Water Quality of the World’s Largest Mangrove...

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Borderless Science Publishing 141

Canadian Chemical Transactions Year 2013 | Volume 1 | Issue 2 | 41-156

ISSN 2291-6458 (Print), ISSN 2291-6466 (Online)

Research Article DOI:10.13179/canchemtrans.2013.01.02.0018

Water Quality of the World’s Largest Mangrove Forest

Mohammad M. Rahman

1, Mir T. Rahman

1*, Mohammad S. Rahaman

2, Farzana Rahman

1,

Jasim U. Ahmad1, Begum Shakera

3, and Mohammad A. Halim

4

1Department of Chemistry, Jahangirnagar University, Savar, Dhaka, Bangladesh

2Department of Chemistry, Comilla University, Comilla, Bangladesh

3Chemistry Branch, Geological Survey of Bangladesh, Dhaka, Bangladesh

4Department of Chemistry, The University of Western Ontario, London, Ontario, Canada

*Corresponding Author, Email: [email protected] Phone: +880-1732436083

Received: April 25, 2013 Revised: June 7, 2013 Accepted: June 10, 2013 Published: June 13, 2013

Abstract: This study reports some physico-chemical parameters (BOD, COD, DO, TDS, TSS, EC, Eh,

hardness, alkalinity, temperature, and content of Na+, Mg

2+, K

+, Ca

2+, Cl

-, SO4

2-, and HCO3

- of the Passur

river in the Sundarbans, the world’s largest mangrove forest. The study was conducted over four sampling

points: Mongla, Dangmari, Koromjol, and Koromjol Creek of the Passur river during the year of 2008-

2009 at rainy, winter, and summer seasons. Overall, the water quality parameters of the river were

acceptable during rainy season; however, moderate to high values of these parameters were appeared for

winter and summer seasons. Result indicated that the concentration of TSS (10.8 -19.7 g/L) during

summer and TDS (3.5-53.3 g/L) in all the season exceeds the recommended concentration for

Bangladesh. The highest DO concentration (6.0-7.33 mg/L) was observed in winter; nonetheless, the

highest BOD (20.2-28.0 mg/L) was obtained in summer season. Moreover, in summer, almost all

sampling points showed high value of COD (19.0-38.0 mg/L). The alkalinity and hardness of river water

was gradually increased in winter and summer seasons than that of the rainy season. Sea water intrusion

and industrial discharge may contribute the high concentration of Cl- (12.5-4672 mg/L), SO4

2- (9.02-968.3

mg/L), HCO3- (116-203.3 mg/L), Mg

2+(4.86-583.2 mg/L), Na

+ (329-8839 mg/L) and K

+ (45.15-992.0

mg/L) ions. Throughout the year, for all sampling locations, the measured average temperature, pH, EC,

salinity of Passur river water were well matched with studies performed in this area and other regions.

Keywords: Physico-chemical Parameters, Sundarbans Mangrove Forest, Water Pollution, Environmental

Chemistry

mailto:[email protected]

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Canadian Chemical Transactions Year 2013 | Volume 1 | Issue 2 | Page 141-156


1. INTRODUCTION

Productive wetlands, which support biodiversity and ecosystems, are essential parts of the

environment. They are important breeding grounds for fish and game. The wetlands also regulate

water quality, quantity, nutrient cycling and act as a buffer between terrestrial and aquatic systems

[1]. Mangrove ecosystems are productive wetlands found in tropical and subtropical regions which

provide suitable shelter for both marine and terrestrial organisms [2]. Human interferences with the

landscape have widespread influence on wetlands [3-4] and global warming [5]. Consequently, water

temperature [6] and in-stream biogeochemical processes are altered [7]. The healthy aquatic ecosystem

depends on the physico-chemical and biological characteristics [8]. Therefore, identifying spatial and

temporal changes in water quality in river basins has been a major focus of many studies. Various studies

have already been conducted on different Indian sub-continental rivers linked to coastal regions [9-23].

Rivers included in other studies are the Han River in South Korea [24], the Amu Darya River in Central

Asia [25], the Struma River in Bulgaria [26], and the Bagmati River in Nepal [27]. These studies

confirmed that anthropogenic activities not only severely degrade water quality in the downstream of the

major rivers, but also result in cumulative effects from upstream and in the small river due to inadequate

wastewater treatment facilities.

The Sunderbans, the largest halophytic mangrove forest located in the southern part of

Bangladesh and West Bengal, is a center for economic activities, such as the extraction of timber and fuel

wood, fishing and collection of honey and other forest products. During the last two decades, commercial

shrimp farming inside the polder areas has emerged as a key economic activity. Within the Sundarbans,

there are three wildlife sanctuaries and one national park covering 27% of the area; all of these are

included as a World Heritage Site by the United Nations Educational, Scientific, and Cultural

Organization (UNESCO) [28]. The plant and vegetation of Sundarbans Reserve Forest (SRF) provide

food and shelter for fish, crustaceans, mollusks, and others aquatic lives. The entire Sundarbans and its

surrounding areas of brackish and marine water are also used as breeding, nursery and feeding habitats by

fishes, mollusks and crustaceans [29].

The Sundarbans ecosystem is mainly dependent on the availability of adequate fresh water.

However, the landscapes began to change during the early 19th century when part of the Sundarbans

began to lose the saline and fresh water balance. The already degraded environment became further

imbalanced when India constructed the Farakka Barrage on the Ganges, which is 17 km upstream of

Bangladesh boarder [1]. The water quality in the ecosystem is largely affected by water pollution from

industries located in the upstream areas and possible oil pollution from nearby ports of Khulna and

Mongla. There are about 165 industries in the immediate upstream Khulna district. Loading of municipal

waste (raw sewage and solid domestic wastes) from Khulna and Mongla Port to the Pussur River is

estimated to be on the order of 2.2 tons of BOD/day [30]. About 60% of the area remains in higher

salinity (>20 mg/L) for at least 1.5 months in a year. In the study area, the average dissolved oxygen (DO)

concentration is around 5.99 mg/L, which is permissible according to the Environmental Quality Standard

(EQS) of Bangladesh. The content of total ammonia, nitrate (NO3–N), and phosphate (PO4–P) level is

sufficient for the survival of aquatic life; however, heavy metal concentrations occasionally exceed the

EQS limit, particularly along with the large barge routes in the western part of the forest [30]. Moreover,

some studies [31-33] revealed that unplanned exploitations of natural resources imposed extreme threats

to the ecosystems of this forest.

Although this forest is very crucial for its plants, animals, and mangrove fishery; to our best

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(A)

(B)

Figure 1. A) Google satellite map of the Sundarbans, B) The locations of the sampling areas.

100 km Imagery ©2013 TerraMetrics, NASA

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knowledge very few studies are available on the physico-chemical parameters in the Sundarbans river

systems, particularly in the largest forest region located in Bangladesh. The primary objective of this

study is to determine the contamination level as well as measure the water qualities, both are essential

components for maintaining an ecological balance between flora and fauna of the world largest

mangrove forest.

2. Materials and Methods

2.1 Study area

The Sundarbans, shared between Bangladesh and India, is the world’s largest, continuous coastal

wetland. It covers an area of about one million hectares in the delta of the rivers Ganges, Brahmaputra,

and Meghna. Enormous amounts of sediments carried by the river system contribute to the expansion

and dynamics of this delta. The Sundarbans area experiences subtropical monsoonal climate with an

annual rainfall of 1,600–1,800 mm and occasional severe cyclonic storms [2]. The maximum elevation

within the Sundarban is only 10 m above the mean sea level. The western and eastern limits of the

Sundarbans are defined by the course of the River Hooghly (a distributary of river Ganges) and River

Baleshwar, respectively. About 60 % of the mangrove forests lie in the Khulna District of Bangladesh

and the rest in the 24-Paragnas District of West Bengal (India). A large number of channels and creeks

flow into larger rivers in the Sundarbans. The largest of these rivers are the remains of the Ganges which

has shifted eastward and is named the Gorai river. This river, the main tributary of the Ganges, is

connected to Passur River and also has an indirect link to the Sibsa river. These two rivers play an

influential role in the Sundarbans ecosystems. In the eastern part of the Sunderban, the Baleswar River is

connected to the Ganges, and thereby receives abundant fresh water from it [34]. The south of the forest

meets with the Bay of Bengal; to the east it is bordered by the Baleswar River. To the north of the

Sunderban there is a sharp interface with intensively cultivated land. Rivers in this forest are meeting

places for salt water and freshwater. Thus, it is a region of transition between the freshwater of the rivers

originating from the Ganges and the saline water of the Bay of Bengal [30]. The maps in Figure 1 show

the satellite image and the sampling stations of the study area.

2.2 Sampling of the samples

The water samples were collected from the Passur River at Sundarbans. The water samples were

collected from four stations at three different times. The distance between sampling points was

approximately 1 km. The sampling stations were (a) Mongla, (b) Dangmari, (c) Koromjol, and (d)

Koromjol Creek, and collection times were (i) September 2008 (Rainy Season), (ii) January 2009

(Winter Season) and (iii) May 2009 (Summer Season). Three liter water samples were collected from

each sampling station at an approximate 2 m depth from the surface of the water. The standard water

sampler (Hydro Bios, Germany) were used with six pre-cleaned 500 mL volume polyethylene bottles to

collect the water from each sampling site. Then waters from six bottles were mixed together to obtain a

composite sample. Then the composites were filtered through 0.45 μm membrane filters.

2.3 Analytical methods

Before sampling all instruments were checked for efficient analysis. A set of environmental

variables, such as temperature (accuracy: ±0.3°C), pH (accuracy: ±0.01), conductivity and total dissolve

solid (TDS), Salinity (accuracy: ±0.5% of reading) were measured in situ using a Portable Mercury

Thermometer (0-500C), HANNA instruments (Model: HI 98106, Portugal), TDS meter (H1-9635,

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portable water proof Multirange Conductivity/TDS meter), portable Refractometer (HQ40d), respectively.

Dissolved Oxygen (DO) concentration was also determined in situ according to Azide Modification

Method following Standard Methods of Water and Waste Water Analysis [35]. BOD, COD, Calcium,

Magnesium, Hardness, Alkalinity, Sulfate concentration were determined in the laboratory according to a

5-Day BOD test method, Closed Reflux titrimetric method, EDTA Titrimetric method, Calculation

method, EDTA Titrimetric method, Titrimetry Method, Turbidimetric method, respectively, following

Standard Methods of Water and Waste Water Analysis [35]. Carbonate and Bicarbonate were measured by

titrimetric method [36]. Sodium and Potassium were measured by using Flame Photometer (Models PEP

7, Voltage 230/115V, Power 13VA). The concentration of chloride was determined by the Ion selective

electrode method (Cole-Palmer chloride electrode, model no.27502-13).

3. Result and Discussion

3.1 Temperature, pH, Redox Potential (Eh), Electrical conductivity (EC), Hardness and Alkalinity

The water temperature at four sampling stations ranged from 30 C to 31

C in rainy, 22

C to 23

C in winter and 30.6

C to 31.1

C in summer season. As expected, the highest temperature (31.1

C) was

observed during the summer season whereas the lowest temperature (19 C) was common in winter

season. A similar trend of temperature also appeared from inland to sea for rainy and winter seasons, as

presented in Figure 2A. In the River Bhavani, Tamilnadu, India, the water temperature was varied from

22 °C to 29.5 °C over the whole year [37]. The average water temperature recorded at four stations was

more or less uniform in the river; however, slightly higher temperature was found in the sea. This

temperature profile is very common in a typical sub-tropical aquatic system [38].

The maximum pH of 8.1 was observed at Dangmari point in rainy season; however, the minimum

pH of 7.1 was observed at Mongla in winter season. Comparing among different seasons, the maximum

pH was frequently found in the rainy season and is slightly alkaline, and the minimum pH was appeared

in the winter season, as depicted in Figure 2B. Slightly acidic pH (6.25-6.95) was found in the water of

Lala, Yobo and Agodo Rivers of Ogun State, Nigeria [39]. A previous study conducted by Rahman et al.,

[40] revealed that the pH of the water of the Sundarbans Reserve Forest (SRF), Khulna, seasonally varies

from 7.0 to 8.4, which this is very similar to the present study. Low and high pH value is very harmful for

the ecosystems of the river, and the recommended pH level of river water is around 7.4. The finding of

Shaikh et al. [41] revealed that pH is the most important factor which controls the growth of the green

algae. The relatively high pH (8.1) in the rainy season may be contributed by the local discharges which

contain alkaline effluents from the surrounding paper and pulp industries, textile and dyeing industries,

and rayon mills located in the Bhairab-Rupsa-Passur belt. These chemically enriched effluents are also

mixed with the river water by the rainwater in the form of local precipitation as well as floodwater

flowing from upstream regions

The redox potential (Eh) at four sampling stations was observed to be 38 to 107 mV in rainy

season, 94 to 96 mV in winter and 41 to 76 mV in summer season (Figure 2C). The value of Eh is very

high at the koromjol point during the rainy season because of excessive rainfall and significant tidal

action. In addition, the Eh value gradually increases towards the sea due to the fresh–saline water

intermixes.

The electrical conductivity (EC) of surface water collected from the four stations varied in the

order summer < Rainy < winter. During the whole season, the value widely varied from 4.11 to 33.2

mS/cm and the variations of EC at four sampling stations are shown in Figure 2D. The EC of the Passur–

Sibsa water around the Sundarbans Reserve Forest (SRF) was 12.7‒47.9 mS/cm [40]. Muhibbullah et al.,

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(A) (B)

(C) (D)

(E) (F)

Figure 2: Season variation of parameters assessed in this study: A) Temperature B) pH C) Redox

Potential (Eh) D) Electrical conductivity (EC) E) Hardness F) Alkalinity

0

10

20

30

40

Temperature (ºC)

Mongla

Dangmari

Koromjol

Koromjol creek

6.60

7.10

7.60

8.10

pH

Mongla

Dangmari

Koromjol

Koromjol creek

0

30

60

90

120

Eh (mV)

Mongla

Dangmari

Koromjol

Koromjol creek

0

50

100

150

200

Alkalinity (mg/L)

Mongla

Dangmari

Koromjol

Koromjol creek

0

1000

2000

3000

4000

Hardness (mg/L)

Mongla

Dangmari

Koromjol

Koromjol creek

0

10

20

30

40

EC (mS/cm)

Mongla

Dangmari

Koromjol

Koromjol creek

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[29] suggests that EC in the water was found to be 0.27, 0.30, 7.80 and 12.30 (mS/cm) in the river

Rupsha, Passur, Shipsa and Arpangasia, respectively. A study carried out by Adejuwon and Adelakun [39]

found the EC value in Lala, Yobo and Agodo Rivers are ranged from 0.125 mS/cm to 0.593 mS/cm. In

most cases, EC depends on the presence of ions, their total concentrations, and their mobility along with

the changing temperature [42]. This is mostly influenced by dissolved salts including sodium chloride and

potassium chloride. EC of River Passur water differed significantly among all the seasons. Higher values

of summer season may possibly be due to the decrease of fresh water flow and high evaporation rate,

which significantly increase the concentration of the dissolve conducting minerals.

During the rainy season, the average water hardness in the sampling stations of Passur river was

found to be 98.47 mg/L (ranged 96.09-100.1 mg/L), whereas water hardness remarkably increased to

1750 mg/L (ranged 1650-1850 mg/L) and 2850 mg/L (ranged 2800-2900 mg/L) in winter and summer

seasons, respectively (Figure 2E). However, a previous study performed by Adejuwon et al., [39]

disclosed a low level (35-168 mg/L) of water hardness in Lala, Yobo and Agodo rivers of Ogun State,

Nigeria. A similar water hardness data (43.8 to 302 mg/L) was also observed in boreholes of the Yola-

Jimeta Metropolis, Nigeria [43]. For the standard case, the hardness of river water remains moderately

high (75-100 mg/L) for the rainy season; however, the values are excessively high (300 mg/L) for the

winter and summer seasons [44]. Hardness of seawater is reasonably higher than that of river water.

Typical seawater has a calcium hardness of 1000 mg/L, magnesium hardness of 5630 mg/L and total

hardness of 6630 mg/L [45]. A high level of hardness is occurred due to the divalent cations such as Ca+2,

Mg +2,

Sr+2

etc. Cations higher than divalent can contribute low level of hardness and mono-valent cations

cannot produce any hardness. The high level of hardness of the Passur river water during the winter and

summer seasons at all stations may be due to the seawater intrusion from Bay of Bengal to the fresh

ground water. In order to prove this, from the perspective of hydrogeology further detailed studies are

required.

The alkalinity of water samples was in the range of 100 to 150 mg/L for all stations, and the

highest alkalinity was observed in Mongla point during winter and summer seasons. The variation in the

alkalinity of the Passur River water throughout the all seasons is shown in Figure 2F. Our findings related

to alkalinity are in good agreement with the results obtained by Bava and Seralathan [46] for the other

part of Sundarbans in West Bengal, India and by Adejuwon et al., [39] for Lala, Yobo and Agodo rivers of

Ogun State, Nigeria. The acceptable level of river water alkalinity, recommended by WHO, is 100 mg/L

[47-48]. During the rainy season, the values of alkalinity remain the same for all stations due to adequate

river discharge. Nevertheless, the gradual increase in alkalinity during the winter and summer seasons

may be due to industrial discharges, as well as low rainfall, high evaporation, sea water intrusion.

3.2 Content of Biochemical Oxygen Demand (BOD), Total Dissolved Solids (TDS), Chemical

Oxygen Demand (COD), Salinity, Total Suspended Solid (TSS), and Dissolved Oxygen (DO)

The average BOD in the water of the Passur river collected from four sampling stations was 11.7

mg/L (ranged 9.7-13.5 mg/L) during rainy season, 16.5 mg/L (ranged 11.9-20.7 mg/L) in the winter, and

24.1 mg/L (ranged 20.2-28 mg/L) for summer seasons, as depicted in Figure 3A. Mongla and Koromjol

creek showed relatively higher BOD values compare to the other two sampling stations. The rainy season

contributed low level of BOD than that of the winter and summer seasons; however, the BOD level of all

seasons exceeded the limit (10 mg/L) of Environment Quality Standards (EQS) recommend for

Bangladesh [49]. This high level of BOD observed in the Passur River is also comparable with the results

obtained by Wahid et al. and Boyd et al., [30, 50]. The reasons for the high level of BOD include (i)

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0

10

20

30

TSS (g/L)

Mongla

Dangmari

Koromjol

Koromjol creek

(A) (B)

(C) (D)

(E) (F)

Figure 3: Season variation of parameters assessed in this study: A) BOD B) TDS C) COD D) Salinity E)

TSS F) DO

0

10

20

30

BOD (mg/L)

Mongla

Dangmari

Koromjol

Koromjol creek

0

20

40

60

TDS (g/L)

Mongla

Dangmari

Koromjol

Koromjol creek

0

10

20

30

Salinity (g/L)

Mongla

Dangmari

Koromjol

Koromjol creek

0

4

8

DO (mg/L)

Mongla

Dangmari

Koromjol

Koromjol creek

0

20

40

COD (mg/L)

Mongla

Dangmari

Koromjol

Koromjol creek

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changing the temperature and nutrient content during the year and (ii) high contributions of waste load

from the surrounding industries. In the rainy season, the waste load was lower compared to other two

seasons due to high water flow, and thus contributed less BOD. However, in winter and summer seasons,

the waste load was significantly high and contributed a high level of BOD. Moreover, the water

temperature was lower in winter seasons than that of summer, which in turn decreases the bacterial and

microbial activities and contributes a low level of BOD than in the summer. This study thus confirmed

that warmer water (the highest temp. 31.1 º C) in summer contributed high BOD levels compared to the

cold water in winter seasons (the highest temp. 23 º C). Several studies revealed that elevated water

temperature promotes the rate of photosynthesis which in turn blooms the primary products (e.g. algae or

phytoplankton) in the water. The increased temperature has very pivotal influences on the total aquatic

environment changing seasonal patterns related to primary products and modifying the food cycles [51,

52 and ref. herein]. The fast photosynthesis helps plants to grow and die rapidly. The dead plats are

decomposed by bacteria and they require more oxygen for this process. Due to the shortage of oxygen,

BOD level goes up at this certain location [53]. Therefore, it is relevant that high water temperature can

contribute high level of BOD. However, despite the high temperature in rainy season the BOD level was

relatively lower than the other seasons may be due to the significant amount of fresh water contributed by

rainfalls frequently appeared in the tropical regions.

The concentration of TDS at four sampling stations ranged from 40.0 to 53.3 g/L in rainy season,

3.5 to 3.68 g/L in winter season, and 22.2 to 24.4 g/L in summer season, as summarized in Figure 3B.

These results confirmed that the TDS concentrations of the Passur river are comparatively higher than the

recommended level of TDS in Bangladesh and in WHO (1000 mg/L) [47-48]. The TDS concentration in

the river Yobo, Agodo and Lala of Ogun State, Nigeria varies with downstream and upstream flows [39].

For example, in the month of October, the TDS concentration was 45 and 60 mg/L in the downstream and

upstream segment of river Yobo, respectively. A nearly similar TDS concentration was observed for the

downstream and upstream part of river Lala. However, the downstream and upstream TDS concentration

was 130 and 125 mg/L for the river Agodo, respectively, which are slightly higher than the river Yobo and

Lala. In the month of December, the downstream TDS concentrations were relatively higher than the

upstream concentration of the river Yobo and Agodo; however, the downstream concentration was slightly

lower in the river Lala. An earlier study on the Passur–sibsa river system showed a low level of TDS

concentration of 8.9-42.2 g/L which is comparable with our findings [40].

The Chemical Oxygen Demand (COD) ranged between 10 and 38 mg/L with an average of 22.67

mg/L. The high level of COD was observed in summer; on the other hand, low COD was found in the

rainy season. In all seasons, the Mongla sampling point showed a high level of COD compare to other

points. The increasing trend of COD with different seasons and sampling locations is presented in Figure

3C. According to the Environmental Quality Standard (EQS) for Bangladesh, the permissible limit of

COD is 4-8 mg/L [49]. In most cases, the acceptable concentration of COD for unpolluted surface water

is less than 20 mg/L; however, for polluted water the tolerable concentration is greater than 20mg/L. It is

interesting to note that the range of COD concentration (5-255.4 mg/L) measured by Wahid et al. [30] in

the Sundarbans mangrove ecosystem is high compared to the range (10.0-38.0 mg/L) obtained in this

study. The concentration of COD found in both studies exceeds the permissible limit set by EQS.

The salinity at four sampling stations ranged from 1 to 20 g/L throughout the whole year, and the

maximum salinity (20 g/L) was observed in summer (Figure 3D). The average salinity (8.2 g/L) found in

this study is well supported by those of other studies conducted in the region [1]. The trend observed in

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the variability of salinity at the four sampling stations follows summer < winter < rainy, signifying that

these water parameters are controlled by the ratio at which the seawater mixes with the fresh water flow.

In an ecosystem understudy, salinity is controlled by three major factors: i) the river discharge at the head,

ii) the tidal input at the mouth, and iii) the topography of the area [38]. These three factors should be

considered in order to obtain precise interpretation of the salinity data.

Total Suspended Solid (TSS) includes solid materials of organic and inorganic origins that are

suspended in the water. There are many suspended matters in the water of the Sundarbans. The particles

may be sand, clay, silt and loam. The Total Suspended Solid (TSS) obtained from this study was 0.428-

1.18 g/L during the rainy season. The TSS the in rainy season was fairly low because of freshwater

predominantly influences the water flow in the river. The highest value (10.8-19.7 g/L) of TSS appeared

in summer season, whereas the lowest level (0.06-0.16 g/L) was found in winter season, as shown in

Figure 3E. During summer season the TSS value increases, probably due to less freshwater flow, urban

runoff, industrial wastes, bank erosion, bottom feeders (such as carp), algae growth or wastewater

discharges. The year round average value of TSS was 4.76 g/L, which is significantly higher than the

standard value (150 mg/L) suggested for Bangladesh [49].

The Dissolve Oxygen (DO) at four sampling stations ranged within 4.2-7.33 mg/L for the whole

year, and the maximum DO was observed in Mongla and Dangmari for winter season (Figure 3F). This

result is in good agreement with the findings of Wahid et al. [30] and Adejuwon et al., [39]. In natural

water, the DO concentration is highest at 0C and decreases with increasing temperature; moreover, the

solubility of oxygen decreases with increasing salinity of water [50]. DO levels in natural water body

depend on the physical, chemical, and bio-chemical activities occurring at surface and subsurface levels.

3.3 Content of Magnesium (Mg2+

), Calcium (Ca2+

), Sodium (Na+), Potassium (K

+), Chloride (Cl

-),

Sulfate ( SO42-

, and Bicarbonate (HCO3-)

In this study, magnesium concentration ranged between a low level of 4.86 mg/L to the high level

of 583.2 mg/L, with an average of 258.05 mg/L (Figure 4A). There is no specific WHO standard available

for magnesium concentration in drinking-water [54]. However, the recommended limits cited in different

literatures range between 50 and 150 mg/L [55-56]. The concentration of Magnesium (Mg) and Calcium

(Ca) are both responsible for the hardness of water. In the rainy season, the concentration remains quite

low related to winter and summer. In particular, the Mg concentration is very high in the summer.

Magnesium may mix up with river water from many different sources, such as chemical industries,

fertilizer application, and cattle feed. During the summer, this mixed up Mg cannot be washed out due to

lack of water flow which in turn increases the hardness in water.

The calcium concentration ranged between 26.05 and 208.42 mg/L, with a mean of 105.42 mg/L

throughout the whole year (Figure 4B). Seawater contains approximately 400 mg/L Ca. One of the main

reasons for high level Ca in water is its natural occurrence in the earth's crust. Rivers generally contain 1-

2 mg/L Ca, but the lime areas of rivers may contain Ca concentrations as high as 100 mg/L. The

experimental values also indicate that the gradually increasing amount of Ca towards the estuarine region

may be due to seawater intrusion. The Ca concentration is very high during the summer season at all

locations. The soil of Oligohaline zone of Sundarbans mangrove forest is rich in calcium followed by

magnesium and potassium [29], which may be the most probable cause of excessive calcium in river

water.

The concentration of sodium (Na+) ranged between 329 and 398 mg/L in the rainy season, 899.05

and 915.9 mg/L in the winter, 7948 and 8839 mg/L in the summer, with an average of 3209.29 mg/L. The

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(A) (B)

(C) (D)

(E) (F)

Figure 4: Season variation of Mg2+

, Ca2+

, Na+, K

+, Cl

-, SO4

2-, and HCO3

- (mg/L) assessed in this study.

0

100

200

300

Bicarbonate (mg/L)

Mongla

Dangmari

Koromjol

Koromjol creek

0 200 400 600 800

Mg (mg/L) Mongla

Dangmari

Koromjol

Koromjol creek

0

500

1000

1500

K (mg/L) Mongla

Dangmari

Koromjol

Koromjol creek

0 2000 4000 6000 8000

10000

Na (mg/L)

Mongla

Dangmari

Koromjol

Koromjol creek

0

2000

4000

6000

Chloride (mg/L)

Mongla

Dangmari

Koromjol

Koromjol creek

0

500

1000

1500 Sulfate (mg/L)

Mongla

Dangmari

Koromjol

Koromjol creek

0

100

200

300

Ca (mg/L) Mongla

Dangmari

Koromjol

Koromjol creek

(G)

Ca




highest level of Na was measured in summer, whereas the lowest concentration was found in the rainy

season (Figure 4C). The recommended level of Na+ in fresh water is 6.3 mg/L [57]. Rivers contain only

around 9 mg/L of Na+. Drinking water usually contains about 50 mg/L sodium; however, this value is

clearly higher for the mineral water. Seawater contains a very high level (11,000 mg/L) of sodium.

Bergman et al., [58] observed that seawater contains 10,800 mg/L of Na+. This investigation showed the

concentration of sodium (Na+) was always very high compare to the standard value. In the rainy season,

the concentration remains quite low than in the winter and summer seasons. In summer, the Na

concentration turned to the highest point, which may be due to seawater intrusion and industrial

discharge.

The concentration of Potassium (K+) obtained was between 45.15 and 57.8 mg/L in the rainy

seaason, 651.35 and 664.3 mg/L in the winter, 951 and 992 mg/L in the summer, with an average of

559.38 mg/L. The highest concentration appeared in the summer and the lowest concentration was

observed in the rainy season (Figure 4D). According to surface water standard, the K+ in fresh water

should be 2.3 mg/L [57]. Seawater contains about 400 mg/L potassium. A study conducted by Bergman

[58] disclosed that seawater contains 392 mg/L of K. Potassium tends to settle and consequently ends up

in sediment mostly. Rivers generally contain about 2-3 mg/L of potassium. This difference is mainly

caused by a large potassium concentration in oceanic basalts. Calcium rich granite contains up to 2.5%

potassium. In water, this element is mainly present as K+

(aq) ions. In the present study, the potassium

values in all stations and all seasons were very high. This happen may be due to the geographical location.

All the sampling stations are located near to the coast. Therefore, the concentration of salt gradient is

increased towards the sea. There is a significant amount of industrial discharge, which may be an

additional cause of high potassium concentration.

The chloride concentration was 13-4381 mg/L in Mongla, 13-4381mg/L in Dangmari, 12.5-

4672mg/L in Koromjol, and 11.99 to 4422 mg/L in Koromjol Creek during the three seasons. The

seasonal variations among all the stations are shown in Figure 4E. Muhibbullah et al., [29] also observed

a high level of dissolved chloride 200, 550, 2800 and 7200 mg/L in the river water of Rupsha, Passur,

Shipsa, and Arpangasia, respectively. In this study, the chloride (Cl-) concentration gradually increased

from Mongla towards the sea. In the rainy season, chloride (Cl-) concentration remains the same as the

standard value (7.8 mg/L) for surface water. Nevertheless, due to seasonal variations the chloride (Cl-)

concentration was gradually elevated and in the summer the values reached to the highest level. In the

summer, various types of chlorinated industrial waste-disposal occurred from river side’s industries and

cannot be washed away due to the low water flow. Moreover, the seawater contributes very high amount

of chloride, which mixes up with fresh water due to semi-diurnal tidal variation.

The sulfate concentrations at four sampling stations during the whole year ranged from 9.02-

968.3 mg/L, and the highest sulfate concentration was observed in the summer (Figure 4F). The typical

and optimal range of sulfate in natural waters is about 5-30 mg/L, with an average of about 11 mg SO4 2-

per liter [59]. If SO4 2-

is converted to sulfur, then the range becomes 1.67-10 mg/L, with an average of

about 3.67 mg/L. In comparison with these data sets, the present results indicate a high level of sulfate

concentration observed in the Sundarbans mangrove forest. In addition, the sulfate concentration

significantly varied with the studied seasons. For instance, in the rainy season, SO42-

concentration was

below the standard value at two stations, Dangmari and Koromjal and slightly higher than the standard

value at Mongla and Koromjal creek. However, in winter, the SO42-

concentration gradually increased and

reached to a high value of 199.36- 245.5 mg/L. A significantly high SO42-

concentration (900 mg/L) was

observed in summer season. High level of SO42-

concentration has been linked to the fresh water flow and

Ca




tide ebb relationship. Beside these reasons, biogenic causes are also accountable as the supply of nutrient

and food is low at winter and summer seasons, which in turn reduces the growth of living biota.

Therefore, their decay increases the SO42-

concentration in river water.

The bicarbonate concentration at four sampling stations during the whole period ranged within

116.0-203.3 mg/L, with an average of 163.53 mg/L (Figure 4G). According to surface water standard, the

HCO3- concentration in freshwater should be 58.4 mg/L [57]. Bicarbonate concentration in boreholes in

the metropolis ranged from 57.0 mg/L to 399 mg/L, with a mean of 195 mg/L [43]. The present result

matches well with the findings of Abubakar et al., [43]. However, HCO3- concentration observed in this

study disclosed a high level of HCO3-

in every station compare to the recommended value. The most

probable cause is the industrial discharge to the river water. The presence of high HCO3- indicates very

hard water which is very hazardous for ecosystems.

4. CONCLUSION

The present study concludes some important points related to the water quality of the Sundarbans,

the world largest mangrove forest. Our results highlight that there is a pronounced variation of most of the

water quality parameters with variation in season and geographical location. Moderately high level of

hardness and alkalinity was observed for the first set of water quality parameters discussed in this paper.

However, significantly high concentration was appeared for BOD, TDS, and TSS. The average values of

these parameters are noticeably higher than the standard values recommended for the water quality of

Bangladesh. The average concentration of Mg2+

(258.05 mg/L), Na+ (3209.29 mg/L), K

+ (559.38 mg/L),

and HCO3-

(1163.53 mg/L) also exceeded the recommended concentrations for river water. In addition,

the present results indicate a high level of sulfate and chloride concentration observed in the Sundarbans

mangrove forest.

This study and some other studies confirmed that the water quality of Sundarbans coastal area on

average is deteriorating continuously. There are numerous causes including increasing numbers of

industries in the neighboring region, uncontrolled deforestation, and global climate change, which are

responsible for deteriorating the water quality of this large and diverse forest. In natural systems, water

always flows from upstream to downstream and it is very important to maintain this flow in order to

retain a good ecological balance. However, due to various reasons the water flows in all the Bangladeshi

rivers are being hampered. The main reasons for the shortage of fresh water flow in the rivers are

diversion (building Farakka barrage) of upstream water by the neighboring country, India, lacking of

proper dazing of the rivers beds, and planning of unwanted structures on the bank of the rivers. The

adverse impact of industrialization is reflected by the value obtained for the various water quality

parameters in this study. There are large quantities of untreated sewage present in the Khulna and

Bagherhat municipalities, which are responsible for contributing pollution to the Bhairab-Rupsha-Passur

river system. As the Sundarbans area spread over in both Bangladesh and India, the policy planners of

both countries should come forward with urgent strategies to protect this mangrove forest from the

impending danger of severe pollution.

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