Influence of chemical fertilizers on arsenic mobilization in the alluvial Bengal delta plain: a critical review

Two Himalayan rivers, the Ganges and Brahmaputra, fall into the Bay of Bengal as a collective river transporting the largest sediment load. These rivers join in the central part of Bangladesh, with one more non-Himalayan River, the Meghna, which has created the largest delta in the world known as the Bengal delta. The Ganges–Brahmaputra River systems carry the largest sediment load in the world, about 80% of which is transported during the four rainy months (Goodbred & Kuehl 2000). More than 200 rivers and streams run through Bangladesh, with a mean annual discharge of water of about 38,000 m³/s (Haque et al. 2016), carrying over 2.4 × 10⁹ MT of sediments every year before discharging into the Bay of Bengal (Talchabhadel et al. 2018). The huge alluvial sediment is carried away and deposited in basin areas and finally creates a large arable region in the world. Now this region has become the largest arsenic-affected area of the world.

Bangladesh is a densely populated (2,890/mile², positioned tenth in the world) and agrarian country. Almost 90% of its lands are cultivable and 55% of the inhabitants are engaged directly in the agricultural sector (BBS 2019). Presently, the application of chemical fertilizers has risen above 1,000% compared to the 1950s and has become a great threat to the environment and human health (Faruq 2018). The hydraulic conductivity of the deltaic alluvial and sandy land is sufficiently high and the residual portion of fertilizers can easily penetrate the topsoil by leaching. For this reason, it can be assumed that through various chemical conversions, some water quality parameters such as pH, NO₃, NO₂, PO₄, NH₄, K, Ca, etc. may increase in the sub-surface water. Several studies have shown that those parameters could influence the release of arsenic from arsenic-rich sediment in aquifers (Anawar et al. 2003; Uddin & Kurosawa 2010; Kurosawa et al. 2013).

The groundwater resource is the key factor for agricultural production in this fertile delta basin. There are 97% rural and 85% of urban people who directly use raw groundwater for drinking and other household purposes in Bangladesh (Mostafa et al. 2017; BBS 2018; MICS-B 2018). But this resource is under increasing threat from over-exploitation, population growth, rapid urbanization, and pollution from industries, domestic, and agricultural sources. Arsenic pollution is one of Bangladesh's most severe environmental problems. According to the WHO, about 35–77 million people were regularly exposed to arsenic (≤50 μg/L) via drinking groundwater, and that was marked as the largest mass poisoning in history (Flanagan et al. 2012; Radfard et al. 2019; Saleh et al. 2019). The major source of arsenic in Bangladesh is geogenic since it is contained in the sediments of the shallow Holocene aquifers of the Ganges delta basin (Ravenscroft et al. 2005). Millions of shallow tube-wells were drilled in this zone to supply drinking water, but the water was contaminated with a higher concentration of arsenic. Arsenic causes harm to the human body with various carcinogenic and non-carcinogenic problems. So far, approximately 40 thousand arsenicosis patients have been identified in Bangladesh (Johnston & Motaleb 2007).

Several studies have been conducted on groundwater arsenic contamination in Bangladesh and neighboring West Bengal of India (Acharyya et al. 2000; Ravenscroft et al. 2001; UNICEF 2001; Chakraborti et al. 2010; CSISA-MI 2015; MICS-B 2018; World Bank Group 2019). Numerous studies, not only in Bangladesh but also in other Asian countries, have been conducted, including China (Sun 2004; Jiang et al. 2019; Sanjrani et al. 2019), India (Paul et al. 2015; Chandrashekhar et al. 2016; Shaji et al. 2020; Alsubih et al. 2021), Vietnam (Glodowska et al. 2020), and Japan (Hossain et al. 2016; Vongphuthone et al. 2017), where arsenic contamination in the environment reached an alarming position. However, the results have not produced the mitigation of arsenic poisoning. The mechanisms through which arsenic is released from soil or sediment into groundwater are still unknown and debatable. There are some hypotheses about the common mechanism of arsenic release in groundwater, i.e., the oxidation of arsenic-rich pyrite in the sediment, the reduction of iron oxo-hydroxide (FeOOH), and desorbed arsenic from the sediment particles, and the ion exchange of adsorbed arsenic with phosphate from fertilizers. The bacterial activity in sediment was considered a driving factor to create reducing the environment in anoxic groundwater through oxidation of dissolved organic matter to the above mechanisms (Wang & Mulligan 2006). Further, soil conditions, pH, Eh, NO₃, HCO₃, SO₄, and NH₄ of sediment and groundwater are considered to be influencing factors along with the above concept. Acharyya et al. (1999, 2000), Anawar et al. (2003), and Brömssen et al. (2014) reported that agricultural fertilizers may encourage arsenic mobilization by ion-exchange with P and N bearing ions resulting from fertilizers. According to the lab experiments of Uddin & Kurosawa (2010) and Mahin et al. (2008), the NO₃ and PO₄ concentrations in sediment and groundwater were high in an arsenic-affected area in Bangladesh, and the source of these ions would be inorganic fertilizers. The mode of incidence and mobility of arsenic in sedimentary aquifers may be controlled by a complex interaction of microbially facilitated reactions and hydro-geochemical processes sensitive to site-specific hydrology and sediment type, as well as anthropogenic activities, such as application of chemical manure. Several studies were conducted in the last few decades but failed to explore a clear concept on the mechanisms of arsenic mobilization in the Ganges, Brahmaputra, and Meghna (GBM) delta aquifers and the role of chemical fertilizer in mobilization are not yet clearly understood. Hence, critical review studies are imperative to explore the geochemical mechanism controlling arsenic mobilization in the Bengal delta area. The study aimed to investigate the impact of chemical fertilizer on arsenic mobilization in the sedimentary deposition of the GBM delta system.

The application of chemical fertilizer is the major anthropogenic cause of arsenic contamination in groundwater of the Bengal delta plain (BDP).

• 1.

  What is the country and district wise pooled concentration of arsenic in groundwater and what districts are highly affected or relatively less affected?

• 2.

  What is the correlation of arsenic concentration with pH, nitrate, ammonium, phosphate, sulfate, calcium, and potassium in groundwater and how is their presence in the aquifer system due to the use of chemical fertilizer?

• 3.

  What is the role of those parameters in the mechanism of arsenic mobilization at certain lithological conditions?

• 4.

  Is the chemical fertilizer the main anthropogenic cause of high arsenic concentration in groundwater?

The mechanisms and influencing factors of arsenic mobilization from sediment to groundwater have been investigated worldwide since the 1980s, but a significant difference of those mechanisms between the papers before 2012 and after that time was observed. Thus, reviewed papers were divided into two groups: before and after 2012. Initially, the study identified over 150 published papers on arsenic contamination in aquifers but finally included 60 articles, 42 before 2012 and 18 after 2012. Here, the study follows the PRISMA statement (Moher et al. 2009) for every event of inclusion and exclusion criteria.

Bangladesh has 19 administrative districts (old). The study pooled the district-wise value/concentration of pH, NO₃, NH₄, PO₄, SO₄, Ca, and K of groundwater from secondary data and the pooled values of those parameters are included in Table 1. The study assumed that these ion concentrations were increased by inorganic fertilizer leaching and influenced the arsenic dissolution process in groundwater aquifers along with the geological processes. At the same time, the district-wise concentration of arsenic is shown in Table 1. It shows the build-up of correlation/association between those parameters with arsenic concentration. Also, the study selected the ten most arsenic-affected districts out of a total of 64 (new), and their soil physiography with other necessary data is shown later. Subsequently, Table 1 acts as referral material to find out the actual mechanism of arsenic mobilization in sediment and/or water body of the BDP. In addition, the two least affected divisions, Rajshahi and Rangpur of Bangladesh, are considered in the research hypothesis. For any statistical measurement in this study, XLSTAT and SPSS software was applied.

Tropical monsoon is a vital climatic feature of the BDP and has humic meteorological conditions. Average yearly rainfall is lowest in the north-west (1,430 mm) and increases both to the south-east (2,745 mm) and the north-east (4,178 mm). Despite the heavy rainfall, about 90% of river flows in Bangladesh originate in India, Nepal, and China, and those create a widespread floodplain. The Bengal delta, situated in Bangladesh, is the largest alluvial sedimentary basement of the world. It is typically made up of tertiary and quaternary deposits (Goodbred & Kuehl 2000). Sediment deposited on the Ganges–Brahmaputra river has created one of the largest deltas in the world (Mukherjee & Bhattacharya 2001). The minerals of these areas are dominated by detritus quartz and feldspar; and the main mineralization processes are Fe(OH)₃ reduction, silicate weathering, and mixing with salty water (Mukherjee et al. 2009).

The country has promising hydrogeologic conditions and tropical monsoon weather indicates potential storage of groundwater in aquifers. The unconsolidated near-surface Pleistocene to recent estuarine and fluvial sediments underlying most of the country usually form productive aquifers and deep semi-contracted to the uncontracted fluvial-deltaic deposit of Miocene age to the recent forms of many aquifers. The floodplains of the active/inactive delta plain and the major rivers of the GBM delta complex occupy 82% of the country (Figure 1) (BGS-DPHE 2001). The existing sub-surface geological studies indicate that the bulk of the good aquifers occur between 25 and 130 m depth but the upper portion of this layer is at high risk for arsenic contamination. The Pleistocene aquifers are typically free of arsenic, but this is not true for the Holocene aquifers, which are rich in arsenic concentration. The highest arsenic was detected in those Holocene aquifers, which are roughly 3,000 years old (Dowling et al. 2002). The water of the upper Holocene aquifers is nearly 100 years old and holds less arsenic concentration. However, the Holocene deposits are not homogenous and steady and are characterized by gaps and holes allowing the vertical extension of arsenic contamination (BGS-BGS 2001). This explains the marked depth-dependence of the arsenic concentration. The maximum concentrations are found at 20–55 m, which correspond to young and shallow aquifers and the depth of aquifers is not enough to measure for waters being free of arsenic. In general, deeper aquifers (>120 m) produce groundwater with arsenic levels below the WHO standard of 10 μg/L (Bhattacharya et al. 2000). Arsenic is transported to the BDP by the phases adsorbed on suspended particulates because of the oxidation of primary arsenic-sulfide minerals. It is typically adsorbed and co-precipitated with secondary Fe, Mn, and Al, solid phases in the form of arsenic (+5) and finally deposited in the delta depending on the topography of the basin (Nickson et al. 1998, 2000). One of the problems met in Bangladesh is the extreme inconsistency in the groundwater arsenic content between boreholes only 100 m away from each other (van Geen et al. 2011).

Based on chemical fertilizer consumption in the agricultural sector, Bangladesh is placed 18th out of 86 of the most worldwide consumer countries and 7th out of 32 Asian consumer countries (Index-mundi 2020). Fertilizer consumption (% of fertilizer production) in Bangladesh was 392.38 from 2016 to 2002. Its highest value over the past 14 years was 476.95 in 2015, while its lowest value was 121.21 in 2002. On the other hand, 175 kg/hectare chemical fertilizer was used in 2002 and this amount increased by 300 kg/hectare in 2016, but worldwide fertilizer consumption decreased by 2% during this same period (Figure 2) (WB 2017). The description of the most used fertilizers in Bangladesh is presented in Table 2. Some raw materials of fertilizer production contain arsenic minerals, and those types of fertilizer contribute to the arsenic in the soil, which then leaches to groundwater. For example, phosphate fertilizers are possible sources of arsenic. The concentration of arsenic in the fertilizer will vary with the source rock for phosphate to produce the fertilizer. The arsenic content of phosphate fertilizer, widely used by farmers in Bangladesh, ranges from 2.42 to 6.79 mg/kg (Mahin et al. 2008; Uddin & Kurosawa 2010). Campos (2002) reported that NPK fertilizer carries 6.12 ± 0.04 mg/kg arsenic. Thus, chemical methods contribute to arsenic in the soil system as well as in the groundwater body. Chemical fertilizers that contain toxic pollutants or species leach through the soils and enter into groundwater and can increase the arsenic mobilization in aquifers (Table 2).

Arsenic, a category-1 carcinogenic element (Driscoll et al. 2000), occurs naturally in groundwater supplies through all parts of south-east Asia. About 25% of water wells of shallow aquifers in the Ganges basin of Bangladesh and West Bengal of India are severely contaminated by naturally occurring arsenic (Ravenscroft 2007a, 2007b). It was first recognized as a thought-provoking problem in Bangladesh in the 1990s. The arsenic concentration in most of the areas exceeded the maximum level of 10 μg/L suggested by the WHO. Over 75 million people, from 59 out of 64 districts, were thought to be at risk of drinking water contaminated by arsenic in Bangladesh (Safiuddin 2011). Every year, an assessed 43,000 people are affected by arsenic poisoning in the country (Jahan 2016). The authority has taken several steps and made strategies to try to address the calamity. Nevertheless, a countrywide movement and social mobilization activities by the government and NGOs to raise consciousness and knowledge levels among the public remain far below expectations. Several studies have been conducted in Southeast Asia and other countries of the world, including Bangladesh, but arsenic mobilization towards groundwater aquifers is not well understood. The complete scenario of the countrywide arsenic level of groundwater is summarized in Table 3.

The mobility of arsenic in soils to groundwater depends on several factors including soil mineralogy, redox potential, pH, Eh, and the presence of other components that compete with arsenic for soil retention sites. Arsenic contamination in aquifers can arise both naturally and anthropogenically and the method of the mechanism of arsenic mobilization be completely regional. This contamination in Bangladesh arises because of an unfortunate combination of four natural aspects: a source of arsenic (arsenic present in the aquifer sediment), adsorption of arsenic (adsorbed with metal in sediment), mobilization (arsenic release from the sediment to neighboring water), and transport (arsenic circulating in the groundwater) (Ravenscroft et al. 2005; Mostafa et al. 2010). In Bangladesh, two prevailing hypotheses expressing the mobilization of arsenic into groundwater are the oxidation of pyrite and the reduction of metal oxy-hydroxide (Mostafa et al. 2011). A study performed in Bangladesh showed a decline in groundwater level at 0.1–0.5 m year⁻¹, indicating a reduction of the storage in aquifer resulting from the unsustainable withdrawal of groundwater for both irrigation and domestic use (Zahid 2015). The over-extraction of groundwater and extensive application of inorganic fertilizer and arsenic-rich pesticides were the main suspects for the anthropogenic cause of arsenic contamination in Bangladesh, but recent investigations strongly oppose this thought by respecting the facts that are discussed below.

• 1.
  Oxidation of pyrite: Several studies conducted in the last two decades illustrated that iron-pyrite and arsenopyrite in the aquifer sediments oxidized to As³⁺ and As⁵⁺, and were released into the groundwater aquifer (Chowdhury et al. 1999; Mazumder et al. 2000; Nickson et al. 2000; Mukherjee & Bhattacharya 2001; Ravenscroft et al. 2001; Hasan et al. 2007; Das et al. 2015). Furthermore, the oxidation might have occurred due to the entry of atmospheric oxygen into the aquifers during groundwater extraction through deep and shallow tube-wells.

  

  

• The rationality of this hypothesis has been keenly criticized by researchers since the groundwater in this region is: (i) anoxic type, with negligible dissolved oxygen, (ii) near-neutral pH or moderate alkaline, (iii) categorized by low SO₄ concentrations with no relationship to arsenic, and (iv) characterized by very low concentration of NO₃ (Ravenscroft et al. 2001; Pal et al. 2009). This mechanism was postponed at that time, and the following processes demonstrated later.

• 2.

  FeOOH reduction: Bacterial metabolism of dissolved organic matter in the sub-surface leading to anoxic conditions, thus leading to reduction of lepidocrocite mineral, FeO(OH), and then resulting in the release of adsorbed arsenic into the groundwater (McArthur et al. 2004; Swartz et al. 2004; Harvey et al. 2005).

• 3.

  Redox process: Arsenic is released by the reductive dissolution of arsenic-rich FeO(OH), and this is a redox process that occurs after microbial oxidation of organic matter has consumed DO, NO₃, and NH₄ (as the nutrient of microorganisms) (Zheng et al. 2004; Harvey et al. 2006; Mukherjee et al. 2006; Pal et al. 2009).

• 4.

  Ion exchange: Arsenate anion adsorbed to aquifer minerals is being displaced into the aqueous medium by a competitive exchange of phosphate anion resulting in arsenic contamination in groundwater (Mahin et al. 2008; Anawar et al. 2013; Shankar et al. 2014).

The above mechanisms of 2, 3, and 4 were established after the 2000s. According to recently published papers, the availability of dissolved organic matter, sufficient microbial community, an abundance of highly positive charge baring metal-minerals, high phosphate loads, and low Eh with moderately high pH in sediment or aquifer are the major criteria to boost arsenic concentration in a shallow groundwater system. Rather than anthropogenic causes, it is assumed that the geogenic condition is a major cause for arsenic mobilization in the study regions. Leached chemical species such as nitrate, phosphate, and ammonium, which originate from inorganic fertilizer, can affect the above mechanisms 2, 3, and 4. Also, due to the application of fertilizer, pH and Eh may change indirectly in sediment. How the arsenic adsorption-desorption mechanisms in the Bengal delta aquifer are affected by chemical fertilizers are critically discussed in the next sections.

About 85% of irrigation and drinking water is being extracted from shallow aquifers in Bangladesh (BGS 2017). These aquifers mainly consist of clayey, silt-clay, or sandy sediment. Several studies have confirmed that aquifers enriched in Fe-, Mn-, Al-oxides, PO₄, NH₄, and NO₃ with organic matter and deposited in the middle part of shallow aquifers contain moderately high concentrations of arsenic, whereas the sediments of deep aquifers contain a low level of arsenic (Nath et al. 2008; Anawar et al. 2011; Uddin et al. 2011; Mohammadi et al. 2020). Among the total 64 administrative Districts (new) of Bangladesh, over 60% of groundwater samples of ten Districts exceed the national arsenic standard concentration of 50 μg/L (Table 3), and those Districts are considered as reference sites of the present study. These study sites represent three of five major geological units of the GBM delta: young Holocene terrace, fluvial flood plain, and delta plain (BBS 2018). The soil physiography and land types of most of the vulnerable areas are of relatively young sediment formation (∼3,000 years old) and these follow the order clayey > silt-clay > silt-loam, and most of the areas are river basins with a low-lying area having clay-muddy land (Table 4). Thus, it can be seen that high arsenic levels in groundwater or sediment are often associated with geologically young sediments (Holocene aquifer) and flat low-lying zones where groundwater flow is slow. Such conditions favor the reductive dissolution of iron oxides and/or desorption of arsenic from metal oxides associated with sediments. Several components are leached into the soil through the decomposition of chemical fertilizers. For this reason, the concentration of NO₃, NH₄, PO₄, SO₄, Ca, and K in shallow groundwater is found in elevated levels because of fertilizers used in agriculture to increase crop yields. In Bangladesh, silt-clay, clayey, peaty-clay, and peat sediments are mainly located within 10–15 m of the surface, followed by sandy sediment going deeper. This part of the soil layer is typically important to desorption of arsenic and around this layer the impacting factors are applying their actions (Uddin & Kurosawa 2010). Stollenwerk et al. (2007), Mahin et al. (2008), Uddin & Kurosawa (2010), and Anawar et al. (2011) have shown by their experiments conducted in a separate region of the country that the N and P containing ions generated from nitrogen and phosphate fertilizer accumulate mainly in peat or clayey zone of the soil. They also found the concentration of arsenic in this part of the soil profile is significantly higher, and the arsenic loads are strongly correlated with those ions.

The present study and previous reports (e.g., Sutton et al. 2009) show that the groundwater in the upper aquifers of the Holocene delta is characterized by high levels of As, Fe, PO₄, NH₄, NO₃, K, total organic carbon with a high pH, EC, and low value of Eh, whereas the deeper aquifers in the Pleistocene terrace deposits are characterized by a low level of arsenic, dissolved carbon, NH₄, Fe, and PO₄, along with a relatively low pH and EC. Like the present study, many prior investigations have noticed similar occurrences of high and low arsenic concentrations in groundwater depending on the depth of the aquifer and the redox conditions in the different areas of Bangladesh (Harvey et al. 2006; Van Geen et al. 2011). Arsenic was inversely correlated with Eh values in the upper aquifer, whereas no relationship was found in the deeper aquifer (Zheng et al. 2004; McArthur 2018). The results suggested that arsenic mobilization was linked to the improvement of reducing conditions.

Here, the study developed a linear correlation matrix of arsenic with such parameters as pH, NO₃, NH₄, PO₄, SO₄, Ca, and K in shallow aquifer water of Bangladesh, and this is shown in Table 5. The results indicated that the values of the correlation coefficient (r) are about 0.5 or higher, and the concentration of these ions is strongly correlated with arsenic at p= <0.05, except for the pH. As well, an inter-component correlation was measured among the eight different variables through the principal component analysis (PCA) method (Table 6). The result showed a total variance of 65.35% and 17.159% for PC1 and PC2, respectively, with an Eigenvalue >1, as determined by two PCs of R-mode. The positive and negative values in PCA clarified that the water samples were affected or unaffected by the presence of extracted loads on a specific constituent. Except for the pH, PO₄, NO₃, As, K, Ca, and NH₄ showed a strong association (bold font) with PC1. However, only NH₄ and pH are strongly loaded for PC2. Again, this result confirmed the strong association among these parameters that revealed the inter-dependency with each other. Hence, the study assumed that the above-mentioned ions were responsible for the desorption of excess arsenic from its minerals into groundwater. However, various factors such as Fe, Al, and Mn load along with Eh, organic carbon, and microbial activities in the sediment can affect this process and are discussed earlier. The mechanism of influencing the above parameters (generated through fertilizer leaching) on arsenic dissolution is discussed below.

Arsenic mobility in groundwater is a function of P concentration. The pooled concentration of PO₄ in a countrywide shallow aquifer is shown in Table 1. A strong positive correlation (r= 0.69; p= <0.05 at 95% CI) between arsenic and PO₄ was observed according to Figure 3. Nonetheless, the actual r value is greater than this observed value because some portion of free PO₄ in groundwater can be settled out by metal-PO₄ precipitation, but did not occur for arsenic. Campos (2002) and Anawar et al. (2011) conducted studies in a region of Brazil and Bangladesh, respectively, and they also found the same correlation between arsenic and PO₄ in the groundwater. Under certain conditions (high pH and/or low iron and Eh), it may increase arsenic solubility in soils contaminated with P-containing fertilizers because of competitive PO₄-AsO₄ exchange (Roberts et al. 2004; Tyrovola et al. 2006). An experimental study has shown that a 0.1M KH₂PO₄ solution can leach as much as 150 mg/kg of arsenic from Fe-coated sediments (Loeppert et al. 2002). Most likely, arsenic (+5) was desorbed from the surfaces of the Fe coatings and replaced by the sorption of HPO₄²⁻ or H₂PO₄⁻. The elemental P and As are placed in the same group of the periodic table, and these have similar electronic configuration and properties. Besides, phosphates and arsenates showed almost a similar physicochemical behavior in soils. These two ions compete for sorption sites on the sediment or soil granules. Phosphate may significantly reduce the adsorption of arsenic because it typically presents greater concentrations than arsenates. Also, the charge density of PO₄ is higher than arsenate and for this reason, PO₄ has a greater attraction to highly positive Fe, Al, and/or Mn ions. Thus, it can easily replace the adsorbed arsenate from these metal mineral/ore surfaces. Manning & Goldberg (1997) stated that phosphorus and arsenic adsorption envelopes exhibited a similar adsorption at pH 5.0 on kaolinite minerals and failed suddenly at pH > 6.5. The transport of PO₄ in the sub-surface probably depends on the physical and chemical conditions and depth of the aquifer (Stollenwerk et al. 2007). For instance, higher pH in groundwater causes decreased sorption of PO₄ on aquifer sediments. Reza et al. (2010) and Uddin & Kurosawa (2010) confirmed that if PO₄ was coming from fertilizers, its concentration decreased as the soil depth increased. The capacity of the sub-surface soils to adsorb PO₄ applied as fertilizers depends, in part, on the PO₄ load applied (Uddin & Kurosawa 2010).

From the above discussion, it may be concluded that in the ten most arsenic-affected Districts of Bangladesh, phosphates fertilizer may be one of the major causes of heavy arsenic loads in groundwater. It is not the only cause but also depends on local soil characteristics. If the soil is peaty, muddy, or clayey, then it can retain the phosphates, otherwise not. Sandy soil cannot adsorb this ion. This argument was supported by several reports (Reza et al. 2010; Uddin & Kurosawa 2010). The soils of the other two Divisions, Rajshahi and Rangpur of Bangladesh, are sandy/sandy-loam (Table 7). Despite relatively more application of chemical fertilizer in both these divisions, they are much less affected by arsenic. In these areas, phosphate-fertilizer did not accumulate to the upper layer of the soil and cannot participate in the process of arsenic mobilization. Reza et al. (2010) also created a plot with r =0.7, p < 0.05 for the same GBM floodplain areas, which means high arsenic concentrations are associated with the higher range of phosphate concentration. Phosphorus is able to replace arsenic in soils and sediments, and thus the groundwater of the areas can have a higher possibility of contamination with arsenic. Another report showed that phosphate-fertilizer uses lower amounts in the ten highly arsenic-affected Districts of Bangladesh (BBS 2018). Therefore, it can be said that the geogenic conditions are the primary and vital cause of arsenic contamination in the groundwater of the Bengal delta plain rather than phosphate concentration.

It is well known that nitrate may be engaged in numerous microbe-mediated anoxic redox reactions in arsenic-rich soils and aquifers. Nitrate can reduce into nitrite and then ammonium, which coupled with the oxidations of several electron donors such as Fe(+2), As(+3), sulfide, lactate, pyruvate, acetate, and other organic/inorganic materials (Jiang et al. 2013; Zhang et al. 2017). The desorption of arsenic from FeO(OH) mineral in soil or sediment, nitrate, nitrite, and ammonium has a complex, complicated, and debatable functioning capacity. Some influencing factors such as organic matter, microbial community, soil profile, pH, and Eh also affect the efficiency of the functioning capacity of these ions. Nitrate and ammonium may accumulate in subsoil by the leaching of nitrogen-rich fertilizer through various chemical decomposition. Like phosphate, the pooled concentration of both these ions is significantly correlated (NO₃: r= 0.48, and NH₄: r= 0.71 at p < 0.05; 95% CI) with arsenic in groundwater of the Bengal delta plain (Figure 3). Nickson et al. (2000), Kiyoshi et al. (2008), Kurosawa et al. (2008, 2013), and Uddin & Kurosawa (2010) carried out their investigations in separate regions of Bangladesh and found a positive correlation of NO₃, NH₄, and pH with arsenic in groundwater, but the relationships among them were not easy to understand. They assumed that a rise in the NO₃ and NH₄ concentration enhances microbial activity through the consumption of dissolved oxygen and then arsenic was released from FeO(OH)-rich sediments to the surrounding water body in a reducing condition. In this procedure, the NO₃ and NH₄ acted as a nutrient for microbes and the groundwater became a reducing environment. This reducing condition of the Fe (Fe³⁺ to Fe²⁺) was boosted by microbial metabolism of sedimentary organic matter and makes arsenic mobilization easier. This assumption may support the findings of the present study that found the significantly positive correlation of both NO₃ and NH₄ with arsenic in the countrywide groundwater samples. They have also identified the source of these ions in groundwater mainly as N-fertilizer, based on the δ¹⁵N analysis. Therefore, the application of N-fertilizer appears to have a positive effect on the arsenic concentration in groundwater. Uddin & Kurosawa (2010) stated that the deposition of NH₄ in peat/peat-clay sediment of shallow aquifer (10–15 m) was higher than deep aquifer like PO₄. They measured up to 60% organic matter in the shallow layer and assumed that this large amount of organic carbon may be able to raise the total nitrogen. Also, in the peaty-clay layer, organic matter can oxidize in the presence of micro-organisms and serve electrons which are received by nitrate to produce nitrite ion and then NH₄ with N₂ by the complete redox process. This combined amount of NH₄ may enhance the arsenic release in sediment.

In the recent past, several studies established the different rules of NO₃ and NH₄ to the mobilization of arsenic in soil or water bodies by their experiments (Smith et al. 2017; Park et al. 2018; Shakya & Ghosh 2019; Yao et al. 2019; Zhu et al. 2020). The result of these investigations differs from the above mechanism. Mayorga et al. (2013) stated that the arsenic in water samples of Spain decreased along with the increase in nitrate concentrations and assumed that the increased use of nitrogenous fertilizer and pig manure in agricultural activities increased the nitrate content in groundwater, which would favor the precipitation of FeO(OH) and arsenic adsorption. As well, Smith et al. (2017) reported that the accumulation of nitrate in anoxic groundwater improved bacterial mediated processes in the aquifer that ultimately result in arsenic immobilization. Another study illustrated that the NO₃ concentration increase in groundwater results in a decrease in the total arsenic concentration through adsorption of arsenic and iron sulfide precipitation (Shakya & Ghosh 2019), although, there was a fixed amount of metal-oxyhydroxides (MOOH) yielded following the nitrate additions, which resulted in an increased arsenic level like those measured in the groundwater before nitrate addition. The nitrate itself was removed in the process, being reduced first to N₂O gas and then to N₂ gas.

Kurosawa et al. (2013) and Smith et al. (2017) stated that NO₃ became a progressively abundant potential electron acceptor for the oxidation of Fe(+2) in an aquifer, but this redox pair has not been well characterized inside subsoil settings. To explore this reaction and its implications for redox-sensitive aquifer contamination, Smith et al. (2017) led an in-situ field study in contaminated groundwater on Cape Cod Bay (USA) with variables Fe(+2), phosphate, nitrate, arsenic(+3), and arsenic(+5) containing groundwater. The experimental results showed that Fe(+2) oxidized to Fe(+3) and released an electron that was accepted by NO₃ and reduced to N₂O followed by N₂. Both Fe(+2) and NO₃ steadily decreased in the aquifer and, finally, Fe(+3) would be precipitated as FeO(OH) in sediment (Figure 4). Arsenic (+3 and +5) could be adsorbed on the surface of FeO(OH). The study revealed that nitrate-dependent Fe(+2) oxidation at anoxic conditions can occur in groundwater, and this process can affect the mobility of other species, e.g., arsenic and phosphate not directly involved in the redox reaction. This mechanism indicates that nitrate can reduce the arsenic in the groundwater system, i.e., it is inversely correlated to arsenic in the groundwater system, but in this process, bacterial activity plays a vital role in the reductive environment that does not consider the above process.

Recently, the impacts of nitrate on the bacterial catalyzed reductive dissolution of arsenic in contaminated aquifer soils have been investigated by many researchers (Chen et al. 2016; Zhang et al. 2017; Shakya et al. 2018; Li et al. 2019; Shakya & Ghosh 2019; Vishvas et al. 2020). The common findings from those studies indicated that due to inhibition of the bacterial arsenate respiring activity, nitrate/nitrite dramatically inhibits the microbe-catalyzed reductive dissolution of arsenic and iron in the arsenic-contaminated soils. The above investigators found that nitrate could significantly resist the genetic factor of the reductase protein in respiring bacterium cells of microbial. Throughout the microbial reactions, Fe(+3)/nitrate/arsenic(+5) reduced to Fe(+2)/nitrogen/arsenic(+3), and finally, organic matter oxidized to CO₂ or bicarbonate (Figure 5) with favorable ΔG^o values. These studies showed that the arsenic adsorbed either as arsenite (AsO₄³⁻) or arsenate (AsO₃³⁻) in the surface of FeO(OH). It formed through nitrite-driven Fe(+2) oxidation by microorganisms under nitrate-reducing conditions and, finally, these ions precipitated in sediment. In this connection, Zhu et al. (2020) showed that denitrifying bacteria could facilitate the reduction of aqueous arsenic (+3) and/or As(+5) through indirect Fe(+2) oxidation in the solid-phase surface of FeOOH-mineral under certain lithological conditions. The denitrifying bacteria could be found in peaty and clayey/muddy soil. Figure 4 represents the above mechanism of arsenic dissolution through microbial activity under the NO₃Fe(+2) redox environment.

Thus, NO₃ and NH₄ would act as a nutrient to microbes that enhanced the potentiality of microbes to participate in the above redox reactions as catalysts. Again, it also summarized that nitrate is involved in multi-dependent microbial mediated anoxic redox reactions in the arsenic-laden soil/sediments and aquifers in which the arsenic dissolution process was inhibited significantly. The effects of these reactions on the arsenic mobilization from the mineral phase are complicated and poorly understood so far. Therefore, the function of nitrogen-fertilizers to arsenic contamination in aquifers of BDB is not acknowledged, and a further thorough study would be needed.

As with other influencing factors, SO₄ significantly correlated with countrywide arsenic concentration (r= 0.55 at p < 0.05; 95% CI). It acts as a weak electron acceptor (⁠G^o = −47.6 kJ/mole acetate), which accepts an electron from organic materials and forms sulfide then it adds to Fe(+2) and As(+3) (Figure 5). Finally, the sulfate is removed by co-precipitation of sulfide of arsenic with sulfide of iron (Altun et al. 2014; Shakya & Ghosh 2019). Like NO₃, SO₄ reduction generates H⁺ and high acidity retards the formation of arseno-sulfide precipitation (Henke 2009). But then, SO₄ has an insignificant contribution to the countrywide arsenic dissolution process in groundwater.

Urea (NH₂CONH₂) fertilizer is the major source of nitrogen of all types of agricultural activities, and it may increase the acidity (lower pH) in soil. Like other results of various studies, Figure 3 show the slightly negative correlation between arsenic and pH. However, experiments indicated that high pH values of the groundwater might also favor the arsenic desorption processes. The sorption of arsenic to metal was usually pH-dependent (Mostafa & Hoinkis 2012; McArthur 2018), and As(+5) tends to be strongly sorbent with metal oxo-hydroxide minerals at near-neutral to acidic pH situations. At alkaline pH, mineral surfaces become negatively charged, hence promoting arsenic desorption (O'Shea et al. 2007). The experiment of Smedley & Kinniburgh (2002) and Romero et al. (2004) showed that the existence of PO₄ and HCO₃ in a solution with increasing pH could change the nonlinear adsorption isotherm of arsenic, and thus affect its mobilization in groundwater. The processes that may result from varying pH are H⁺ uptake by mineral or rock dissolution and ion-exchange reactions. As well, the weathering of carbonate minerals may occur by reducing pH, which may, in turn, mobilize arsenic that had arrived at the carbonate lattices at higher pH (Romero et al. 2004). Most studies showed that in oxidizing systems pH is positively correlated with dissolved arsenic.

Some metals such as K and Ca were released and accumulate in sub-soil or groundwater from inorganic fertilizer. Potassium could release from MP fertilizer, which has been excessively used in Bangladesh. A strong positive correlation (Figure 3) was observed between arsenic and K in Bangladesh's shallow groundwater. Thus, the study expected that K has no participation in the arsenic dissolution mechanism in the sediment or water phase. We cannot find any literature about this matter and it demands future research.

The above discussion proves that the function of NO₃, SO₄, and NH₄ in arsenic release to groundwater depends on several factors and conditions. Recent investigations have reported the negative impact of those ions on arsenic mobilization in sediment and/or aquifer water, but countrywide data on those components of groundwater have strongly positive relations with the arsenic concentrations. Some researchers also stated that the role of these ions fully depends on the local lithological conditions. In the case of PO₄, pH, Ca, and K show the expected correlation with the arsenic dissolution rate, but the causes of an excess concentration of arsenic in the most affected areas in which both the cultivable land and application of chemical fertilizers are relatively lower than the other less affected areas of Bangladesh are not clear (Table 7). Thus, it is assumed that chemical fertilizer leaching is not the major (or any) cause of groundwater arsenic contamination. Other drivers may be controlling the action of fertilizer-borne influencing factors. This phenomenon will be clear if the situation of the most arsenic-affected areas with the least affected areas of Bangladesh is compared.

Rajshahi and Rangpur Divisions are in the northwest of the country and have very fertile cropland relative to other regions of Bangladesh. There is 72% of total land cultivable, of which, nearly about 30% is triple cropped area. Almost all cultivable lands are loamy, sandy, or sandy-loam whereas clayey or silt-clay formation is not found in either division (Reza et al. 2010). The sub-soil of these regions is neutral to extremely acidic with a pH of 7.2–4.5. The consumption of urea, phosphate, and potash fertilizers in both divisions are the highest in the country (MSUK 2010). The irrigable area of the Rajshahi and Rangpur Divisions is 85% out of total cultivable land. These areas' soils contain fewer organic matters, and Eh values of soil are relatively high. Both these areas are free from any arsenic contamination, although the chemical manure consumption rate is very high (Reza et al. 2010). In these areas, except for the heavy application of fertilizer, other factors such as soil pH, dissolved organic carbon (DOC), and Eh, which are the main drivers of arsenic dissolution, favor less arsenic contamination. Comparative information between the ten highly arsenic prevalence Districts and relatively safer two Divisions of Bangladesh is listed in Table 7 (MSUK 2010; Reza et al. 2010; BBS 2019).

Some investigators think that water levels are drawn down by heavy water mining allowing the atmospheric oxygen to enter the aquifer, which may accelerate the release of arsenic by oxic-oxidation of pyrites (Chowdhury et al. 2010). However, the credibility of this observation is not acceptable for the Rajshahi and Rangpur Divisions. The irrigation activities are very common in both areas in the dry period, with the groundwater level significantly declined due to over-exploitation of water (Zahid 2015; Hasan et al. 2020). Nickson et al. (2000), Nath et al. (2008), and Reza et al. (2010) described the relative function of mobilizing agents in the most and least affected areas of Bangladesh which are included briefly in Table 8. This table shows that, in the least affected areas, the lower concentration of arsenic does not depend on PO₄, dissolved organics, and alkalinity of soil and arsenic released from the soil with low concentration due to direct desorption of arsenic or precipitation of FeO(OH). In the case of the ten most affected Districts, the reversed result was observed. It seemed that the main cause of this reversed result is the variation of soil physiography, i.e., fully geogenic, not anthropogenic.

The review identified two major anthropogenic causes of arsenic contamination in groundwater along with the geogenic cause. These are over-use of chemical fertilizers and pesticides; and over-exploitation of groundwater. The exact cause of arsenic dissolution in an aquifer is very complex and doubtful. The study explained the possible mechanisms and causes of arsenic mobilization in groundwater. The analysis results showed that the ten most arsenic-affected Districts of Bangladesh have less agrarian and irrigable land and use relatively less quantity of chemical fertilizers. The land of these areas consisted of clayey, peaty, silt-clay types of soil. Conversely, the northern part of the country with a negligible arsenic concentration in groundwater has the opposite situation of the highly affected areas. The investigation indicated that the anthropogenic causes were not considered for arsenic contamination in the Bengal basin area, whereas soil/sediment physiography was the major cause of the contamination.

Hence, the study summarized the findings as follows:

1. The countrywide pooled concentration of arsenic was 109.75 (52.59, 166.91) μg/L at 95% CI with a standard deviation of ±118.56, which was greater than double the national standard (50 μg/L).

2. The concentrations of arsenic were significantly correlated with NO₃, NH₄, PO₄, SO₄, Ca, and K in nationwide groundwater, and these components may be released from chemical fertilizer leaching.

3. The role of PO₄ in arsenic mobilization was understandable but not in the case of NO₃, SO₄, and NH₄ at certain lithological conditions.

4. The fertilizer leaching could affect the arsenic mobilization in groundwater if lithological conditions are clayey, peaty, silt-clay in nature, and have a rich microbial community with sufficiently organic carbon load.

5. The main anthropogenic cause of high arsenic concentration in groundwater of the BDP was not caused by using chemical fertilizers in this area, but it is geogenic.

6. The overall lithological conditions are the main reason for the high arsenic load in groundwater in the basin area, and the anthropogenic source is insignificant in the study areas.

All relevant data are included in the paper or its Supplementary Information.