296 REVIEW Arsenic in groundwater in the southern lowlands of Nepal and its mitigation options: a review Barbara 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 are unsafe to humans using the groundwater as drinking water. Whereas Bangladesh has received much international attention concerning the As crisis, Nepal was more or less neglected. The first report about As contamination of the groundwater above toxic levels in Nepal was published in 1999. Twenty-four percent of samples analyzed (n = 18 635) from the Terai Basin exceeded the WHO guideline of 10 ␮g/L. Since the first overall survey from 2001, only sporadic information on the situation has been published. The geological and geochemical conditions favour the release of the contaminant as As can be easily solubilized in groundwaters depending on pH, redox conditions, temperature, and solution composition. The thin alluvial aquifers of the Terai are some of the most severely As contaminated. These sediments constituting a hugh proportion of the Terai aquifers are derived from 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 low 2− 3− − SO4 and high DOC, PO4 , and HCO3 concentrations in groundwater signify ongoing microbial-mediated redox processes favoring 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 chemical powder) and later iron (Fe)-assisted biosand filters were commonly used to remove As and Fe from well water in Nepal—these two options were believed to be the best treatment option at household levels. This review focus on the description of the overall situation, including geogenic issues, occurrence of As in the sediments of the Terai, mechanisms for the release of As to the groundwater, 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, le Viêt-Nam, le Cambodge) les concentrations d’arsenic dans les eaux souterraines des plaines du Népal (connues sous le nom de Terai) peuvent atteindre des concentrations qui sont dangereuses pour les humains qui utilisent l’eau souterraine comme eau potable. Tandis que le Bangladesh a reçu beaucoup d’attention internationale concernant la crise d’arsenic, le Népal a été plus For personal use only. ou moins négligé. Le premier rapport sur la contamination des eaux souterraines par l’arsenic au-dessus des niveaux toxiques au Népal a été publié en 1999. Vingt-quatre pour cent des échantillons analysés (n = 18 635) du bassin de Terai excédait la ligne directrice de l’OMS, soit 10 ␮g/L. Depuis la première enquête globale de 2001, on a publié que des informations sporadiques sur la situation. Les conditions géologiques et géochimiques favorisent le rejet du polluant puisque l’arsenic peut être facilement solubilisé 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 sédiments constituant une très grande partie des aquifères de Terai proviennent de deux sources principales, soit (i) des sédiments déposés par les grandes rivières qui érodent les roches cristallines du Haut-Himalaya, (ii) des sédiments métamorphisés météorisés portés par de plus petites rivières prenant leur source dans les contreforts du Siwalik. Le potentiel d’oxydo-réduction généralement bas, 2− 3− − les concentrations faibles de SO4 , et élevées de COD, de PO4 et d’HCO3 dans les eaux souterraines signifient qu’ilyades processus continus d’oxydo-réduction par intermédiaire microbien favorisant la mobilisation d’As dans l’aquifère. D’autres processus géochimiques, par exemple, la réduction des oxydes-hydroxydes de fer et la dissolution de carbonates sont aussi responsables 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 fer de l’eau de puits au Népal—ces deux options étaient censées être la meilleure option de traitement au niveau des ménages. Cette revue se penche sur la description de la situation globale (des questions géogéniques, la présence d’arsenic dans les sédiments du Terai, les mécanismes de rejet d’arsenic dans les eaux souterraines, les options d’atténuation). [Traduit par la Rédaction] Environ. Rev. Downloaded from www.nrcresearchpress.com by ETH Zuerich Gruene Bibliothek on 10/13/17 Mots-clés : arsenic, contamination a` l’arsenic, rejet d’arsenic dans les eaux souterraines, élimination de l’arsenic, atténuation. Introduction are hazardous to human health if geological and geochemical condi- In the groundwaters of several countries of Southeast Asia tions favour the release of this contaminant. The World Health (namely Bangladesh, India, Nepal, Myanmar, China, Vietnam, and Organisation (WHO) has imposed a drinking water guideline with Cambodia), arsenic (As) can naturally reach concentrations that a 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: [email protected]. Copyright remains with the author(s) or their institution(s). Permission for reuse (free in most cases) can be obtained from RightsLink. Environ. Rev. 25: 296–305 (2017) dx.doi.org/10.1139/er-2016-0068 Published at www.nrcresearchpress.com/er on 23 December 2016. Mueller 297 are likely to occur. Excess uptake of As causes a range of adverse stitutes less than 20% of Nepal’s surface, it contains over half of health effects like characteristic skin lesions including pigmenta- the total arable land and is home to about 50% of the Nepalese tion changes, mainly on the upper chest, arms, and legs, and population, i.e., 30 million inhabitants. Groundwater is the main keratoses of the palms of the hands and soles of the feet, and as source of water for drinking and irrigation in the Terai area. Over the most severe effect, cancer (Smith et al. 2000; Adhikari and 90% of the Terai population draws groundwater from tube wells Ghimire 2009). for drinking, household use, and irrigation (Guillot et al. 2015). Arsenic itself is not found in high abundance in the Earth’s According to some publications, 25 058 tube wells in the Terai continental crust; it is less abundant than several of the “rare-earth” region have been tested for As, of which 5686 tube wells (22.7%) elements. Unlike the rare-earth elements, however, As is com- exceed the WHO As guideline (As = 0.01 mg/L) and 1916 tube wells monly concentrated in sulphide-bearing mineral deposits, and it (7.6%) exceed the Nepal Interim As Standard (As = 0.05 mg/L) has a strong affinity for pyrite, one of the more ubiquitous min- (Panthi et al. 2006). It is estimated that there are perhaps 200 000 erals in the Earth’s crust. Arsenic is also concentrated in hydrous tube wells in the Terai region and that 3.5 million Nepalese have iron (Fe)-oxides and clay minerals. Arsenic can be easily solubi- no access to drinking water that does not exceed the WHO As lized in groundwaters depending on pH, redox conditions, tem- guideline (Mahat and Shrestha 2008; Mahat and Kharel 2009; perature, and solution composition. Many geothermal waters Pokhrel et al. 2009). In the most recent report from the National contain high concentrations of As. Natural As in groundwater at Arsenic Steering Committee/National Red Cross Society (NASC-NRCS ␮ concentrations above the drinking water guideline of 10 g/L is 2011), the total database covers 1.1 million wells tested between the not uncommon. A small number of source materials are now recog- years 2003 and 2008. Approximately 1.73% showed values above nized as significant contributors to As in water supplies: organic-rich the Nepal drinking water standard of 50 ppb, while approxi- or black shales, Holocene alluvial sediments with slow flushing mately 5.37% of tube wells contain 11–50 ppb of As concentration. rates, mineralized and mined areas (most often gold deposits), volca- The percentage of all tube wells exceeding 50 ppb varies from nogenic sources, and thermal springs. Two other environments can 0.05% of the wells in the district of Jhapa to 11.69% in the district of lead to high As: (i) closed basins in arid-to-semi-arid climates (espe- Nawalparasi. cially in volcanogenic provinces) and (ii) strongly reducing aqui- The most severe As contamination is prevalent in several dis- fers, often composed of alluvial sediments but with low sulphate tricts of the Terai, namely Nawalparasi, Bara, Parsa, Rautahat, concentrations. Young sediments in low-lying regions of low hy- Rupandehi, and Kapalivastu (Shrestha et al. 2014). Maharjan et al. draulic gradient are characteristic of many As-rich aquifers. Ordi- (2005) reported that 29% of more than 20 000 tube wells had As nary sediments containing 1–20 mg/kg (near crustal abundance) of concentrations exceeding the WHO guideline (10 ␮g/L), that the As can give rise to high levels of dissolved As (>50 ␮g/L) if initiated prevalence of arsenicosis varied between 1.3% and 5.1% (average of by one or both of two possible “triggers”—an increase in pH above 2.6%; see NRCS–ENPHO 2002; Yadav et al 2011) among four inde- 8.5 or the onset of reductive Fe dissolution.