater samples of middle Pleistocene aquifer, locating at Tran De estuary and the northwestern part of Ke Sach district have high salinity in comparison with other samples, possibly displayed expanding the saline boundary from surrounding areas to these areas due to tremendous groundwater drawdown.

The results of WQI and IWQIs reveal that fresh water resources in coastal areas of Soc Trang province seem to be finite to drinking and irrigation purposes. Most of available fresh groundwater from middle and lower Pleistocene aquifers, whereas the rest of water resources cannot used for drinking and irrigation due to WQI and IWQIs values exceeding acceptable values. According to the WQI values, more than 90% of groundwater samples of middle Pleistocene (98.15% and 90.48% of samples in Cu Lao Dung and in Soc Trang City, respectively), 40% of lower Pleistocene and 30% of deep aquifers present good to excellent quality for drinking purpose, with WQI values ranging from 22.27 to 135.97.

Meanwhile, WQI values of shallow groundwater samples exceeding 100, and cannot be suitable for both drinking and irrigation uses. Importantly, surface water in Soc Trang province was significantly influenced by seawater intrusion in the dry weather, in particularly in river and canal systems that are close to the coast. In the dry season, only 4.76% of surface water samples, located northwestern parts of Soc Trang province can be used for drinking purpose. In the wet weather, most of surface water samples (75%) surrounding Ke Sach and Soc Trang City can be an alternative source for drinking demand. This is an optional choice in the context of groundwater degradation that has been occurred in Soc Trang province.

Irrigation water quality indexes (SAR, KR, RSC, SSP and PI) indicate very differences of water quality between groundwater and surface water. Interestingly, 92.59% and 57.14% of groundwater samples of middle Pleistocene aquifer in Cu Lao Dung Island and Soc Trang areas, respectively can be used for irrigation, while groundwater of Holocene, upper Pleistocene and deeper aquifers (Pliocene and Miocene) were not suitable for irrigation purpose. More than 60% surface water in the rainy season can be used for irrigation, whereas only 4.76% surface water samples in the dry season can be used for that purpose, indicating seawater intrusion strongly affects surface water in the dry season rather than the rainy season in this area.

Nitrate contamination also was highly detected at all shallow aquifers and some deep groundwater samples surrounding Soc Trang City. These samples have very high nitrate concentration, ranging from 32.86 mg/L (G3) to 256.31 mg/L (Q59801Z ). It is due to influences of waste water and fertilizers from agricultural activities.

Table 4 - Results of water quality assessment for drinking and irrigation purposes

Indexes Range values Aquifers: Shallow -> Deep aquifers River water

qh qh3 qp23 CLD

qp23 ST

qp1 n13 Dry season

Rainy season

WQI

Max 1665.52

2319.25

1665.52

135.97

441.18

895.00

1415.58 743.99

Mean 888.29 1413.40

888.29 72.00 129.25

254.93

534.93 88.77

Min 111.06 227.65 111.06 22.27 60.37 80.35 31.53 24.02 Desirable value < 100.0

Drinking Percent. A.V 0.00 0.00 98.15 90.48 40.00 30.00 4.76 75.00

SAR

Max 73.72 39.29 73.72 9.59 30.77 46.17 38.71 28.19 Mean 40.07 26.65 40.07 2.98 7.70 32.18 20.12 4.30 Min 1.77 1.57 1.77 0.48 0.77 3.42 0.62 0.37 Acceptable value

< 26.0

KR


< 1.0

RSC


<2.5

SSP


<50

PI


>25

Irrigation Percent. A.V 0.00 0.00 92.59 57.14 0.00 0.00 4.76 60.00

7

Groundwater quality in Soc Trang province should be concerned in terms of increasing seawater intrusion both directly from East Sea and saline diffusion as well as nitrate pollution in somewhere. It is concluded that groundwater quality deterioration in Soc Trang province was driven from many intricate factors, including: (i) inappropriate environmental management in the delta, leading to water pollution; (ii) over extraction inducing seawater intrusion and mixing contaminants and (iii) poor wells construction and lack of sustainable exploitation and effective management, which create a direct pathway for exacerbating water quality of aquifers and seeping pollutants from surface to good quality groundwater layers.

Therefore, an integrated management practices of water resources is urgently needed for sustainable use. Particularly, an appropriate management practices and regulation on groundwater extraction should be carefully considered in coastal areas which have were strongly affected by salt intrusion, wastewater of households, industrial and agricultural activities.

ACKNOWLEDGEMENTS

The authors would like to express their gratefulness to Prof. Maki Tsujimura, Chair of Doctoral Program in Sustainable Environmental Studies, Faculty of Life and Environmental Sciences, University of Tsukuba for providing necessary facilities for this research. We also would like to thank Department of Natural Resources and Environment, Center for Rural Water Supply and Sanitation of Soc Trang Province, especially Mr. Dong Thong Nhat, Mr. Nguyen Van Chanh, Mr. Lam Huynh Kien and Mr. Thach Hoang Linh for their support during the field surveys between 2013 and 2014. Our thankfulness also is extended to Dr. Bui Tran Vuong, Vice Director of Division for Water Resources and Planning for South of Vietnam, and Mr. Hoang Dai Phuc, a hydro-geological specialist at Improvement of Groundwater Protection projects in Vientam for their kind support and providing relevant data to this study. Particularly, we would like to address special thanks to Japanese Grant Aid for Human Resources Development Scholarship (JDS program) for completing this study fruitfully.

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