Groundwater Pollution in Bangladesh: A Review

Bangladesh relies mainly on groundwater for irrigation and drinking purposes. Groundwater, however, continuously polluted, is a major obstacle. Nowadays, Bangladesh is moving towards industrial revolution in a considerable speed. As part of this paper's attempt to analyze the groundwater pollution scenario in Bangladesh, specifically in the past two decades, about 100 articles, conference papers, and reports published in national and international journals and books were reviewed, as well as issues concerning pollution sources, health impact assessment, and future perspectives were discussed. The groundwater is contaminated by different contaminants, such as physico-chemicals, trace metals, and microbes. Human health is at great risk from arsenic (As) contamination; it is one of the biggest threats. The cancer risk and non-cancer risk of ingesting water are increased. On the other hand, a large number of peoples were affected due to waterborne diseases governed by microbial contamination. Geophysical and anthropogenic sources, the depth of wells, and geographical factors may influence groundwater pollution. It is recommended that policy makers should address the issue immediately and precautions should be taken wherever necessary.


Introduction
Human metabolic systems and other life sustaining activities require water (Dkhar et al., 2014).A natural and renewable asset that is essential to life, water experiences natural and continuous processes within the hydrological cycle (Iscen et al., 2008).Life on earth began with water (Moe and Rheingans, 2006), however, groundwater is perhaps the most valuable resource, which has been misused (Arumugam and Elangovan, 2009;Chaudhary and Satheesh kumar, 2018).Globally, groundwater sources supply 43% and 40% of total water used for irrigation and drinking purposes, respectively (Salman et al., 2018).Over the last few decades, water demand has increased rapidly with the development of energy, industry, urbanization, agriculture, improvements in living standards, and construction of environmentally friendly homes (Ravikumar and Somashekar, 2017).Besides, lack of water is a major issue in many countries because of the disparity in rainfall caused by global warming (Mahaqi et al., 2018).Practically, 1.8 billion individuals around the globe, may face absolute water shortages by 2025 (UNESCO, 2012).
In Bangladesh, amount of withdrawn groundwater is approximately 32 km 3 per year where 90% of water is used for irrigation and rest 10% is utilized for industrial and domestic purposes, which is equivalent to 4% of the world's withdrawal of groundwater (Shamsudduha et al., 2019).Bangladesh has between 6 and 11 million tube wells, and 98% of population use groundwater as their main source of drinking water (Islam et al., 2020a;Gaus et al., 2003).During 1970s, Department of Public Health Engineering (DPHE) of Bangladesh and the United Nations Children ' s Fund (UNICEF) worked together to establish handpumped tube wells for providing fresh drinking water among the rural population in Bangladesh to prevent waterborne diseases (Haque, 2018); nevertheless, the number of deaths caused by waterborne diseases is still 8.5% (UN-Water, 2013).Bangladesh's biggest challenge is to preserve its groundwater sustainably (Saha et al., 2020).Water quality and quantity in Bangladesh are affected by several factors, either directly or indirectly (Islam et al., 2020a).The groundwater pollution by arsenic in Bangladesh has been associated with health issues (Mukherjee and Bhattacharya, 2001).Arsenic contamination occurs in groundwater in 61 districts of Bangladesh, and 20 million people drink this water with levels exceeding the national standard limit for arsenic (Ghosh et al., 2020).Lead (Pb), cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe) and zinc (Zn) are other common metals that contribute to groundwater contamination in Bangladesh (Zakir et al., 2020).Additionally, bacteria and pesticides play a significant role in groundwater pollution (Anwar and Yunus, 2013;Sarker et al., 2020).According to BNDWQS (Bangladesh National Drinking Water Survey), As concentrations exceeded Bangladesh standard in 8% of sampled water samples and WHO standards in 18% of cases (BNDWQS, 2009).They reported that 97.8% of Bangladesh's population used safe drinking water.Nevertheless, many studies in recent decades have indicated the hazard of drinking groundwater over the long term, including cancer-causing and non-cancer risks which actually drove us to review the groundwater pollution in Bangladesh.1).In addition to measuring mineralization, EC determination can be used to tell if water quality is changing in natural and wastewater quickly (Arulbalaji and Gurugnanam, 2017).Permissible limit of EC value is 500 µS/cm based on WHO and Bangladesh standard and testing institution (BSTI) standards.There have been 25 studies reporting above the acceptable limit (Table 1).In Satkhira district, for example, EC levels were reported to be too high (7,135.67 ± 3,433.58µS/cm) (Rakib et al., 2020).Besides, BSTI and WHO do not specify a range of salinity for drinking water.As a result of field sampling from 113 different locations in Bangladesh, Shahid et al., (2006) found that 8.31% of the groundwater had high salinity levels (Shahid et al., 2006).According to a study, groundwater in Faridpur, Netrokona, Madaripur, Khulna, Shatkhira, Barguna, Patuakhali and Chittagong is highly salinized (Akter et al., 2016).Water hardness has no known adverse effects; but some evidences denote its role in heart diseases, kidney problems, unpleasant taste and decreases the ability of soap to make lather (Ali and Ali, 2018).For drinking, the WHO recommends a limit of 300 mg/L and only few study areas have exceeded the limit.Kushtia has the highest concentration (432.85 mg/L), while Dinajpur has the lowest concentration (22.74 ± 20.44 mg/L).pH can indicate the acidity or alkalinity of the water and the strength of the H + ions in that water (Tiwari et al., 2017).pH range 6.5-8.5 indicated the water as safe for drinking.Except for Khagrachari, Rangamati, and Satkhira, most studies reported pH values within the range that is suitable (Table 1).Besides, less than 6.5 (6.26 ± 0.11 and 6.36 ± 0.33 during winter and summer, respectively) pH value is observed in Chittagong (Rifat et al., 2021).Acidic water may damage the mucous membrane cell and cause of irritation in eyes and skins as well as metal corrosion (WHO, 1986, Popoola et al., 2019).The turbidity of water is measured by the presence of tiny-sized suspended particles, which tint and cloud the water (Solangi et al., 2019) and has potential health risk when it is consumed (WHO, 1996).BSTI determines turbidity at 10 Nephelometric turbidity unit (NTU), whereas WHO specifies 5 NTU.The standard limit for turbidity has exceeded only in Chittagong, Noakhali, and Dinajpur districts (Table 1).

Ion characteristics
Table 2 summarizes the major cations and anions in the groundwater in different regions of Bangladesh.Despite its essential role in human function, excessive levels of sodium (Na) may result high blood pressure and kidney failure in the body (Ameen, 2019).WHO and BSTI established 200 mg/L of Na in drinking water as the prescribed limit.Only Gopalganj, Satkhira, Jessore, and Barisal districts have exceeded the recommended level (Table 2).In human and animal tissues, potassium (K) is essential to life; it is especially abundant in plant cells (Meride and Ayenew, 2016).However, excess potassium may cause some health issues like nausea, vomiting, diarrhea, hyperkalaemia, shortness of breath, and heart failure (WHO, 2009).In every study except Satkhira, Jessore, Barisal and Sunamgonj districts, potassium concentrations were less than 12 mg/L (Table 2).Humans need calcium (Ca) for strong bones and for a healthy nervous system, but too much calcium causes kidney stones and digestive problems (WHO, 2009;Verma et al., 2020), forms scale in pipeline (Saraswat et al., 2019).Two studies have found levels above the WHO standard for calcium concentrations in drinking water (200 mg/L); however, many areas have exceeded the BSTI standard (75 mg/L) (Table 2).The body requires magnesium for over 300 biochemical reactions (Jamal et al., 2020) and also helps maintain normal nerve and muscle function, and supports a healthy immune system (Solangi et al., 2019;Verma et al., 2020).Magnesium (Mg) concentrations were predominantly below the permissible limit (30-35 mg/L) compared to BSTI standards in the majority of the studied areas.On the other hand, Satkhira and Jessore districts reported excess magnesium based on WHO standards (150 mg/L) (Table 2).Chloride makes a salty taste in water and higher consumption may cause hypertension, stroke risk, renal stones, left ventricular hypertension and asthma in human body (McCarty, 2004).Water should have a chloride concentration of no more than 250 mg/L and 150-600 mg/L according to WHO and BSTI, respectively.In Satkhira district, 2,940.78± 1,563.53 mg/L chloride ion concentrations were recorded.Besides, excess content of chloride ion was reported in Gopalganj, Noakhali and Barisal (Table 2).Naturally, no more than 10 mg/L of nitrates are present in water, but anything greater indicates man-made pollution (Rao and Rao, 2010) and may be associated with different diseases, such as methemoglobinemia or blue baby syndrome, thyroid disease, gastric cancer and diabetes specially in pregnant women and bottle-fed children (Asamoah and Amorin, 2018;Kumar et al., 2015;Kumar and Puri, 2012).In Manikganj, the nitrate concentration was 253.18 mg/L, 25 times the permissible limit (Rahman et al., 2016a); however, nitrate concentration was below the standard limit in most of the areas (Table 2).Sulfate is formed by the oxidation of the ore and hydrogen sulfide (H2S) by a variety of bacteria including Rhodothiobacteria, Chlorothiobacteria, etc. (Mkadmi et al., 2018), whereas extra sulphate may cause diarrhea and gastro-intestinal irritation (Bashir et al., 2012;Marghade et al., 2012).All studies have recorded sulphate concentrations below WHO (250 mg/L) and BSTI (400 mg/L) standards.Another dominant ion is bicarbonate, however, no recommended intake level for it has been established (Hasan et al., 2019a).There is lowest bicarbonate concentration in Panchagarh district (0.02 mg/L), highest in Faridpur district (542.37 ± 130.71 mg/L) (Table 2).

Trace metals
Table 3 summarizes the trace metals in different parts of Bangladesh.Arsenic (As) is the 20th most abundant element in the earth's crust (Huq et al., 2020).For As, there are generally 4 oxidation forms: −3, 0, +3, and +5.As (III) and As (V) is the most durable form among those (Zhao et al., 2010).Groundwater may become contaminated by arsenic through industrial and natural processes (Safiuddin and Karim, 2001).Tolerable limit of As in Bangladesh is 0.05 mg/L and WHO established this value as 0.01 mg/L.Many study areas exceeded the BSTI standard, and at almost all study sites the As value exceeded the WHO standard.Das et al., (2009) collected groundwater samples from 50,515 tube-wells in 64 districts of Bangladesh.According to his report, As was found above 0.01 mg/L in 60 districts and above 0.05 mg/L in 50 districts (Das et al., 2009).Furthermore, Chakraborti (2010) and his group published a report on As after their 14 year survey in 338 Upazilla (sub-district) where 52,202 groundwater samples were analyzed.
In 197 Upazillas (sub-districts), concentrations were greater than 0.05 mg/L.15.8% samples were crossed the WHO standard and 7.1, 12.4, 4.3, 1.8, 1.0 and 0.6 percent samples fell in 0.05-0.1,0.1-0.29,0.3-0.49,0.5-0.69,0.7-1.0 and >1.0 mg/L ranges respectively (Chakraborti et al., 2010).In another study Chakraborti and his group survey, As concentration in four major geomorphological regions (Tableland, Flood Plain, Deltaic Region including Coastal region and Hill Tract) in Bangladesh (Chakraborti et al., 2015).In Flood Plain and Deltaic (including Coastal) regions, As value crossed in 10,085 and 6,932 samples out of 19,845; and 12,128 and 7,255 samples out of 22,113 based on WHO and BSTI standard, respectively, whereas in Tableland region, maximum samples and, in hill tract region, all the samples were below tolerable limit (Chakraborti et al., 2015).As affects the circulatory system, gastrointestinal tract, liver, kidneys, skin, nervous system and heart and results even death.Moreover, inorganic arsenic increases the cancer risk in human body (Ahmad et al., 2018;Smith et al., 2000;Tchounwou et al., 1999).Inhalation of hexavalent chromium (Cr-VI) can cause cancer of lung and stomach in human body (Smith and Steinmaus, 2009).Only one study reported excess chromium (>0.05 mg/L) in Narayanganj district (0.071 ± 0.03 mg/L) (Table 3).Iron (Fe) is an essential element in human nutrition, and is also important to good health because it transports oxygen in the blood (Kumar et al., 2010).Safe limit of Fe in drinking water is prescribed as 0.3 mg/L and 1.0 mg/L according to WHO and BSTI, respectively.It was reported that Fe concentrations were above the permissible limit in most study areas in the country, where Kurigram observed the highest concentration (16.6 mg/L) (Table 3).Of note, Fe can make oxidized taste in water and lead to stain clothes, discolor plumbing fixtures.In human body, excess Fe may lead to heart disease, liver problems, diabetes and organ dysfunction (Kohgo et al., 2008).
Manganese (Mn) occurs naturally in ores and rocks (Popoola et al., 2019).Maximum allowable limit of Mn in drinking water is 0.1 mg/L.Except for Dhaka, Panchagarh, Kushtia, and Patuakhali, all other locations (30 study sites) had Mn concentrations above the standard limit (Table 3).Excess amount of Mn affects nervous system, heart, liver and may cause cancer and pancreatic damage (Mukanyandwi et al., 2019).Humans require copper (Cu) and zinc (Zn) as essential nutrients like iron and manganese.But excess amount of these elements can also cause adverse health effects (Akoto and Adiyah, 2007;Javaid et al., 2008;Zahra et al., 2015).Permissible range of Cu and Zn are 1 mg/L and 5 mg/L, respectively; however, unfortunately content of Cu and Zn were observed in all places below acceptable limit (Table 3).Excess lead (Pb) can damage the brain, nerves, and kidneys of children.Pregnant women, infants and children are more susceptible to the toxic effect of Pb (Barua et al., 2016;Javaid et al., 2008).Tolerable limit of Pb in drinking water is 0.05 and 0.01 mg/L based on BSTI and WHO, respectively.Maximum Pb concentration was observed in Tangail (0.307 ± 0.15 mg/L).In terms of long-term environmental effects, cadmium (Cd) is another concern (Fernández-Luqueño et al., 2013).Cd can cause headaches, nausea, cough and vomiting even in a little dose intake (Burke et al., 2016).In addition, Cd may cause cardiovascular diseases, cancer, liver and kidney failure.Fortunately, most of the study sites in Bangladesh have recorded below the permissible limit of Cd based on WHO (0.001 mg/L) and BSTI (0.005 mg/L) standards.Surprisingly, only one report in Chuadanga district contains the highest Cd concentration (0.33 ± 0.14 mg/L) (Table 3).

Bacteriological contamination
Waterborne diseases are primarily caused by microbial contamination.In Bangladesh, it is undoubtedly shocking to see a high percentage of pathogenic microorganisms in groundwater that can lead to disease (Datta et al., 2014).Total coliform bacteria are very common in nature and a large group of bacteria are found.With some exceptions, most of them are harmless (USEPA, 2013).Bacteria known as faecal coliforms (FC) reside in the faeces, while Escherichia coli belong to FC (Karim et al., 2016).Escherichia coli and Fecal coliforms (FC) bacteria are important indicators detecting the level of health risk and potential water borne diseases in drinking water (Saha et al., 2019b).Presence of these microorganisms in water may cause several diseases like diarrhea, cholera, dysentery, gastroenteritis, typhoid fever, nausea, vomiting, headaches and fatigue (Okullo et al., 2017).Most study areas contained coliform bacteria and Escherichia coli was detected in Khulna, Barguna, Jessore and Narayangonj districts (Table 4).Islam et al., (2001) found the existence of microbial contamination in the deep way back at Chandpur district (Table 4).Also noteworthy, other groups reported bacterial presence in Jamalpur, Tangail, Netrokona, and Kishoreganj in 2018 as well as Rajshahi in 2020 and Narayunganj, Chittagong, Noakhali and Patuakhali in 2021(Table4).

Pesticide pollution
Throughout the world, pesticides are needed to control undesirable organisms like weeds, fungi, insects as well as to increase the yield of crops (Shammi et al., 2020).Due to the indiscriminate application of pesticides on land, groundwater can easily become contaminated by rain or runoff (Hasanuzzaman et al., 2017).The availability of data on pesticide contamination of groundwater in Bangladesh is very limited.This could be attributed to a lack of facilities (funding and laboratory) in the country (Hasan et al., 2019a).
Malathion was detected at extreme concentration which was 78 times greater than permissible limit in Rangpur.Fortunately, other pesticides such as DDT, DDE, DDD, Diazinon and Chloropyrifos were not detected at all (Ara et al., 2014).Like Rangpur, malathion was detected in Dhaka but below the acceptable range (Hasanuzzaman et al., 2017).However, in 1997, 0.0015 mg/L DDT was observed in Nayarhat, Dhaka and that was the breakthrough in the groundwater of Bangladesh and the value is slightly higher than tolerable limit (0.001 mg/L) (Rahman, 1997).Next year (1998), trace amounts of DDT were found at the same location (Matin et al., 1998).Matin et al., (1998) collected 144 groundwater samples and found DDT concentrations ranging from 0.051 to 1.653 g/L and heptachlor concentrations between 0.025 and 0.789 g/L.As for DDT, all samples were lower than the permissible limit, and as for heptachlor except only one sample all were above the limit (Matin et al., 1998).Matin and his group concluded that the groundwater was not contaminated by pesticides (Matin et al., 1998).

Human health risk assessment of trace metals in groundwater
When significant amounts of metals containing water are ingested, it may cause health effects ranging from cancer to non-cancer (Karim, 2011;Kavcar et al., 2009).Several villages in Pabna, Kushtia, Chuadanga, Meherpur, and Jessore districts were surveyed by Chakraborti et al., (2015) presented in table 6.During 1996-1999, some people with arsenical skin lesions and drinking highly arsenic contaminated water from local hand tube wells were selected.In 2009, they observed that 15.66, 15.0, 27.40, 17.51 and 15.77 percent of their registered patients died in respective districts (Pabna, Kushtia, Chuadanga, Meherpur, and Jessore).In another study, 70 spontaneous abortions, 48 stillbirths, and 67 neonatal deaths per 1000 live births were observed (Milton et al., 2005).Yunus and his team published a review on the impact of As on health in Bangladesh that reported cancer-causing and non-cancer effects (Yunus et al., 2016).Both carcinogenic and non-carcinogenic effects of As were observed in Satkhira district (Rahman et al., 2019).Rangpur and Gopalganj districts have carcinogenic effects for As and non-carcinogenic effects for Mn, As, Fe, barium (Ba), Zn, and aluminium (Al); and for As, Fe, Mn, and boron (B), respectively (Rahman et al., 2018a;Islam et al., 2019).The Haripur gas blowout area of Sylhet district presents a cancer risk for Pb, Cd, and nickel (Ni) (Howladar et al., 2021).In addition, another study was conducted in Sylhet district, where 11 Upazilla were considered under Surma basin, and carcinogenic health risk was determined for As.Non-carcinogenic effect was also observed due to As, Mn, Fe and nitrate (NO3 -) (Bodrud-Doza et al., 2019b).The presence of cadmium and manganese in the groundwater did not lead to cancer risk in Chuadanga District, where Pb was the only carcinogen (Bodrud-Doza et al., 2019b).Furthermore, in Dhaka city, non-carcinogenic health effects were found for metals (Fe, Mn and Zn) and anions (F and NO3 -) (Bodrud-Doza et al., 2020).In Khulna and Jessore districts, Fe and Mn played a key role in non-carcinogenic health risks (Ghosh et al., 2020;Hossain and Hassan, 2020).

Geogenic and anthropogenic factors
Groundwater contamination sources are represented in a scheme (Figure 2).There are typically two factors that contribute to groundwater contamination: geological and anthropogenic (Islam et al., 2020a).
Various natural or geogenic processes such as evaporation, mineral dissolution, precipitation of secondary minerals, cation and anion exchange, redox reactions, microbial processes, erosion, ore formation, weathering of rocks and mixing of waters influence groundwater quality (Bodrud-Doza et al., 2019a;Bodrud-Doza et al., 2019b;Wu et al., 2016).The contamination of groundwater is exacerbated by anthropogenic sources, including raw sewage, urban waste, medical waste, mining, smelting, wastewater treatment, industrial effluent, and agricultural activity (Bodrud-Doza et al., 2020;Bodrud-Doza et al., 2019b;Hasan et al., 2019b;Saha and Rahman, 2020).In terms of bacterial contamination, nearby latrines and septic tanks may be responsible for infiltrating wastewater into the tube well.Moreover, seepage of polluted surface water through leaky tube well seals is a risk factor.Drinking water also becomes contaminated by secondary microbial organisms at the time of collection, handling, and storage in households (Dey et al., 2017;Rahman et al., 2019b).Several researchers used principal component analysis (PCA) and cluster analysis (CA) to identify pollution sources in Bangladesh that are both geogenic and anthropogenic in origin (Bhuiyan et al., 2016;Bodrud-Doza et al., 2016;Bodrud-Doza et al., 2019b;Bodrud-Doza et al., 2020;Rahman et al., 2018a;Rifat et al., 2021;Islam et al., 2017b).

Geographical factors
Geographically, Bangladesh is located between 20 o 34'N and 26 o 38'N; and 88 o 01'E and 92 o 41'E (Shahid et al., 2006).The Bengal Delta Plain (BDP) is one of the largest deltas in the world, formed by sediment deposition carried by the Ganges, Meghna and Brahmaputra (GMB) rivers (Mukherjee and Bhattacharya, 2001).There are two different mechanisms of contamination by arsenic (As) in the Ganges delta plain: (i) The lowering of the water table allows arsenic-rich pyrite to oxidize and enter the groundwater (ii) Sedimentary geology reduces iron oxyhydroxide by organic matter (Anawar et al., 2003).Arsenic was also detected in Bangladesh due to its fluvial-sedimentary history (Tareq et al., 2003).Fe and Mn are also naturally generating metals found in minerals and rocks in an insoluble form.The occurrence of Fe in water is due to the weathering of iron-rich rocks or the interaction of rock with water (Islam et al., 2017d).
Moreover, southern part of the Bangladesh is located on the bank of the Bay of Bengal (Mukherjee and Bhattacharya, 2001).Due to the infiltration of seawater into the coastal area, it has a high potential of contaminating groundwater (Rahman et al., 2011), which was observed in different individual studies (Naus et al., 2019).Furthermore, as the ship breaking industries were located close to the sea, wastes release from ship breaking industries severely threaten for water contamination in southern part of Bangladesh (Kutub et al., 2017;Patwary and Bartlett, 2019).Polluted groundwater has been observed in Chittagong's ship breaking industrial area (Hasan et al., 2013).

Industrialization
Industrialization can also affect groundwater quality by discharging toxic effluents (Hossain et al., 2020).Dhaka, Chittagong, Gazipur, Khulna, Sylhetand Narsingdi are the main industrial city of Bangladesh.Noticeably, industries are normally established on the bank of river or sea; therefore, industrial effluents are easily released into the river or sea.For instance, a survey showed that river water around Dhaka is highly contaminated and 60% of that contamination occurred due to industrial effluent (Ahsan, 2019).
Polluted seawater and river water can infiltrate into the groundwater of industrial areas (Arefin and Mallik, 2017).

Depth of groundwater level
The BNDWQS 2009 survey covered both shallow (<150 m) and deep (>150 m) tubes wells, and it found that metal concentrations of deep tubes wells were lower than shallow tubes wells.According to BGS and DPHE (2001), shallow wells had higher Mn concentrations than deep wells.Deep wells are also relatively free of arsenic pollution (Hasan and Ali, 2010).According to Mukherjee and Bhattacharya (2001), the concentration of arsenic decreases with depth (Mukherjee and Bhattacharya, 2001;Tareq et al., 2003).A low amount of arsenic was detected in shallow wells, while a high amount of arsenic (>450 m, 0.25 mg/L, and >375 m, 0.37 mg/L) was detected in deep wells (Tareq et al., 2003).Fluvial-sedimentary history may have caused it (Tareq et al., 2003).To add, a study by Parvez and his co-authors found coliform bacteria in 81.2% of tube well samples (<140 feet) and 0% of deep tube well samples (>300 feet) from 37 Bangladeshi districts (Parvez et al., 2016).

Synthesis of the existing scenario and future recommendation
There are several physical, chemical, and even trace-metal pollutants in groundwater in coastal areas.High salinity is observed in coastal districts (Akter et al., 2016;Islam et al., 2017a;Rahman et al. 2018a;Sarker et al., 2020).TDS and EC were found to be proportional to salinity (Mahmud et al., 2020).Moreover, the groundwater surrounding ship dismantling and industrial areas was found to be contaminated by trace metals (Chowdhury and Ahmed, 2019;Hasan et al., 2013;Hiroshiro et al., 2009;Rahman et al., 2020b;Islam et al., 2017d).As a result, this review suggests policy makers and the relevant authorities should focus on the coastal zone and industrial zone for sustainable management.The most common trace metal contamination problem in Bangladesh is arsenic (As).Iron has a correlation with arsenic, where iron concentrations increase with an increase in arsenic levels (Ahmed et al., 2010a;Bhuiyan et al., 2016;Bibi et al., 2008;Halim et al., 2009;Islam et al., 2017d).Most of the study areas had excessive concentrations of Fe and Mn.Conversely, vital nutrients such as Cu and Zn remained below the prescribed levels throughout the country.Several studies found bacterial contamination in very few locations and found it to be alarming.However, extensive investigations on microbial contamination were missing.Because the investigations of pesticides in groundwater are very limited, concluding remarks are impossible.There have been no reports of residues of pesticides in groundwater in the past few years except for Rangpur (Ara et al., 2014) and Dhaka (Hasanuzzaman et al., 2017).Despite being banned, DDT is still available illegally in the market in Bangladesh (Rahman et al., 2019a;Shammi et al., 2017).The health risks were also assessed, but only at a few places.
On the whole, the quality of groundwater has been assessed in 55 districts out of 64 in Bangladesh.Major concern is that most studies looked at the contents of physico-chemicals parameters or trace metals only or both of them.There would be a need for background investigations in Gazipur, Madaripur, Narshingdi, Kishoreganj, Feni, Lalmonirhat, Meherpur, Jhalokati, Habiganj districts.Importantly, future studies should analyze the detection of microorganisms and pesticides against the water height.An assessment of health risks from groundwater is warranted, especially in the untouched area.This review suggests comprehensive research on another untouched area: exposure to mixed metals/metaloids in residents (urine/blood) living in an industrial zone in Bangladesh.Further, the regions in which ingestion of water caused cancer risk such as (Satkhira, Sylhet, Gopalganj, Rangpur and Chuadanga districts) to the residents should be re-evaluated by comparing the metal content in the groundwater to the bio-fluids.Finally, contaminants of emerging concern (CESs) such as bisphenol A (BPA), nonylphenol (NP), benzophenones (BPs), and benzotriazole (BT); disinfectant by products (DBP), pharmaceuticals, pe-and polyfluoroalkyl substances (PFAS) need to be addressed in future study for sustainable management of groundwater.

Conclusion
This paper on Bangladeshi groundwater pollution is the first ever to focus on this topic.There are various contaminants polluting the groundwater in most parts of Bangladesh.Coastal as well as industrial zone of Bangladesh is worrying.Among the trace metals, As content shows alarming situation throughout the country.Besides, Fe and Mn also observed in most of the areas.Moreover, Pb, Cd and Cr were found in few studies only and Cu, Mn and Zn were detected below the acceptable limit.Major cations and anions were found below permissible limit throughout the country.Carcinogenic and non-carcinogenic effects were observed at Dhaka, Gopalganj, Rangpur, Sylhet, Chuadanga, Jessore, Satkhira and Khulna districts.Geogenesis, anthropogenic activities, changes in water height, unplanned industrialization, and geography all influence groundwater pollution.We recommend that policy makers and appropriate authorities should take proper measures to protect groundwater, i.e., monitoring groundwater pollution and implementing laws-using adequate human resources, installing modern treatment and supply systems, educating the public about water usage and safety.

Table 4 :
Summary of microbiological data within Bangladesh.(Mean values with standard deviations were represented)

Table 5 :
Summarized data of pesticides content within Bangladesh (Mean values were represented)

Table 6 :
Arsenic (As) contamination effect in different districts (Chakraborti et al., 2015) Sl.No. Location Registration year Number of patients registered Died before 2009 % of death