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Arsenic is a widespread contaminant of drinking and groundwaters in the world. Even if these contaminations have a geogenic origin, they often are exacerbated by anthropogenic activities. This is particularly true for the Bengal delta. Millions of people in Bangladesh are consuming drinking water with arsenic concentrations ≥ 50 µg/L. Their drinking water supply is based on groundwaters extracted by pumping wells, which were part of a well-drilling program by the United Nations. The intention was to provide the people with groundwater instead of surface water due to its critical hygienic conditions. Unfortunately, many wells extract the groundwater at depths where arsenic concentrations are highest. Arsenic is being dissolved from the aquifer by biogeochemical processes that are fueled by the presence of high amounts of organics in the Bengal delta sedi- ments. This problem was not encountered at the time due to a lack of chemical analyses of the waters. Pollution of groundwaters by arsenic is widespread through- well-drilling program to provide the people with ground- out the globe. Very often, it is of anthropogenic origin water instead of surface water (Fig. 1). Indeed, the pumped (Raessler et al. 1999, 2000). Of particular concern in some waters were free of cholera germs. On the other hand, they regions, however, is the geogenic presence of arsenic in often contained considerable amounts of arsenic, which was drinking water. Presumably, more than 100 million peo- dissolved from the soils—a problem that was also true for ple worldwide suffer from varying amounts of arsenic in neighbouring regions of India, i.e., the province of West their drinking water resources. This is particularly true for Bengal. Due to a lack of chemical analyses of the waters, Asian countries, such as Bangladesh, India, China, Mongo- this problem was not encountered at the time. lia, Nepal, Vietnam, Thailand, and Cambodia, but also for The pumping of groundwaters and intrusion of surface other regions, such as Argentina, Chile, Mexico, and parts waters led to the penetration of organic matter into the of the United States. Additionally, European countries, such groundwaters, which fueled biogeochemical processes, as Hungary and Romania, are concerned. The consequences resulting in the dissolution of arsenic from the soil sediments are most pronounced in Bangladesh where huge parts of the (Neumann et al. 2010; Polizzotto et al. 2008). population have been exposed to high amounts of arsenic for Today, more than 3 million of 11 million wells in Bang- a long time (Smith et al. 2000). ladesh are supposed to be affected, i.e., these wells contain Until the 1970s, people used to take surface waters from > 10 µg/L arsenic—a concentration that the UN recom- rivers and lakes for their drinking water. The tropic climate mends as a threshold value and which must not be exceeded and flooding of the plain countryside during monsoon rain - in German drinking waters as ruled by the German drinking fall combined with hygienic deficiencies fueled the propa - water directive (Trinkwasser-Verordnung 2001). gation of cholera and other diarrhoea diseases. To face this In this study, we closely look at both the hydrogeology problem, the United Nations and UNICEF initiated a huge and the biogeochemistry of arsenic in the Bengal Delta to better understand the mechanisms and the reasons for the mobility of arsenic in Bangladesh’s groundwaters and dis- * Michael Raessler cuss the consequences for the health of the people living firstname.lastname@example.org in these regions. Finally, recommendations to mitigate the 1 situation will be given. Max-Planck-Institut für Biogeochemie, Hans-Knoell-Strasse 10, PF 100164, 07745 Jena, Germany Vol.:(0123456789) 1 3 2 Archives of Environmental Contamination and Toxicology (2018) 75:1–7 water supply of the capital city of Dhaka), this is not the case for the holocenic aquifers, which are rich in arsenic. The highest arsenic contamination is observed in those holocenic aquifers, which are approximately 3000 years old (Dowling et al. 2002). They make up the main part of drinking water supplies in Bangladesh. The waters of the upper holocenic aquifers are approximately 100 years old and contain less arsenic. However, the holocenic layers are not homogenous and settled but are characterized by gaps and holes enabling the vertical extension of arsenic con- tamination. This explains the marked depth-dependence of the arsenic contamination. The highest concentrations Fig. 1 Usage of a pumping well provided by UNICEF are found in 20–70 m, which corresponds to shallow and young aquifers. On the other hand, the depth of aquifers is not a sufficient criterion for waters being free of arse- Hydrogeology of the Bengal Delta nic. Moreover, it is possible that arsenic is dissolved into the waters from deeper aquifers by the pumping activity The Bengal Delta, most of which is part of Bangladesh, of wells for drinking water production (Mc Arthur et al. is one of the world’s largest sediment basins. It mostly 2016). consists of tertiary and quaternary sediments (Umitsu 1993; Goodbred and Kuehl 2000). These were deposited by action of the rivers Ganges, Brahmaputra, and Meghna. Biogeochemistry of Arsenic in the Bengal The different layers of the sediment are made from sand, Delta alluvial sand, and clay minerals. The lower layers of the sediment are partly disrupted by gravels. Glacial variations Arsenic of geogenic origin in the Bengal delta can be of sea level led to formation of deep river gorges that were dissolved from sediments into the ground waters by bio- filled with new sediments. These processes strongly influ- geochemical processes. Dissolution is determined by both enced the present properties of the aquifers and waters. chemical transformations and the properties of the arsenic The pleistocene sediment layers of the Madhupur com- compounds. Basically, arsenic is present in waters in two plex are oxidized and strongly weathered offering a very different oxidation states: As(III) (arsenious acid or arsen- good water quality. Concentrations of arsenic are mostly ite) and As(V) (arsenic acid or arsenate), respectively. The < 10 µg/L, in agreement with UN recommendations, degree of protonation of the arsenic species is ruled by the although arsenic concentrations in pleistocene sediments acidity of the water, i.e., its pH value, as shown in Fig. 2 can range up to 100 µg/L beneath deep paleo-channels at (Sharma and Sohn 2009). depths between 120 and 180 m (Mc Arthur et al. 2016). On It is well known today that a variety of different factors is the other hand, the holocenic sediments made of grey clay influencing the mobility of arsenic. Among them are chemi- minerals and sand are not weathered. They often contain cal phenomena fostering the mobility, such as the leaching of peat organic matter, which severely impairs water qual- arsenic by the carbonate ion, and others impairing its mobil- ity. These sediments were formed in a transgressive phase ity, such as the formation of solid arsenic sulfide phases and approximately 6000 years ago while the coastline was also the competition of the arsenic ion with the analogous inundated by sea water (the sea level was approximately phosphate ion for sorption sites on ferric oxyhydroxides. 2 m above today’s sea level). These holocenic sediments Ravenscroft et al. (2009) distinguished the following four are highly productive and quickly renewed. Their aquifers mechanisms of arsenic mobilisation: are anoxic, favouring the mobilisation of arsenic (Smedley and Kinniburgh 2002) and characterized by high levels of Reductive dissolution. calcium, magnesium, and iron (Ravenscroft et al. 2009). • Alkali desorption. Both the pleistocenic and the holocenic sediments contain Sulfide oxidation. the most important aquifers of the Bengal delta, extend- • Geothermal waters. ing from 30 to 130 m in depth. Individual layers cannot be distinguished horizontally or vertically over longer In the humid and tropical climate of the alluvial Bengal distances. These sandy and muddy layers can be consid- delta, reductive dissolution is the dominating mechanism. ered as aquifers of limited size. Whereas the pleistocenic The primary source of arsenic contamination in Bangla- aquifers are mostly free of arsenic (they serve as drinking desh is arsenic-bearing minerals from the Himalaya, such 1 3 Archives of Environmental Contamination and Toxicology (2018) 75:1–7 3 Fig. 2 Distribution of As(III)- and As(V)-species depending on pH (Sharma and Sohn 2009) as arsenopyrite FeAsS, oripigment As S , or realgar AsS. different ways with arsenic by either oxidizing arsenite or 2 3 As(III), Fe(II), and sulfate formed from sulfide had been reducing arsenate (Oremland and Stolz 2005): chemoauto- released from the ore during weathering by action of oxygen trophic arsenite oxidizing bacteria (CAO) oxidize dissolved or by oxygen dissolved in water, respectively. Within a short As(III) to As(V) using either oxygen or nitrate as electron range of time, there was further oxidation of Fe(II) to Fe(III), acceptors, while incorporating inorganic carbon (C O ) in which subsequently precipitated as ferric oxyhydroxide. At the cell material. Heterotrophic arsenite oxidizing bacteria the same time, there was slow oxidation of As(III) to As(V), (HAO), on the other hand, use organic carbon as energy which, in turn, was adsorbed to the ferric oxyhydroxides. source and cell material. These processes are favoured by As(V) was thus immobilized in the sediments and did not the constructed well systems, which will deliver the oxi- contribute to the pollution of groundwaters. The soluble sul- dative agents needed, i.e., oxygen, nitrate (Oremland and fate ions were eroded (Anawar et al. 2003). Stolz 2003). The constant supply of organic matter maintains Hydrochemical “on-site” studies performed in Bang- the microbial respiration, resulting in anoxic conditions by ladesh gave incidence that the mobility of arsenic in the consumption of oxygen. The anoxic conditions are used by groundwaters is strongly related to the availability of organic dissimilatory arsenate respiring prokaryotes (DARP) reduc- matter (Selim Reza et al. 2010). Organic carbon serves as ing As(V) to As(III). As(V) serves them as electron accep- foodstock of microorganisms able to decompose deep soil tor during anaerobic cell respiration. DARP oxidize several layers leading to the dissolution of arsenic (Harvey et al. organic and inorganic electron donators. Organic carbon is 2002). It is not clear whether the organic matter originates present in various compounds (Bisutti et al. 2004). Based from peat layers in the holocenic aquifers or whether it is of on recent findings, the mobilization of arsenic is strongly anthropogenic origin, caused by agriculture (fertilization), fueled by the fraction of organic carbon that is biologically manure, waste disposal, or even measures taken for arse- degradable. Biologically degradable organic carbon (BDOC) nic decontamination and removal, respectively. The organic often originates from pond waters (Polizzotto et al. 2008). matter is metabolized by the microorganisms, causing anoxic On the other hand, the organic carbon resulting from the conditions by exhaustive consumption of oxygen. The ferric irrigation of paddy fields is mostly recalcitrant and will not oxyhydroxides are reduced by action of the microorganisms, be metabolized by the microorganisms. Consequently, this releasing Fe(II) and As(V), which is reduced to As(III). This organic carbon does not contribute to arsenic mobilization. process is characteristic of the Bengal delta and gives rise to This hypothesis is supported by relatively low arsenic con- high amounts of dissolved iron which very often correlate tents found in ground waters from paddy fields. Moreover, with As(III) concentrations. The microorganisms react in incubation and column experiments with pond waters after 1 3 4 Archives of Environmental Contamination and Toxicology (2018) 75:1–7 action of oxygen led to a reasonable decrease in organic car- Even worse are skin maligna (Guha Mazumder et al. bon which was not observed with organic carbon of paddy 1998). Additionally, senso- and vaso-motoric disorders are fields ground waters (Neumann et al. 2010). The impact of observed, which can result in a collapse of the patient. BDOC on the mobilization of arsenic also was observed in Both mobility and accumulation of arsenic strongly sediments of the Mekong delta in Cambodia (Seyfferth et al. depend on the presence of its compounds in the environ- 2014). This is of particular interest, as both the geological ment, i.e., its chemical speciation. Examples are the inor- and anthropogenic settings are quite different. Due to the ganic compounds arsenite [As(III)] and arsenate [As(V)], much higher population density in Bangladesh, anthropo- as well as the organic compounds monomethlylarsenite genic influences do have a much greater impact than in the [MMA(III)], monomethlylarsenate [MMA(V)], dimethyl- Mekong delta. Further and intense research activities will arsenite [DMA(III)], and dimethylarsenate [DMA(V)]. The be necessary to clarify the relationship between the nature latter is transformed from the inorganic arsenic compounds and the impact of organic carbon on the mobilization and by biomethylation (Cullen and Reimer 1989). The order of thus the biogeochemistry of arsenic in Bangladesh and other toxicity of the compounds is as follows: MMA(III) > DMA contaminated regions. (III) > As(III) > As(V) > MMA(V) > DMA(V). The Toxicity of Arsenic Groundwater Contamination and Implications for Human Health The toxicity of arsenic has been known for centuries. Promi- nent examples are the arsenic poisonings of Napoleon Bona- The problem of groundwater contamination by arsenic also parte and Descartes, as well as the German murderer Gesche concerns many other people in developing countries in Gottfried who was executed in Bremen in 1831 for several South East Asia where nonmonitored groundwaters serve crimes committed by diarsenic trioxide As O . The lethal as the main source of drinking water supply. 2 3 dose for ingested As O in humans is 70–180 mg (Léonard 2 3 1991). The Arsenic Contamination of Rice The consequences of longer exposer to arsenic are numer- ous. Arsenic is a capillary and enzyme poison due to its high The arsenic contamination of rice, which is the most affinity for sulfur-containing proteins. Chronic arsenic poi- important feedstock of the region, has become more severe soning—arsenicosis—often is accompanied by a keratosis, (Meharg et al. 2009). Rice is an important path of ingestion which is a painful skin disease with eczemas and ulcers. of inorganic arsenic compounds into the human organism. Inorganic arsenic compounds originate the formation of The rice plant takes up the arsenic from the soil to accu- the characteristic nodes on the hands and soles of the foot mulate into its grain. The arsenic contamination of the rice (shown in Fig. 3). plant is not only threatening human beings but also to the Fig. 3 Nodes on hands and soles of the foot caused by arse- nic poisoning (keratosis) 1 3 Archives of Environmental Contamination and Toxicology (2018) 75:1–7 5 cattle that often are fed with rice straw. The high rice con- Bangladesh. Consequently, arsenic concentrations detected sumption in South East Asia has become the preponderant in cow milk are generally low (Ghosh et al. 2013). However, arsenic source for people not suffering from contaminated there seems to be the risk of adultering the raw milk by dilut- groundwaters. The average daily rice consumption for an ing it with contaminated water. Due to the limited number adult in Bangladesh ranges from 400 to 650 g. This is one of of studies from Bangladesh and, more importantly, the lim- the highest per capita rice consumption figures in the world ited sample sizes in almost all of these studies, a conclusive (Joseph et al. 2015; Ahsan and Del Valls 2011). Irrigation of species-specific observable range for arsenic in edible plant rice fields is mandatory in the dry season, also called “boro”. parts (except rice) and animal-origin food cannot be defined It was estimated that almost 1000 metric tons of As were as yet (Joseph et al. 2015). cycled with irrigation water during the dry season each year This also applies to the data on the arsenic content of due to the fact that boro rice needs huge amounts of water breast milk samples of women from Bangladesh. Arsenic (Saha and Ashraf Ali 2007). In a recent study, more than content was found to be low (Islam et al. 2014) and conse- 900 polished rice samples collected from several districts of quently, exclusive breast-feeding of infants was considered Bangladesh were analyzed for their arsenic content. While to be a protection against arsenic exposure (Fängström et al. mean and median values of total arsenic concentrations were 2008). 126 and 107 µg/kg, respectively, a maximum concentration However, this conclusion suffers from very limited data. of arsenic of 680 µg/kg, dry weight, was measured (Islam Moreover, the situation is worsened by the fact that the num- et al. 2017). ber of studies on arsenic in human breast milk samples is In 2014, the World Health Organization (WHO) recom- generally much lower than compared with other elements, mended a maximum level for arsenic of 0.2 mg/kg rice. such as cadmium or mercury, where many more data are Under anaerobic conditions such as those found in flooded available. With respect to Bangladesh, meaningful data on rice paddies, arsenite predominates which may cause severe a possible contamination of human breast milk are almost problems. The rice plant is unable to distinguish between completely missing (Rebelo and Caldas 2016). arsenite and silicic acid. In general, silicon is beneficial to the rice plant as it strengthens the rice plant’s stem and protects it against pathogens. It is noteworthy that rice can Outlook and Recommendations accumulate 10 times more silicon than other grasses which means that rice also can accumulate 10 times more arsenic. Although the problem of arsenic contamination of ground In case of aerobic conditions, arsenic will mostly be present and drinking waters in Bangladesh came to public attention as arsenate, which cannot be distinguished from phosphate more than two decades ago, it is far from being resolved. by the rice plant, which is an important nutrient to it. Cur- According to a new report by Human Rights Watch released rently, the arsenic accumulation of different rice cultivars is on April 6, 2016, in 2013 approximately 12% of rural house- examined. Those cultivars revealing lowest accumulation holds, which corresponds to approximately 20 million peo- of arsenic could be used as stock for breeding programs. On ple in Bangladesh, were still consuming waters with arsenic the contrary, rice cultivars that accumulate less arsenic also concentrations > 50 µg/L. Although arsenic occurs naturally will accumulate less silicon and, consequently, will be less in Bangladesh’s groundwater, the deadly contamination of stable and more prone to diseases. An easier way to reduce the drinking water of many millions of Bangladesh’s rural the arsenic content in rice seems to change the way rice is poor is a disaster that humans have caused and perpetuated. boiled. Using excess water to cook rice and pouring off the The same report states that the government of Bangladesh extra water decreases the amount of arsenic in cooked rice. is failing to address this predicament of the rural poor, but Using an off-the-shelf coffee percolator can remove up to on the contrary, is expending considerable resources in areas 85% of arsenic from the rice (Carey et al. 2015) and up to where the risk of arsenic contamination is relatively low and 96% of arsenic from rice bran (Signes-Pastor et al. 2017). where water coverage is relatively good. It provides evidence of how the positioning of tube wells can sometimes represent a political manoeuvre by government officials keen to garner Other Risks of Arsenic Contamination support, rather than the prioritization of safe water provision to the areas of greatest risk of arsenic contamination. There are not many data available with respect of arsenic Despite government reports stating that the government contamination of milk and meat. In Bangladesh, cattle should do a better job of targeting arsenic mitigation options are generally fed by rice straw and rice husk, respectively. in areas where there are most needed, it inexplicably fails to There is almost no grassing land for cattle due to the high do so. Additionally, there is a serious lack of monitoring and population density. Cattle also are exposed to contaminated quality control in arsenic mitigation projects. Although in water; however, dairy cows typically access surface water in 2005, the country adopted a “pro-poor strategy” to prioritize 1 3 6 Archives of Environmental Contamination and Toxicology (2018) 75:1–7 poor villagers when new water points were allocated, there References was no substantial improvement of the situation. 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