Scenario, Perspectives and Mechanism of Arsenic and Fluoride Co-occurrence in the Groundwater: A Review

Journal Pre-proof Scenario, Perspectives and Mechanism of Arsenic and Fluoride Co-occurrence in the Groundwater: A Review

Manish Kumar, Ritusmita Goswami, Arbind Kumar Patel, Medhavi Srivastava, Nilotpal Das PII:

S0045-6535(20)30319-2

DOI:

https://doi.org/10.1016/j.chemosphere.2020.126126

Reference:

CHEM 126126

To appear in:

Chemosphere

Received Date:

27 September 2019

Accepted Date:

04 February 2020

Please cite this article as: Manish Kumar, Ritusmita Goswami, Arbind Kumar Patel, Medhavi Srivastava, Nilotpal Das, Scenario, Perspectives and Mechanism of Arsenic and Fluoride Cooccurrence in the Groundwater: A Review, Chemosphere (2020), https://doi.org/10.1016/j. chemosphere.2020.126126

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Scenario, Perspectives and Mechanism of Arsenic and Fluoride Cooccurrence in the Groundwater: A Review

Manish Kumar1#, Ritusmita Goswami2, Arbind Kumar Patel1, Medhavi Srivastava1, Nilotpal Das3

1Discipline

of Earth Sciences, Indian Institute of Technology Gandhinagar – 382355, Gujarat, India

2Department

of Environmental Science, The Assam Royal Global University, Guwahati, Assam 781035, India

3Department

of Civil Engineering, Indian Institute of Technology Guwahati, Assam - 781039, India

*Corresponding author: Manish Kumar, Ph.D. Assistant Professor | Discipline of Earth Sciences | Room No. 336A, Block 5| Indian Institute of Technology Gandhinagar – 382355, Gujarat, INDIA E-mail: [email protected]; [email protected]

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Abstract Arsenic (As) and fluoride (F-) are the two most conspicuous contaminants, in terms of distribution and menace, in aquifers around the world. While the majority of studies focus on the individual accounts of their hydro-geochemistry, the current work is an effort to bring together the past and contemporary works on As and F- co-occurrence. Co-occurrence in the context of As and F- is a broad umbrella term and necessarily does not imply a positive correlation between the two contaminants. In arid oxidized aquifers, healthy relationships between As and F- is reported owing desorption based release from the positively charged (hydr)oxides of metals like iron (Fe) under alkaline pH. In many instances, multiple pathways of release led to little or no correlation between the two, yet there were high concentrations of both at the same time. The key influencer of the strength of the co-occurrence is seasonality, environment, and climatic conditions. Besides, the existing primary ion and dissolved organic matter also affect the release and enrichment of As-Fin the aquifer system. Anthropogenic forcing in the form of mining, irrigation return flow, extraction, recharge, and agrochemicals remains the most significant contributing factor in the cooccurrence. The epidemiological indicate that the interface of these two interacting elements concerning public health is considerably complicated and can be affected by some uncertain factors. The existing explanations of interactions between As-F are indecisive, especially their antagonistic interactions that need further investigation. “Multi-contamination perspectives of groundwater” is an essential consideration for the overarching question of freshwater sustainability.

Keywords: Arsenic; Fluoride; Groundwater; Co-occurrence; Correlation; Health;

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1. Introduction: Groundwater is one of the most critical resources of a nation, and it is currently under threat. Due to the significant dependence of the population on groundwater, the problem of its contamination becomes grave. Groundwater formations or aquifers are typically impermeable and thus are not polluted easily; at the same time, groundwater also gets filtered naturally to some extent during infiltration. These qualities have resulted in much higher dependence on groundwater for drinking as compared to surface water (UNEP, 2002). Aquifers all around the globe are running dry because of over-exploitation, and the little available water is becoming more ill-suited for use due to contamination. In hindsight, the low recharge rates of groundwater also mean that once polluted, it cannot be reverted to its earlier “purer state”.

Groundwater quality of a region depends on various essential features. Natural or geo-genic factors like the sediment type, native geomorphology, and minerals are primary considerations (Verma and Mukherjee, 2015), and the prevalent anthropogenic conditions, which include land-use patterns cannot be ignored either (Chan, 2001). In recent times, two of the most common inorganic groundwater pollutants, As and F-, are widely popular due to their detrimental and omnipresent menace. A sizeable proportion of the global population is under the threat of one or both of these contaminants through groundwater. Figure 1 shows the relative percentage of people exposed to groundwater arsenic and fluoride contamination in countries from across the globe.

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Arsenic causes arsenicosis, which is a global threat. In drinking water, the provisional permissible limit for Arsenic is 10µgL-1(WHO, 1993). But the population exposed to the threat of arsenic contamination is vast and familiar. As seen in Figure 2, the population exposed to arsenic contamination is disturbingly high in India. Due to the absence of alternative drinking sources in India, people profoundly depend on groundwater highly contaminated with arsenic. Considering the above factor, the permissible limits were kept 0.05mg/l in some developing countries like India (WHO, 1993). Very high concentrations of As have been detected in Mexico ( Bundschuh et al., 2012; Armienta and Segovia, 2008), USA (Haque and Johannesson, 2006), China (Guo and Wang, 2005; Yang et al., 2012), India (Kumar et al., 2010; Shah, 2012, 2010), Bangladesh (Halim et al., 2009; Kamal and Parkpian, 2002), Vietnam (Berg et al., 2007; Winkel et al., 2011) and Pakistan (Arooqi et al., 2007; Farooqi et al., 2007; Muhammad et al., 2010). Recently, groundwater in Japan (Yoshizuka et al., 2010) and Korea (Ahn, 2012), was also found to be contaminated with As. Worldwide, more than 140 million people in around 70 countries are exposed to As contamination through groundwater (Ravenscroft, 2007). Specifically, in South Asia, a large population is exposed to As menace, as shown in Figure 1 and 2.

The permissible limit for F- in drinking water is 1.5 mgL-1 as prescribed by WHO (WHO, 2017). A little amount of Fluoride is necessary for the strength of bones and teeth (Rao, 2011), but an excess of F- in drinking water can cause dental and skeletal fluorosis. Excess amounts of F- causes weakening of the enamel of the teeth by converting the two forms of hydroxyapatite to fluorapatite (Grynpas and Cheng, 1988). Fluoride forms Hydrofluoric acid, 4

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which is highly corrosive to the bones and acts on the carbonated hydroxyapatite forming an insoluble salt (CaF2), which results in the weakening of the bones (Everett, 2011). According to the recent estimations, about 200 million people in 25 nations worldwide are under the grasp of fluorosis (Ayoob and Gupta, 2006). However, China (Chaoke et al., 1997) and India (Arveti et al., 2011) are the worst affected countries based on the data of the population exposed to the contaminant. Figure 2 (a) depicts the fluoride threat in India and China to nearly 26 and 35 million people, respectively. A hefty population is exposed to both As and F- in countries like India, China, and Pakistan where these contaminants co-occur (Fig. 2a). The probable interactions that the two might exhibit in the human physiology are not understood. Even though there are a lot of separate reports on groundwater contamination due to As and F-, the need of the hour is a better understanding on co-occurrence and the toxicity effects due to co-exposure. Further, a concomitant review becomes very imperative when there is need of detailed future study in this aspect. There is also a dire need for a database of existing study, directions and purposes.

Therefore, the current article reviews the perspective of As and F− co-contamination around the world, followed by understanding the process of co-contamination. This work intends to address the problems of co-contamination and guide towards the future research directions on this topic. Finally, it summarizes the background story of the co-occurrence of these obnoxious groundwater contaminants and sheds light on their intertwined fates as well as combined effects on the health and socio-economic status of the affected region.

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2. Sources of arsenic and fluoride The problem with As and F- widespread distribution becomes more complicated because they are released from both geogenic as well as anthropogenic sources (Smedley and Kinniburgh, 2002; Alarcón-Herrera et al., 2013). As is commonly found to occur in several minerals, out of which oxides and hydroxides of metals (Mn, Al, and Fe), elemental arsenic, sulfides, arsenides, and arsenites are common. In the ore zones, sulfides like Arsenopyrite (FeAsS), Orpiment (As2S3) and Realgar (AsS) are common As bearing minerals. The most important mineral source of As is arsenian pyrite [(Fe(S,As)2], which most commonly contributes to its geogenic contamination. Generally, the existence of pyrite in the aquifer sediments makes As available for release into the environment. However, pyrite is stable under reducing conditions, other than if exposed to aerobic conditions it oxidizes and forms iron oxides that release As (Patel et al. 2019). Arsenic is found to adsorb strongly to metal oxides (e.g. Al and Mn) more commonly on to iron oxides; thus, arsenic can again be taken up by metal oxides depending on the environmental conditions. If reducing conditions occur when arsenic is sorbed to metal oxides, then the metal oxide redox chemistry is changed, allowing for mobility of arsenic into the water bodies (Smedley et al. 2002). It is also found as adsorbed species on many metal hydroxides which forms a major source. Its adsorption onto Fe oxides/ hydroxides contributes to be the source of its particularly high concentrations in groundwater (Smedley and Kinniburgh., 2002). They are also released as oxidation products of As-bearing Iron sulfide minerals. Other sources include local anthropogenic inputs and geothermal systems. In groundwater, Inorganic As exists as oxyanions under different oxidation states, the oxidation states, in turn, are governed by the existing pH and ORP of the aquifer. Thus, the chemical properties of the groundwater 6

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environment like the pH and ORP are important factors controlling the release of As (Smedley and Kinniburgh, 2002).

Arsenic is released under oxidizing as well as reducing conditions, the former is more common in arid to semi-arid conditions (Alarcón-Herrera et al., 2013;Nicolli et al. 2010). Although As seems to be the problem of an humid area like Bangladesh, Vietnam, Taiwan and others, some arid and semi-arid environments may also cause the genesis of As-enriched groundwater due to the evaporative enrichment of As (Nicolli et al.2010). Moreover, under oxidizing conditions, As is released by the oxidation of As bearing sulfide minerals like arsenopyrite (FeAsS) (Smedley and Kinniburgh, 2002; Kim et al., 2012; Alarcón-Herrera et al., 2013; Das et al., 2018; Kumar et al., 2016). In reducing aquifers, the key mode of As release is by reductive hydrolysis of metal (hydr)oxides (Berg et al., 2007; Kinniburgh and Kosmus, 2002; Macur et al., 2004). As is released under weakly alkaline and strongly reducing environments by desorptive release from iron oxides in such environment (Wen et el., 2013; Wang et al., 2019; Xie et al., 2014). The desorption is brought about by the increase in pH of groundwater resulting in the onset of the “point of zero charge” (PZC) for these metal (hydr)oxides, ultimately leading to the release of As in groundwater. The presence of organic matter prominently assists the process as it results in bacterial degradation resulting in depletion of dissolved oxygen, ultimately creating an anoxic groundwater condition (Macur et al., 2004). However, it is challenging to classify aquifers as completely oxidizing or entirely reducing; most of the time, they exist as a mixture of the two conditions (Das et al., 2018). There is also the possibility of change from an oxidizing to a reducing state with increasing

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depths due to a reduced influx of oxygen (Patel et al., 2019). The chemical reactions below elaborate on the most common pathways of arsenic release. Aerobic degradations → CH2O (aq) + O2 (aq) → CO2 (g) + H2O (aq) Anaerobic degradation → 2CH2O (aq) → CH4 (g) + CO2 (g) Nitrate and sulfate ions act as e- acceptors: NO3- → NO2- → N2O → N2 → NH4+ SO42- → SO32- → S → S2- → HS- → H2S OC(aq) + As(V) → As(III) (aq) (in the presence of microbes) As(V)-Fe(III) (s) → As(III) (aq) + Fe (II) (aq) (in the presence of microbes) Fluoride release depends a lot on rock-water interaction or mineral dissolution, the most common minerals which bear fluorine are fluorite (CaF2), apatite (Ca5(PO4)3(F,Cl,OH)) and micas (Brunt et al., 2004). Fluorite and micas are quite common in sedimentary deposits. Moreover, minerals such as topaz, fluorite, biotites, and their analogous host rocks such as syenite, granite, basal, and shales contain fluoride that can be released into the surrounding groundwater under favourable conditions (Amini et al. 2008). High groundwater fluoride has also been reported from the areas dominating igneous and metamorphic rocks. Fluorine substitutes hydroxyl positions in mineral structures of biotites and amphiboles (K2 (Mg, Fe4 (Fe, AL)2 [Si6Al2O20](OH)2 (F, Cl)2). When the bedrock is rich in biotite, it becomes the primary source of Fluorine in groundwater through weathering (Edmunds and Smedley, 2013). The problem of high groundwater Fluoride has been detected in igneous and metamorphic 8

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rocks as well. Fluoride is also found in hydrothermal deposits in igneous environments, here the release of F- also depends on the solubility of CaF2 (Brunt et al., 2004). Concerning rock type, Fluoride concentrations have been found highest in groundwater associated with metamorphic rocks as they have the highest Fluoride content. Adsorption of Fluoride forms its prevalent source in groundwater. Clays minerals (Kaolinite, bentonite, illite), active alumina and Fe(III) oxyhydroxides (goethite) are common and preferential adsorbates of Fluoride. Therefore, Fluoride in groundwater may be a result of hydrolysis of Fluorine bearing minerals and/or Fluoride desorption from metal oxides (Edmunds and Smedley, 2013). Its occurrence is also associated with geothermal systems and anthropogenic contributions (Hossain et al., 2016).

Throughout the world, high groundwater F- has been associated with deeper aquifers and sluggish groundwater movement. Moreover, arid to semi-arid conditions influence the release of F- as well. Under drier conditions, groundwater flow rates are low, and reaction time with rocks is much longer; such an environment also promotes enrichment of F- through evaporation (Brunt et al., 2004; Frencken et al., 1992). Chemistry of groundwater also influences F- levels. Na-HCO3 type water with low Ca2+ and Mg2+ levels has been found to increase F- in groundwater. Under such conditions, minerals like CaF2 may undergo dissolution to release F- into groundwater as observed from the reactions (1, 2 and 3) below. When calcite (CaCO3) is present in the system, it could also favor the dissociation of F- rich minerals like CaF2 (reaction 2) (Saxena and Ahmed, 2001).

CaF2 → Ca2+ + 2 F CaF2 + H2O → CaO + 2HF HF + OH <=> F- + H2O

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Despite the differences mentioned above in the hydro-geochemistry of each of these contaminants, there is a narrow set of conditions that lead to the co-occurrence of As and Fin an aquifer. This work attempts to shed light on these very factors and update the current understanding of the subject.

3. Process of co-occurrence The co-occurrence of both contaminants has been observed and documented in many countries all over the globe (Figure 3). Many factors have been highlighted to be responsible for this including reduction of Fe(III), As(V), SO42− and desorptive release of both. The solubility of arsenic and fluorine bearing minerals is another factor. Groundwater conditions, as well as the geology, equally contributed to the co-occurrence. The presence of precipitating or complexing ions also affects the concentrations. As the alkalinity, pH and ORP greatly affect the concentrations of both As and F-individually, they also play an important role in their co-occurrence in groundwater. Many studies investigated the groundwater of aquifers with co-occurring As and F- and came up with several probable reasons which are listed in Table 1. Most studies indicate that Fe (hydr) oxides play a major role in the co-occurrence as they are common hosts for both. The natural geological cooccurrence has been reported in many studies- particularly, some areas of Latin America prevailing under semi-arid to arid conditions and characterized by certain conditions such as mixed type of sediments i.e. calcareous and volcanoclastic or Na-HCO3- type alkaline groundwater sources (Pauwels and Ahmed 2007), co-occurrence of these two elements were investigated with high probability. Alarcon-Herrera et al. (2013), studied co10

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contamination and found geology to be the most crucial cause. Additionally, some other responsible factors identified for the co-contamination were aridity, poor monsoon, and over-exploitation of groundwater (Noyola-Medrano et al., 2009). Asia: India: Both As and F- have been widely reported from various parts of India- As being native to the floodplain regions of north (Ganga flood plain) and eastern (Brahmaputra flood plain) India (Ahamed et al., 2006; Kumar et al., 2010; Shah, 2010, 2012; Patel et al., 2019; Mukherjee and Fryar, 2008; Sailo and Mahanta, 2014; Rahman et al., 2005; Chakraborti et al., 2003), While F- is endemic to the drier areas of western and southern India, the problem is most severe in the states of Rajasthan and Andhra Pradesh (Suthar et al., 2008; Sarma and Sunil, 2010; Handa, 1975; Mamatha and Rao, 2010; Paya and Bhatt, 2010; Rao, 2011). In the case of As release, the most common pathway in India is through the dissolution of metal (hydr)oxides, especially Fe (Smedley and Kinniburg, 2002; Kumar et al., 2010; Shah et al., 2010, 2012). Additionally, desorptive release of As(V) from Fe (hydr)oxides has also been mentioned in many studies on aquifers (Das et al., 2018, 2016; Kumar et al., 2016). Fluoride release is mainly dependent on rock-water interaction and the presence of F bearing minerals in hard rock aquifers (Ahmed, 2003). India is a climatically diverse country with relatively varying spatial distributions of the two contaminants individually. However, many Indian states share the problem of both As and F- contaminations but are limited in the number of studies. Data from works of Chakraborti et al., 2016; Bhattacharya et al., 2011; Mukherjee et al., 2016 and Das et al., 2016, shows the extent of both As and F- distribution across the country (Fig. 4). 11

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Despite the less number of co-occurrence studies, it can be observed that there is potential for co-occurrence in hotspots along the Ganga Brahmaputra flood plain (Fig.4) (Das et al., 2018, 2016; Kumar et al., 2016). Several studies have revealed that high F- content in groundwater and fluorosis is a problem in many districts of Assam (Das et al., 2003; Kanta and Bhabajit, 2010), which incidentally is also a region with a very high monsoonal rainfall of 2000mm on average (Jain et al., 2012). Thus, it is likely that hard rock aquifers with already high F- content could also be a source of As if there is deposition of sediment with As minerals on such formations. In such aquifers, desorptive release of As and F- under drier conditions appears to be a possibility. Such co-occurrence could also follow a seasonal pattern with prominence during the drier pre-monsoon season (Das et al., 2017). Pakistan: Pakistan has a much drier climate compared to India, which seems to be favourable for F- rich groundwater (Shah and Danishwar, 2003). High As concentrations have been found in groundwater from alluvial plains of the region (Farooqi et al., 2007). There are three major zones of high co-occurrence in Pakistan. One of them is the Tharparkar district and the groundwater of the Nagarparkar and the Mithi sub-districts show very high levels of both As and F- (Brahman et al., 2013). Fluoride from associated minerals was found to replace OH- in the groundwater leading to high concentrations, however, the exact hydrochemistry and strength of the correlation between the two were not accounted in these studies. The second region consists of the Lahore and Kasur districts. The sources of As and F- in these studies were found out to be anthropogenic, viz fertilizers (Diammonium phosphate), and local stock of coal (Farooqi et al., 2007). The correlation between the two 12

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was low. The third region is Mailsi Tehsil in the Vehari district (Rasool et al., 2015). The contaminants in groundwater show a medium level correlation (r=0.3), multivariate statistical analysis shows that the sources are both geological as well as anthropogenic. Coal combustion and agrochemicals, especially PO43- from fertilizers interfered with the adsorption of As. Overall, we can say that most studies show the alkaline nature of the groundwater to be the reason for the co-release of both As and F-. China: Arsenic and F- co-occurrence in China is an endemic problem in the northern arid regions and mostly confined to three basins (Huhhot Basin, Datong Basin, and Yuncheng Basin) and two plains (Hetao Plain and Songnen Plain) (Yanfeng et al., 2005; Wen et al., 2013). In China, co-occurrence for most affected regions is due to different pathways of mobilization of the two contaminants. Most of the High As regions (Huhhot Basin, Datong Basin, Hetao Plain, and Songnen Plain) have reducing conditions and the causative mode of As release is reductive hydrolysis (Lin et al., 2002; Smedley et al., 2003; Mukherjee et al., 2009; Wang et al., 2009; Currell et al., 2011; Li et al., 2012; Xie et al., 2013; Wen et al., 2013). Highest As and F concentrations were found to be 1550 µg/l and 10.4 mg/l, respectively, in Datong basin attributing to F- enrichment due to soda water and high pH leading to As desorption from oxyhydroxide surfaces (Wang et al., 2008). F- in these regions is enriched through mineral dissolution in the presence of a Soda rich (NaHCO3) type groundwater, pH was also uniformly high (Zhang et al., 2003; Gao et al., 2007; Wang et al., 2009; Wang et al., 2009; Li et al., 2012; Su et al., 2013; Wen et al., 2013). Despite the co-existence of both As and F-, the correlation between the two was found to be non-existent or low (Wen et al., 2013). However, in Yuncheng Basin, the drier oxic conditions were found to result in desorption

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based release of both As and F- resulting in a positive correlation between the two (Gao et al., 2007; Currell et al., 2011; Wen et al., 2013). Japan: The co-occurrence of As and F- in Japan is isolated to certain regions with geothermal activities (Yoshizuka et al., 2010). A positive correlation was found in one aquifer in the studies of Hossain et al. in the western part of Kumamoto area, but much of the studies show scattered distribution (Hossain eta al., 2016). South Korea: Release mechanisms of As and F- showed a unique pattern in the Republic of Korea (Geumsan County), As was observed to originate from sulfide mineral oxidation in “metasedimentary rocks and mineralized zones”. The highly alkaline pH maintained high As levels, most importantly, the affected region was the shallow depth aquifers. Fluoride, on the other hand, was released at much greater depths due to interactions with the fluoride-rich granite rocks, after release, F- was carried to shallower depths due to upward flow, while the Ca-HCO3 water type in the shallow layer maintained the elevated levels of F- (Ahn, 2012).

Africa: One of the highly affected regions of the African continent is the Main Ethiopian Rift (MRE) area. The source of As and F- was found out to be the rocks of volcanic origin abundant in the region. These rocks were mainly silicic, weathering of which led to the development of Narich clays and enhanced pH in the natural waters of the region. All these, along with the “devitrification” of the native volcanic glass, was found to form clays and oxides of metals (Fe and Al), which are sinks of As and F-. The mobilization of F- was traced to ion exchange which 14

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leads to the replacement of Ca2+ by Na+ resulting in no precipitation of CaF2 and enhanced mobility of F- ions. Whereas, As release was associated with silicate weathering which results in high NaHCO3 in water thus high pH. (Rango et al., 2010). Apart from the MRE, other regions where both As and F- have been reported include Tanzania, Ghana, Nigeria and South Africa (Ahoulé et al., 2015).

America: Although As and F- have been detected in many North as well as South American countries, the problem of co-occurrence is localized to three countries which include Mexico (Armienta and Segovia, 2008; Gonzalez-Horta et al., 2015; Reyes-Gómez et al., 2013), Argentina (García et al., 2014; Gomez et al., 2009) and recently Chile (Alarcón-Herrera et al., 2013). Their cooccurrence in Mexico is localized into three main hydrogeological environments; these are: 1) Geothermally active areas, 2) Alluvial aquifers towards the north of Mexico and 3) Mining areas over north-central Mexico (Alarcón-Herrera et al., 2013). In geothermal areas, there is the upwelling of water leading to high As and F- in the aquifers. In some of the affected regions, there is also the influence of industrialization in addition to the geothermal activities. For example, in Los Azufres, evaporative ponds and re-injection of wastewater into the aquifers are part of the operations of a power plant that affect the co-occurrence. The climate in these regions is arid, and in some of the geothermal regions (Las Tres Vírgenes), alluvial formations overlay a “granular basement” (Alarcón-Herrera et al., 2013).

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Mexico: Co-occurrence of As and F- is very prominent in the alluvial regions of northern Mexico, with high correlation between the two observed in Sonora (r=0.92), Chihuahua (r 0.55-0.66, p < 0.001), San Luis Potosí (r =0.68, p < 0.001), and geothermal waters (r = 0.68, p < 0.001). In these alluvial regions, the As was traced to rhyolite, while fluorapatite was found to concentrate in shale, high abstraction rates of deeper groundwater along with evaporative influence the co-occurrence (Alarcón-Herrera et al., 2013). A mixture of geothermal and alluvial conditions at some places in Durango-Coahuila has led to high concentrations of As and F- which have been called “oligo-elements” (Cebrian et al., 1983; Del Razo et al., 1990; M Tsresa Alarc6n-Herrera et al., 2001). In the mining regions of Mexico, mine tailings along with the hydrothermal mineral deposits contribute to As contamination of groundwater (Gutierrez et al., 2009; Smedley et al., 2002; Alarcón-Herrera et al., 2013). High Na+ and HCO3- levels are prominent and associated with eminent levels of the contaminants in some of the mining regions (Mahlknecht et al., 2004; Alarcón-Herrera et al., 2013). Argentina: The Chaco Pampean region of Argentina is well known for high concentrations of both As and F- in the groundwater. Here the aquifers mainly have a geology where watersoluble volcanic glass is the primary As source (Alarcón-Herrera et al., 2013). Arsenic and Fshowed positive correlation in Córdoba province (r = 0.72, p<0.001) and Robles county, (Argentina), and in almost all the affected regions the groundwater type was NaHCO3 type with alkaline water (Nicolli et al., 1989; Alarcón-Herrera et al., 2013). Both As and F- were also found to show prominence mainly in the shallower aquifers- e.g. in Los Pereyras, Tucumán province, the concentration was much higher in shallow loess deposits and decreased with depth (Bhattacharya et al., 2006; Alarcón-Herrera et al., 2013). Also, the 16

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discharge zones close to river mouths and “endorheic lakes or lagoons” have higher levels, thus local geology, as well as geomorphology, played a crucial role in co-occurrence. Hydrogeochemical processes like evaporation and cation exchange of alkaline earth ions (Ca2+, Mg2+) by alkali ions like Na+ and adsorption of K+ on the clay minerals result in conditions favourable for the increased solubility of As and F- (Alarcón-Herrera et al., 2013). Chile: Co-occurrence in Chile is attributed to geothermal activities in the affected sites (Fernandez-Turiel et al., 2005; Alarcón-Herrera et al., 2013). In these regions the high levels of dissolved silica results in high As levels, because such elevated silica leads to the formation of siliceous coatings on ferric (hydr)oxides, thereby inhibiting the adsorption of As(V) (Landrum et al., 2009). The phenomenon of F- dissolution has not been appropriately studied, but the evaporative concentration is thought to be a reason behind high concentrations (Perrez-Carrera and Fernandez-Cirelli, 2005). Because of these unique environmental conditions, the As and F- levels are high even in the Rio Loa river, which is the discharge zone of the El Tatio geothermal region (Romero et al., 2003).

Europe: Cases of As and F- contamination are not very well documented in Europe. There are reports of high As levels in some countries like Hungary, Greece, Croatia, Romania, Serbia, Spain and Turkey (Katsoyiannis et al., 2008); while F- has been detected in Slovakia, Hungary, Moldova and Ukraine (Fordyce et al., 2007). The European countries where co-occurrence has been observed so far are Slovakia and Finland. Hydro-geochemical study of both As and F- are

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limited in the region, and therefore the exact trend of co-occurrence remains unknown to date.

4. Co-exposure to arsenic and fluoride The prevalence of inorganic As in groundwater may become even greater danger than Fhazards owing to its high toxicity at a low concentration that sometimes undetectable particularly in As3+ state (Camacho et al., 2011). The possibility of age-specific effects is based on the daily intake in volume per unit of body weight (mL/kg/d) which is three to four times more in infants than that of an adult, making them much more vulnerable (NRC 1999, Sun et al. 2007). Particularly in Asian countries, where more than 130 million people are exposed to arsenic-contaminated groundwater, and among them, at least 20 million are children below 11 years of age (Bencko, 1977; Chakrabarti et al. 2004, WHO 2003). Usually, children do not show skin lesions up to this age, but their blood, urine and hair samples contain high levels of arsenic (Chakraborti et al. 2004). However, exceptions are observed when As content in water consumed is extremely high (> 1000 μgL-1) and As content in drinking water is moderately high (around 100 μgL-1) but the children’s nutrition is poor. Children are also more prone to arsenic affecting their lungs and nervous systems (NRC 1999).

Even though there are several reports on individual toxicity effects of As and F-, information on the combined effects of these pollutants is still limited. In a recent study by Zeng et al. 18

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(2014), it was found that co-contamination can also affect at genetic level affecting the expression of genes. Armienta & Segovia (2008) explored the prevalence of As and F- cocontamination in Mexico and the epidemiological studies in the areas affected showed a positive correlation between adverse health effects and intake of water having high concentrations of As and F-. Many such studies indicate that the interface of these two interacting elements concerning public health is considerably complicated and can be affected by some uncertain factors. So, the existing explanations of interactions between fluoride and arsenic are indecisive as some experiments also show the antagonistic nature of these contaminants on the cellular level but many such hypotheses need further investigations.

4.1 Individual and Combined effects Epidemiological data have shown that inorganic As is a grave toxicant resulting in various diseases like cancer and other disorders in the circulatory and nervous systems ( Lin et al., 1998; Golub et al., 1998; Ahamed et al., 2006). Concurrently, the concentrations > 1.5 mgL-1 of F are harmful, causing fluorosis in teeth and bones and finally leading to death (Miretzky and Cirelli, 2011). Moreover, arsenicosis and fluorosis are non-reversible, and the disorders have no medical treatment. Infant and children have often been found to be more affected by pollutants like As and F-, the likely reason is exposure to higher amounts of water through ingestion compared to adults because of their low body weight (Kumar et al., 2016).

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When we talk about the co-occurrence of such dangerous contaminants, they may act either independently after ingestion, or antagonistically or synergistically in each other’s presence (Chouhan and Flora, 2010). There is an extensive study on the toxic effects of As and Findividually, but the co-exposure of them has given little interest. It was reported that coexposure to As and F- may affect the integrity of the genetic material of cells more in comparison to the individual exposure (Rao and Tiwari 2006). Among animal studies, simultaneous exposure of As and F- were found to cause detrimental effect on kidney and liver and decreased comet tail in rats even at lower concentrations (Flora et al., 2009; Mittal and Flora, 2006).

A study by Flora et al. (2012) was carried out to assess (i) mechanism of damage at the cellular level in mice caused due to combined effect of As and F-and (ii) oxidative stress in mice due to combined chronic exposure to As and F- via potable water. From this study, combined exposure to As and F- evidenced fewer effects in comparison to their individual effects, detected based on DNA damage, histopathological observations and biochemical changes. It was reported that intelligence and growth of children (below 12 years) were severely affected due to intake of water highly contaminated with As and F-(Mittal and Flora 2006; Wang et al. 2007). In spite of the necessity to understand this issue, very limited data is available on populations co-exposed to these toxic elements. The combined toxicity effect of fluoride and arsenic on living organisms can be seen in Figure 5.

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5. Socio-economic implications of groundwater arsenic and fluoride pollution Access to safe drinking water is one of the most significant issues for health and socioeconomic development (Cvjetanovic 1986) of the populace, however about one billion people all over the world do not have access to safe potable water. Inadequate access to potable water has become the most vital and challenging environmental issue all over the world. Thus, the water quality may be tainted by many natural constituents and As and F are the most dangerous among them. Therefore, studies on their prevalence and associated impacts on health and wellbeing are of great interest. The adverse effects of As and Fcontamination can be classified into two types i.e. primary and secondary. Different symptoms arising due to the consumption of contaminated water are categorized into primary effects, whereas the secondary effects are the outcome of the primary results leading to reduced productivity of the affected mass. Thereby, it leads to social barring and ultimately, socio-economic impacts on the victims.

The socio-economic impacts can be divided into three categories as health problems, agricultural problems and others (Fig.6). The foremost effect can be observed on the health of the exposed population. Skin lesions, cancer in different organs, and finally mortality are the health troubles due to this. Contaminated groundwater leads to decreased soil fertility and thereby agricultural productivity and grounds health problems with pollutants inflowing the food chain. The problem of arsenicosis was also found to have a significant impact on the socio-economic structure. The symptoms of arsenicosis are often mistaken for some contagious skin diseases or leprosy as a result of which employment, marriage, and 21

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even social interactions become difficult for the sufferers. According to the WHO report, social troubles raised due to arsenic calamity put pressure on the economic status of the affected regions (WHO, 2000). Simultaneously, fluorosis is also observed to have an influential result on the poverty and economic status of the victims. Unlike arsenicosis, it is also severe and chronic among the semi-urban as well as the rural habitations afflicted with malnutrition. Due to suffering and disquiet causing psycho-sociological dilemmas to self and family, the person with these disorders eventually develops inferior complex and depression. It is also observed that the patient’s mental health is affected as well by the ‘loss of self-esteem’ (EPA, 1985). Thus, the socio-economic aspects of arsenicosis and fluorosis reiterate the urgency and concern of the scientists and technologists for a sustainable solution to this problem.

6. Co-contamination investigation perspectives Co-contamination of As with F- in groundwater was found to be possible by two most common processes i.e. reductive dissolution and desorption under oxidizing and reducing conditions of the aquifer. The probable arsenic and fluoride interactions in the ecosystem can be seen in Figure 7, which results in co-contamination. Various tools and techniques have been used to identify the controlling factors of the co-contamination and the studies are summarised in the following sections.

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6.1 Modelling approach Modelling approaches are an indirect method of predicting, validating, and understanding the processes which could lead to the co-occurrence of As and F- in the aquifers. Most of the models which have been applied in this field can be classified into two categories, which are: 1) hydrogeochemical models and 2) statistical models; however, most of the time, these techniques have usually been combined in the same work.

6.1.1 Hydrogeochemical Modelling Hydrogeochemical modelling helps to understand processes like the evolution of the groundwater by precipitation (of mineral phases), dissolution and its relation with As and Fmobilization in groundwater. Different speciation models like PHREEQC, visual MINTEQ, can help to illustrate the interactive mechanism of various ions in the groundwater system. The saturation index (SI) calculations has been extensively used to trace the factors responsible for release of contaminants like As and F- in groundwater and to evaluate the interrelationship between environmental conditions and contaminant concentrations (Brahman et al., 2013; Kumar et al., 2016; Sracek et al., 2004). Generally, minerals like anhydrite, goethite, dolomite, calcite, Fe(OH)3, FeS, AsS, and FeCO3 are chosen for speciation modeling to check the prospect of solubility control for As (Sracek et al., 2004). On the other hand, aquatic phases which have been analysed for the stability of F- minerals and the subsequent release of F- in groundwater include CaCO3 and CaF2 (Li et al., 2012; Rasool et al., 2015, Kumar et al., 2016, Das et al., 2016).

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In an As and F- co-occurrence study, Rango et al (2010) applied PHREEQC (using the WATEQ4F) to calculate the SI of As adsorbing “Fe and Mn oxy-hydroxides” and reported the supersaturation of the groundwater for ferric oxides and hydroxides (hematite, magnetite and goethite), implying stable nature of these minerals in the aquifers that could act as the potential As adsorbers. However, under high pH, there was a plausible desorption of the As from these (hydr)oxides. The dissolution of volcanic glass was considered to be the primary mode of As release, which also released F-. While Rango et al., (2010) did not calculate saturation indices for F- phases, Rasool et al (2015) applied SI calculations for the F- bearing phases and reported the As and F- co-occurrence in oxidized alkaline conditions. Arsenic was found to be released specifically due to the evaporative concentration of phosphate in the surface sediments in the shallow aquifers. Phosphate, an As analog was found to replace it from the sediment sorption sites.

Our group has published two works, viz Kumar et al. (2016) and Das et al. (2018) that utilized MINTEQ v 3.1 for speciation of the aquatic phases to study the co-occurrence of As and F-. Kumar et al., 2016 reported super-saturated conditions for aragonite and hydroxy-apatite, calcite, and dolomite, indicating a mineral dissolution-based F- release in the aquifer, while As was found to be released through the change in redox and saturation conditions. It concluded that mobilization and enrichment of the As and F depended on “chemical interactions and individual affinities”. Das et al., 2018 reported saturated groundwater for Fe (Hydr)oxides like ferrihydrite and goethite in the Brahmaputra floodplains (BFP), along 24

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with this data the supporting results of the EDX also revealed the presence of As in the sediment matrix as a significant constituent along with Fe. Therefore, the (hydr)oxides of Fe were the primary sources of As in the BFP. The study concluded that fluoro-apatite is the primary mineral-bearing F- in the aquifer, but it was anthropogenic factors like groundwater abstraction induced drawdown that was the ultimate influencers leading to secondary changes like fluctuations in ORP and mineral equilibrium and thus causing co-contamination.

6.1.2 Multivariate Statistical Modelling Multivariate statistical tools like principal components analysis (PCA), factor analysis (FA), and hierarchical cluster analysis (HCA) have been utilized as modelling tools that can expose the hidden interrelationships between the different hydrogeochemical parameters. Most of the times these techniques have been used alongside hydrogeochemical analyses to validate the findings or to generate the hypotheses pertaining to the environmental (both geogenic and anthropogenic) forcings controlling the co-release of As and F-.

Li et al (2012) integrated PHREEQC with HCA to study the co-occurrence of As and F- in the Datong Basin region of China. The speciation modelling revealed that the groundwater of the region was super-saturated for calcite while at the same time undersaturated with fluorite, this along with the high evapotranspiration rates led to elevated levels of F-. Groundwater was mainly Na-HCO3 type, and desorption of As and F- from Fe (hydr)oxides under alkaline pH were the main mode of co-occurrence. HCA was utilized in this study for the “spatial 25

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distribution” of As and F-. High As was found to be relegated to the fluvial plains of the Senggan River in Shanyin county, while F- was confined to the deeper groundwater of the mountainous region and in the shallow aquifers in the discharge zones of the basin.

Brahman et al (2013) utilized PCA and cluster analysis to study the interrelations and sources of total As, inorganic As and F- in the Mithi and Nagarparkar subdistricts of Tharparkar, Pakistan. This particular study also utilized an integrated approach of multivariate statistical assessment and speciation modelling using PHREEQC. The HCA methodology was not used for understanding the co-occurrence pathways, instead, it helped in the zonation of the groundwater into low, medium and high As and F- groundwater pairs. PCA did hypothesize the involvement of lithological influences on As and F- release, however, the nature of the lithological influences was not explained properly. Further investigation through speciation modelling revealed a lot of crucial details, the groundwater was found to be universally saturated with both calcite as well as fluorite, thus the calcite and fluorite equilibrium in the groundwater controlled the F- released. The highly saturated states of both the minerals was attributed to extensive water logging which also promoted the release of As. The majority of the inorganic As was found out to be in the form of As(V), which was released due to desorption from Fe (hydr)oxides.

Ain et al (2017) combined PCA with multiple linear regression (MLR) for the source apportionment of As and F- in the study area. During the initial PCA step it was revealed that

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anthropogenic F- was released from agrochemicals like phosphate fertilizers. While As which was grouped with Fe, Mn and PO43-, was released due to the desorption of As(V) from Fe (hydr)oxides when the pH was alkaline, at the same time PO43- (from fertilizers) also worked to compete with As(V) and thus influenced its desorption from the Fe (hydr)oxides sorption sites. Application of MLR after the PCA reinforced the involvement of anthropogenic influences on F- release, while the release of As was a mixture of both geogenic and anthropogenic influences, 6.2 Isotopic and ionic ratio tracing Experimental methods that have investigated As and F- co-behavior can be broadly categorized into two different approaches: 1) hydrogeochemical process tracing and source apportionment using isotope and ionic ratios of the sample and 2) understanding the mechanism of co-occurrence by simulating the aquifer conditions in a laboratory setting through batch, column or fractionation technique.

Farooqi et al (2007b) studied the sources of As and F- in east Punjab, Pakistan using an integrated hydrogeochemical and isotopic (δ34S, δ18O, δD and δ15N ) approach. Analyses of the S isotopes suggested that apart from desorption/reduction of FeOOH; oxidation of Asbearing sulphide minerals was the other geogenic mode of As release, especially under alkaline pH. Isotopic investigations further indicated the insignificant roles of evaporation and condensation in enriching F-. Hydrogeologically, the generation of Na-HCO3- type water due to the exchange of Ca2+ in the aquifer matrix by Na+ resulted in concentration of F- in the

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groundwater. Both F- and As were also found to reach the groundwater from anthropogenic sources like fertilizers. A more recent study from the Datong Basin (Pi et al., 2015) illustrated the application of isotope (δ18O signature) and Cl/Br molar ratios to investigate the corelease of As and F- at aquifers of varying depths. The study reported low arsenic levels in the shallow aquifer coupled with a widely different Cl- concentration with too little changes in δ18O values and Cl/Br values, implying recharge, and influence of evapotranspiration causing the development of an oxidizing condition, drop in pH and the precipitation of Fe (III) (hydr)oxides and thus weakening the As migration (equation 1). O2 +4Fe2 + +2HAsO24 ― +2H2O→4FeOHAs4(s) + 4H + -- recharge related weakening of As migration Interestingly, Fe (III) (hydr)oxides also immobilize the As (V) through adsorption. Fluoride behaves the opposite of arsenic as evaporation facilitates F- enrichment in the aquifer. On the other hand, high levels of aqueous HCO3- led to the over-saturation of calcite and dolomite, ultimately resulting in the dissolution of CaF2 and subsequent F- release. The same study reported considerable variation in δ18O and Cl/Br values for deeper aquifer with little change in the Cl- concentrations, indicating the much lesser influence of salt flushing and higher occurrences of the mixing of “laterally infiltrating water with connate groundwater”. As such, the influence of air influx was minimal, high As under such conditions was thus attributed to reductive dissolution of As bearing Fe (hydr)oxides. Fluoride, on the other hand, was attenuated in the deeper aquifers as there was a shift towards CaF2 precipitation from calcite/dolomite. Extremely high reducing conditions under which FeS precipitated

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and HCO3- was low, both As and F- were found lower than the WHO limit as the former sequestered by the sulfides while F- formed CaF2. 6.3 Fractionation and Laboratory Batch Investigations The essence of laboratory-based tracing mechanisms lies in the simulation of the native environments of high arsenic-fluoride aquifers. A major plus point of this approach is the ability to “tweak” the conditions to recreate the original environment, or to try many other permutations and combinations to create hypothetical situations to better understand As and F- co-occurrence and co-evolution. Sorption/leaching based experiments (Kim et al 2012; Kumar et al 2016) have been used recently for simulating As and F- co-release. Long term leaching experiments were used to study the release of both As and F- in the rhyolitic rocks and fluvio-lacustrine sediments from the Main Ethiopian Rift (MER) region of Africa (Rango et al., 2010). It was found that high levels of both As and F- in the volcanic rocks were traced to alkaline pH and Na-HCO3 type water. “Fractional crystallization” led to the concentration of incompatible elements like halogens (F-) and As (Rango et al., 2010). Fluoride release was traced to reverse ion exchange removal of Ca2+ from aquifer matrices by Na+, which does not allow the formation of CaF2 and thus the mobility of F-. Arsenic release, on the other hand, was a more complicated process, following a dual release pathway: 1) due to the dissolution of the silicic volcanic rocks and 2) through the desorption from secondary clay minerals and metal (Al, Fe) oxyhydroxides under the alkaline pH brought about by the formation of Na-HCO3 type water. Leaching studies involving both the elements are rare and in this particular case, a deeper analysis of the As and F- bearing fractions in the sediment and rocks was not conducted. 29

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In recent years the understanding of “operationally defined fractions” of As in aquifer material through sequential extraction has led to the further development of leaching and sorption based methods. The most widely used methodologies for sequential extraction of As (Wenzel et al., 2001; Romero et al., 2003; Bhattacharya et al., 2006; Selim Reza et al., 2010; Sailo and Mahanta 2014; Das et al., 2015; Kumar et 2016) revealed that the Fe (hydr)oxides fraction (both amorphous and well crystallized) was the “key” in desorptive release of As. Kim et al (2012) was one of the first studies to make use of this knowledge to design a batch sorption experiment showcasing the co-release of both As and F-. It was found from the study that, sediments with Fe (hydr)oxides released both As and F- at alkaline pH after initial spiking with a mixed solution of As and F- (Na2HAs(V)O4.7H2O–NaF). While sediments from which Fe (hydr)oxides had already been removed, did not release significant As nor F-, as these second class of sediment samples was not able to adsorb As or F- in the first place. Alkaline pH (around 8.5) was responsible for inducing charge neutrality (point of zero charge) in the Fe (hydr)oxides which resulted in co-release of As and F-. Similarly, another study (Kumar et al., 2016=) further combined both SEP as well as batch desorption to expand upon the results of Kim et al (2012). Kumar et al (2016) found that in alluvial sediments, Fe (hydr)oxides were the main sink/source of As and a secondary source of F-. Under alkaline pH (10) both F- as well As release was the maximum from raw sediments after spiking, while Fe (hydr)oxides removed sediments did not adsorb or release any significant amounts of both contaminants. Under neutral and acidic conditions, the volume of As and F- released decreased gradually for both raw as well as Fe (hydr)oxides removed sediments.

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7. Co-occurrence versus Co-contamination: Strength and Mechanism Co-occurrence and co-contamination are determined by the correlation between the two contaminants. As reported earlier that we intend to report the co-occurrrence of As-F in the groundwater where any four conditions may exist i.e. ii) Both As and F are not above than the WHO limit for drinking water; ii) Only As is higher than the WHO limit; iii) Only F is higher than the WHO limit, and iv) both As and F are higher than the prescribed limit. Out of these four scenarios, the only fourth condition should be called co-contamination. Herewith we are discussing the strength and mechanism of co-occurrences.

7.1.

Correlation of As and F with various factors

In accordance with redox potential, the correlations between the two contaminants are dissimilar (Kim et al., 2012). The adsorption capacity of Fe hydroxides for both fluoride and arsenic decreases with an increase in pH, releasing them into groundwater. This indicates that Fe-hydroxides are a correlating factor and play crucial role in their co-occurrence (Streat et al., 2008). As seen in the Chaco–Pampean plain, arsenic and fluoride concentrations in groundwater are inured by natural sources. Many natural causes attribute to the co-contamination like local geology. (Nicolli et al., 1989, 2001; Bundschuh et al., 2004; Gómez et al. 2009). Figure 2 (b, c) shows the correlation between the risk to fluoride and arsenic pollution worldwide which was found to be R2= 0.43 overall and R2= 0.41 and 0.20 for the Asian and African countries respectively. Overall, the correlation is not very

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significant which may be due to the diverse and varied distribution of these two contaminants across the globe. But some specific correlation studies have shown that there is a positive correlation in the occurrence of the two contaminants (Table 1).

Other factors leading to a correlation between the contaminants include the oxidising conditions of the groundwater aquifer (Table 1). Despite what might be expected, the reductive dissolution of Fe (hydro)oxides expands to increase in F- concentrations along with As concentration, especially in the aquifers with reducing the environment under a very strict ORP range. The geochemical behavior of an area can account for the deviation from the perfect correlation between As and F-. The reductive dissolution of As in alluvial aquifers keeps F- concentration intact with the reduction, here the correlations between As and F- are good to some extent. However, as the conditions become exceedingly reducing, As may precipitate out as sulfide minerals while F- continues to remain in the solution (Smedley and Kinniburgh, 2002; Kim et al., 2012), resulting in a diminished or no correlation between the two. Very high reducing conditions can occur with increasing depths therefore also decreasing the probability of correlation between As and F- at such depths. As levels decrease with increasing depth in reducing the environment due to sulfate reduction and is removed in the form of arsenopyrite (Hossain et al., 2016). This leaves high fluoride in cocontaminated systems and hence the correlation decreases with depth.

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Arid and humid climate seems to show high correlation in many areas across the globe (Table 1). Many places like Chichuahua, Mexico and Assam, India show that a strong correlation coefficient is found where the climatic conditions are humid and sub-tropical. Many studies show a positive correlation between the two in the Yuncheng basin of Northern China. fluoride is mostly found associated with arid and semi-arid conditions (Fuhong and Shuqin., 1998). This is attributed to the slow groundwater infiltration and flow rates which allows prolonged reaction time for fluoride release and build-up (Edmunds and Smedley., 2013). High salinity is also a driver of As desorption from minerals (Amini et al., 2008) and enhances fluoride leaching and migration in groundwater with soluble salts (Fuhong and Shuqin., 1998). Both As and F- are reported to get affected by other factors like HCO3- and pH. Many studies have a common conclusion that the desorption of As and F- from positively charged metal oxide surfaces caused at higher pH can be the reason of co-occurrence. These findings are also confirmed experimentally by some studies, which suggest that the cooccurrence was attributed to the dissolution of Fe oxy/hydroxides under alkaline and reducing conditions. The effects of these factors on the individual release of the two contaminants and the conducive environment for their co-occurrence is summarised in Table 2 and Figure 8.

7.2 Role of local geochemistry on As and F- behaviour The local geochemical properties of the region greatly impact the co-occurrence of As and F. the higher correlation found in arid and semi-arid conditions is because of the local geochemistry which creates oxidised groundwater (Currell et al., 2011). Dynamics of pH 33

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alter the adsorption capacity of metal hydroxides causing the release of F- and As groundwater (Streat et al., 2008). It is suggested in numerous studies that co-contamination may be associated with desorptive release and due to a change in pH (Bhattacharya et al., 2006; Currell et al., 2011). Although, the most collective and practical reason of cocontamination till date is believed to be the desorptive release from Fe-(hydr)oxides worldwide, the definite cause for many good correlations is yet to be comprehended. As fluoride and arsenic both have anionic natures in groundwater, As mostly as oxyanions such as arsenite and arsenate, both are attracted to the positive surface of Fe-(hydr)oxides (Smedley and Kinniburgh, 2002; Tang et al.,2009). Furthermore, an increase in pH results in their detachment from the metal hydroxide and subsequent release in groundwater.

Arsenic concentrations are found to be high in groundwater with high pH, DOC, alkalinity, Fe, and NH4–N levels and low NO3, and SO42-levels. Irrespective of insignificant participation of F in redox reactions, its associations with many other parameters were like that of As (Kim et al., 2012). However, F was found to show a stronger positive correlation with pH. In many aquifers under strong reducing conditions, a strong correlation between F- and pH was observed along with a weak correlation between As and F (Guo et al.,2008; He et al., 2013;Smedley et al., 2003). High F concentrations in As contaminated groundwater is shown to be a result of weathering of fluorine-bearing minerals under reducing environments (Das et al., 2003; Deng et al.,2009). In another study, this was attributed to the exchange of OH ions in clay minerals or micas at high pH (Guo and Wang, 2005). Nonetheless, dissolution related release of F- is not essentially dependent on pH, it is also associated with calcite 34

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precipitation by common ion effect (Kim and Jeong, 2005). Calcite precipitation which is generally increased in alkaline conditions enables further dissolution of fluorite if the groundwater is saturated with both fluorite and calcite simultaneously (Figure 8).

7.3 Anthropogenic contribution A number of reports are available worldwide, showing high concentrations of both As and F- in groundwater. Exposure to both the pollutants occurs through natural as well as anthropogenic causes. Human activities like mining, industrial and pharmaceutical waste generation, pesticide application, etc. contribute a significant amount to the environment. Such anthropogenic activities may also be the cause of combined exposure to both As and F. Additionally, there is a possibility of co-occurrence of these two pollutants in the groundwater from the discharge of wastewater as well as combustion of coal. In southern China and Inner Mongolia, concurrent exposure to As and F- sourced from coal combustion, were found to cause severe health hazards covering vast areas (Finkelman et al., 2002; Smedley et al., 2002; Wang et al., 1999).

Many studies show the anthropogenic contribution of fluoride and arsenic contamination individually. Phosphate fertilizers have high fluoride content (Kabata-Pendias and Pendias, 1984) and Fluoride concentrations are found high in groundwater due to runoff and leaching of phosphate-based fertilizers (Podgorski et al., 2018). Locally, fertilizers, sewage, pesticides, and mining runoff contribute largely as anthropogenic sources of fluoride in groundwater 35

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(Wen et al., 2013). The anthropogenic source of arsenic is mining effluent, coal burning, and groundwater pumping. Intensive groundwater pumping and land-use changes deeply impact the hydrogeochemistry of groundwater, altering the arsenic concentrations. It is evident that heavily populated regions are prominent As affected regions too, owing to these perturbations (Wang et al., 2017; Neumann et al., 2010). Groundwater pumping and irrigation change the flow patterns and hydrologic gradients which lead to altered geochemical conditions controlling As mobilization (Wang et al., 2017). Flushing of irrigational redox-active compounds like organic matter, increasing of dissolved oxygen, nitrate and sulfate further effects the As mobilization. Pollution in groundwater increases other chemical species which impact the release of these contaminants with unknown feedbacks (Geen et al., 2006) and also become an additional source of As in groundwater (Farooq et al., 2010). Other anthropogenic activities like sediment excavation, pond construction, the building of dams and levees also cause Spatio-temporal alterations which affect the rate of mobilization and transport of arsenic (Neumann et al., 2010). Mukherjee et al. (2006), gave several case studies from across the world of the anthropogenic source of arsenic contamination. Leaching from landfill sites close to aquifers may also be a source of As and F- in groundwater (Das et al., 2018). Mining effluents have highly oxidizing acidic discharges which may be capable to mobilize As (Smedley and Kinniburgh., 2002).

8. Future Scenario and Research Directions The concurrent release of As and F- in groundwater is controlled by several factors, most of which are yet to be known. Adequate information on the hydrogeochemistry of the 36

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contaminants is deficient, specially in the regions with high concentrations of both. Future research should be directed towards the understanding of the probable interactions between the co-occurring contaminants and among other parameters with the cocontaminants. The extent to which the local environmental, hydrological and geological conditions as well as the geochemistry of their host aquifers effect their co-occurrence is still obscure. How these anions act in the presence of the other, and their synergistic/ antagonistic relations are yet to be determined, which may further depend upon and vary under dissimilar conditions. Moreover, it opens a much broader avenue which invites further research on their behavior and nature of co-occurrence.

This work attempts to ring up the curtain on the problems like (i) The co-existence and interactions of As and F- in a natural setting as well as in controlled conditions, (ii) Release of As and F- into aqueous solution from solid and adsorbed species, and (iii) Controlling factors behind such release and the fate of their aqueous species in presence of each other. Co-occurrence of consequential contaminants like arsenic and fluoride in the groundwater is a less understood subject. This review paper has attempted to understand scenario, mechanism (Insight) and perspectives of As and F- co-occurrence in groundwater all around the world, emphasizing the recent reports from Asian countries. The probable reasons given for the co-occurrence at specific sites are summarised from various existing literature.

Presumably, there are areas where this problem remains unrecognized. The biological, as well as hydro-geochemical aspects, need to be considered more to understand the co37

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contamination. The concentrations of these contaminants in groundwater needs to be quantified and the anthropogenic sources need to be traced. The burden of the health effects of these pollutants are globally bearded and can only be stopped by a deeper insight into the subject. Therefore, the priority is to remediate the crisis by a better understanding of probable sources of co-contamination and early identification of the affected regions. Only then it is possible to mitigate the contamination or come up with another source of drinking water for the affected population.

Acknowledgment: We thankfully acknowledge the financial assistance provided by WIN foundation under the program to facilitates innovation in the areas of water and sanitation (WatSan) and Maternal and Child Health (MCH), primarily in India. We are indebted to the people involved in developing the partnership between IIT Gandhinagar and WIN foundation. References Acharya, G. and Barbier, E.B., 2000. Valuing groundwater recharge through agricultural production in the Hadejia-Nguru wetlands in northern Nigeria. Agricultural Economics, 22(3), pp.247-259.Ahmed, V.K.S.Æ.S., 2003. Inferring the chemical parameters for the dissolution of fluoride in groundwater 731–736. https://doi.org/10.1007/s00254-002-0672-2 Ahamed, S., Kumar Sengupta, M., Mukherjee, A., Amir Hossain, M., Das, B., Nayak, B., Pal, A., Chandra Mukherjee, S., Pati, S., Nath Dutta, R., Chatterjee, G., Mukherjee, A., Srivastava, R., Chakraborti, D., 2006. Arsenic groundwater contamination and its health effects in the state of Uttar Pradesh (UP) in upper and middle Ganga plain, India: a severe danger. Sci. Total Environ. 370, 310–22. https://doi.org/10.1016/j.scitotenv.2006.06.015 Ahn, J.S., 2012. Geochemical occurrences of arsenic and fluoride in bedrock groundwater: A case study in Geumsan County, Korea. Environ. Geochem. Health 34, 43–54. https://doi.org/10.1007/s10653-011-9411-5 Ahoulé, D.G., Lalanne, F., Mendret, J., Brosillon, S., Maïga, A.H., 2015. Arsenic in African Waters: A Review. Water. Air. Soil Pollut. 226. https://doi.org/10.1007/s11270-01538

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V., S., S., A., 2001. Dissolution of fluoride in groundwater: a water-rock interaction study. Environ. Geol. 40, 1084–1087. https://doi.org/10.1007/s002540100290 Vega, M., Pardo, R., Barrado, E. and Debán, L., 1998. Assessment of seasonal and polluting effects on the quality of river water by exploratory data analysis. Water research, 32(12), pp.3581-3592. Verma, S., Mukherjee, A., 2015. Geomorphological Influence on Groundwater Quality and Arsenic Distribution in Parts of Brahmaputra River Basin Adjoining Eastern Himalayas, in: Geostatistical and Geospatial Approaches for the Characterization of Natural Resources in the Environment. https://doi.org/10.1007/978-3-319-18663-4 Wang, Y., Shvartsev, S.L., Su, C., 2009. Genesis of arsenic/fluoride-enriched soda water: A case study at Datong, northern China. Appl. Geochemistry 24, 641–649. https://doi.org/10.1016/j.apgeochem.2008.12.015 Wen, D., Zhang, F., Zhang, E., Wang, C., Han, S., Zheng, Y., 2013. Arsenic, fluoride and iodine in groundwater of China. J. Geochemical Explor. 135, 1–21. https://doi.org/10.1016/j.gexplo.2013.10.012 WHO, 2017. Guidelines for Drinking-Water Quality, 2nd edition, Addendum to Volume 1 – Recommendations, World Health Organisation, Geneva, 1998, 36 pages, World Health Organization. https://doi.org/10.1016/s1462-0758(00)00006-6 WHO, 1993. Guidelines for drinking water quality. Geneva, Switzerland. Winkel, L.H.E., Trang, P.T.K., Lan, V.M., Stengel, C., Amini, M., Ha, N.T., Viet, P.H., Berg, M., 2011. Arsenic pollution of groundwater in Vietnam exacerbated by deep aquifer exploitation for more than a century. Proc. Natl. Acad. Sci. 108, 1246–1251. https://doi.org/10.1073/pnas.1011915108 Xie, X., Wang, Y., Su, C., Duan, M., 2013. Effects of Recharge and Discharge on δ2H and δ18O Composition and Chloride Concentration of High Arsenic/Fluoride Groundwater from the Datong Basin, Northern China. Water Environ. Res. 85, 113–123. https://doi.org/10.2175/106143012x13373575831196 Yanfeng, S., Dianjun, S., Xinhua, Z., Guangqian, Y., 2005. Screening report in areas of endemic arsenism and high content of arsenic in China. Zhongguo di fang bing xue za zhi = Zhongguo difangbingxue zazhi = Chinese J. Endem. 24, 172—175. Yang, X., Hou, Q., Yang, Z., Zhang, X., Hou, Y., 2012. Solid-solution partitioning of arsenic (As) in the paddy soil profiles in Chengdu Plain, Southwest China. Geosci. Front. 3, 901–909. 51

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52

भारतीय प्रौद्योगिकी संस्थान गाँधीनगर पालज, गांधीनगर, गुजरात 382 355 INDIAN INSTITUTE OF TECHNOLOGY GANDHINAGAR PALAJ, GANDHINAGAR, GUJARAT 382 355

E-Mail Web Tel Office

: [email protected] : www.iitgn.ac.in/academics/es/ : +91-863-814-7602 | : 07923952531 | Ext: 2531(O)|

Japan Society for the Promotion of Science (JSPS) alumni Associate Editor, Hydrological Research Letter Associate Editor, Groundwater for Sustainable Development https://www.researchgate.net/profile/Manish_Kumar138

Declaration: We declare to have no competing financial interest. We declare no conflict of interest.

Dr. Manish Kumar (Corresponding author)

IITGN

Dr. Manish Kumar Assistant Professor, Earth Sciences, Block-5 Journal336A, Pre-proof

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Figure 1: Population exposed to groundwater Arsenic and fluoride pollution in some countries (modified from Fewtrell et al., 2006 and Ravenscroft, 2007) 120

Fluoride Contamination

Arsenic Contamination

12

100

Fluoride Contamination

Arsenic Contamination

10

Number of people at risk (in million)

Number of people at risk (in million)

(a) 80

60

8

6

4

2

40

0

Ethiopia

Saudi Arabia

Egypt

Kyrgyzstan

Eritrea

20

0

India

(b)

China

Pakistan

Ethiopia

Saudi Arabia

Egypt

Kyrgyzstan

Eritrea

(c)

Figure 2: Plot of estimated population (×106) exposed to groundwater Arsenic and fluoride pollution in some countries (a); Overall correlation between Population at risk of F and Population at risk of As (b); Correlation between Population at risk of F and Population at risk of As in Asia and Africa (c); (modified from Fewtrell et al., 2006 and Ravenscroft, 2007)

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Af gh

Ne

kis tan

China

pa

Pa

Pa

kis tan

China

l

Ne

pa

Bhutan

I N D I A Bay of Bengal Arabian Sea

Bhutan

Bangladesh

Myanmar

Bangladesh

l

I N D I A Bay of Bengal

Myanmar

Af gh

an

an

ist an

ist an

Figure 3: Major Co-occurrence and co-relation studies of As-F- around the globe.

Arabian Sea

Sri a Lank

India Ocean

Sri a Lank

India Ocean

Figure 4: Arsenic and fluoride contamination in India (modified from Chakraborti et al., 2016; Bhattacharya et al., 2011; Mukherjee et al., 2016)

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As

F

_

Binding with _ OH _ H &_H_N

Binding with _ SH group

• • • •

GSH metabolism inhibition Metabolic enzyme inhibition Imbalance of trace metal Disruption of protein structure Antioxidant status catalase, GPx , GST, GSH, SOH

Free radicals . .. . . OH , O2 , NO , NOO , . . RO , ROO

Oxidative stress Protein oxidation Lipid peroxidation Damage DNA base

Dysfunction of protein Membrane damage Impaired DNA repair

Figure 5: Combined toxicity effect of fluoride and arsenic on living organisms.

-

As – F induced Socio-economic and health problem

Dermatological Effects • Dermal • Cardiovascular • Respiratory • Gastrointestinal • Endocrinological (diabetes mellitus) • Neurological • Reproductive and • Developmental • Cancerous Neurological Effects • Peripheral neuropathy symptoms including o Limb pain o Hyperpathia o Distal paresthesia o Hypesthesia o Calf tenderness o Distal limb symptom o Diminished tendon reflexes

First signs • Stomach • Nausea • Abdominal pain • bloody vomiting • diarrhoea

Followed by

Fluoride

Arsenic

Health

• • • • • • • • •

Respiratory Effects • Cough, • Shortness of breath, • Noisy chest while breathing, • Non-malignant • Malignant lung diseases Hepatological Effects • Noncirrhotic portal fibrosis • Hepatomegaly Gastrointestinal Effects • Dyspepsia • Nausea • Diarrhea • Anorexia Cardiovascular Effects • Ischemic heart disease, • Peripheral arterial disease • Systemic arteriosclerosis • Gangrene • Hypertension

Paleness Weakness Shallow breathing Weak heart sounds Wet, cold skin Cyanosis Dilated pupils Hypocalcaemia Hyperkalemia

Non-skeletal • Muscular problems • Degeneration of kidney • Melanosis and keratosis • Cancer susceptibility • Changes in blood pressure

Agriculture

Reproduction and Developmental Effects • Minimum exposed woman o Congenital malformation o Repeated childbirth malnutrition • Chronically exposed woman o Spontaneous abortions o Stillbirths o Preterm births o Low birth weight o Neonatal deaths Cancerous Effects • Skin o Squamous Cell Carcinoma o Multiple Basal Cell Carcinoma o Impending Skin Cancer o Arsenical Keratosis • Lung • Liver • Urinary tract • Bladder • Kidney • Other types of cancers

Skeletal • Dental fluorosis • Dental caries • Dental decay • Skeletal fluorosis • Pain in the bones and joints

Other possible effects • Muscle paralysis • Carpopedal spasms • Extremity spasms

Other

Increased per unit cost

Production cost increment

Indirect effect of pollution

Cost of improvement or prevention

Social Hazards and Poverty • Social uncertainty • Social injustice • Social isolation • Social rejections Superstition • “An act of the devil/impure air” • “A curse of god” • “The work of evil spirits”

Quality

Reduction in the product value

Social Instability • Social stigmatization and discrimination • Ostracized

Quality

Marketing

Marriage Related Problems • Divorced • Remain unmarried • Abandoned by their husbands • Marital tie becomes weaker • Dowry • Physical torture • Polygamy

Reduced demand for farming appliances and amenities

Food chain

Economic problem

• •

Social problems

Wage loss due to sick days Money lending for treatment

Figure 6: Socio-economic implications of groundwater arsenic and fluoride pollution and their concurrent implications.

• • • • • • •

Chlorella pyrenoidosa Ankistrodesmus braunii Chaetoceros gracilis Marine algae Fish Mushrooms Microbial action

• • • • • • • • • • • • • • •

Cryolite Topaz Bastnaesite Sellaite Villiaumite Fluorite Fluorapatite Carobbite Hornblende Apatite Fluorspar Igneous rocks of mafic origin Magmatic differentiation Limestone formation Disodiummono fluoro-phosphate

Arsenic

Biogenic • • • • • •

Marine algae Fish Mushrooms Microbial action Mussels Crustaceans

• • • • • •

Geogenic • • • • • • • • • • • •

Pyrite Arsenopyrite Arsenolite Olivenite Cobaltite Proustite Enargite Orpiment Realgar Tennantite Volcanic emissions Geothermal activity

Coal burning Oil refining Steel production Brick-making industries Phosphatic fertilizers Animal and urban waste

Anthropogenic • • • • • • • • • • • • •

Glass industries Smelting of metals Combustion of fuels Use of pesticides Insecticides Wood preservatives Pharmaceutical Leather preservatives Poisonous baits Manufacture of alloys Semiconductor devices Cattle and poultry feed Cotton defoliant

Exposure Hazard and Cancer Incidence

Fluoride

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Figure 7: Schematic flow diagram of probable As-F- interactions of three origins leading to the health hazard.

Holocene Deposit

Potential Risk

Shallow Groundwater

Vulnerable Zone

Evapotranspiration Well

Coal Combustion Mining

Proximity to River

Drinking Groundwater

Arsenical Fungicides Herbicides & Insecticides

High Arsenic

NO 3 - , PO 43leaching

High Fluoride

FeAsS Na 3 AlF6 CaF2 Volcanic Sediment

(Ca 5 (PO 4)3F) Magmatic process and deep groundwater movement concentrate fluoride

Volcanic Glass

Co-occurrence of Arsenic and Fluoride

[(Fe(S,As)2] Fe-OOH

As2 S3 Organic Matter Microbes Fe-OOH Organic Matter

CaCO 3

Na-HCO 3

Desorption under Alkaline condition Competetive adsorption High Salinity

Microbes AsS

Microbes

Cation Exchange with Clay Minerals

Aquitard

Precipitation of Ca2+

Anthropogenic Intrusion Water Table

Arsenic Contaminated Zone

Co-Contaminated Zone

Fluoride Contaminated Zone

Volcanic Sediment

Fe-OOH

Figure 8: Conceptual model depicting the source, process, and conducive environment of As, F, and their co-occurrence.

Journal Pre-proof Highlights: o First review on co-occurrence perspectives of Arsenic and Fluoride in the groundwater. o As and F co-occurrences is quite common yet concealed owing to one being dominant. o Arid areas are more suitable for co-occurrence with few reports in the Alluvium plains. o Local hydro-geochemical processes and seasonality greatly affect co-contamination. o Co-contamination can have severe health and socio-economic impacts around the World.

Table -1 Major co-occurrence of arsenic and fluoride around the globe: Range, Reasons and Strength of Co-occurences with reference. Table -1 Major co-occurrence of arsenic and fluoride around the globe

China

Asia

India

Continent- Country and Region studied

1. Assam 2. Bihar 3. Delhi 4. Jharkhand 5. Uttar Pradesh 6. West Bengal

1. Xinjiang 2. Shanxi province 3. Datong Basin 4. Yuncheng Basin 5. Songnen Basin

Highest Highest total total Arsenic in Fluoride in GW GW (µg/L) (mg/L) 0.68 23.4 0.118 8.32 0.100 32.0 1 14.32 12.8 3.19 3.20 14.47 11.15 0.97 8.5 0.4 6.3 0.73 25.1 14 22.1(pre14.4 (premonsoon), monsoon), 17.3 (post0.45 (postmonsoon) monsoon) 4.44 10.0 1.932 22 0.027 6.6 0.179 10 250 9.2 1550 10.4 469 22 2680 3.32 1860 6 27 6.6

Correlation and Co-occurrence

Probable reasons for co-occurrence

Co-occurrence observed Co-relations found: 0.79 ( Darrang, Assam) 0.73 (Lakhimpur, Assam)

 Fluoride was found to be prevalent in drier aquifers away from the river, whereas Arsenic was found closer to the river.  As-F co-occurrence was observed owing to desorption under alkaline pH.  Fluoride release by the dissolution of F bearing minerals, arsenic released by reductive hydrolysis of Fe (hydr)oxides  Desorptive release of both from Fe (hydr)oxides at elevated pH.  Seasonal variation also observed in the release of fluoride and arsenic.  Arsenic release is ascribed to reductive hydrolysis under the presence of organic matter whereas Fluoride release is dependent on evapotranspiration.

Reference

Gupta et al., 1999; Singh, 2004; Lalwani et al., 2004; Ahamed et al., 2006 ; Nayak et al., 2008; Borah et al., 2010; Kumar et al., 2010, 2016; Ray et al., 2013 ; Hazarika and Bhuyan, 2013; Das et al., 2016, 2017, 2018; Singh Patel et al., 2017 Patel et al., 2019 Co-occurrence observed.  The reason behind Fluoride release was CaF2 solubility and Wang et al., 1999; A positive correlation ion exchange with OH- whereas Arsenic release was attributed Smedley et al., 2003; was found in some to desorption from (hydr)oxides under elevated pH conditions. Zhang et al., 2003; studies. Wang et al., 2007, 2009; A positive correlation was linked with the high sodic Guo et al., 2012; environment. Murcott, 2012;  Reductive hydrolysis of Fe (hydr)oxides was found Li et al., 2012; responsible for Arsenic release in most of the studies.  Co-occurrence was predominant in high alkaline groundwater. Bian et al., 2012; Wen et al., 2010, 2013;  Arsenic and fluoride concentrations in groundwater were K Pi et al., 2015; controlled mainly by the dissolution-precipitation of CaChen et al., 2017 arsenate and fluorite under weak alkaline conditions.

Pakistan Bangladesh SouthKorea Argentina Mexico

America

1. Kalalanwala 2. Mailsi 3.Tharparkar 4. Rahim Yar Khan

1.9 1900 2400 4100 3960 683 507 107.23

21.1 21.1 22.8 60.5 56.4 35.4 29.6 26.4

Co-occurrence observed and some studies show a positive correlation. Many correlations were found to be low or not significant. R2= 0.63, 0.64, 0.39, 0.37

1. Barishal 2. Dhaka 3. Khulna 4. Pabna 5. Rajshahi 6. Lakshimpur

1.0 0.049 0.499 1.0 0.299 404

1.76 1.60 2.32 1.63 1.94 0.45

Co-occurrence observed but correlation not studied

100 113

2.74 7.54

0.535 >5 0.758 >3 3810 760 250 535 5300

14.2 >25 8.3 >6 6.3 8.3 12 14.2 10

Co-occurrence observed and correlation was found to be strong in arid conditions and weak in humid conditions. Co-occurrece observed and Strong positive correlation calculated (r=0.7, R2=0.51, r=0.8)

0.12 0.344 24.0 0.2451 134 149 39 73600 (geothermal wells) 344

16.0 9.7 17.0 11.10 3.6 17.8 4.55 17 (geothermal wells) 9.71

1. Mankyeong river floodplain 2. Geumsan County

1. La Pampa Province 2. La Pampa Province 3. Tucuman province 4. Cordoba Province

1. San Luis 2. Chihuahua 3. Michoacan 4. Guanajuato 5. Sonora 6. Durango

Strong positive correlation (r=0.72) in some studies whereas crrelation not clear in others. But co-occurrence is commonly observed.

 Both contaminants have few common sources. Some Farooqi et al.2007, 2009; anthropogenic sources like coal combustion were found Brahman et. al, 2013, 2014; responsible for high concentrations of both.  High alkalinity and salinity were found to stabilize and retain Rasool et al., 2015; Quarat-ul-Ain et al., 2017 fluoride and arsenic in groundwater.  High fluoride is maintained by the enhanced alkalinity as OHin solution replaces F-.  Reductive hydrolysis of Fe (hydr)oxides has been suggested as the arsenic releasing process.  Most of the existing correlation is between fluoride and arsenate (AsV), indicating that desorption could play a role.  The sources were found to be geogenic as well as Dhar et al., 1997, Hoque et al., 2003, anthropogenic.  The regions where co-occurrences were observed were Bhuiyan et al., 2016 predominantly hot and humid which enhances the release of the contaminants.  Heavy deposition of Holocene sediments and severe surface erosion for As. Desorptive release of both arsenic and fluoride from Fe Kim et al., 2012, Ahn 2012 (hydr)oxides at elevated pH. Desorption process under high pH conditions may control arsenic mobility whereas F seemed to be enriched by deep groundwater interaction with granitic rocks.  Sources were predicted to be of volcanic glass and Fluoride and Nicolli et al., 1989; arsenic released from Fe (hydr)oxides due to desorption under Smedley et al., 2002; Warren et al., 2005; the influence of alkaline pH.  Dissolution of volcanic glass in a NaHCO3 rich environment Perez, 2009; releases both fluoride and arsenic. This is accompanied by calcite Alarcón-Herrera et al., 2012 ; dissolution which releases more F-.  Fluoride and arsenic originate from volcanic sediments. Rise in Alcaine et al., 2013; pH, oxidation/reduction, adsorption/desorption, co-precipitation along with surface complexation with arsenic bearing minerals mobilized arsenic in shallow groundwater. Birkle and Merkel, 2000;  High groundwater withdrawals and elevated temperatures Westerhoff et al.2004; could lead to concentration of both pollutants. Mahlknecht et al., 2008;  Fluoride release is a function of dissolution of acid volcanic rocks whereas Arsenic release is due to desorption from Fe and Amadora et al., 2011; Gonzalez, 2011; Mn oxides and evaporation.  Magmatic processes concentrate both fluoride and arsenic in Alcazar et al., 2012; Herrera et al., 2013; rocks. Heavy groundwater abstraction ultimately leads to coVictor et al., 2013; occurrence. Gomez et al., 2013;  Both found in volcanic sediments, geothermal and mining Victor et al.2013, areas. Desorption likely to be a common pathway of coPrado et. al, 2013 occurrence.

USA Finland Slova kia Ethiopia West Africa

Europe Africa

1. Texas 2. Washington 3. Wisconsin 4. West Virginia

Co-occurrence observed  Co-occurrence either due to a hydrothermal system or due to Frost et al., 1993; and some studies show desorption of As retained on clay as well as desorption release of Reddy et al. 2011 (Report of Texas water positive, highly Fluoride. significant correlation  Mostly due to the local geology and characteristics of bedrock development Board) Camacho et al., 2011; (ρ= 0.58, p < 0.001). but anthropogenic contribution cannot be ignored. Luczaj and Masarik, 2015;  Significant correlation in geothermal waters Law et al., 2017 Co-occurrence observed  These geogenic contaminants are mainly derived from sulfide Backman et al., 1998; but correlation not Karro and Lahermo, 1999 mineralizations. studied.  High concentrations attributed to local geology- found in hydrothermally altered domains in Precambrian bedrock and volcanic sediments. Co-occurrence observed  The local geology and characteristics of bedrock. Backman et al.1998 but correlation not studied.

6.018 18000 1500 10

38.7 8.9 7.6 4.0

1. Uusikaupunki-Ylane 2. Kymi Province

11.5 138

3.95 3.0

1. Brezno

887

3.0

1. Main Ethiopian Rift (MER)

220

7.6

Co-occurrence observed  Oxidizing conditions and high pH conditions lead to desorption Rango et al., 2010, 2012 but correlation not of As and F- from stable phases like Fe (hydr)oxides in studied. fluvio/volcanic-lacustrine sediments.

1. Bolgatanga, Ghana 2. Burkina Faso

Several hundred

3.8

Co-occurrence observed  Areas of granite and of the crystalline basement could carry Edmunds et al., 1996; but correlation not Smedley, 2002 groundwater with unacceptably high fluoride concentrations. studied.

Table -2 Conducive environmental conditions for the occurrence and co-occurrence of Arsenic and Fluoride. Factors

Conducive condition for Fluoride release Sources o Minerals like Fluorides (NaF, CaF2, SnF2,) (Geogenic) Fluorspar, Fluorapatite, Cryolite, Amphiboles, and Micas. o F adsorbed on clay minerals (Kaolinite, Bentonite, Illite), active alumina, and Fe(III) oxyhydroxides (Goethite). o High-temperature environment including hydrothermal solutions. Sources Mining activities, phosphate fertilizer (Anthropogenic) effluents

Conducive condition for cooccurrence  Presence of oxides and hydroxides of metals especially Fe  Presence of a bedrock with the source minerals for both.  In the groundwater of the geothermal system

pH

High pH conditions

Organic matter Salinity Redox conditions Ionic Concentration Climate

Conducive condition for Arsenic Release o Oxides and hydroxides of metals (Mn, Al and Fe), elemental Arsenic, Sulfides, Arsenides, Arsenites, Arsenopyrite (FeAsS), Orpiment (As2S3) and Realgar (AsS), arsenian pyrite [(Fe(S, As)2], o As adsorbed on iron oxides and hydroxides o Active geothermal systems and associated groundwater. Mining, burning of fossil fuels, use of arsenical fungicides, herbicides and insecticides in agriculture, and wood preservatives High pH favors high Fluoride content in High pH is the driver of Arsenic desorption groundwater through leaching and from minerals desorption Fluoride is more enriched in groundwater High organic matter is a driver for microbial with a higher content of organic matter activity which enhances the As release in reducing conditions High salinity is reported to be associated with High salinity enhances As desorption from high fluoride in groundwater minerals Reducing environment results in F release Both reducing and oxidizing conditions enhance As mobilization High Bicarbonate, Sodium, Magnesium, and High Bicarbonate, Phosphate, Chloride, low Calcium, Iron and Aluminium are found Manganese, and Iron are often reported with with high F concentration in the high As groundwater. Semi-arid to arid climate Arid, semi-arid and also humid

Mining zones or as a result of fertilizer applications

High organic matter High salinity Reducing environment High bicarbonate content

Arid/Semi-arid climate or extensive groundwater pumping