Groundwater Quality of Some Parts of Coastal Bhola District, Bangladesh: Exceptional Evidence

Abstract

The composition of groundwater governs the drinking and irrigation water suitability. A large part of the coastal region of Bangladesh is affected and is responsible for changing the composition of the groundwater. This research attempted to observe the groundwater quality of the Bhola Sadar and Char Fasson upazilas in coastal Bangladesh. Twenty-eight (28) water samples, 27 at depths of 260–430 m (850–1400 ft) and 1 from a crop field, were collected and analyzed. The quality of water samples was determined through the evaluation of odor, color, turbidity, electrical conductivity, pH, total dissolved solids, nitrate (NO₃⁻), ammonium (NH₄⁺), sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn) and arsenic (As) ions. An Atomic Absorption Spectrophotometer was used for heavy metal analysis. The outcomes were compared with the drinking water quality of Bangladesh and the World Health Organization. The results showed that the average values of nearly all of the parameters were underneath or within the standard level, representing that the groundwater was appropriate for drinking purposes. The water quality parameters were also compared with the irrigation water quality of Bangladesh and the Food and Agriculture Organization. It was found that the collected samples were also suitable for irrigation. To do this, the soluble sodium percentage, sodium adsorption ratio, magnesium adsorption ratio, Kelley’s ratio, and total hardness were calculated. The novelty of this research is that, despite being in a coastal district, the deep aquifer water of Bhola was appropriate for drinking and irrigation purposes.

1. Introduction

The coastal part of Bangladesh is surprisingly susceptible to disasters [1,2,3], while climate change is having a new depressing effect on life and agronomy [4,5,6,7,8,9]. The BICZMP states that 20 districts (Bagerhat, Barguna, Barisal, Bhola, Chandpur, Chittagong, Cox’s Bazar, Feni, Gopalganj, Jashore, Jhalokati, Khulna, Lakshmipur, Madaripur, Narail, Noakhali, Patuakhali, Pirojpur, Satkhira, and Shariatpur) comprise the coastal zone of Bangladesh. Nearly 147 upazilas of these 20 districts are considered coastal upazilas, of which 48 upazilas are coupled to the coast and 99 upazilas are on the inner side of the beach [10]. Climate changes constantly [4,5], which may be responsible for the increasing salinity in the coastal districts of Bangladesh [11,12,13]. A report shows that about 6.69% of the area of the Bagerhat Sadar, Fakirhat, and Rampal upazilas is high in saline, about 85.43% has moderate saline, and around 7.88% of the area is low in saline [14]. Climate change is predicted to have a devastating impact on agriculture and the environment [6,14]. The reasons behind the salinity intrusion are disasters, mounting temperatures, and lessening rainfall in the southwestern parts of Bangladesh [1,5,15]. Despite being a coastal district, the salinity condition is the opposite in Bhola. Bhola is a district located in the middle south part of Bangladesh as well as part of the coastal region (Figure 1). It is one of the most vulnerable districts to climate change [2]. Due to stumpy salinity intrusion, the monsoon season in Bhola is encouraging for agriculture from June to October [16,17].

The changing climate may have impinged on the cropping pattern of Bangladesh from the primeval period [6]. In Bhola, the cyclone that took place on 3 November 1970 was a caustic tropical cyclone that smashed crops on around 17% of the land area and destroyed around 30% of food production, resulting in escalating poverty. More than a few offshore islands were destroyed by the storm surge, wiping out villages and smashing crops across the country. Among the seven upazilas in the Bhola district, lots of people from Char Fasson and Tazumuddin lost their lives during the distressing Bhola cyclone. The social environment was hampered pessimistically by the horrific impact of natural calamity, which are definitive effects of climate change, especially cyclones and storm surges [1]. The cyclone also deteriorated both the surface and GW of the Bhola district. The water and soil were also contaminated by some industries [18,19].

In Bangladesh, around 97% of the residents depend on GW sources for drinking [20]. Around 20 million people in Bangladesh live along the coastal expanse, which is affected by anecdotal degrees of salinity in the drinking water [11,12,21]. In Bangladesh, there is an amplified flood frequency, shifting rainfall patterns, more frequent droughts, and infiltration of salinity. Around 20 million people are affected by salinity in the coastal parts of Bangladesh, and they are facing a dearth of pure drinking water [22,23]. The intrusion of saltwater comes from the flow of saline water from side-to-side freshwater aquifers, which may result in the pollution of sources of drinking water [12]. However, there are different results also. Rock weathering is the central resource of ions in GW in the coastal district Satkhira, while seawater intrusion and precipitation have trifling effects on it [24].

It has been shown that salinity is mounting progressively and continually in the GW and in soils of some parts of the coastal region of Bangladesh [12,17,25], which eventually decreased plant growth brutally [26,27]. The salinity and GW contamination problem in the coastal region is not only a problem of Bangladesh but also a global problem.

Recently, numerous types of research have been performed to verify the water quality in different coastal parts of Bangladesh, e.g., Satkhira [8,24,28], Khulna [29,30], Patuakhali [31], Gopalganj [32,33], Jashore [7,34,35,36,37,38,39,40], and central south parts of Bangladesh [31,41]. Not only has those, but also the water quality been determined in other parts of Bangladesh to determine the actual concentrations of the elements [42,43,44]. In most cases, it has been reported that the GW of the DTW of the coastal zone is mostly saline, and the inhabitants are using this supply or other sources of water for drinking and other daily purposes. In the case of the Bhola district, people usually use the DTW for their daily use. However, there has been no solitary report regarding the composition or quality of the GW in the important coastal district of Bhola. Therefore, this research was conducted. Bhola is very close to the Bay of Bengal, and it is often affected by natural disasters, such as cyclones [1,2]. Despite this, agricultural production is good in Bhola [2]. Mostly, DTW water is used for irrigation and drinking purposes in Bhola, but river water is not used for irrigation [2]. Considering the above fact, it was necessary to identify the water quality of the GW used for drinking and irrigation purposes in Bhola. The BSU was chosen because most of the people in the Bhola district live there, and this is the prime upazila in the Bhola district. The CFU was chosen because this upazila is situated in the southern part of the district and is directly connected to the Bay of Bengal. Considering the background stated here, the main objectives of this research were to (1) determine the physical characteristics, and (2) determine the chemical composition of the DTW water in Bhola to find out whether the GW has a suitable composition for drinking and irrigation purposes or not.

2. Hydrogeological Characteristics of the Study Area

The aquifer system of the coastal region of Bangladesh is very intricate, because the distribution of the aquifer aquitard is extremely variable within a very short distance [45]. Bangladeshi researchers have divided the GW aquifer system in different ways. The shallow and deep aquifers were divided at 150 m [42,44]. This is because reports show that the water chemistry changes the most at the depth of 150 m [42,46]. The BWDB-UNDP [47] primarily classified the aquifers based on lithology and region. The three aquifers were identified as (1) the first or shallow aquifer, which extends from 50 to 100 m in depth. In some places, this aquifer is composed of an upper clay and silt unit with a substantial thickness; (2) the second or main aquifer is classically overlain and underlain through aquitards that extend from depths of 250–350 m. Mostly, this layer consists of fine to very fine sands and/or is rarely interbedded with clay lenses. This main aquifer is slightly leaky, in the semiconfined category, and consists of stratified inter-related sandy formations. This aquifer forms the key aquifer of the study area from where the GW was abstracted; and (3) the third or deep aquifer is located underneath the clay or silty clay aquitards, ranging from depths of 300–350 m. This aquifer comprises grayish-colored fine sand with variations of clay or silty clay lenses. However, this characteristic is absent in many places [45].

This classification is not always fixed. It depends on the geographical position. Based on isotopic characteristics, [48] classified aquifers into three categories based on the age of the GW, e.g., (1) the first aquifer: extends from 70 to 100 m, (2) the second aquifer extends from 100 to 200–300 m, and (3) the third aquifer extends from >300 m. The chronological ages of just the above-mentioned GW are ≤100, about 3000, and 20,000 years, correspondingly [49]. The HC values of the three aquifers measured from slug tests were 1–25, 1–9, and 1–9 m/day, respectively [50]. The GW levels are found within a few meters from the surface and fluctuate in the monsoon and dry seasons. In the monsoon period (July–September), the aquifers become recharged, receiving more than 80% of the annual precipitation. Mostly, floods inundate a large portion of the country every year, and inundating floodwater also enhances the recharge activity [51]. As a result, in the monsoon period, water tables rise gradually to levels of around 1–2 m of the ground surface. This expresses a dynamic equilibrium that happens among the surface water, water table, and deep-rooted plants [52]. In the study area, the GW flows horizontally from north to south, but the rate of flow is prolonged. Furthermore, the intensity of the horizontal movement is much lower compared to the vertical flow of the water [51].

3. Materials and Methods

3.1. Description of the Study Area

Bhola is the largest island in Bangladesh and is positioned in the coastal part. Mostly, it is surrounded by water bodies (Figure 1). It is located in the southern part of the Barisal division and has an area of about 3737.21 km². There are 5 municipalities, 9 thanas, 66 unions, and 473 villages in this district. It was a fraction of the Noakhali district which was then attached to the Barisal district in 1869 [43]. There are seven upazilas in the Bhola district, and the number of residents is about 2,037,201. The agricultural production rate is about 63.64%, and the literacy rate is about 47% in Bhola. Bhola is encircled by the Bay of Bengal to the south, by Lakshmipur and Noakhali districts, the lower Meghna river, and the Shahbazpur channel to the east, Lakshmipur, and Barisal districts to the north, and the Patuakhali district and the Tetulia river [43] to the west (Figure 1). Despite being a coastal district, the agricultural production is reasonable, and it is perceived that the GW of the DTW in Bhola is more or less fresh for drinking and irrigation [2]. This interesting phenomenon led us to choose Bhola as the study area for analyzing GW.

3.2. Data Collection

The principal records were composed of field explorations, face-to-face surveys, and focus group discussions. The survey was conducted to obtain the primary data on the construction of embankments around the Bhola district. The survey was also conducted to determine the perceptions of people regarding the temperature and rainfall. It is believed that the constructed embankments prevent the entrance of saline water inland of Bhola from neighboring water sources, e.g., the river or Bay of Bengal. Water samples were collected from 2 to 15 September 2019 on clear and sunny days. During sampling, the humidity was average, and the temperature was around 20–25 °C. September was chosen, because this month is more or less representative of both the premonsoon and postmonsoon seasons.

Again, various journals, legislative and nonlegislative proceedings, managerial data, newspapers, research papers, and unpublished thesis papers were used in this study. Images of the research areas were downloaded from Google Earth on 29 September 2019 to identify the sample spot. These specific data were collected because, in Bangladesh, systematic and scientific data are not recorded, especially in the case of the Bhola district.

3.3. Water Sample Collection and Preservation

The DTW water was collected from the BSU and CFU by following typical etiquette [53]. The sampling time was from 9:00 a.m. until 7:00 p.m. A total of 27 DTW water samples at depths of approximately 260 to 430 m and one sample from a mixed-crop field were collected from 28 unions in untainted bottles (

Table 1. Sample locations with global positioning system coordinates of the Deep Tube Wells in Bhola district. Samples were collected from the Bhola Sadar upazila and Char Fasson upazila.

Sample ID		Sampling Site		GPS Coordination		Depth
		Union	Location	Latitude	Longitude	(ft)	(m)
S1	01 BSU	Bhola Sadar	*	22°41′4.82″	90°38′50.49″	950	290
S2	02 BSU	Illisha	Nabokumar	22°44′32.22″	90°37′1.98″	1050	320
S3	03 BSU	Kachia	Porangonj	22°43′41.83″	90°38′3.81″	950	290
S4	04 BSU	Bapta	Hazirhat	22°42′27.88″	90°38′25.8″	1200	366
S5	05 BSU	Dhonia	*	22°43′33.32″	90°39′19.99″	1050	320
S6	06 BSU	Alinagor	Madrashabazar	22°41′24.80″	90°37′13.47″	1400	427
S7	07 BSU	Samaiya	*	22°39′43.69″	90°35′33.66″	850	260
S8	08 BSU	Veduria	Pulgora	22°39′54.44″	90°33′4.76″	850	260
S9	09 BSU	South Dighaldi	*	22°35′38.97″	90°38′58.41″	1050	320
S10	10 BSU	North Dighaldi	*	22°36′55.93″	90°38′50.42″	950	290
S11	11 BSU	Veduria	Launchghat	22°42′4.51″	90°33′55.24″	1350	411
S12	12 CFU	CMS	CM School	22°9′47.64″	90°47′24.48″	1300	396
S13	13 CFU	CMN	CMGS	22°9′31.76″	90°46′9.87″	1250	381
S14	14 CFU	Char Hazarigonj	Hazarigonj	22°8′ 24.62″	90°45′45.81″	1050	320
S15	15 CFU	Zahanpur	Zahanpur	22°3′11.92″	90°26′44.56″	1350	411
S16	16 CFU	Ewazpur	Minabazar	22°0′50.84″	90°37′41.24″	1050	320
S17	17 CFU	Aichabazar North	Aichabazar	22°4′1.57″	90°37′8.91″	850	260
S18	18 CFU	Rosulpur	*	22°2′32.36″	90°40′56.36″	*	*
S19	19 CFU	Jinnahghar	Kashemgonj	22°5′42.08″	90°41′42.48″	*	*
S20	20 CFU	Aslampur	Vuyerhat	22°13′55.19″	90°45′40.74″	950	290
S21	21 CFU	Janotabazar	*	22°14′12.04″	90°46′1.55″	*	*
S22	22 CFU	Osmangonj	Janotabazar North	22°12′22.44″	90°47′2.88″	1250	381
S23	23 CFU	Aminabazar	Rodrerhat	*	*	1250	381
S24	24 CFU	Aminabazar North	Majirhaat	22°10′11.76″	90°43′30.99″	1050	320
S25	25 CFU	Nilkamal	Dulahat	22°11′38.59″	90°38′38.76″	*	*
S26	26 CFU	Nurabad	Hajirhat	22°10′16.23″	90°40′23.62″	1400	430
S27	27 CFU	Char Kolmi	Anjurhat	22°6′30.64″	90°38′24.61″	1400	430
S28	28 CFU	CMMC	CMGS	22°12′39.75″	90°45′11.28″	*	*

NB: BSU = Bhola Sadar Upazila; CFU = Char Fasson Upazila; CM = Char Mandraz; CMN = Char Mandraz North; CMS = Char Mandraz South; CMGS = Char Mandraz Girl’s School; CMMC = Char Mandraz Mixed Cropping; GPS = Global Positioning System. S = Sample. * Indicates the data were not collected. Sample no. 28 CFU was collected from a mixed-crop field.

). Depths from 10 to 150 m are considered shallow aquifers, and depths > 180 m are considered deep aquifers [42]. Fairly accurate profundity on DTWs was recorded from the owners’ information. The water samples were collected after pumping the DTW for about 10 min until reaching a secure chemical condition. This was conducted to collect the delegate samples. The samples were collected randomly from each unit of the two upazilas to represent the study area. Samples were collected with three replications. Uncertain sources of error, such as the water quality determination, sampling technique, and instrumental accuracy were determined where needed. The repetition of the standard solution was performed after the analysis of seven samples. In every case, the analytical grade standard solution was used. Deionized water was used as a blank control. Detailed information on the sample’s location and the depth of the DTW are presented in Table 1. A “Redmi Note 5 A1” mobile phone was used to determine the GPS coordinates.

The samples were kept in an ice box and brought to the laboratory quickly. Then, they were stored in the refrigerator. Before storing, some important primary data, especially odor, color, turbidity, pH, EC, TDS, NO₃⁻, and NH₄⁺ were determined. Some parts of the samples were also taken to the Environmental Laboratory of the Asia Arsenic Network, Jashore 7400 to analyze some parameters. All samples were eventually stored in the refrigerator (Walton, Model: WNH-3H6-0101, Kaliakoir, Gazipur, Bangladesh) and treated with two to three drops of concentrated HCl.

3.4. Sample Grounding for Investigation

All protocols were followed for sample preparations [53]. Before analysis, it was confirmed that the types of equipment were competent. Licensed analytical-grade customary materials were used throughout the research. The instruments were calibrated with accredited standard solutions or supplies.

3.5. Determination of Physico-Chemical Parameters

The customary process was practiced throughout the investigation [53]. The odor of the samples was checked simply by the nose, color was observed only by the naked eye, and taste was measured by the tongue. Turbidity was determined with a HACH turbidity meter by the method of USEPA 180.1. The pH was measured instantaneously with a digital pH meter (EZODO; Model 6011; Waterproof IP5; Taiwan). The EC and TDS were determined with an EC/TDS/Temperature tester (Model HI 98312; HANNA; Waterproof IP57; Woonsocket, RI, USA) [54]. Na and K were measured with a flame photometer (JENWAY; Model PEP7/C; UK) [55]. Ca and Mg were measured with the Titrimetric Method [55]. NH₄⁺ and NO₃⁻ were determined with a spectrophotometer (Model DR 3900; HACH; USA) with the Nessler and Cadmium Reduction Method [54].

3.6. Determination of Heavy Metals and Metalloids

Iron was measured with a spectrophotometer (Model DR 3900; HACH; USA) using the Powder Pillow Process technique [54]. The quantities of three additional heavy metals and one metalloid—Mn, Cu, Zn, and As—were measured in the Laboratory of Asia Arsenic Network, Jashore 7408, Bangladesh. Heavy metals were measured with the sophisticated Atomic Absorption Spectrophotometer instrument (AAS; Model: AA-7000; Shimadzu, Japan City, Japan). Arsenic was determined with the help of the AAS by using the hydride generation atomic absorption spectroscopy method [56].

3.7. Methods Used to Evaluate the Irrigation Water Quality

The subsequent parameters were calculated to appraise the quality of the water to be used for irrigation. The SSP was calculated to identify the existence or nonexistence of Na hazards in GW to be applied for irrigation by using Equation (1) [57]:

$$\mathrm{Na}\%=\frac{\left({\mathrm{Na}}^{+}+{\mathrm{K}}^{+}\right)\times 100}{\left({\mathrm{Ca}}^{2+}+{\mathrm{Mg}}^{2+}+{\mathrm{Na}}^{+}+{\mathrm{K}}^{+}\right)}$$

(1)

The Na⁺ or alkaline vulnerability articulated by the SAR is extensively applied to estimate the quality of water to be used for irrigation [29,41]. If the water sample is rich in Na⁺ and has a low concentration of Ca²⁺, then the ion substitute complex may become saturated with Na, which may be responsible for destroying the soil structure [57]. The SAR value was calculated to determine the quality of GW to be applied for irrigation by using Equation (2) [58], where the ions were measured in meq L⁻¹

$$\mathrm{SAR}=\frac{{\mathrm{Na}}^{+}}{\sqrt{\frac{{\mathrm{Ca}}^{2+}+{\mathrm{Mg}}^{2+}}{2}}}$$

(2)

In Equation (3), Na⁺, Ca²⁺, and Mg²⁺ represent the concentrations of the above-mentioned ions [59].

The MAR was calculated to determine the existence or nonexistence of the Mg hazard in GW by using the Equation (3) [60]:

$$\mathrm{MAR}=\frac{\left({\mathrm{Mg}}^{2+}\times 100\right)}{\left({\mathrm{Ca}}^{2+}+{\mathrm{Mg}}^{2+}\right)}$$

(3)

The KR was calculated by using Equation (4) [61]:

$$\mathrm{KR}=\frac{{\mathrm{Na}}^{2+}}{\left({\mathrm{Ca}}^{2+}+{\mathrm{Mg}}^{2+}\right)}$$

(4)

The determination of the Na level against the Ca and Mg levels in water samples is known as Kelley’s Ratio. Based on KR, the irrigation water can be rated [61]. The presence of excess Na in irrigation water makes the water unsuitable for crop production.

The hydrochemical arrangement and GW appraisal were discussed by using the Wilcox diagram [62]. The ionic concentrations were articulated in milli equivalents per liter (meq L⁻¹). Every one of the calculated values was compared with nationalized and intercontinental standard values to appraise the GW fitness. SPSS software (version 16.00) was used for statistical correlations among cations and anions in the GW.

The TH expressed in ppm in the water sample was calculated by using Equation (5) [57,60]. It was determined to determine whether the water sources were suitable for homestead uses.

TH = 2.497 Ca²⁺ + 4.115 Mg²⁺

(5)

3.8. Data and Statistical Analysis

Before performing the research, a reconnaissance survey was conducted. For every point, three water samples were assessed, and the average result was used. The standard deviation was considered. Data were evaluated by means of the Ryan–Einot–Gabriel–Welsch multiple range tests (p = 0.05; [63]). The data were analyzed by using SPSS 20.0, MS Word 2013 and MS Excel 2013 computer software. The water quality parameters of the DTW were matched with the basic standards recommended for drinking water by the BNDWQS and WHO [64,65]. Additionally, the parameters of the DTW were matched with the standards recommended by the DoE and FAO for irrigation [66,67] where necessary.

4. Results

4.1. Physicochemical Properties

The Odor, Color, and Taste: The DTW water samples did not have any odor. The odor was very simple, as with the other samples normally found in GW in Bangladesh [36]. Similar to odor, the color of the samples was also very simple, as found in other samples of GW reported recently [36]. The taste of the collected samples was as usual. No abnormalities were found regarding odor, color, and taste.

Turbidity: The minimum turbidity of 0.30 NTU was found in sample no. 19 CFU (

Table 2. Physical and chemical parameters of deep tube well water samples collected from the Bhola Sadar upazila and the Char Fasson upazila. The values were compared with the values of the Bangladesh National Drinking Water Quality Survey and the World Health Organization.

Sample ID		Turbidity	EC	pH	TDS	NO3−	NH4+
		NTU	µS cm−1		-------------------mg L−1----------------
---------------------------------------------------Drinking Water Quality-----------------------------------------------
BNDWQS 2011		10.0	300–1500	6.5–8.5	1000.0	10.0	0.5
WHO 1984		5.0	750	6.5–8.5	1000.0	50.0	1.50
S1	01 BSU	0.47	570.0	7.97	270.0	7.00	0.42
S2	02 BSU	0.50	650.0	8.03	330.0	7.00	0.46
S3	03 BSU	1.47	620.0	7.21	300.0	7.00	0.35
S4	04 BSU	0.78	510.0	7.80	250.0	6.00	0.32
S5	05 BSU	1.04	590.0	8.06	290.0	5.00	0.17
S6	06 BSU	0.64	630.0	7.81	350.0	6.00	0.55
S7	07 BSU	0.44	510.0	7.73	250.0	4.00	0.68
S8	08 BSU	1.32	700.0	7.70	360.0	7.00	0.42
S9	09 BSU	0.45	640.0	7.71	310.0	4.00	0.36
S10	10 BSU	0.85	910.0	7.83	430.0	7.00	1.46
S11	11 BSU	1.49	900.0	7.64	450.0	5.00	1.12
S12	12 CFU	2.52	270.0	8.12	130.0	6.00	0.78
S13	13 CFU	1.77	490.0	7.85	250.0	4.00	0.77
S14	14 CFU	0.38	300.0	7.36	130.0	5.00	1.06
S15	15 CFU	3.49	1020.0	7.60	510.0	4.00	1.42
S16	16 CFU	0.71	780.0	7.45	360.0	7.00	0.87
S17	17 CFU	0.77	280.0	8.00	170.0	3.00	1.59
S18	18 CFU	0.88	520.0	7.90	260.0	4.00	1.26
S19	19 CFU	0.30	410.0	7.55	200.0	5.00	1.46
S20	20 CFU	2.57	420.0	7.77	210.0	6.00	0.71
S21	21 CFU	1.29	570.0	7.72	280.0	6.00	0.96
S22	22 CFU	0.76	710.0	7.46	360.0	4.00	1.36
S23	23 CFU	1.18	420.0	8.02	210.0	5.00	0.76
S24	24 CFU	1.10	610.0	8.14	300.0	6.00	1.05
S25	25 CFU	0.60	580.0	7.83	280.0	4.00	1.00
S26	26 CFU	0.40	620.0	7.79	300.0	7.00	2.17
S27	27 CFU	1.40	650.0	7.95	330.0	4.00	1.75
S28	28 CFU	28.6	310.0	7.89	150.0	6.00	0.22
--------------------------------------------------Descriptive Statistics----------------------------------------------
Maximum		3.49	1020	8.14	510.0	7.00	2.17
Minimum		0.30	270	7.21	130.0	3.00	0.17
Mean		1.11	588.15	7.78	291.48	5.40	0.94
Median		0.85	590	7.80	290	5.00	0.87
SD		0.76	181.96	0.23	89.56	1.30	0.50
---------------------------------------------------Irrigation Water Quality-------------------------------------------
DoE 1997		**	2250	6.5–8.5	2100	**	**
FAO 1976		**	700–3000	6.5–8.5	0–2000	**	**
MRV *		**	1500	6.5–8.4	1000	50.0	1.50

NB: BNDWQS = Bangladesh National Drinking Water Quality Survey; BSU = Bhola Sadar Upazila; CFU = Char Fasson Upazila; EC = Electrical Conductivity; DoE = Department of Environment; FAO = Food and Agricultural Organization; MRV = Maximum Recommended Value; NTU = Nephelometric Turbidity Unit; S = sample; SD = standard deviation; TDS = Total Dissolved Solids; WHO = World Health Organization. * Indicates that the highest value to be practiced for agricultural uses and ** indicates that the data were not collected. Sample no. 28 CFU was collected from a mixed-crop field.

; Kashemganj, Jinnahghar, Char Fasson), and the maximum turbidity of 3.49 NTU was in sample no. 15 CFU (Table 2; Zahanpur, Char Fasson). This means that the turbidity of the collected samples in the study area varied widely.

Electrical Conductivity (EC): The smallest EC value was 270.0 µS cm⁻¹, and the highest EC value was 1020.0 µS cm⁻¹ with a mean value of 588.15 µS cm⁻¹ (Table 2). This result shows that the EC of the samples depends on the geographical position of the research area.

Hydrogen Ion Concentration (pH): The lowest pH value was 7.21, and the maximum value was 8.14 with a mean value of 7.78 (Table 2). The BNDWQS and the WHO guidelines for pH give values of 6.5–8.5 for drinking purposes (Table 2). The different pH values in different places govern the chemical characteristics of GW.

Total dissolved solids (TDS): The lowest TDS was 130.0 mg L⁻¹, the highest value was 510.0 mg L⁻¹ and the average was 291.48 mg L⁻¹ (Table 2). According to the BNDWQS and WHO guidelines, the standard value of TDS in drinking water is 1000.0 mg L⁻¹ [64,65]. This means the turbidity was not the same for all of the water samples.

4.2. Nonmetallic Ion Concentrations

Nitrate (NO₃⁻) and ammonium (NH₄⁺): In DTW water, the NO₃⁻ concentration ranged from 3.0 to 7.0 mg L⁻¹ (Table 2). The smallest value of NH₄⁺ in the DTW water sample was 0.17 mg L⁻¹, and the highest value was 2.17 mg L⁻¹ (Table 2). The BNDWQS permissible concentration of NH₄⁺ is 0.50 mg L⁻¹, and the WHO permissible limit is 1.50 mg L⁻¹ (Table 2).

4.3. Metallic Ion Concentrations

Sodium (Na), potassium (K), calcium (Ca), and magnesium (Mg): The Na concentration of the GW varied from 25.0 to 85.0 mg L⁻¹ in the BSU and CFU (

Table 3. Concentrations (mg L⁻¹) of some elements in Deep Tube Well water samples of the Bhola Sadar and Char Fasson upazilas. The values were compared with the values of the Bangladesh National Drinking Water Quality Survey and the World Health Organization (WHO).

Sample ID		Na	K	Ca	Mg	Fe	Mn	Cu	Zn	As
		-----------------------------------------mg L−1-------------------------------------------------
BNDWQS 2011		200.0	12.0	75.0	30–35	0.3–1.0	0.10	1.00	5.00	0.05
WHO 1984		200.0	30.0	100.0	150.0	0.30	0.40	2.00	3.00	0.01
S1	01 BSU	65.0	5.0	80.16	46.06	0.16	BDL	BDL	BDL	BDL
S2	02 BSU	65.0	10.0	48.09	27.66	0.20	BDL	BDL	BDL	BDL
S3	03 BSU	60.0	10.0	64.13	36.85	0.68	BDL	BDL	BDL	BDL
S4	04 BSU	55.0	5.0	96.19	55.27	0.20	BDL	BDL	BDL	BDL
S5	05 BSU	65.0	5.0	64.13	36.85	0.09	BDL	BDL	BDL	BDL
S6	06 BSU	75.0	15.0	48.09	27.66	0.90	BDL	BDL	BDL	BDL
S7	07 BSU	45.0	5.0	96.19	55.27	0.18	BDL	BDL	BDL	BDL
S8	08 BSU	65.0	10.0	48.09	27.66	0.36	BDL	BDL	BDL	BDL
S9	09 BSU	55.0	10.0	48.09	27.66	0.06	BDL	BDL	BDL	BDL
S10	10 BSU	85.0	10.0	80.16	46.06	1.07	BDL	BDL	BDL	BDL
S11	11 BSU	80.0	15.0	64.13	36.85	0.03	BDL	BDL	BDL	BDL
S12	12 CFU	50.0	10.0	64.13	36.85	0.07	BDL	BDL	BDL	BDL
S13	13 CFU	25.0	5.0	96.19	55.85	0.01	0.50	BDL	BDL	0.002
S14	14 CFU	45.0	10.0	80.16	46.06	0.03	BDL	BDL	BDL	BDL
S15	15 CFU	80.0	15.0	48.09	27.66	0.17	BDL	BDL	BDL	BDL
S16	16 CFU	45.0	10.0	96.19	55.27	0.55	BDL	BDL	BDL	BDL
S17	17 CFU	75.0	5.0	96.19	55.27	1.02	BDL	BDL	BDL	BDL
S18	18 CFU	70.0	10.0	48.09	27.66	0.13	BDL	BDL	BDL	BDL
S19	19 CFU	60.0	5.0	64.13	36.85	0.04	BDL	BDL	BDL	BDL
S20	20 CFU	40.0	5.0	80.16	46.06	0.32	BDL	BDL	BDL	0.003
S21	21 CFU	50.0	10.0	48.09	27.66	0.03	BDL	BDL	BDL	BDL
S22	22 CFU	65.0	15.0	64.13	36.85	0.12	BDL	BDL	BDL	BDL
S23	23 CFU	50.0	5.0	64.13	36.85	0.02	BDL	BDL	BDL	BDL
S24	24 CFU	60.0	5.0	80.16	46.06	0.07	BDL	BDL	BDL	BDL
S25	25 CFU	40.0	5.0	64.13	36.85	0.56	BDL	BDL	BDL	BDL
S26	26 CFU	75.0	20.0	48.09	27.66	1.05	BDL	BDL	BDL	BDL
S27	27 CFU	70.0	15.0	64.13	36.85	0.78	BDL	BDL	BDL	BDL
S28	28 CFU	55.0	15.0	128.26	73.70	0.22	BDL	BDL	BDL	BDL
------------------------------------------Descriptive Statistics-------------------------------------------
Maximum		85.0	20.0	96.19	55.85	1.07	NA	NA	NA	NA
Minimum		25.0	5.0	48.09	27.66	0.01	NA	NA	NA	NA
Mean		59.81	9.26	68.28	39.27	0.33	NA	NA	NA	NA
Median		60.00	10.00	64.13	36..85	0.17	NA	NA	NA	NA
SD		14.51	4.32	17.56	10.12	0.36	NA	NA	NA	NA
---------------------------------------------------Irrigation Water Quality--------------------------------------------
DoE		200	12.0	**	**	**	**	**	**	0.05
FAO		0–40	0–20	**	**	**	**	**	**	**
MRV *		230.0	20.0	**	**	5.0	0.20	0.20	2.0	0.10

NB: BNDWQS = Bangladesh National Drinking Water Quality Survey; BDL = Below Detection Limit; BSU = Bhola Sadar Upazila; CFU = Char Fasson Upazila; DoE = Department of Environment; FAO = Food and Agricultural Organization; MRV = Maximum Recommended Value; NA = Not Applicable; S = sample; SD = standard deviation; WHO = World Health Organization. * Indicates that the highest value to be practiced for agricultural uses and ** indicates that the data were not collected. Sample no. 28 CFU was collected from a mixed-crop field.

). The BNDWQS and WHO recommended concentration of Na in drinking water is 200.0 mg L⁻¹ [68,69,70]. Again, the lowest K concentration was 5.0 mg L⁻¹, and the highest value was 20.0 mg L⁻¹ (Table 3). The BNDWQS guideline for K in drinking water is 12.0 mg L⁻¹ and the value for WHO is 30.0 mg L⁻¹ (Table 3). The Ca concentration in DTW samples varied from 48.09 to 96.19 mg L⁻¹ (Table 3). The standard value of Ca in drinking water is 75.0 mg L⁻¹ for BNDWQS and 100.0 mg L⁻¹ for WHO [64,65]. The Mg concentration in the DTW water samples varied from 27.66 to 55.85 mg L⁻¹ in the BSU and CFU (Table 3). The recommended value of Mg in drinking water in Bangladesh is 30.0–35.0 mg L⁻¹, and the WHO recommended value is 150.0 mg L⁻¹ [64,65].

Iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn): The Fe concentration ranged from 0.01 to 1.07 mg L⁻¹ in the collected samples (Table 3). The acceptable BNDWQS value for the Fe concentration in drinking water is 0.3–1.0 mg L⁻¹, and the WHO recommended value is 0.30 mg L⁻¹ [68,69]. The experimental results showed that only 13 samples from the CFU contained more than 0.50 mg L⁻¹ Mn (Table 3). The leftover samples did not retain any detectable concentration of Mn (Table 3). The Cu concentrations in all composing samples were BDL (Table 3). Considering the Cu concentration, the present data unmistakably confirm that all DTW water samples were safe for drinking purposes. Similar to Mn and Cu, no Zn was found in the collected water samples in the study area (Table 3). It is important to mention that the GW of two upazilas in the Bhola district is free from excess Zn (Table 3). The low concentration of Zn in deep GW is a natural phenomenon [65].

4.4. Arsenic Concentration

No As was found in the collected water samples of the two upazilas of the Bhola district (Table 3). The BNDWQS limits the As concentration in drinking water to 0.05 mg L⁻¹, and the WHO limits the value to 0.01 mg L⁻¹ [64,65].

4.5. Parameters Related to the Irrigation Water Quality

Soluble sodium percentage (SSP): The SSP ranged from 17.12 to 39.20% with a mean value of 30.34 ± 6.57% in the BSU (from Figure 2). However, the SSP values ranged from 10.25 to 40.75% with a mean value of 26.62 ± 8.12% in the CFU (from Figure 3). The average value in the two upazilas was 28.14 ± 7.89%.

Sodium adsorption ratio (SAR): The SAR value varied from 1.28 to 3.02 with a mean value of 2.28 ± 0.5 in the water samples of the BSU (from Figure 2). Similarly, the values were from 0.71 to 3.22 with a mean value of 1.96 ± 0.67 in the CFU (from Figure 3). This means that the SAR values did not change abnormally.

Magnesium adsorption ratio (MAR): The average value of the MAR in the water samples of the BSU was 48 ± 0.01 (from Figure 2), and in the CFU, the value was 48.68 ± 0.06 (from Figure 3). The total value of the MAR ranged from 48.65 to 48.91 with a mean value of 48.67 ± 0.05. It seems that the MAR values did not change abruptly.

Kelley’s ratio (KR): In the BSU, the KR in the water samples ranged from 0.21 to 0.70 with a mean value of 0.47 ± 0.14 (from Figure 2). Similarly, the values were 0.12–0.74 with a mean value of 0.40 ± 0.17 in the CFU (from Figure 3). The obtained result shows that the KR ratios were almost similar in the two sampled upazilas.

Total hardness (TH): The TH in the water samples ranged from 15.36 to 30.70 ppm with a mean value of 21.40 ± 5.69 ppm in the BSU (from Figure 2). Similarly, in the case of the CFU, the values were 15.36–30.90 ppm with a mean value of 22.08 ± 5.36 ppm (from Figure 3). The total values ranged from 15.36 to 30.90 ppm with a mean value of 21.80 ± 5.51 ppm.

Wilcox (1955) value: The Wilcox diagram shows that nearly 96% of the water samples were excellent to good, and 4% were good to permissible (Figure 4). From this result, it can be said that the GW of the BSU and CFU is fit for irrigation purposes.

5. Discussion

Construction of the embankment and people’s perception: Bhola is the largest island in Bangladesh and is on the north face of the Bay of Bengal. Bhola is encircled by the giant Meghna and Tetulia rivers. Most often, Bhola is affected by severe tides and cyclones, as it is on the north face of the Bay of Bengal. Soil erosion is a common feature.

To protect Bhola island, the then government of Pakistan constructed embankments around the Bhola district in 1965 (people’s perception). After that, a super cyclone (Bhola Cyclone) took place on 3 November 1970 [1]. The embankment was destroyed severely and almost 0.5 million people died. The soil was submerged completely in the saline water, and people lost their fertile soil [2]. After that, the salinity decreased naturally and gradually. Unfortunately, Bhola was again submerged by a tropical cyclone in 1991. Many people died, and the soil and water became saline once more. To overcome this situation, the broken embankments were again reconstructed in 1996 to protect the agricultural soil [2]. These embankments protected Bhola from saline water entering the land [71]. At present, precipitation has increased as compared to previous years [2], which may be responsible for washing out the soil salinity (which took place during the cyclone). The minimum temperature has also decreased in the Bhola region over time [2]. The three combined factors (embankment, increased rainfall, and decreasing minimum temperature) might be responsible for decreasing the soil and GW salinity.

At present, we do not have any clear data in hand about the soil and water salinity in the Bhola district. Unfortunately, the cyclone in 1970 increased the water and soil salinity abruptly in the Bhola district, but after that, it decreased gradually. Again, the salinity increased in both soil and water after the cyclone in 1991. People strongly believe that the soil and water of the DTW are fresh for drinking and irrigation purposes. However, the river and pond water are still saline, which led to the present research. The geological characteristics, evaporation, dissolution due to rainfall, and rock–water interactions are the major processes controlling the GW quality in the study area [72].

Physicochemical properties: The water samples were collected at depths of 260–430 m. Generally, microbes are absent in the DTW, and therefore, the possibility of producing odor is scarce [73]. In most cases, the taste of the water samples was good (tasteless). This is because soil can act as a natural filter for GW [73] and can be drunk without any disinclination [35,36,65].

The tested samples did not cross the mentioned standard value. In most cases, the turbidity of the GW samples was within the permissible limit. This was because the samples were collected from the DTW, and the DTW might be set in a good aquifer in the water layer. It is also true that the soil acts as a natural filter, which keeps the water less turbid at greater depths [73]. Sample no. 28 CFU was collected from a mixed-crop field, and the turbidity was measured. The value was 28.6 NTU, which indicates a higher concentration of turbidity. This was because the field is watered frequently with GW. The water passing from one channel to another channel might be responsible for the higher turbidity.

For drinking purposes, the BNDWQS permissible limit value of EC is 300.0–1500.0 µS cm⁻¹, and the WHO permissible limit is 750.0 µS cm⁻¹ (Table 2). If we consider the WHO permissible limit value, then only four samples marginally exceeded the permissible limit, but the maximum number of samples was within the permissible limit (Table 2). This finding is in agreement with the recently reported finding for inland GW where the mean EC value was reported to be 270.5 µS cm⁻¹ in the Jamalpur Sadar upazila of Bangladesh [74]. However, it is not in agreement with the other existing literature on the coastal region, because the GW near the coast is unsuitable for drinking purposes due to the higher EC value and salinity [75]. The EC/salinity values in the coastal region increase gradually. High concentrations of EC (3018.65 µS cm⁻¹) in the STW [29] and DTW, e.g., 6479.0 µS cm⁻¹ in Koyra, 5950.0 µS cm⁻¹ in Dacope, and 2568.0 µS cm⁻¹ in Batiaghata in the coastal Khulna district were reported [12]. However, a slightly higher than recommended value of EC was found in the Tanguar haor region of Bangladesh (the northeastern part) [76]. These results imply that the EC of the water samples varies from location to location and depth to depth and may be related to other factors, e.g., embankments and cyclones. Generally, GW containing EC values lower than 1000.0 µS cm⁻¹ is considered freshwater, values higher than 1000.0 µS cm⁻¹ and less than 10,000.0 µS cm⁻¹ are believed to be brackish water, values higher than 100,000.0 µS cm⁻¹ and lower than 35,000.0 µS cm⁻¹ are considered saline water, and values higher than 35,000.0 µS cm⁻¹ are considered hypersaline [70,77]. From this result, it is clear that, in spite of being a coastal district, the DTW water of Bhola is not saline; rather, it is suitable for drinking, and this shows the individuality of this research. On the contrary, the GW in the Khulna [12] and Satkhira districts [68] has a higher salinity.

For irrigation purposes, the Wilcox and DoE permissible limit value for the EC is ~2250.0 µS cm⁻¹ [62,66], and the FAO permissible limit value is 700.0–3000.0 µS cm⁻¹ (Table 2). This means that the majority of the samples were within the standard value. The EC value of brackish water is 1000.0–55,000.0 µS cm⁻¹, and the value of seawater is 55,000.0 µS cm⁻¹ [78]. Therefore, considering the above values, it can also be said that the collected samples were not brackish, because the average EC value is 588.15 µS cm⁻¹ (the only exception was for sample no. 15 CFU). It was confirmed that the collected DTW water is appropriate for drinking and irrigation purposes. The EC of mixed-crop culture water was 310.0 µS cm⁻¹ (Table 2).

Before 1965, Bhola was not bounded by the embankment [2], and the saline water from the Bay of Bengal could easily penetrate into the soil or inland water bodies. After introducing the embankment, the saline water could not easily enter the inland region, and therefore, the salinity might not be introduced. Again, rainfall in the Bhola region has increased as compared to previous years. Increased rainfall may wash out soluble salt from the soil or dilute the concentrations of ions, which might ultimately reduce the EC. This is only a speculation that needs to be explained with research.

Considering the pH values, the community-based DTW water samples are safe for drinking purposes. A similar finding was reported regarding the DTW of Jashore municipality [34] and the JUST campus [35]. pH values of 7.4–8.8 in lower-depth samples (20–85 m) were also reported for Joypurhat [79] and for the Pasur river in Bangladesh [40]. The DoE and FAO recommended pH values for irrigation water are 6.5–8.5 (Table 2). Therefore, the GW is safe for agricultural uses.

Considering the guideline values, the TDS content was safe for drinking purposes in all the collected samples. A TDS lower than 1000.0 mg L⁻¹ is considered freshwater. Similarly, TDS values from 1000.0 to 3000.0 mg L⁻¹ represent fresh to brackish water, from 3000.0 to 5000.0 mg L⁻¹, the water is brackish, from 5000.0 to 35,000.0 mg L⁻¹, it is saline, and water with TDS values higher than >35,000 mg L⁻¹ is hypersaline in nature [80,81]. In another report, TDS < 300.0 mg L⁻¹ in drinking water was considered excellent, and the range of 300.0–600.0 mg L⁻¹ was considered good [65]. Considering this criterion, nearly 63% of water samples were excellent, and the remaining 37% were in good condition for drinking purposes. However, 5% of the water samples were excellent, 35% were poor, and 60% were unacceptable for the samples collected from the STW of the coastal Khulna district [29]. Water collected from different sources, e.g., ponds, channels, rivers, and DTW of the Batiaghata, Dacope, and Koyra upazilas (Khulna, Bangladesh) contained very high concentrations of TDS [12,68]. In spite of being an island and coastal district, the DTW water samples of Bhola contained suitable concentrations of TDS. This phenomenon is not normally found in nature, but it was found in this research. This shows the individuality of this current research. The TDS was suitable for drinking purposes; therefore, the water is recommended directly for irrigation. The DoE and FAO recommended values of TDS in irrigation water are 2100.0 mg L⁻¹ and 0.0–2000.0 mg L⁻¹, respectively (Table 2), which are much higher than the observed result.

Regarding nitrate, the BNDWQS permissible concentration of NO₃⁻ in drinking water is 10.0 mg L⁻¹, and the WHO permissible limit is 50.0 mg L⁻¹. Therefore, the NO₃⁻ concentration of the current samples was within the standard level in all samples, indicating that the samples in the study area were safe for drinking purposes. Very low concentrations of NO₃⁻ in water sources were reported, e.g., 1.6 ± 0.48 mg L⁻¹ in STW and 1.67 ± 0.51 mg L⁻¹ in DTW from Rajshahi city [82]. On the contrary, a low concentration of NO₃⁻ was also reported from the STW in the Khulna district, and the average value was 2.61 mg L⁻¹ [29]. The NO₃⁻ concentrations in GW are naturally low, but they can be very high due to leakage from agricultural land and runoff contamination by human waste after rainfall [65].

Regarding ammonia, out of 27 DTW water samples, 20 samples exceeded the BNDWQS standard, indicating that most of the water samples were not suitable for drinking purposes. On the contrary, only three exceeded the WHO permissible limit (Table 2). If we consider the WHO permissible limit, then most of the water samples were safe for drinking purposes. Considering the WHO permissible limit for NH₄⁺ and the geographical position of the Bhola district, it is recommended that the DTW water of Bhola is suitable for drinking purposes.

The sodium concentrations in the water samples of the study reveal that the GW contained a normal concentration of Na. It can be said that in spite of having a high concentration of Na in the river water surrounding the Bhola district, Na might not penetrate easily into the deep GW aquifer. This means that a depth > 260 m is safe for drinking purposes in the coastal Bhola district. It is not actually known what the result will be if the depth is <260 m. It was clearly found that shallow depths are not safe for drinking and irrigation purposes in the Bhola region, and the STWs are out of use [2]. At present, there are hardly any STWs in the Bhola district [2]. The Na concentration in the mixed-crop culture field was 55.0 mg L⁻¹ (Table 3). In this research, the Na concentration in GW was within the DoE, FAO, and maximum recommended values prescribed for irrigation [66,67].

Usually, the K concentration is not excessive in GW. The present result shows that 6 out of 27 samples exceeded the K concentration of the Bangladesh standard limit but were still within the WHO standard value (Table 3). This means that although Bhola is a coastal district, the GW of the study area contains a regular concentration of K at the depth of >260 m. This result clearly indicates that the K concentration might not be the problem in GW for drinking or irrigation purposes in the BSU and CFU. A number of similar reports have been recently documented [68,69,83].

The results for the Ca concentration show that 10 out of 27 samples of DTW exceeded the Bangladesh standard value, but all values were below or within the WHO standard (Table 3). Considering the standard value of WHO, all of the DTW water was good for drinking purposes. Studies have commonly found that Ca has sound effects on the health of its drinkers. Several studies have reported that the degree of hardness becomes greater as the Ca content increases. The calcium concentration in drinking water has a dose-dependent protective effect when it comes to cardiovascular disease [65]. The result clearly demonstrates that, although Bhola is a coastal district, the collected samples from the study area contained the usual concentration of Ca for its users. The permissible concentration of Ca might play a moderate role in the TH or any other related parameters. The Ca concentration of the mixed-crop culture water was 128.26 mg L⁻¹ (Table 3).

It was observed that 19 out of 27 DTW water samples exceeded the Mg concentration value recommended by Bangladesh, but the values were below or within the WHO standard value (Table 3). Considering the standard value of Mg recommended by the WHO, the collected water was safe for drinking purposes and was certainly appropriate for irrigation purposes, as verified by the MAR values in the following section (Figure 2 and Figure 3). The Mg concentration of the mixed-crop culture was 73.70 mg L⁻¹ (Table 3). Diversified Mg concentrations in water samples are reported in the literature from different parts of Bangladesh [13,40]. The magnesium concentrations of samples collected from the DTW were 151.41 mg L⁻¹ in the Batiaghata upazila, 161.65 mg L⁻¹ in the Dacope upazila, and 186.68 mg L⁻¹ in the Koyra upazila of Khulna district [12,68]. Again, the average concentration of Mg in samples collected from the STW (Khunla) was 78.28 mg L⁻¹ [29]. However, the lowest concentration of Mg ranged from 9.89 to 32.8 mg L⁻¹ in the DTW in the Gopalganj Sadar upazila [84].

In the case of Fe, only three samples crossed the standard limit of Fe in the collected samples of DTW (Table 3). The people from these three unions in the study area may suffer from diseases associated with excess Fe. Therefore, people must be aware of the excess Fe. At present, the WHO does not have any guidelines for the standard concentration of Fe in drinking water, because generally the soluble Fe concentration in GW is not at a harmful level. Therefore, it is considered that the concentration of Fe was safe in almost all of the collected water samples. A recent report from the coastal Khulna district showed that the DTW of Koyra, Dacope, and Batiaghata contained a relatively high concentration of Fe [21,70]. The STW contained 3.1 ± 0.64 mg L⁻¹ Fe, and the DTW contained 2.23 ± 5.72 mg L⁻¹ Fe in Rajshahi city in Bangladesh [82]. About 73% of hand tube wells in the Jashore region contain higher than permissible concentrations of Fe [37]. Considering the above fact, it can be said that the low concentration of Fe in the DTW of the BSU and CFU is mostly due to the depth of the source water.

Regarding Mn, the BNDWQS acceptable value of Mn in drinking water is 0.1 mg L⁻¹, and the WHO recommended value is 0.40 mg L⁻¹ (Table 3). Generally, the DTW water of Jashore municipality and other parts of Jashore contain higher concentrations of Mn [34]. This result shows that the collected samples were safe for drinking purposes regarding the Mn concentration. The presence of a low or no detectable concentration of Mn in the present research is mostly due to the depth of the DTW (Table 3).

Regarding As, fortunately, the concentration of As was BDL in all samples collected from the two upazilas of the Bhola district (Table 3), indicating that As was not the problem in the GW water of these two upazilas. Shallow aquifers are generally enriched with As [32,44], but the deep aquifer is generally As-free [34,66]. Considering As, the people of the study area can drink or could use the water for irrigation without any hesitation. A similar finding was reported for the surface water bodies of the JUST campus and its surrounding areas [35,69]. Again, the sedimentary aquifers of Bangladesh contain almost two times higher elemental concentrations as compared to the concentrations of the basement aquifer of Tanzania and Ghana [85]. A high concentration of As was found in the sedimentary aquifer of Bangladesh [85]. A high concentration of elements was found in the pond water of the Jashore Sadar upazila, Bangladesh [40]. All of these dissimilarities are most probably due to the differences in geological positions and the depths and sources of water bodies [20,51].

Soluble sodium percentage (SSP): The result showed that the SSP of CFU is better than that of the BSU. The categories of SSP (Na%) are excellent (0–20%); good (20–40%); permissible (40–60%); doubtful (60–80%); and unsuitable (>80%) (

Table 4. Classification of groundwater in the Bhola Sadar and Char Fasson upazilas on the basis of hydrogeochemical properties. A total of 28 samples were collected, including one from a mixed-crop field. However, in the following table 27 groundwater samples are considered.

Category	Grade	n = 27	%	Category	Grade	n = 27	%
EC (µS cm−1; Wilcox 1955)				SSP (Wilcox 1955)
Excellent	<250	0	0.0	Excellent	0–20	5	18.0
Good	250–750	23	85.0	Good	20–40	21	78.0
Permissible	750–2250	4	15.0	Permissible	40–60	1	4.0
Doubtful	2250–3000	0	0.0	Doubtful	60–80	0	0.0
Unsuitable	>5000	0	0.0	Unsuitable	>80	0	0.0
EC (µS cm−1; WHO 2004)				SSP (Eaton 1950)
Low salinity	0–250	0	0.0	Safe	<60	27	100.0
Medium salinity	251–750	23	85.0	Unsafe	>60	0	0.0
High salinity	751–2250	4	15.0	SAR (Vasanthavigar et al., 2010)
Very high salinity	2251–6000	0	0.0	Suitable	<2	10	37.0
Extensively high salinity	6001–10,000	0	0.0	Unsuitable	>2	17	63.0
Brine	>10,000	0	0.0
TDS (mg L−1; WHO 2004)				MAR (Kacmaz and Nakoman 2010)
Excellent	<300	17	63.0	S uitable	<50	27	100.0
Good	300–600	10	37.0	Unsuitable	>50	0	0.0
Fair	600–900	0	0.0	KR (Kelley 1963)
Poor	900–1200	0	0.0	Suitable	<1	27	100.0
Unacceptable	>1200	0	0.0	Unsuitable	>1	0	0.0
				TH (mg L−1; Sawyer and McCarthy 1967)
				Soft		27	100.0
				Moderately hard	75–150	0	0.0
				Hard	150–300	0	0.0
				Very hard	>300	0	0.0

NB: EC = Electrical Conductivity; KR = Kelley’s Ratio, MAR = Magnesium Adsorption Ratio, SAR = Sodium Adsorption Ratio, SSP = Soluble Sodium Percentage; TDS = Total Dissolved Solids; TH = Total Hardness, WHO = World Health Organization.

). Again, the categories of SSP are also classified as safe <60% and unsafe >60% [86]. Considering all of the mentioned values, it could be concluded that the SSP of the collected samples was within the safe zone. People can apply the sources of water for agricultural uses confidently in the present situation. In the BSU, out of 11 samples, only 1 sample (nearly 9.09%) was in the excellent category, and the remaining 10 samples (nearly 90.01%) were in the good category. Similarly, in the CFU, out of 16 samples, 4 samples (25%) were in the excellent category and 12 samples (75%) were in the good category. This means that, considering the SSP value, water sources from both upazilas can be used for irrigation.

Recently, it was reported that the average SSP of the DTW water samples was 46.84% in Batiaghata, 51% in Koyra, and 51.48% in the Dacope upazila of the coastal Khulna district, Bangladesh [39], values which are the permissible category [62]. The reason behind these anomalies in SSP values might be that the Bhola is bound by big embankments [2].

Sodium is an imperative cation in irrigation water, which, in excess amounts, breaks down the soil structure and negatively hampers the crop yield [73]. The application of irrigation water with high Na substitutes for Ca and Mg may damage the soil structure [73].

The SAR values presented in Figure 2 and Figure 3, indicate that the DTW water from the CFU is better than that from the BSU. The total average value of SAR was 2.09 ± 0.64. SAR values < 2.0 are classified as suitable and those >2.0 are classified as unsuitable [87] for irrigation (Table 4). Considering the designed value, out of 11 samples, only 2 samples (18.18%) were suitable for agricultural uses, and the remaining 9 samples (81.81%) were not suitable for crop production in the BSU. On the contrary, out of 16 samples, 8 samples (50%) were suitable, and the remaining 50% samples were not suitable for irrigation in the CFU. This result indicates that, depending on the percentages of the SAR values, the GW in the CFU is better than that in the BSU. It is strongly believed that in spite of crossing the limit value (>2.0), the water samples in the BSU might not be so bad, because the average value just crossed the designed value (slightly higher than 2). Moreover, the total Na concentration and SSP were within the permissible values (Table 3 and Figure 2 and Figure 3).

Regarding the MAR, the result showed that none of the samples collected from the upazilas crossed the MAR value of 50. The lower MAR value in the DTW water was most probably due to the depth of the water source. In this research, the water samples were collected at depths of 260.0–430.0 m (Table 1). MAR values < 50 are considered appropriate for irrigation, but values >50 are considered inappropriate [88]. Considering the designed value of MAR, all the collected samples were in the safe zone and could be used for agricultural purposes. However, controversial findings have also been reported for another coastal district, Khulna. Nearly 70% of the samples exceeded the designed MAR value (>50) in STW collected from the Khulna district [29]. Similarly, the average MAR value in the DTW was reported to be 63.64 in samples from the coastal Khulna district [12]. It was suggested that the MAR value in water samples varies due to the depth of water sources and the geographical location of the source water.

Kelley’s Ratios < 1.0 in irrigation water are appropriate for crop production, but ratios > 1 are inappropriate for irrigation [61]. Considering the value of KR, the collected water samples were appropriate for irrigation. In spite of being a coastal district, the values of KR were <1.0, and this indicates the distinctiveness of the present research. A recent report showed that based on the KR, nearly 95% of STW samples in the Khulna district were not suitable for irrigation uses [29]. Although Bhola is a coastal district, the deep aquifer water is suitable for irrigation. The causes behind the outcome might be the difference in the depth of the water sources and the differences in the geographical position. The other cause might be that Bhola is protected by embankments, protecting it from the intrusion of saline water in the land masses [2].

Based on the TH, GW is categorized as soft, moderately hard, hard, and very hard. TH values < 75.0 mg L⁻¹ are soft, values of 75.0–150.0 mg L⁻¹ are considered moderately hard, values of 150.0–300.0 mg L⁻¹ are hard, and values > 300.0 mg L⁻¹ are considered very hard [89]. The research results showed that the DTW water samples from Bhola were 100% soft even though Bhola is a coastal district (Table 4). However, a report showed that nearly 45% of the samples were hard, and nearly 55% of samples were very hard in the STW water samples collected at depths of 21.0–54.0 m from Khulna district. In the current research, the depth of the water sources was 260.0–430.0 m. Considering this fact, it is suggested that the depth of the water sources might be responsible for the variation in the TH.

Regarding the Wilcox data, the reason for the freshness of the water might be related to the depth of the tube well. In Bhola, most of the tube wells are deep tube wells (Table 1). Another cause might be the embankment constructed around the Bhola district which might prevent the saline water moving inland from the sea or river [2]. Moreover, the higher precipitation rate and lower temperature might also be involved.

The fitness of GW for irrigation is related to the salinity and concentration of sodium ions in relation to others (Rao et al. 2012). The main concerns about the irrigation water quality are the salinity hazard and sodium hazard. In the current research, the fitness of GW for irrigation was determined by the Wilcox diagram [62], SSP [57], SAR [58], MAR [60], and KR [61].

6. Conclusions

The originality of this research is that, in spite of being a coastal district, the GW of the BSU and CFU is saline-free or fresh. The main reason for the lower EC value in the DTW water samples of the two upazilas is mostly related to the embankment constructed around the Bhola district. It is possible that the embankment does not permit the easy penetration of saline water from the river to the inland soil or GW. The freshness of the GW might also be related to the depth of the collected samples. Not only the EC, but the other parameters were also below or within the permissible limit recommended by the BNDWQS or WHO for drinking purposes. Additionally, the values of SSP, SAR, MAR, and KR were also within or closely around the permissible limit values for irrigation water. The Wilcox diagram showed that about 96% of the water samples were suitable for irrigation purposes. The overall results explain that the DTW water from the Bhola region is safe for drinking and irrigation purposes. The TH results prove that the GW is more than safe for homestead uses. People from the BSU and CFU use the community-based DTW, whose average depth is 260 m (>850 ft).

7. Limitations of the Study

Considering the study area, the total number of samples was not sufficient. For extensive research, a bigger sample size is required in the near future. Other heavy metals also need to be determined in the future to gain a clear idea. Spatial range and seasonal variations are also strongly suggested for future research. The Bhola is a totally isolated district of Bangladesh surrounded by water bodies; therefore, protection is needed by managing the embankments around Bhola.