Nitrate Pollution in the Groundwater of Bangladesh: An Emerging Threat

Abstract

Access to safe potable water is one of the most significant challenges in an environmentally vulnerable country like Bangladesh. The presence of high concentrations of nitrate in groundwater can deteriorate its quality and pose serious health threats. A review was conducted to evaluate the current status of overall nitrate concentrations in different districts (35 out of 64 districts) of Bangladesh based on available published data. Human Health Risk Analysis (HHRA) and nitrate pollution index (NPI) were calculated to illustrate the level of chronic risk and degree of pollution among the population of the studied districts. The HHRA model predicted that the adult population of 48.57% districts and the child population of 56.25% districts were found to be facing potential health risks associated with elevated nitrate consumption. The NPI results revealed that at least 45.72% of the studied districts were characterized by light to very significant nitrate pollution loads. It can be concluded that the outcomes of this systematic study would draw the attention of policymakers and the population of all districts and enable them to take effective measures in preserve groundwater resources in Bangladesh and prevent long-term, complex diseases.

1. Introduction

Groundwater contamination due to anthropogenic activities has increasingly become a concern in recent years with the rapid development of modern civilization [1]. Different types of toxic and carcinogenic materials have been found in groundwater, and nitrate is one of them [2]. Continuous and extensive usage of agrochemicals is one of the major sources of nitrate in groundwater [3,4]. One of the toxic characteristics of nitrate in groundwater is that it can remain there for many years and increase in concentration with time (Wellman and Rupert, 2016 [5]). Drinking water sources are increasingly contaminated by nitrate in developing countries, including Bangladesh [6]. USEPA, 2004 [7] reported that long-term high nitrate exposure could lead to different gastrointestinal diseases, including cancer. Groundwater is mainly used as potable water in Bangladesh for drinking. This water is consumed as raw water without any treatment in almost all districts of the country [8]. Thus, there is a potential relationship between human health and the presence of nitrate in groundwater. This relationship can be obtained through human health risk analysis [9]. Most of the studies from the perspective of groundwater nitrate pollution in Bangladesh focused solely on the oral ingestion pathway. In this study, we have also included dermal exposure data for both adult and child populations based on available peer reviewed literature. Due to the unavailability of total district data for 64 districts, the human health risk assessment did not cover the complete evaluation of the health risk index system of nitrate pollution from the perspective of Bangladesh. Nitrate pollution in groundwater is a vital health problem in Bangladesh. Literature studies revealed that the groundwater of the central districts (e.g., Manikganj), south-west district (e.g., Satkhira), south-central district (e.g., Jessore, Gopalganj), south district (e.g., Barishal, Bagerhat, Barguna, Patuakhali, Pirojpur), south-west district (Chandpur), central-west district (Pabna), western district (Rajshahi, Naogaon), north-western district (e.g., Thakurgaon), northern district (Kurigram, Gaibandha), and north-eastern district (Sunamganj) were high to moderately loaded with nitrate concentrations. The child population in these districts was considered to be more susceptible to groundwater nitrate-induced health hazards [10,11,12,13,14].

Based on these findings, the aim of this study was to make an extensive review of available groundwater-mediated nitrate data collected through a comprehensive literature search. To achieve this, a literature review was first conducted to evaluate the current status of groundwater nitrate pollution in Bangladesh. Then, a complete human health risk assessment was subsequently conducted by focusing on two population groups, adults and children. The exposure pathways considered in this study included both oral and dermal. To minimize potential anomalies, the equations and standards used in this study were retrieved from native articles. This study produced a spatial distribution of groundwater nitrate concentrations (NPI) and total hazardous quotients (THQs) for both the adult and child populations of Bangladesh. Thus, the findings of this study are expected to visualize the nitrate-induced groundwater pollution problem and its associated health risks in order to make more effective and sustainable decisions.

2. Methodology

A literature search was conducted for peer-reviewed articles on groundwater nitrate pollution in Bangladesh. We utilized the Web of Science and Scopus databases to carry out the literature search. Selected keywords included ‘nitrate pollution’, ‘groundwater nitrate pollution’, ‘nitrate occurrence in groundwater’, and “groundwater nitrate and Bangladesh’ in different combinations. In addition, reference lists of identified papers and books were reviewed and manually searched for additional data. Research papers that were not in the English language were excluded. Based on this process, a total of 35 districts were found and mean concentrations of groundwater nitrate were considered for further analysis.

2.1. Nitrogen Pollution Index (NPI)

The presence of nitrate is one of the vital parameters for groundwater quality analysis [15,16]. Nitrate contamination is an important issue to consider since it can affect human health [17,18]. To evaluate the nitrate pollution in groundwater, Panneerselvam et al. 2020 [19], introduced the nitrate pollution index (NPI). The NPI level can be measured by the following equation:

$$NPI=\frac{Cs-HAV}{HAV}$$

(1)

The NPI results indicated the degree of nitrate pollution levels in the groundwater of the studied areas. The NPI value according to pollution type is shown in

Table 1. Degree of nitrate pollution index (NPI).

NPI Value	Pollution Level
<0.0	Clean
0.1–1.0	Light
1.1–2.0	Moderate
2.1–3.0	Significant
>3.0	Very significant

[19]. Where Cs it the concentration of groundwater nitrate in different districts and HAV is the human acceptable nitrate level according to the Department of Environment, Bangladesh (10 mg/L) [5,20].

2.2. Human Health Risk Assessment (HHRA)

Human health risk assessment exhibits pollutants that mediate health risks, especially from groundwater [21]. USEPA studies from 2001 and 2004 [7,22] revealed that drinking water could be one of the major routes of direct pollutant consumption. However, exposure through dermal adsorption cannot also be neglected [23]. The HHRA model can be applied to analyze the probability of occurrence of any given magnitude of adverse health effects over a certain duration [24]. In this study, the HHRA model was applied to calculate the health risk in two groups of population (adults and children) in the studied areas found in the literature so far by the following Formulas (2)–(7). All the parameters needed to calculate the HHRA model were listed in Table 2.

$$CDI=\frac{CW\times IR\times EF\times ED}{BW\times AT}$$

(2)

$$DAD=\frac{CW\times Ki\times SSA\times ED\times EF\times EV\times C}{BW\times AT}$$

(3)

Chronic risk, which is the non-carcinogenic risk, was calculated as the Hazardous Quotient (HQ) for both oral and dermal exposure as:

$$HQ oral=\frac{CDI}{RfD}$$

(4)

$$HQ dermal=\frac{DAD}{RfD}$$

(5)

Total hazardous quotients (THQs) was calculated for both adult and child populations according to the following equations [23]:

$$THQ Adult=HQ oral+HQ dermal$$

(6)

$$THQ Child=HQ oral+HQ dermal$$

(7)

Based on the resultant THQ values, the chronic risks were labelled as negligible chronic risk to high chronic risks (

Table 3. Chronic risk level [27].

THQ Value	Chronic Risk
<0.1	Negligible
>0.1 < 1.0	Low
≥1.0 < 4.0	Medium
≥4.0	High

) [24,25]. For statistical analysis, Microsoft Excel 2016 was used, and for spatial analysis, ArcGIS 10.8 was applied.

Table 2. Parameters used to calculate the HHRA model.

Parameters	Description	Values		Unit	References
		Adult	Child
CDI	Chronic daily intake of nitrate			mg/kg/day	[26]
CW	Concentration of groundwater nitrate			mg/L	[26]
IR	Ingestion rate	2.2	1.0	L	[27]
EF	Exposure frequency	365	365	Days/year	[28]
ED	Exposure duration	70	10	years	[29]
BW	Body weight	70	25	kg	[29]
AT	Average time in days	25,550	3650	days	[27]
DAD	Dermal adsorption dose of nitrate			mg/kg/day	[23]
Ki	Dermal permeability coefficient	0.001		cm/h	[23]
SSA	Skin surface area	16,600	12,000	cm2	[30]
EV	Bathing frequency	1		time/day	[9]
CF	Conversion factor	0.001		-	[30]
RfD	Reference dose	1.6		mg/kg/day	[31]

Table 2. Parameters used to calculate the HHRA model.

Parameters	Description	Values		Unit	References
		Adult	Child
CDI	Chronic daily intake of nitrate			mg/kg/day	[26]
CW	Concentration of groundwater nitrate			mg/L	[26]
IR	Ingestion rate	2.2	1.0	L	[27]
EF	Exposure frequency	365	365	Days/year	[28]
ED	Exposure duration	70	10	years	[29]
BW	Body weight	70	25	kg	[29]
AT	Average time in days	25,550	3650	days	[27]
DAD	Dermal adsorption dose of nitrate			mg/kg/day	[23]
Ki	Dermal permeability coefficient	0.001		cm/h	[23]
SSA	Skin surface area	16,600	12,000	cm2	[30]
EV	Bathing frequency	1		time/day	[9]
CF	Conversion factor	0.001		-	[30]
RfD	Reference dose	1.6		mg/kg/day	[31]

3. Biogeochemistry of Nitrate in Groundwater

Nitrate concentrations have been found in 35 districts, and many of them are characterized by high concentrations of nitrate pollution. Most of these districts’ groundwater bodies have direct or indirect hydraulic continuity with dead animals or plants, fertilizers, sanitary latrine sewage, areas of high-density confinement, or geologic materials containing soluble nitrogenous compounds. Since nitrate, nitrite, and clay contain negative charges, they can be adsorbed by each other. However, the ammonium ion carries a positive charge and can be adsorbed by clay particles with the help of microorganisms. This will result in negatively charged nitrite or nitrate ions being free to migrate with percolating soil waters. Studies found that this percolation or leaching of nitrates can build up a layer of nitrate in shallow groundwater tables [32]. The literature cited in this study also indicated that deep aquifers (deeper than 50 feet) showed little or no evidence of nitrate contamination in Bangladesh.

The biogeochemical processes controlling this nitrate attenuation in groundwater are very complex [33]. Denitrification is considered to be the dominant nitrate attenuation process in groundwater. The limiting factors for denitrifying bacteria are oxygen and electron donor concentrations and availability. Other environmental factors like nitrate concentration, nutrient availability, pH, temperature, the presence of toxins, etc. are considered secondary influential factors in nitrate attenuation in groundwater. [33].

A groundwater-based nitrogen cycle is illustrated in Figure 1. This Figure 1 involves the illustration of the reduction of nitrate via a chain of microbial reduction reactions to nitrogen gas [34]. However, nitrate can also be reduced to nitrite and nitrous oxide by abiotic reactions, but in the subsurface, these reactions were found to be very rare [35]. In brief, the presence of organic carbon, the absence of oxygen, and reduced iron or sulfur facilitate the initiation of denitrification. The nitrification process can be written as the following half reaction [35]:

2NO₃⁻ +12H⁺ + 10e⁻ = N₂ + 6H₂O

This nitrogen reduction process is controlled by the local biogeochemical conditions of the subsurface environment [36,37]. These biogeochemical conditions include the presence of an unsaturated zone, superficial deposits, a groundwater-surface water interface, and a confined-unconfined aquifer interface (Figure 2).

4. Drivers That Affect the Variability of Nitrate in Groundwater

There are some driving factors that make Bangladesh prone to groundwater-based nitrate pollution. Along with the geological settings, examples of other driving factors are changes in land use cover, extensive agricultural practices, and over extraction of groundwater for industrial, irrigational, and drinking purposes [8,29,38]. However, limitations in nitrate pollution prediction and spatial distribution remain. Ketchmen-Tandia et al., 2017 [39] revealed that discharge from sanitary latrines and sewage could be one of the driving factors of nitrate pollution in groundwater. Thompson et al., 1986 [40], reported that nitrate leaching in shallow aquifers could be associated with water table level and vertical gradients. Young, 1992 [41] found that fertilizers containing toxic elements, including arsenic, could act as one of the factors that drives nitrate leaching in groundwater, especially in areas of high permeability [42]. Again, since ambient redox profiles and availability of electron donors could influence nitrate loading in groundwater, pH and HCO₃⁻ could also be considered as minor factors that drive nitrate contamination [43]. Islam et al., 2021 [8], also stated that salinity, F⁻, Ca²⁺, PO₄²⁻, and SO₄²⁻ had less influence on groundwater nitrate concentrations. Again, it was also revealed that the piling up of heterogeneous solid waste could also influence the nitrate level in shallow aquifer systems [44,45]. Excessive usage of chemical fertilizers, irrigation, and precipitation together wash away the agro-chemical materials, including nitrate, and can infiltrate the groundwater table, especially in the monsoon, without any transformation or retardation [46,47]. A literature-based study on nitrate sources in different districts’ groundwater in Bangladesh stated agricultural input in the form of nitrogen fertilizer and manure contributed the most followed by pit latrines [48]. All of these findings were found to be quite similar to the literature studies from the perspective of Bangladesh. Different sources of groundwater nitrate pollution are summarized in

Table 4. Sources of nitrate in groundwater [49].

General Sources	Specific Sources
Natural sources	Mobilization and leaching of geogenic nitrogen.
Pilling of waste load	Animal manure load, e.g., dairy firm, poultry industry. Industrial or municipal load, e.g., sludge load and effluent mobilization to crop land, forest, inland, etc. Unmanaged septic tank system. Leachates from landfills and unmanaged disposal sites.
Agricultural practice	Excessive fertilizer application can reduce the nitrogen uptake capacity of plants and crops, and reduce soil mineralization capacity, and thus easily penetrate permeable aquifers and make the soil infertile. Unplanned irrigation practices, precipitation and flooding, temperature variation, and tillage practices also influence the presence of nitrate in groundwater.

.

5. Distribution and Magnitude of Nitrate Pollution in Groundwater

Occurrences of nitrate concentrations in groundwater had been retrieved from 35 districts in Bangladesh. According to ESRM, 2022 [50] and USEPA, 2018 [51], the safe consumption limit of nitrate was set at 10 mg/L, while WHO [52], set this limit at 50 mg/L. The distribution of the groundwater-mediated nitrate concentration data set across 35 districts has been displayed in

Table 5. Concentration of groundwater nitrate in different districts of Bangladesh.

S.N	Districts	NO3 Level (mg/L)	References
1.	Bagerhat	5.87	[53]
2.	Bandarban	0.107	[54]
3.	Barguna	8.8	[11]
4.	Barisal	4.23	[55]
5.	Brahamanbaria	0.25	[56]
6.	Chandpur	7.9	[57]
7.	Chittagong	2.47	[54]
8.	Chuadanga	0.03	[58]
9.	Comilla	0.55	[59]
10.	Cox’s Bazar	0.925	[54]
11.	Dhaka	3.6	[12]
12.	Dinajpur	0.35	[60]
13.	Gaibandha	10.27	[61]
14.	Gopalganj	9.96	[62]
15.	Jessore	10.62	[13]
16.	Joypurhat	3.45	[63]
17.	Khulna	2.13	[64]
18.	Kurigram	8.69	[65]
19.	Manikganj	253.18	[8]
20.	Munshiganj	3.93	[10]
21.	Naogaon	7.9	[66]
22.	Narayanganj	0.53	[12]
23.	Nawabganj	3.02	[67]
24.	Noakhali	3.9	[54]
25.	Pabna	31.69	[14]
26.	Panchagarh	3.45	[63]
27.	Patuakhali	11.57	[68]
28.	Pirojpur	13.5	[69]
29.	Rajshahi	8.75	[70]
30.	Rangamati	0.534	[54]
31.	Satkhira	54.44	[71]
32.	Sherpur	0.19	[61]
33.	Sunamganj	5.19	[72]
34.	Sylhet	3.18	[54]
35.	Thakurgaon	6.66	[73]

and Figure 3.

Figure 3 illustrates that out of 35 considered districts, 5 districts (e.g., Gaibandha, Jessore, Pabna, Patuakhali, and Pirojpur, which constitute 14.29% of considered districts) crossed the DoE, 1997 [20] and USEPA, 2018 [51] guidelines, and 2 districts (Manikgonj and Satkhira, which constitute 5.71% of considered 35 districts) exceeded the WHO, 2011 [52] and the other guidelines. The rest of 28 districts (80%) out of 35 considered districts were found to be relatively lower than all the standard parameters.

Both natural and anthropogenic activities were found to be responsible for elevated nitrate concentrations in groundwater [8]. For example, unconfined shallow aquifers are more susceptible to nitrate association than deep, confined aquifers [74]. Another geogenic reason for high nitrate concentrations in the studied areas could be a poor soil profile. Anthropogenic sources of high nitrate concentrations in the groundwater of the studied area were animal and human waste, high cropping, aquaculture practices, and an unmanaged irrigation system [13]. The high levels of nitrate in the groundwater of Manikganj (253.18 mg/L) and Satkhira (54.44 mg/L) could be caused by extensive agricultural practices and the widespread use of unplanned latrines and septic tanks that allow their contents to leak into the soil and then into the groundwater [75,76].

Nitrate Pollution Index (NPI)

The spatial distribution of the nitrate pollution index (NPI) was also prepared on the basis of available data from the literature (Figure 4). Figure 4 depicts that about 54.28% of studied districts (19 out of 35 considered districts) contain clean groundwater with no chronic risk. Though 28.5% of districts (10 out of 35 considered districts, e.g., Bagerhat, Barguna, Barisal, Chandpur, Gopalganj, Kurigram, Naogaon, Rajshahi, Sunamganj, and Thakurgaon) exhibit clean groundwater, there is still a low level of chronic risk in these districts. Again having a low chronic risk level, 11.43% of the studied areas (4 districts out of 35 districts), e.g., Gaibandha, Jessore, Patuakhali, and Pirojpur, were characterized by light nitrate pollution. Satkhira district contained a medium level of chronic risk with a moderate level of nitrate pollution, while nitrate pollution was found to be very significant in Manikganj district with a high chronic risk factor.

6. Human Health Risk

Nitrate itself is not directly toxic to humans. However, it becomes hazardous when this nitrate turns into nitrite in the stomach with amines and amides and forms different types of nitroso compounds [77]. In this study, potential non-carcinogenic impacts of nitrate were calculated in accordance with retrieved data from 35 districts (about 54.69% of the total 64 districts in Bangladesh). (Figure 5). Both oral and dermal exposure pathways were considered to analyze the total hazardous quotients (THQs) of the adult and child populations (Figure 4). The spatial magnitude of the resultant total hazardous quotient (THQ) for both adults and children is illustrated in Figure 6 and Figure 7, respectively. Analysis of THQs for adults (Figure 6) revealed that about 14 out of 35 districts (40% of considered 35 districts) showed a low level of chronic risk, while Satkhira (THQ_adult = 1.068) and Manikganj (THQ_adult = 5.010) possessed medium and high-level chronic risk among adults, respectively.

Again, THQ values for children (Figure 7) revealed that the children of Manikganj were highly vulnerable (THQ_child = 6.329) to nitrate-associated health hazards, while the the child population of Satkhira was found to be mediumly vulnerable (THQ_child = 1.387) to nitrate-induced health hazards. THQ child also suggested that the child populations of Bagerhat, Barguna, Chandpur, Gaibandha, Gopalganj, Jessore, Kurigram, Munshiganj, Naogaon, Pabna, Patuakhali, Pirojpur, Rajshahi, Sunamganj, and Thakurgaon were exposed to low-nitrate induced health risks. The negligible health risk was found in 18 districts (51.42% of the considered 35 districts) for adults and 17 districts (48.57% of the considered 35 districts) for the child population (Figure 7).

7. Potential Health Effects of Groundwater Nitrate

The resultant concentration of groundwater nitrate, nitrate pollution index (NPI), and total hazardous quotient (THQs) data for both adult and child populations revealed that elevated nitrate levels posed a significant health concern in certain districts (e.g., Manikganj and Satkhira), while Thakurgaon, Kurigram, Gaibandha, Naogaon, Rajshahi, Sunamganj, Pabna, Jessore, Gopalganj, Gopalganj, Chandpur, Barisal, Bagerhat, Pirojpur, Patuakhali, and Barguna) tend to pose toxic impacts in the near future. The people of these areas could suffer from two major health concerns related to nitrate toxicity, e.g., methemoglobinemia and gastric cancer. Methemoglobinemia primarily causes toxicity in babies and is referred to as the ‘blue baby syndrome’. According to the USEPA, infants or children who consume nitrate in amounts greater than 10 mg/L are more vulnerable to nitrate toxicity than the rest of the population. The main reason for this vulnerability is the presence of bacteria in the digestive tract of infants that can convert consumed nitrate to toxic nitrite. The main toxicity of this disease is that it can transform blood hemoglobin into methemoglobin, which can reduce the oxygen transportation system in the blood and cause asphyxiation. Studies revealed that the first symptom of methemoglobinemia (cyanosis) became noticeable when the presence of methemoglobin exceeded 5% in the blood, and anoxia (death) could occur at a level of 50% or higher [78,79].

The first study regarding methemoglobinemia was conducted by Comly in 1945 [80]. He revealed that infants who drink water contaminated with 90–140 mg/L are highly susceptible to blue baby syndrome. Literature reviews conducted in Bangladesh so far have revealed that the nitrate level in about 7 districts (out of 35 considered districts) exceeded the 10 mg/L level and posed serious health concerns such as the occurrence of methemoglobinemia, especially in infants and children. However, methemoglobinemia is considered a less notifiable disease in many countries, including Bangladesh. Along with the infants in the highly contaminated study areas, the risk of overexposure to nitrates in the adult population was not zero. Studies revealed that when high amounts of nitrate were consumed by adults, it could turn to nitrite in the stomach and react with secondary and tertiary amines to form nitrosamine, a potent carcinogen [81,82]. Shuval and Gruener, 1977 [83] showed possible carcinogenic, teratogenic, and mutagenic properties of nitrosamines that could theoretically develop in human digestive tracts if one is overexposed to nitrates. However, in 1985, the WHO suggested that the relationship between high levels of nitrate (>10 mg/L) and gastrointestinal cancer is still controversial. Studies in South Australia and England noticed that elevated concentrations of nitrate (>10 mg/L) consumed via oral and dermal pathways could also be responsible for congenital malformations and cardiovascular diseases [84].

Besides these, ruminants in these 35 districts could face health problems due to excessive nitrate consumption via drinking water, which is yet to be revealed. Studies found that sheep and cattle suffered from high nitrate contamination from birth to adulthood [49]. Horses, pigs, and chickens were also susceptible to the negative health effects of nitrate overexposure [85]. Some of the common health effects due to high nitrate concentrations among livestock and other animals were loss of milk production, shortness of breath, accelerated heartbeat, staggered gait, frequent urination, convulsions, or even death [49,86].

8. Recommendations for Preservance of Groundwater Aquifers

The extraction of groundwater in Bangladesh is perfomed by numerous dispersed pumpers who typically use small pumps to irrigate small sections of land. In Iran, for instance, fewer than 500,000 tubewells are used to pump 45 Km³ of groundwater, but in Bangladesh, more than 1.5 million tubewells are in operation to siphon only 28 Km³ of groundwater. The fact that 85 percent of Bangladeshi natives live in village areas and rely on agriculture for a livelihood further complicates the situation [86].

In Bangladesh, flood irrigation remains the predominant method of boro cultivation despite significant advancements in scientific research. Notably, studies have indicated that maintaining moisture levels in boro fields for a period of 3–4 days following the disappearance of standing water did not result in reduced yields [87,88]. As per the research carried out by the Bangladesh Rice Research Institute [89], the average quantity of water utilized for the production of 1 kg of boro in farmers’ fields is approximately 4.0 m³, whereas in researcher-managed trials, the quantity of water used is comparatively lower, i.e., 2.0 m³. According to Rashid (2008) [90], farmers tend to apply water in quantities that surpass the crop’s actual needs in order to account for the seepage and percolation rates in rice fields that utilize flood irrigation practices, typically ranging from 4 to 8 mm per day. According to Mollah et al. (2009) [91], research conducted in Bangladesh has demonstrated that bed planting methods can result in a reduction of up to 40% in water usage when compared to flood irrigation while also yielding higher gross margins. The primary hindrances in the implementation of bed-planting methods for rice-wheat systems in Bangladesh are perceived to be insufficient funds and limited equipment accessibility, as stated by Krupnik et al. (2013) [92], despite the benefits associated with this approach. The alternate wetting and drying (AWD) technique presents a promising approach to enhancing water use efficiency in rice cultivation. This method involves allowing the ponded water to gradually infiltrate the field for a period of several days until the perched field water table attains a depth of 15–20 cm. The implementation of All-Wheel Drive (AWD) technology in Bangladesh has the potential to conserve 20–30% of water consumption during flooded rice cultivation, resulting in an estimated irrigation cost savings of US$ 73.5 million. Another approach to preserving the groundwater aquifer is rainwater harvesting, which could be implemented in public and community wells located near slums and villages in Bangladesh. This involves collecting rainwater from nearby roofs and streets, particularly during the monsoon season. The potential benefits of implementing rainwater harvesting on a large scale must be assessed with caution, taking into account its effects on the challenges, opportunities, and policy suggestions for maintaining a sustainable water balance in the basin [93], as well as the accessibility of water for farmers downstream [94]. Again, in the primary boro-cultivating districts of Bangladesh, groundwater serves as the primary source of irrigation water, accounting for over 90% of the total supply. Consequently, implementing limitations on rice cultivation in regions that are susceptible to groundwater depletion may alleviate the strain on this vital resource. The potential decline in rice yield resulting from spatial limitations could be compensated for by cultivating alternative grain crops that require less water, such as wheat, maize, legumes, and various other crops. According to BADC’s 2013 [95] report, wheat and maize are cultivated on 400,000 and 170,000 hectares of land, respectively. However, their combined yield falls significantly short of meeting the country’s annual demand. According to BARI’s report in 2014 [96], Bangladesh’s national demand for wheat and maize is met through the importation of 2.0 million tons and 1.0 million tons, respectively, on a yearly basis. The adoption of these two crops could potentially contribute to the stabilization of aquifers and reduce the expenditure of valuable foreign exchange that is currently allocated towards the importation of these two grains.

The implementation of groundwater use rights and permit systems to regulate access to groundwater is unlikely to be a feasible solution for Bangladesh, given the considerable number of users and inadequate institutional frameworks to enforce legal and regulatory measures. Consequently, it is imperative to devise a comprehensive and practical approach that exhibits prudence, perseverance, and patience to tackle the challenge of groundwater management. Several factors that could contribute to success include active user engagement, improvements in water pricing models, significant investments in contemporary water and agricultural technology, incentives for farmers to shift towards less water-intensive crops, and the establishment of supportive policies and decision-making frameworks. The focus of policy research ought to be on determining the optimal approaches for groundwater governance in Bangladesh in the future. With this in mind, the following specific management options could be followed as aid to preserve the groundwater aquifers of different districts of Bangladesh from further contamination:

• Sustainable disposal and management of industrial and municipal solid waste in lined landfill sites to avoid nitrate leaching into groundwater aquifers.
• Introducing sustainable sewage system management, especially in urban areas, to manage the sewer properly
• Sustainable agricultural practices like proper application of agrochemicals and manures in the crop field and vegetative land to prevent nitrate leaching in aquifers
• Initiation of a thorough survey and site characterization to establish new well sites
• Adopting artificial recharge methods such as piloting the rainwater harvesting technique

Since deep aquifers contain a trace amount of nitrate, new deep tube wells can be utilized for drinking purposes. However, the other parameters, including the presence of fluoride, must also be considered.

• Establishing buffer zones with high vegetation cover that could prevent flows from crop lands and disposal sites.
• Finally, prioritizing awareness creation for improving sanitation and groundwater quality protection among the people of affected districts.

9. Conclusions

Exposure to nitrates via groundwater in Bangladesh poses or is likely to pose a health hazard to adults and child. The root causes of this nitrate pollution in groundwater are mainly considered to be a result of natural factors, the pilling of waste disposal, as well as unplanned extensive agricultural practices. However, groundwater-based nitrate loads in the other districts, which could not be revealed yet, remain unclear, and the potential health hazards for the population in these districts cannot be neglected. Thus, this study illustrates insights into the present groundwater nitrate pollution scenario in a single review paper that would assist scientists and authorities to perform further evidence based research along with taking further steps to sustainably protect and manage groundwater nitrate pollution.