REVIEW

Arsenic in groundwater in the southern lowlands of Nepal andits mitigation options: a reviewBarbara Mueller

Abstract: As in several other countries of Southeast Asia (namely Bangladesh, India, Myanmar, China, Vietnam, and Cambodia)arsenic (As) concentrations in the groundwater of the lowlands of Nepal (the so called Terai) can reach concentrations that areunsafe to humans using the groundwater as drinking water. Whereas Bangladesh has received much international attentionconcerning the As crisis, Nepal was more or less neglected. The first report about As contamination of the groundwater abovetoxic levels in Nepal was published in 1999. Twenty-four percent of samples analyzed (n = 18 635) from the Terai Basin exceededthe WHO guideline of 10 �g/L. Since the first overall survey from 2001, only sporadic information on the situation has beenpublished. The geological and geochemical conditions favour the release of the contaminant as As can be easily solubilized ingroundwaters depending on pH, redox conditions, temperature, and solution composition. The thin alluvial aquifers of the Teraiare some of the most severely As contaminated. These sediments constituting a hugh proportion of the Terai aquifers are derivedfrom two main sources: (i) sediments deposited by large rivers that erode the upper Himalayan crystalline rocks, and (ii) weath-ered meta-sediments carried by smaller rivers originating in the Siwalik forehills. The generally low redox potential and lowSO4

2− and high DOC, PO43−, and HCO3

− concentrations in groundwater signify ongoing microbial-mediated redox processesfavoring As mobilization in the aquifer. Other geochemical processes, e.g., Fe-oxyhydroxide reduction and carbonate dissolu-tion, are also responsible for high As occurrence in groundwaters. Originally, gagri filters (a two-filter system with chemicalpowder) and later iron (Fe)-assisted biosand filters were commonly used to remove As and Fe from well water in Nepal—these twooptions were believed to be the best treatment option at household levels. This review focus on the description of the overallsituation, including geogenic issues, occurrence of As in the sediments of the Terai, mechanisms for the release of As to thegroundwater, and mitigation options.

Key words: arsenic, arsenic contamination, release of arsenic to the groundwater, removal of arsenic, mitigation.

Résumé : Comme dans plusieurs autres pays de l’Asie du Sud-Est (a savoir le Bangladesh, l’Inde, le Myanmar, la Chine, leViêt-Nam, le Cambodge) les concentrations d’arsenic dans les eaux souterraines des plaines du Népal (connues sous le nom deTerai) peuvent atteindre des concentrations qui sont dangereuses pour les humains qui utilisent l’eau souterraine comme eaupotable. Tandis que le Bangladesh a reçu beaucoup d’attention internationale concernant la crise d’arsenic, le Népal a été plusou moins négligé. Le premier rapport sur la contamination des eaux souterraines par l’arsenic au-dessus des niveaux toxiques auNépal a été publié en 1999. Vingt-quatre pour cent des échantillons analysés (n = 18 635) du bassin de Terai excédait la lignedirectrice de l’OMS, soit 10 �g/L. Depuis la première enquête globale de 2001, on a publié que des informations sporadiques surla situation. Les conditions géologiques et géochimiques favorisent le rejet du polluant puisque l’arsenic peut être facilementsolubilisé dans des eaux souterraines en fonction du pH, de la condition d’oxydo-réduction, de la température et de la compo-sition de solution. Les aquifères alluviaux minces du Terai sont parmi les plus sévèrement contaminés par l’As. Ces sédimentsconstituant une très grande partie des aquifères de Terai proviennent de deux sources principales, soit (i) des sédiments déposéspar les grandes rivières qui érodent les roches cristallines du Haut-Himalaya, (ii) des sédiments métamorphisés météorisés portéspar de plus petites rivières prenant leur source dans les contreforts du Siwalik. Le potentiel d’oxydo-réduction généralement bas,les concentrations faibles de SO4

2−, et élevées de COD, de PO43− et d’HCO3

− dans les eaux souterraines signifient qu’il y a desprocessus continus d’oxydo-réduction par intermédiaire microbien favorisant la mobilisation d’As dans l’aquifère. D’autresprocessus géochimiques, par exemple, la réduction des oxydes-hydroxydes de fer et la dissolution de carbonates sont aussiresponsables de la présence élevée d’As dans les eaux souterraines. À l’origine, on utilisait généralement des filtres Gagri(système a deux filtres avec poudre chimique) et par la suite des filtres de sable bio aidés de fer pour éliminer l’arsenic et le ferde l’eau de puits au Népal—ces deux options étaient censées être la meilleure option de traitement au niveau des ménages. Cetterevue se penche sur la description de la situation globale (des questions géogéniques, la présence d’arsenic dans les sédimentsdu Terai, les mécanismes de rejet d’arsenic dans les eaux souterraines, les options d’atténuation). [Traduit par la Rédaction]

Mots-clés : arsenic, contamination a l’arsenic, rejet d’arsenic dans les eaux souterraines, élimination de l’arsenic, atténuation.

IntroductionIn the groundwaters of several countries of Southeast Asia

(namely Bangladesh, India, Nepal, Myanmar, China, Vietnam, andCambodia), arsenic (As) can naturally reach concentrations that

are hazardous to human health if geological and geochemical condi-tions favour the release of this contaminant. The World HealthOrganisation (WHO) has imposed a drinking water guideline witha value of 10 �g/L for As. When this value is exceeded, health risks

Received 3 August 2016. Accepted 16 December 2016.

B. Mueller. Deptartment of Environmental Science, University of Basel, 4056 Basel, Switzerland.Email for correspondence: bar.mueller@unibas.ch.Copyright remains with the author(s) or their institution(s). Permission for reuse (free in most cases) can be obtained from RightsLink.

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Environ. Rev. 25: 296–305 (2017) dx.doi.org/10.1139/er-2016-0068 Published at www.nrcresearchpress.com/er on 23 December 2016.

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are likely to occur. Excess uptake of As causes a range of adversehealth effects like characteristic skin lesions including pigmenta-tion changes, mainly on the upper chest, arms, and legs, andkeratoses of the palms of the hands and soles of the feet, and asthe most severe effect, cancer (Smith et al. 2000; Adhikari andGhimire 2009).

Arsenic itself is not found in high abundance in the Earth’scontinental crust; it is less abundant than several of the “rare-earth”elements. Unlike the rare-earth elements, however, As is com-monly concentrated in sulphide-bearing mineral deposits, and ithas a strong affinity for pyrite, one of the more ubiquitous min-erals in the Earth’s crust. Arsenic is also concentrated in hydrousiron (Fe)-oxides and clay minerals. Arsenic can be easily solubi-lized in groundwaters depending on pH, redox conditions, tem-perature, and solution composition. Many geothermal waterscontain high concentrations of As. Natural As in groundwater atconcentrations above the drinking water guideline of 10 �g/L isnot uncommon. A small number of source materials are now recog-nized as significant contributors to As in water supplies: organic-richor black shales, Holocene alluvial sediments with slow flushingrates, mineralized and mined areas (most often gold deposits), volca-nogenic sources, and thermal springs. Two other environments canlead to high As: (i) closed basins in arid-to-semi-arid climates (espe-cially in volcanogenic provinces) and (ii) strongly reducing aqui-fers, often composed of alluvial sediments but with low sulphateconcentrations. Young sediments in low-lying regions of low hy-draulic gradient are characteristic of many As-rich aquifers. Ordi-nary sediments containing 1–20 mg/kg (near crustal abundance) ofAs can give rise to high levels of dissolved As (>50 �g/L) if initiatedby one or both of two possible “triggers”—an increase in pH above8.5 or the onset of reductive Fe dissolution. Other important fac-tors promoting As solubility are high concentrations of phos-phate, bicarbonate, silicate, and (or) organic matter in the groundwaters. These solutes can decrease or prevent the adsorption ofarsenate and arsenite ions onto fine-grained clays and especiallyFe-oxides. Arsenite tends to adsorb less strongly than arsenate,often causing arsenite to be present at higher concentrations. Thegeologic and groundwater conditions that promote high As con-centrations are now quite well known and help identify high-riskareas (Nordstrom 2002; Smedley and Kinniburgh 2002). The watertable within the Indo-Gangetic Basin, including the Terai, alluvialaquifer is typically shallow (<5 m below ground level). Abstractionof groundwater can also influence As flux: it can flush aqueous Asfrom the aquifer; irrigation pumping protects deeper groundwa-ter in some instances, by creating a hydraulic barrier, but it seemsthat high-capacity deep pumping may draw As down to levels inthe Bengal aquifer system that are otherwise of good quality(MacDonald et al. 2016).

Whereas Bangladesh has received much international atten-tion concerning the As crisis (e.g., Hug et al. 2011 and referencestherein), Nepal was more or less neglected, though the populationof the southern lowlands of Nepal (the so called Terai, the Indo-Gangetic Plain of southern Nepal) face the same As contaminationof the groundwater (Nakano et al. 2014). The study of As concen-trations in the groundwater in Nepal began only after the severityof the As contamination problem in the Bengal delta was recognizedin 1998. The first report of As contamination in groundwater abovetoxic levels in Nepal was made from the Terai Basin (Sharma 1999).Twenty-four percent of samples analyzed (n = 18 635) from the TeraiBasin exceeded the WHO limit of 10 �g/L (Shrestha and Shrestha2004). Since the first overall survey conducted by WHO (2001), onlysporadic information on the situation has been published. Avail-able documents later indicated that the region of As contamina-tion extends into 24 districts in Nepal, including all 20 Teraidistricts and four hill districts (Bhattacharya et al. 2003; Neku andTandukar 2003; Shrestha et al. 2003; FAO 2004; Tandukar et al.2005; Panthi et al. 2006; Maharjan et al. 2006; Pokhrel et al. 2009;Emerman et al. 2010; Thakur et al. 2011). Although the Terai con-

stitutes less than 20% of Nepal’s surface, it contains over half ofthe total arable land and is home to about 50% of the Nepalesepopulation, i.e., 30 million inhabitants. Groundwater is the mainsource of water for drinking and irrigation in the Terai area. Over90% of the Terai population draws groundwater from tube wellsfor drinking, household use, and irrigation (Guillot et al. 2015).

According to some publications, 25 058 tube wells in the Terairegion have been tested for As, of which 5686 tube wells (22.7%)exceed the WHO As guideline (As = 0.01 mg/L) and 1916 tube wells(7.6%) exceed the Nepal Interim As Standard (As = 0.05 mg/L)(Panthi et al. 2006). It is estimated that there are perhaps 200 000tube wells in the Terai region and that 3.5 million Nepalese haveno access to drinking water that does not exceed the WHO Asguideline (Mahat and Shrestha 2008; Mahat and Kharel 2009;Pokhrel et al. 2009). In the most recent report from the NationalArsenic Steering Committee/National Red Cross Society (NASC-NRCS2011), the total database covers 1.1 million wells tested between theyears 2003 and 2008. Approximately 1.73% showed values abovethe Nepal drinking water standard of 50 ppb, while approxi-mately 5.37% of tube wells contain 11–50 ppb of As concentration.The percentage of all tube wells exceeding 50 ppb varies from0.05% of the wells in the district of Jhapa to 11.69% in the district ofNawalparasi.

The most severe As contamination is prevalent in several dis-tricts of the Terai, namely Nawalparasi, Bara, Parsa, Rautahat,Rupandehi, and Kapalivastu (Shrestha et al. 2014). Maharjan et al.(2005) reported that 29% of more than 20 000 tube wells had Asconcentrations exceeding the WHO guideline (10 �g/L), that theprevalence of arsenicosis varied between 1.3% and 5.1% (average of2.6%; see NRCS–ENPHO 2002; Yadav et al 2011) among four inde-pendent surveys, and that approximately 0.5 million people inTerai were at risk of consuming water with an As concentra-tion >50 �g/L, the maximum permissible limit for Nepal (Shresthaet al. 2003). It was found that overall prevalence of arsenicosisamong the subjects ≥15 years old was 6.9%, which was comparablewith those found by the same examiner in As-contaminated areasin Bangladesh, and that males had prevalence twice as high asfemales, which could not be explained by the difference in theexposure level Maharjan et al. (2005). These reports have alertedthe decision makers of the government as well as non-governmentalagencies involved in controlling the water supply. As a conse-quence, in 2003 the National Arsenic Steering Committee (NASC)was formed, involving major stakeholders from the drinking wa-ter and sanitation sectors (Shrestha et al. 2003). The NASC workedin collaboration with the Environment Public Health Organiza-tion (ENPHO) to perform testing on 18 635 tube wells in 20 Teraidistricts, under a program called the State of Arsenic in Nepal2003. All data collected revealed that the concentration of Asvaried both spatially and seasonally, suggesting the possibility ofspatial variation owing to geospatial conditions such as latitude,longitude, and depth of tube well. The temporal distribution of Asshowed seasonal dependence with lower concentration in winterand higher concentration in summer (Yadav et al. 2012).

Geological situation of the Terai regionNepal is a landlocked country in South Asia, located between

Tibet to the north and India to the south, east, and west. With atotal land area of 147 181 km2, the country is characterized by adiverse, rugged, and undulating topography, geology, and in gen-eral by a cold climate. Nepal is predominantly mountainous, withelevations ranging from 64 m above sea level to 8848 m at the peak ofthe world’s highest mountain, Sagarmatha (Everest), within a span of200 km. Approximately 6000 rivers and rivulets, with a totaldrainage area of about 194 471 km2, flow through Nepal, whereof76% of this drainage area is contained within Nepal. The topo-graphic variations in Nepal are largely controlled by geology (BGSReport 2001; Thakur et al. 2011). The geology of Nepal marks the

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transition where the Southern Gondwanaland collided with theNorthern Eurasianland, lifting the sediments of the then TethysSea to form the Himalayas. As a consequence, the southern andnorthern parts of Nepal differ widely in their formations. The Archeancrystalline formations deep beneath the Alluvium of the Terai, as wellas the marine sedimentary deposits forming the high Himalayas,and the Siwalik formation formed by the then east–west flowingrivers can be found within this confined space (Yadav et al. 2015).

The prominent mountain chain in Nepal—the Himalayas—isbuilt up by four major Himalayan tectonic units: (1) the TethysHimalaya, delimited at the base by the South Tibetan Detachmentsystem; (2) the Higher Himalayan Crystallines, delimited at thebase by the Main Central Thrust I; (3) the Lesser Himalaya, dividedinto upper and lower Lesser Himalaya, delimited at the base bythe Main Boundary Thrust; and (4) the Siwaliks, delimitated at itsbase by the Main Frontal Thrust and the Quaternary forelandbasin. These units span a wide range of various rocks being met-amorphic, sedimentary, and igneous in origin, making it possiblefor their differential erosion to account for some of the ground-water As heterogeneity we see in the foreland and delta (i.e.,Gurung et al. 2005; Shah 2008; van Geen et al. 2008; Guillot et al.2015). The Terai Plain is an active foreland basin consisting ofQuaternary sediments that include molasse units along withgravel, sand, silt, and clay. Most of the rivers in the Terai flowfrom north to south. All major rivers originate in the high Hima-layas, whereas minor rivers also emanate from the nearby SiwalikHills, and therefore deposit sediments in the form of a fan alongthe flank of the Terai basin. Fine sediments and organic materialare deposited in inter-fan lowlands, wetlands, and swamps (Sharma1995). The Siwalik lithofacies are strongly diachronous, and fur-ther complicated by a variable addition of micaceous sands andarkose, which are locally derived from southward-draining tribu-taries from the emerging Himalayas. The most typical Siwaliklithologies are conglomerates, “salt and pepper” micaceous sand-stones, blue-grey siltstones, clay-stones, red (Fe-rich) shales, andminor lignite. Potential adsorption substrates and co-precipitationhosts for As are common throughout the finer-grained Siwalikfacies, as Fe mineralization, as sulphides, or as clays. In Nepal, thegroundwater As is of relatively local provenance, being deriveddirectly from eroded Siwaliks (Stanger 2005).

High monsoon precipitation (1800–2000 mm) and year-roundsnow-fed river systems recharge the Terai sediments, giving thema high potential for groundwater resources. Shallow aquifers(<50 m) are generally unconfined or semi-confined, whereas deepaquifers (>50 m) are mostly confined by impermeable clay layers.

The aquifer system is highly sensitive to precipitation (Gurunget al. 2005).

The geology of the Terai region of Nepal itself is on the wholecomparable to the Bengal Delta Plain and is a continuation of theIndo-gangetic trough. The Terai Plain covered by recent and olderalluvium comprises channel sand and gravel and outwash depos-its. These fluviatile deposits are cross-bedded, eroded, reworked,and redeposited because of regular shifting of stream channels.Geomorphologically, the Terai Plain is divided into two zones: theBhabar zone in the north and the main Terai zone in the south.They have diverse hydrogeological characteristics and are sepa-rated by a line of natural springs. The Bhabar zone is a narrowextension of a recent alluvial and colluvial fan deposit at thebottom of Siwalik Hills (Kansakar 2004). It consists of thick depos-its of gravel, pebble, and boulder mixed with sand and silt. Sedi-ments in the main Terai zone were deposited by braided rivers,which regularly changed their course. As a result, clay, silt, sand,and gravel deposits of varying thicknesses occur interlayered witheach other. The Terai Plain has a multiple aquifer system (Yadavet al. 2011).

So far the most intensively studied Terai province concerninglocal geology and As-contaminated groundwater is Nawalparasi.This district lies in the Terai Plain as a continuation of the Indo-Gangetic Plain (Fig. 1). It has a gentle slope towards the south froman elevation of 200–300 m in the north to a low of 63 m in thesouth near the Indian border from the mean sea level (Upreti2001). From the Indian border, Nawalparasi district extends north-ward across Narayani River (one of the major rivers of Nepal)alluvium then across the low gradient fan of locally derived allu-vium and finally into the Himalayan foothills (also known as Chu-ria hills) (Hagen 1969). The lithology of the Terai sedimentarybasin belongs to Holocene alluvium, which includes the presentday alluvial deposits, channel sand, gravel deposits, and outwashdeposits (Yadav et al. 2014). The district has three distinct hy-drogeological zones: the Siwalik Hills, the Bhabar recharge zone,and the Terai Plain unconsolidated Holocene floodplain sedi-ments. The northern part of the district is bounded by the steeplysloped Siwalik Hills, which are composed of sedimentary rockssuch as sandstone, siltstone, mudstone, shale, and conglomerates.Immediately south lies the Bhabar zone, which is composed ofunconsolidated sediments that are porous, coarse such as gravel,cobbles, and boulder material, thereby making the Bhabar zonehighly permeable (Kansakar 2004; Shrestha 2007). A major river,the Narayani (also known as Gandaki), which descends from theHigher Himalaya, flows along the eastern boundary of the Nawal-

Fig. 1. Groundwater arsenic testing in districts of various development regions of Nepal (from Thakur et al. 2011).

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parasi district and has had a major influence on the underlyingunconsolidated Holocene fluvial deposits that comprise the flood-plain aquifer system. Unlike other regions of Terai, where finersediments typically increase toward the south, finer sedimentspredominate in the north and sand and gravels are found near theNepal–India border (S.D. Shrestha et al. 2004). In the areas withfine-grained sediments, elevated concentrations of As are typi-cally recorded (Brikowski et al. 2004, 2014; Diwakar et al. 2015).

Small ephemeral rivers originating from the Siwalik frontalmountains disappear upon entering into the Indo-Gangetic Plainand reappear again in Nawalparasi. Hence, small natural pondsand river meanderings were observed as characteristic geomor-phic features of the area. Therefore, close to the frontal mountainchain, the Indo-Gangetic Plain consists of boulder- to gravel-sizedsediments, while soils further south consists dominantly of fine-grained sediments. Guillot et al. (2015) report the lithology ofsledge core samples from five drill holes, showing various coarse(millimetric) to fine-grained (micrometric) sediments in the Narayanibasin. They distinguished light-grey to dark-grey sands; grey,greenish-grey to brown–grey and yellow–brown silts; and light-grey to black–grey, yellow–brown, and black clay with occasionalgravel layers. Macroscopic observations showed that on average,the drilled sediments are composed of 33% silts; 30% grey to blackclays, 27% brown clay, 9% fine-grained silt and sand, and less than1% calcrete. Sands, silt, and clay sediments often contained micasthat were occasionally massive to laminated, bioturbated, and (or)also containing roots and plant debris. Binocular observationsshow that the detrital minerals in the silt fraction are dominatedby quartz, biotite, muscovite, K-feldspar, calcite, and dolomite asmajor phases and garnet, zircon, and monazite as heavy minerals.In the region of provenance of the Narayani basin, the TethysHimalaya includes 10 km of various metasedimentary rocks (lime-stones, calc-schists, shales, and quartzites) ranging from Cam-brian to Jurassic. There is also the Manaslu leucogranite emplacedwithin the Tethyan rocks. The Higher Himalayan Crystallines area metamorphic stack, including from base to top: paragneisses(metapelites and metapsammites), gneisses with calcsilicate min-erals (diopside and amphibole), and orthogneisses representingmetamorphosed Lower Paleozoic granites. The Lesser Himalayaconsists of mostly unfossiliferous metasediments and some dolo-mitic meta-carbonates alternating with dominant black schists,aluminium-rich schists, and quartzites. Amphibolites occur in bothof these groupings. The Siwaliks represent the Cenozoic forelandbasin of the Himalayan belt with local thickness of 6 km in Nepal.They are divided into three units having a typical coarsening-upwardsuccession. The lower unit consists of fluvial channel sandstonesalternating with calcareous paleosols; the middle unit consists ofvery thick channel sandstones with minor paleosols; and the upperunit mainly hosts conglomerates of gravelly braided river deposits(after Guillot et al. 2015).

Arsenic in groundwater in the TeraiThe worst affected districts in Nepal include Nawalparasi (west-

ern region), Rautahat and Bara (central region), and Bardia (mid-western region). These districts, together with Parsa, Rupandehi,Kapilbastu, and Banke were a priority for testing, water-supplymitigation, and health screening (BGS Report 2001). The spatialand temporal distribution of elevated groundwater As in Nepal isunique in South Asia. In the Terai districts, elevated As is foundexclusively in the foreland basin south of the Main Frontal Thrust,on the undisturbed floodplain. Surficial aquifers here are formedfrom material eroded from the thrust wedge (immediately northof the Main Frontal Thrust), which is composed of earlier flood-plain and later debris fan sediments exhumed by thrusting. Arse-nic occurrences are further limited to the areas immediatelydownslope from exposures of the fine-grained Lower SiwalikFormation (Smith et al. 2004), comprised of meandering stream

deposits laid down during the initial uplift of the Himalayas(Brikowski et al. 2014). The thin alluvial aquifers of the Nawal-parasi district are some of the most severely As contaminated inthe Terai region. Diwakar et al. (2015) state that the alluvial sedi-ments comprising the Terai aquifers in this district are derivedfrom two main sources: (i) sediments deposited by large rivers thaterode the upper-Himalayan crystalline rocks, and (ii) weatheredmeta-sedimentscarriedbysmallerriversoriginatingintheSiwalikfore-hills. The aquifer itself is characterized by Ca-HCO3 type water andis multi-contaminated, with the WHO guideline values exceededfor As, Mn, and F in 80%, 70%, and 40% of cases, respectively. Themiddle portion of the floodplain is heavily contaminated with As,predominantly as As(III). The river water displayed some evidenceof reductive processes in the hyporheic zone contributing As, Fe,and Mn to baseflow and also had elevated fluoride Diwakar et al.(2015). Fifty-five percent of water samples collected from streamsthat drain the Terai, sedimentary rocks of the Siwalik Group andcarbonate and low-grade metamorphic rocks of the Lesser Hima-laya, had ≥0.01 ppm of As (Mukherjee et al. 2009).

Provenance of the aquifer sediments is relevant for tracing thesource of As. As already mentioned, there are two possible sourcesfor the Terai sediments—the Siwalik hills and the higher Himala-yas. Sediments carried from the Siwalik hills by the minor riversseem to release more As than those carried by major rivers fromthe higher Himalayas. Rare-earth elements and other charged cat-ion elements like Th, Sc, Hf, and Zr are highly immobile in mostgeological processes, and thus they can be used for provenancestudies. The observed enrichment of incompatible elements isalso indicative of a felsic source. Sediments hosting As-contaminatedaquifers are therefore probably homogeneous mixtures of differ-ent types of rocks, with a felsic source. Studies of As contamina-tion of groundwater of the Bengal delta have demonstrated thegeological control and found that high concentration of As isrestricted to the Holocene sediments rich in organic matter. Av-erage As content of the Terai sediments is within the range ofnormal sediments (9 ppm). Abundances are greater in finer sedi-ments such as black clay (maximum 31 ppm) than in coarsersediments (silt and fine sand, 3 ppm). The sediments representhomogeneous mixtures of a wide range of parent rocks of felsiccomposition. Significant As-leaching rates indicate that the Teraisediments have high potential for As release, and that pH and redoxconditions play crucial roles Gurung et al. (2005). Paudyal (2011) men-tion that at present there exist several possible natural sources ofAs in Nepal. On the basis of chemical and mineralogical analysis ofcollected rock, minerals, soil, and water samples from differentparts of Nepal, several primary sources of As have been identified(Sharma 1999; Sah et al. 2003). The sulphide minerals from thepolymetallic deposit of Ganesh Himal, iron ore of the Phulchaukiarea, ferruginous concretions of Tertiary deposits, bituminouscoal of the Tosh area, Kalimati clay of the Kathmandu Valley, andsediments from hot spring water all show high values of As con-centration. Ferruginous quartzite, sandstone, and mudstone alsoshow comparatively higher values of As. The above mentionedminerals, rocks, and sediments could therefore represent the pri-mary sources of As in Nepal (Paudyal 2011).

In Nawalparasi district, clays contain particularly high amountsof Fe, in the range of 21.9–59.9 g/kg (2%–6% in sediments). To-gether with the high levels of Fe, high concentrations of Al werealso extracted from the sediments (2.75–34.1 g/kg). Fe and Al in thesediments were positively correlated with As, with correlationcoefficients of 0.607 and 0.444, respectively. Arsenic is retainedabundantly in finer particles like clay minerals, where it formsseveral different types of phases including ion exchange phases,carbonate and sulphide phases, ferric or manganous oxide andhydroxide phases, and soil organic matter phases depending onpH and redox potential (Eh) (Nakano et al. 2014). Yadav et al. (2015)describe that As concentration varied from 0.22 to 0.64 ppm (mean0.36 ppm) in sediment samples. Comparatively, a higher concen-

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tration of As was observed mostly in the fine-grained clay sedi-ments (black and yellow) than in coarse-grained sediments. Avariety of Fe minerals in the Nawalparasi aquifer system are keyhost phases for As. Specific examples include goethite (�-FeOOH),authigenic pyrite (FeS2) in deeper organic-clays, and ferrihydriteflocs (Johnston et al. 2015). Vertical distribution of Fe followedsimilar distribution pattern as that of As, showing its higher andlower concentrations in clay and fine-sand, respectively. High As-yielding aquifers also contained higher percentages of Ca, silica,Al, and Fe. Arsenic occurs generally in oxyanionic forms in aque-ous environment. The hydrogeochemical data for groundwater ofthe Terai Alluvial Plain aquifers suggest predominantly reducingcharacter, with high HCO3

− and low SO42− and NO3

− concentra-tions. Elevated HCO3

− levels result primarily from the oxidation oforganic matter, while low SO4

2− levels are a result of sulphatereduction (Bhattacharya et al. 2003). Yadav et al. (2012) found threetypes of tube wells that are used as a source of drinking water inthe Terai region that all vary by depth. These three include shal-low tube wells (<50 m deep), deep tube wells (>50 m deep), anddug wells (up to 20 m or more). A majority of them (98%) wereshallow tube wells. The depth of dug wells displayed various Asconcentrations. The depth of deep tube wells ranged from 1 to183 m. Virtually all (97%) of the tested tube wells that had As levelsexceeding WHO guidelines were of a depth less than 20 m. At thisdepth range, more than 8% of tube wells had As levels above10 mg/L, while only 2% of tube wells had levels above 50 mg/L. At adepth of 21–50 m, 4.7% and 1.3% of the water in tube wells had Asconcentrations that exceeded the 10 and 50 mg/L guideline levels,respectively. Similarly, at a depth greater than 50 m, tube wellshaving an As concentration that exceeded guideline values (10 and50 mg/L) were significantly fewer in number. Therefore, it seemsthat tube wells having a depth less than 20 m had on averagehigher As concentrations. Most of the known wells record a highAs concentration in March, and a low value in May and Septem-ber. A general pattern of low As – low piezometric level, high As –high piezometric level can be observed (S.D. Shrestha et al. 2004).According to Emerman (2005), central Nepal does not contain onegeographically limited source of As and that nearly all riversshowed elevated levels of As. Nearly all rivers also showed elevated lev-els of Cu, Co, Fe, and Ni, while fluvial Zn was very close to theglobal background level. Therefore, As mineralisation may be as-sociated with the mineralisation of Cu, Co, Fe, or Ni, but probablynot with Pb–Zn mineralisation (Pb and Zn are almost always asso-ciated). Bhusal and Paudyal (2014) clearly state that the distribu-tion and occurence of As is controlled by geological material andmuch less by topography and not by land use, artificial fertilizers,pesticides, and other organic additives.

Mechanisms of arsenic release to groundwaterSince the fundamental work by Nickson et al. (2000) some sci-

entific articles about the specific situation and mechanisms of Asrelease to the groundwater in the Terai in Nepal have been pub-lished. As outlined by Nickson et al. (2000), the As in the ground-water derives from reductive dissolution of As-rich Fe-oxyhydroxidesthat exists as a dispersed phase (e.g., as a coating) on sedimentarygrains. The reduction is driven by microbial degradation of sedi-mentary organic matter (O2 consuming, O2 as electron acceptor)and the redox process that occurs after microbial oxidation oforganic matter takes place as soon as dissolved O2 and NO3 aredisappeared. Strong correlation between dissolved organic car-bon (DOC) and As in groundwater suggests that the microbialdegradation of organic matter in the sediment results in an over-all reducing environment and facilitates the release of As in thegroundwater (Halim et al. 2009). Whilst As release by the dissolu-tion of arsenious pyrite is still recognized as a minor contributingbut widespread process, a consensus view emerges in which thedominant process is, initially, the fixation of aqueous As by sorp-

tion onto Fe-oxide, Mn-oxide, or clay surfaces during high-redoxmedium-pH conditions (i.e., about 5.5–6.5). Subsequently, desorp-tive release of As occurs as groundwater becomes more reducingand alkaline (i.e., negative Eh and pH > 6.5), principally as thebyproduct of bacterially mediated FeOOH dissolution. Since thereducing agent is buried organic material, such as peat, man-groves, reed-swaps, etc., and since the predominant adsorptionsubstrate is goethite or its analogues, with clay, high-Fe and high-organic sedimentary environments are evidently prerequisites forthe modern release of As (Stanger 2005).

Arsenic (As, atomic number = 33) is a ubiquitous element, whichranks 20th in the earth’s crust. Arsenic exists in four oxidationstates—+V (arsenate), +III (arsenite), 0 (arsenic), and −III (arsine). Ar-senic is unique among the heavy metalloids and oxy-anion formingelements. Its sensitivity to mobilization largely depends on the pHvalues typically found in groundwater (pH 6.5–8.5) under bothoxidizing and reducing conditions. The valency and species ofinorganic As highly dependent on the redox conditions (Eh) andthe pH of the groundwater. Arsenite, the reduced trivalent form(As (III)), is normally present in groundwater (assuming anaerobicconditions) while arsenate, the oxidized pentavalent form (As (V)),is present in surface water (assuming aerobic conditions). In gen-eral, inorganic As species are more toxic than organic forms of Asfor living organisms. As already mentioned, redox potential andpH basically control As speciation in natural environments. Inor-ganic As primarily occurs as arsenic acid (H3AsO4) under oxidizingconditions, and predominates only at extremely high Eh valuesand low pH (<2). Within a pH range of 2–11 it is replaced byH2AsO4

− and HAsO42−. At low Eh values, H3AsO3 (arsenious acid)

exists up to moderately alkaline pH but is replaced by H2AsO3− at

pH > 9.2 (Thakur et al. 2011; Zakhaznova-Herzog and Seward 2006).Bhattacharya et al. (2003) report that the groundwater in the

Terai is mostly near-neutral to alkaline within a pH range of 6.1–8.1. Redox potential (Eh) levels between −0.20 and −0.11 V suggesta fairly reduced condition in the aquifers. The groundwater ispredominantly of Ca-Mg-Na-HCO3

−-type with HCO3− as the princi-

pal anion and low levels of Cl− and SO42−. Low NO3

− coupled withelevated NH4

+ concentrations in this groundwater reflects thedissimilarity nitrate reduction in the aquifers. Moreover, redoxlevels (Eh < −0.2 V) for sulphate reduction are sufficiently low,which facilitates the reduction of Fe3+ and Mn4+ in the aquifersediments. The source of As in the subsurface environment is geogenic,and principally mobilized through natural interaction of theaqueous phases with the aquifer sediments under anoxic condi-tions. The sequence of redox reactions or terminal electron ac-cepting processes prevalent in the aquifers plays a critical role incontrolling the As chemistry in groundwater. The predominantterminal electron accepting processes in the sedimentary aquifersare O2 reduction (aerobic respiration), NO3

− reduction (denitrifi-cation and dissimilatory nitrate reduction), Mn4+ reduction, Fe3+

reduction, and SO42− reduction with oxygen (O2), NO3

−, Mn4+, Fe3+,and SO4

2− as the prominent electron acceptors. High levels of Feand Mn in the groundwater together with the predominance ofAs(III) in the groundwater suggest that As is mobilized becuae ofthe reductive dissolution of Fe and Mn oxides and hydroxides withsorbed As-oxyanions in the Terai sediments. According to Panthiet al. (2006), the reductive desorption theory is the most likelyexplanation in which As-rich Fe-oxides break down and get dis-solved into water regarding the context of strongly reducingenvironments (Eh −110 to −200 mV) of groundwater in Nepal.Moreover, the As is thought to be closely associated with theoxidation-reduction process of Fe-oxides and pyrite. Evidence ex-ists to support oxidizing/reducing desorption of Fe-oxides andpyrite oxidation theories of releasing As. But negative correlationbetween As and SO4

2− demonstrates the As may not be directlymobilized from sulphide minerals like arsenopyrite. In floodedsoils, As is mobilized into porewater owing to reductive dissolu-tion of FeIII-(hydr)oxides and to arsenate (AsV) reduction to the less

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competitively sorbing arsenite (AsIII). By contrast, As concentra-tions in porewater are markedly lower under oxic conditions andgenerally dominated by AsV (Roberts et al. 2011). Furthermore, theequilibrium of groundwater with respect to carbonate mineralsand their precipitation/dissolution seems to be controlling theoverall groundwater chemistry. The low SO4

2− and high DOC,PO4

3−, and HCO3− concentrations in groundwater signify ongoing

microbial-mediated redox processes favoring As mobilization inthe aquifer. Multiple geochemical processes, e.g., Fe-oxyhydroxides re-duction and carbonate dissolution (pH!), are responsible for highAs occurrence in groundwaters Bhowmick et al. (2013).

The generally sub-oxic conditions, dominance of As(III) and Fe2+

species, and positive correlation between As and both NH3 andUV-absorbance at 254 nm suggests that oxidation of organic mat-ter coupled with microbial-mediated reductive processes areimportant for mobilizing As in the aquifers in the Terai. The gen-erally low redox potential of tube well waters combined with theabundance of reduced species of various redox sensitive elements(i.e., Fe2+, As(III), and NH3) clearly indicates that reductive pro-cesses are important controls on aquifer geochemistry Diwakar et al.(2015). For example, McArthur et al. (2011) proposed that the ab-sence or presence of a palaeo-weathering surface was a key con-trol on As heterogeneity at their study site in West Bengal, India.They suggested that a palaeo-weathering surface formed duringthe last glacial maximum protects the underlying Pleistoceneaquifer from contamination with DOC- and As-enriched water.According to Brikowski et al. (2014), mitigation efforts concerningelevated As in groundwater in Southeast Asia are hindered bypersistent uncertainty about the proximal source of As and mech-anisms for its mobilization. At the core of this uncertainty seemsto be the relative roles of surficial organic clays versus deeperaquifer matrix Fe-oxyhydroxides. Temporal variations in ground-water chemistry can serve to distinguish the contributions of thesetwo sources, and such variation is especially pronounced in headwa-ter areas of the Ganges floodplain immediately adjacent to theHimalayan foothills (e.g., the Terai of Nepal). Monsoon rechargerefreshes these aquifers, temporarily minimizing As concentra-tions. Post-monsoon, average groundwater compositions exhibitincreasing trends in water–rock interaction (higher total dis-solved solids, with cation exchange to form increasingly Na–HCO3waters), as well as in As and Fe concentrations. This cycle can berepeated during dry-season precipitation events as well, revealinga direct correlation between trends in degree of clay interaction(sodium fraction of major cations) and As concentrations. These ob-servations strongly support a model of reductive mobilization of Asfrom adjacent clays into aquifers, tempered by repeated flushingduring periods of appreciable rainfall. Surficial sediments in theTerai exhibit extreme heterogeneity. Highly organic clays pre-dominate in the shallow hydrologic system (the upper 50–100 mof surficial sediments contain >70% clay), and aquifer hydraulicconductivities are two orders of magnitude lower than in thedelta. Low hydraulic conductivity of surficial fines limits infiltra-tion, which likely enhances reducing conditions and mobilizationof As. In the Terai these factors combine to yield highly heteroge-neous groundwater As concentrations both in space and time,providing a valuable setting for exploring the As mobilizationprocess.

Mukherjee et al. (2012) state that information on groundwaterchemistry in the central Ganges basin could provide insights intorecharge, provenance, and fate of solutes in As-affected areas up-stream of the more intensively studied Bengal basin. The areathey studied extends from the northern edge of the Indian cratonoutcrops to the foothills of the Himalayas. Arsenic is probablymobilized by reductive dissolution of Fe–Mn (oxyhydr)oxides inthe alluvium, with possibility of competitive anionic mobiliza-tion. Hence, relative to the Bengal basin, in addition to lowergroundwater abstraction influence, groundwater chemistry in theirstudy area reflects a greater variety of differences in the geological

and geomorphological settings of the aquifers. Redox-sensitiveparameters indicate generally reducing, post-oxic, metal-reducingconditions. However, redox conditions are highly spatially vari-able (oxic to methanic), with no systematic depth variation withinsampled depth of the aquifers. Nakano et al. (2014) assumed thatthe brown color of the sediments in the Terai arises from thepresence of Fe(III) and the gray color can be induced by the reduc-tion of Fe(III). The dissolution of FeOOH seems to be mainly de-rived from microbial fermentation under redox condition. Theyfound that microbial degradation accompanying Fe reduction re-leased As attached on the surface of Fe-bearing solids; however,the released As coupled with dissolution of Fe can be continuouslyresorbed on the surface of solid phases like aluminosilicates (clayminerals) and silty sediments. Another possibility of resorptionare crystalized Fe-bearing minerals, which might be reproducedalong with As during the sediment–water interactions controlledby microbial activity and redox condition. Microbial activity willbe strongly affected by redox and pH changes. Upon saturation ofadsorption sites, the As remains in the groundwater. The dissolu-tion of calcium-related minerals may also play an important rolein the process of releasing As as this dissolution raises the pHlocally, making the environment more alkaline. Alkaline condi-tions favor the desorption of As from As-bearing oxides as well asfrom organic matter. Low concentrations of NO3

− and SO42− to-

gether with high Fe, as found in the geochemical analysis, alsoindicates reducing conditions being prevalent in Terai groundwa-ter. In sequential extraction techniques, chemical leaching by po-tassium chlorate and HCl releases As from sulphide and silicatephases. As exhibited in regression analysis, weak interrelation-ship between As, Fe, and SO4

2− suggests the absence of a pyrite/arsenopyrite oxidation mechanism in the present site. Further, ifpyrite would have been oxidized, then As would have been sorbedonto the resulting Fe-oxyhydroxide rather than getting releasedin the groundwater. The leachable As content was high in organicmatter phase next to sulphide/silicate phase as observed in se-quential leaching analysis. This is an indication of the role ofmicrobial population and organic matter in mobility of As underreducing condition. Moreover, the microbial oxidation of organicmatter consumes dissolved oxygen present in the groundwaterresulting in the formation of HCO3

−. The distribution of grain sizeof the sediments in groundwater may also play a vital role in themobility of As. It is evident from XRF analysis that high As con-centration was mostly associated with fine-grained clay minerals.As the fine grain-size fraction has larger surface area it adsorbs themajor part of As on their surface. Since, Fe, Mn, and Al oxides andhydroxides are the major components of fine-grained particlesand thought to retain high As under specific pH conditions, theirabundant percentage in Terai groundwater also suggests a reduc-tive dissolution mechanism for As release Yadav et al. (2015). Find-ings presented by Johnston et al. (2015) provide direct XAS-basedquantification of solid-phase As and Fe speciation in the alluvialaquifer sediments of the Terai region and help to shed light on keyprocesses controlling spatial patterns of solid-phase As/Fe specia-tion. Their dataset is broadly consistent with the widely invokedhypotheses that reductive dissolution of (near surface) Fe-oxidesand (or) reductive desorption of As(III) coupled with downwardtransport are largely responsible for As mobilization in Gangeticfloodplain aquifers (e.g., Fendorf et al. 2010). The findings alsostrongly affirm the critical role that various Fe minerals can playas host phases for As as it undergoes redox cycling throughout thefloodplain landscape. Most tube wells on the Nawalparasi flood-plain are screened more than 15–20 m below ground level(Gurung et al. 2005) to tap permanently saturated thin sandy lay-ers. Data presented in this article for the various floodplain sitesindicate that at these depths, solid phase As(III) and lower valencyAs-sulphide species are the dominant species, while poorly crys-talline Fe(III)- and Fe-oxides are largely absent. Consistent with thefindings of Polizzotto et al. (2005), the paucity of Fe-oxides at the

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depth of tube well screens suggests that current mobilization ofAs(III) within these sedimentary facies is more likely owing todownward transport or desorption of As(III) rather than contem-porary in situ reductive dissolution of As-bearing Fe-oxides occur-ring at the depth of well screens. Johnston et al. (2015) state thatdownstream transport is likely to be followed by some degree offloc reburial on the floodplain and therefore result in exposure ofFe(III) floc to seasonally fluctuating redox conditions. The materialis freshly precipitated, very poorly-crystalline—hence susceptibleto reductive dissolution—and contains readily exchangeable As atconcentrations well above those of bulk sediments. Arsenic-bearing authigenic pyrite occurs within 12 m of the ground surfaceat various floodplain sites and close (�5 m) to the current range ofseasonal water table fluctuations. While stable under reducing condi-tions, if there is some regional lowering of water tables throughprolonged drought, climate-induced shifts in monsoonal precipi-tation, or excess groundwater abstraction, then these materialsmay be at risk of exposure and oxidation. Although oxidation ofpyrite may simply cause As to shift host phases and become se-questered in the resulting Fe(III)-oxides (Polizzotto et al. 2006), it isconceivable there could be consequences for mobilizing addi-tional As in the aquifer, especially in the short-term.

Mitigation options: types of arsenic removal filtersused in the Terai of Nepal

Following the study of Sharma (1999), several organizations andagencies have conducted surveys into As contamination of wellwater in Nepal. In 2003, NRCS/ENPHO provided the following sixtypes of mitigation options for arsenicosis patients in all VDCs ofRautahat district: (i) two-gagri (water vessel) filter, (ii) innovateddug well, (iii) As–Fe removal plant (AIRP), (iv) tube wells from Asfree aquifer, (v) modified biosand filter, and (vi) awareness pro-gram on nutrition. Of these, the option of two-gagri filter andawareness program has been provided in Bagahi (Pradhan 2006).According to Nakano et al. (2014), gagri filters and Fe-assistedbiosand filters were later commonly used to remove As and Fefrom well water in Nepal, which are believed to be the best treat-ment option at household levels (Yadav et al. 2011). The remainderof this review will focus on the description of these two householdfilter types.

Gagri filtersOne of the first filters employed was the two-gagri filter system

with chemical powder. The system, consisting of two earthen pots(Nepali language: gagri), uses chemical powder (a mixture ofFeCl3, NaOCl, and charcoal). Ferric chloride is the compound thatremoves As present in affected water. The candle filter aids infiltration of the coagulants formed in the upper pot. The secondpot underneath the first one receives water free from As, Fe, bac-teria, and odour. This system is 90% efficient in removing As and isbelow the Nepal interim standard. Further development led to thethree-gagri filter system. This filter replicates the three-kulsi sys-tem of Bangladesh and solves the problem of chemical powderuse. Oxidation, adsorption, precipitation, and filtration are theprocesses for removal of As and Fe in this filter. This filter systemcan remove up to 95% of the As, even when the water is highlycontaminated. Retardation of the filtration process because ofclogging and presence of microbes in the treated water limits thefilter’s performance. Therefore, techniques for improvement ofmicrobiological quality should also have been used while provid-ing this option (B.R. Shrestha et al. 2004). The three-gagri filter is awater container made of copper, brass, steel, tin, and or clay pot.The three-gagri filter unit consists of three clay pots staggeredvertically with a 1 cm diameter hole in the bottom of the middleand top filters. The top and middle filters work as a reactor, andthe bottom filter stores the treated water. The top filter containsthe following, from bottom to top: a layer of polyester cloth, 3 kg

of iron nails (3 cm depth), 2 kg of coarse sand (4 cm depth), andraw water. The middle filter contains the following from bottomto top: a layer of polyester cloth, about 50 kg of brickbats, 2 kg offine sand (3.5 cm depth), 1 kg of charcoal (6 cm depth), 2 kg ofbrickbats (3 cm depth), and filtered water from the top filter. Thisfilter could remove 95%–99% of the As, but there were problemswith high Fe in treated water and filter clogging because of bac-terial growth. This filter was quickly replaced by an As biosandfilter (Neku and Tandukar 2003; Thakur et al. 2011).

Kanchan filtersThese Fe-assisted biosand filters were constructed on the basis

of As removal from water using zero-valent Fe (ZVI) media. Underconditions applicable to drinking water treatment, arsenate re-moval by ZVI media involves surface complexation only and doesnot involve reduction to metallic As. Under the pH and redoxconditions of most groundwaters and surface waters, dissolved Asexists as As(V) (arsenate) species, H2AsO4

− and HAsO42−, and As(III)

(arsenite) species, H3AsO30 and H2AsO3

−. Removal of As occursthrough adsorption and coprecipitation during the formationof Fe(III)-hydroxides. However, acceptable levels of removal areachieved only when there is a filtration step to remove colloidal As(Farrell et al. 2001). Greater attention is required for the removalof As(III) from groundwater owing to its higher toxicity andmobility, which mainly arise from its neutral state (H3AsO3

0)in groundwater as compared with the charged As(V) species(H2AsO4

− and HAsO42−), which predominate near pH 6–9. The

As(III) removal mechanism is mainly due to spontaneous adsorp-tion and coprecipitation of As(III) with Fe(II) and Fe(III) oxides andhydroxides, which form in situ during ZVI oxidation (corrosion).Heterogeneous reactions at the corroding ZVI surface are complexand result in a variety of potential adsorption surfaces for As(III)and As(V). Evidence has been presented showing that As(III) can beremoved by adsorption on nanoscale ZVI (NZVI) in a very shorttime (minute scale) and is strongly adsorbed on NZVI over a widerange of pH and anion environments (Kanel et al. 2005). Investi-gations by Neumann et al. (2013) regarding the SONO householdfilters used in Bangladesh (an other version of an Fe-assisted bio-sand filter) showed that over 95% of the As passing the top sandlayer was removed in the composite Fe matrix (CIM) by sorption,coprecipitation, and incorporation into solids formed during thecorrosion of ZVI. The continued presence of dissolved Fe(II) inthe CIM appears to be important for the long-term operation ofthe filters. While young CIM contained large fractions of As inamorphous or poorly crystalline phases, magnetite was dominantin older CIM, consistent with an invariably deep black color. Thetransformation of As-rich Fe(III)-(hydr)oxides into magnetite is im-portant for the following two reasons: (i) the much denser mag-netite does not lead to clogging of the filter and (ii) magnetite ismore stable toward dissolution than freshly formed amorphousphases and leaches less As during milder extraction steps. Leach-ing tests with spent CIM in a previous study have shown very lowremobilization of As, rendering used CIM nonhazardous. BecauseAs is removed predominantly in the CIM, the other filter compo-nents such as sand, brick chips, and the plastic components canbe disposed of without special care.

The biosand filter (the modified model now used in Nepalis known as a Kanchan filter) as a point-of-use drinking watertreatment option was initially designed by David Manz of theUniversity of Calgary, Canada, in the late 1990s with support ofnumerous organizations and individuals. The biosand filters weremodified to remove As and tested in Nepal jointly by Massachu-setts Institute of Technology (MIT) researchers; ENPHO, Nepal;Rural Water Supply and Sanitation Support Programme (RWSSSP),Nepal; and CAWST, Canada, based on slow sand filtration andFe-hydroxide adsorption principles (Thakur et al. 2011). Now suchfilters can be used for removal of As, Fe, bacteria, and turbidity.This filter uses the process of aeration, adsorption, and filtration.

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As this system has a high flow rate of 30 L/h, the biosand filter hasbecome in high demand in communities, not only for As removalbut also for its higher flow rate. Field test showed that this filterremoves more than 95% of the As on average and up to 99% insome cases (NRCS–ENPHO 2003; Ngai and Walewijk 2003). Thefilter also removes high levels of Fe—up to 99%, with an average of 95%.The microbiological quality of this treated water has been deemedsatisfactory (B.R. Shrestha et al. 2004).

The Kanchan Arsenic Filter™ (KAF), an award-winning house-hold water filter, was constructed for simultaneous As and patho-gen removal. The KAF is constructed using locally available labourand materials and is optimized based on the local socio-economicconditions. The KAF combines the concept of a slow sand filter forintermittent use (i.e., a biosand filter base) with the innovation ofa diffuser basin containing (rusty) iron nails for As removal. Op-erating under the water quality conditions encountered in theTerai region of Nepal (total As < 500 mg/L, phosphate < 2 mg/L,pH < 8) the iron nails can last 3 years before replacement is nec-essary (Ngai et al. 2006). A two-year technical and social evaluationof over 1000 KAFs deployed in rural villages of Nepal determinedthat the KAF typically removes 85%–90% As, 90%–95% Fe, 80%–95%turbidity, and 85%–99% total coliforms. In total, 83% of the house-holds continued to use the filter after 1 year, mainly motivated bythe clean appearance, improved taste, and reduced odour of thefiltered water, as compared with the original water source. Insidethe KAF, non-galvanized iron nails are exposed to air and water,rusting quickly and producing ferric hydroxide on the iron nails’surface. When As-containing water is poured into the filter, As israpidly adsorbed onto the surface of the ferric hydroxide. Thismechanism is similar to As adsorption on ZVI and As adsorptionon hydrous ferric oxides. Some of the As-loaded Fe particles areflushed on to the sand layer below where they are trapped in thetop few centimeters of the fine sand because of straining. As ferrichydroxide particles “exfoliate” from the iron nails, new iron sur-faces are created, providing additional As adsorption capacity. AKathmandu university study found that Fe and As do not migratethrough the sand media over time (Ngai et al. 2007). The filtercontainer can be constructed out of concrete or plastic. Thecontainer is about 0.9 m tall by 0.3 m in diameter (Fig. 2). Thecontainer is filled with layers of sieved and washed sand and

gravel. There is a standing water height of 5 cm above the sandlayer. The diffuser basin is filled with 5–6 kg of non-galvanizediron nails for As removal. In addition, pathogens, Fe, and sus-pended material are removed from the water through a combina-tion of biological and physical processes—mechanical trapping,adsorption/attraction, predation, and natural death. This filtercan treat approximately 10–15 L/h of As-contaminated water. Thefilters are locally available at a cost of about 1400–1800 NRs (aboutUS$20) per filter (Thakur et al. 2011). Figure 3 exhibits one of thesefilters operating in the district of Nawalparasi in October 2015.This ZVI-based filter is able to remove As and other pollutantsfrom drinking water, but its performance depends on the form ofZVI, filter design, water composition, and operating conditions.KAFs use an upper bucket with ZVI in the form of commercial ironnails, followed by a sand filter, to remove As and pathogens. Wenket al. (2014) evaluated factors that influence the removal of Asand uranium (U) with laboratory columns containing iron nailswith six different synthetic groundwaters at pH 7.0 and 8.4 over30 days. During the first 10 days, As removal was 65%–95% andstrongly depended on the water composition. As removal atpH 7.0 was better than at pH 8.4 and high P combined with low Cadecreased As removal. From 10–30 days, As removal decreased to45%–60% within all columns. Phosphate in combination with lowCa concentrations lowered As removal, but in combination withhigh Ca concentrations a slightly positive effect was seen. Thedrop in performance over time can be explained by a decreasingrelease of Fe to solution because of the formation of layers of FeIII

Fig. 2. Diagram of the Kanchan Arsenic Filter, showing the locationand arrangement of its components. Source: Ngai et al. (2005).

Fig. 3. Operating filter in Nawalparasi district, Nepal, October 2015.Photo: B. Mueller.

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phases and calcite covering the Fe surface. Mobile corrosion prod-ucts contained ferrihydrite, Si-containing hydrous ferric oxides,and amorphous Fe–Si–P phases. Comparisons with another typeof ZVI filter (SONO filter, see Hussam and Munir 2007) were usedto evaluate filter design parameters. Higher ZVI surface areas andlonger contact times should lead to satisfactory As removal withKanchan-type filters. Economical and promising methods are co-precipitation of As with naturally occurring or added FeII or FeIII,sorption or adsorption to inexpensive prefabricated sorbents orion exchangers, or As removal with metallic Fe. Phosphate inter-acts strongly with precipitating FeIII-(hydr)oxides and outcom-petes As for sorption and incorporation, such that additional Fe isnecessary to remove both As and phosphate. Removal units usingmetallic ZVI are promising for several reasons, including ZVI fil-ters can be constructed with locally available materials (typicallysand and iron in various forms such as turnings, filings, nails, orcleaned scrap iron). Corroding Fe can potentially produce the larg-est amount of As-sorbing FeIII-(hydr)oxides per mass of startingmaterial. Aerobic Fe corrosion leads to oxidation of AsIII to themore strongly sorbing AsV, without the need of added oxidants.Two measures that could improve the performance of KAFs are(i) larger specific ZVI surface areas (e.g., by use of smaller nails) and(ii) increased contact times by more controlled and restricted flowfrom the upper diffuser bucket. As Singh et al. (2014) state in theirarticle, KAF efficacy in field conditions operating for a long periodhas been scarcely observed. They observed the efficacy of KAFrunning over 6 months in highly As-affected households in Nawal-parasi district. Of 62 tube wells, 41 had influent As concentrationexceeding the Nepal drinking water quality standard value (50 �g/L). Ofthe 41 tube wells having unsafe As levels, KAF reduced As concen-tration to the safe level for only 22 tube wells, an efficacy of 54%.In conclusion, they did not find a significantly high efficacy ofKAFs in reducing unsafe influent As level to the safe level underthe in situ field conditions.

Summary and future perspectiveAs mentioned above, the factors that influence the removal of

As with laboratory columns containing iron nails were evaluated.As stated, the drop in performance over time could be explainedby a decreasing release of Fe to solution because of the formationof layers of FeIII phases and calcite covering the iron surface. In-spection of operating filters in Nawalparasi district during a fieldcampaign in October 2015 often revealed corrosion products cov-ering the nails as well as insufficent contact time with the nails.Higher ZVI surface areas and longer contact times should lead tosatisfactory As removal with Kanchan-type filters. Three criticalmeasures that could improve the performance of KAFs includelarger specific ZVI surface areas (e.g., by use of smaller nails),increased contact times by more controlled and restricted flowfrom the upper diffuser bucket, and immersed and anoxic condi-tions in the nailbed under no-flow conditions. Further improve-ments concerning these questions are under investigation.

AcknowledgementsI am grateful for the assistance of Stephan Hug, Eawag, Düben-

dorf; Christian de Capitani, Department of Environmental Sci-ence, University of Basel; and Marcel Guillong, Earth SciencesDepartment, ETH Zürich. My great appreciation for support is alsoexpressed to Tommy Ngai, Candice Young-Rojanschi, FinnMacdon-ald,andLauraMacDonaldfromCAWST,Calgary,Canada;BipinDangoland Hari Boudhatoki, ENPHO, Kathmandu, Nepal; Gyan PrakashYadav, Parasi, Nepal; and last but not least to Shankar Rai and SomRai, my loyal expedition and trekking guides in Nepal, responsi-ble for logistics over many years.

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