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Uncorrected Proof © 2021 The Authors AQUA — Water Infrastructure, Ecosystems and Society Vol 00 No 0, 1 doi: 10.2166/aqua.2021.043 Influence of chemical fertilizers on arsenic mobilization in the alluvial Bengal delta plain: a critical review Md. Shajedul Islam and M. G. Mostafa* Institute of Environmental Science, University of Rajshahi, Rajshahi 6205, Bangladesh *Corresponding author. E-mail: mgmostafa@ru.ac.bd ABSTRACT Arsenic contamination of alluvial aquifers of the Bengal delta plain causes a serious threat to human health for over 75 million people. The study aimed to explore the impacts of chemical fertilizer on arsenic mobilization in the sedimentary deposition of the alluvial Bengal delta plain. It selected ten comparatively higher affected Districts and the least affected two Divisions as a referral study site. The countrywide pooled concentration of arsenic in groundwater was 109.75 μg/L (52.59, 166.91) at a 95% confidence interval, which was double the national guideline value (50 μg/L). The analysis results showed a strong positive correlation (r 0.5) of arsenic with NO3,NH4,PO4,SO4, Ca, and K, where a portion of those species originated from fertilizer leaching into groundwater. The results showed that PO4 played a significant influence in arsenic mobilization, but the role of NO3,SO4, and NH4 was not clear at certain lithological conditions. It also showed that clay, peat, silt-clay, and rich microbial community with sufficiently organic carbon loaded soils could lead to an increase in arsenic mobilization. Finally, the study observed that the overall lithological conditions are the main reason for the high arsenic load in the study area. Key words: aquifer sediment, arsenic mobilization, Bengal delta plain, fertilizer leaching, lithology, redox reaction HIGHLIGHTS • The groundwater of the Bengal delta basin is highly contaminated with arsenic. • Arsenic significantly correlated with originated fertilizer species: NO3,NH4,PO4,SO4, Ca, and K. • Clayey, peaty, and silt-clay soil with heavy microbial and organic matters enhanced the arsenic mobilization by fertilizers leaching. • Lithological conditions are the major reasons for arsenic toxicity in groundwater. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY-NC-ND 4.0), which permits copying and redistribution for non-commercial purposes with no derivatives, provided the original work is properly cited (http://creativecommons.org/licenses/by-nc-nd/4.0/). Downloaded from http://iwaponline.com/aqua/article-pdf/doi/10.2166/aqua.2021.043/929355/jws2021043.pdf?guestAccessKey=3101e41b-c033-4c5c-a3a1-55da9dd403ae by guest on 23 September 2021 Uncorrected Proof AQUA — Water Infrastructure, Ecosystems and Society Vol 00 No 0, 2 GRAPHICAL ABSTRACT INTRODUCTION 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 Â 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 in the world. Almost 90% of its lands are cultivable and 55% of the inhabitants are engaged directly in the agricultural sector (BBS 2019). Pre- sently, 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, Downloaded from http://iwaponline.com/aqua/article-pdf/doi/10.2166/aqua.2021.043/929355/jws2021043.pdf?guestAccessKey=3101e41b-c033-4c5c-a3a1-55da9dd403ae by guest on 23 September 2021 Uncorrected Proof AQUA — Water Infrastructure, Ecosystems and Society Vol 00 No 0, 3 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 des- orbed 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 micro- bially 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. Hypothesis The application of chemical fertilizer is the major anthropogenic cause of arsenic contamination in groundwater of the Bengal delta plain (BDP). Research questions 1. What is the country and District wise pooled concentration of arsenic in groundwater and what Districts are highly affected or relatively less

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