Ground Water Contamination by Arsenic in Nepal: Lessons to be Learned from Geology

Research Article

Austin Chem Eng. 2019; 6(1): 1064.

Ground Water Contamination by Arsenic in Nepal: Lessons to be Learned from Geology

Mueller B*

¹Bamugeobiochem, Switzerland

*Corresponding author: Barbara Mueller, Bamugeobiochem, Horbenstrasse 4, 8356 Ettenhausen, Switzerland

Received: February 04, 2019; Accepted: March 18, 2019; Published: March 25, 2019


In several countries of South East Asia ground water extracted from Quarternary alluvial sediments exhibits arsenic concentrations exceeding the the World Health Organization (WHO) drinking water guideline of 10 μg/L. According a widely accepted theory a reductive dissolution of Fe-bearing minerals leads to a release of As-oxyanions from these minerals. Yet, a pronounced decoupling or As and Fe concentrations in the ground water in the lowlands of Nepal (Terai) results in a loss of correlation between these two elements mainly being derived from the same minerals. Therefore, it seems obvious that other mineral sources of as have to be considered. Albeit As is mainly associated with iron (hydr) oxides, clay minerals (including micas) cannot be disregarded as s substantial source of As. This lack of correlation between aqueous As and Fe as well as the high temperature of the well water induces severe consequences for water treatment.

Keywords: Arsenic; Clay minerals; Decoupling


The districts Nawalparasi, Bara, Parsa, Rautahat, Rupandehi, and Kapalivastu are among the regions in the lowlands (Terai) of Nepal most severely affected by the adverse effects of arsenicosis (keratosis of the palms of the hands and soles of the feet, skin lesions including pigmentation changes, mainly on the upper chest, arms and legs, and as the most severe effect, cancer of the skin and internal organs [1-3]. The arsenic concentrations in the ground water hosted in Quarternary alluvial sediments and used for consumption regularly exceed the World Health Organization (WHO) drinking water guideline (10 μg/L). The maximum permissible limit of arsenic in Nepal is as high as 50 μg/L. An estimated 0.5 million people in Terai were at risk of consuming water with an arsenic concentration above this limit [4].

Arsenic in ground water is of clear geogenic origin and its elevated concentrations in are considered to be due to natural weathering of the Himalayan belt [5-10]. The sediments are derived from the high Himalayas, carried downward by the Ganga–Brahmaputra river system and lastly build up the foreland basin and the Bengal fan [11,12]. In such regions, young sediments of low elevation and low hydraulic gradient are characteristic of many arsenic-rich aquifers.

Arsenic is mainly concentrated in the mineral pyrite, mostly being found in sulphide-bearing mineral deposits. Pyrite weathering leads to formation of hydrous iron oxides and clay minerals hosting As. In general, sediments containing 1 to 20 mg/kg (near crustal abundance) of arsenic can already give rise to high dissolved arsenic (>50 μg/L) in groundwater if one or both of two possible “triggers”-an increase in pH above 8.5 or the onset of reductive iron dissolution are initiated [13]. Solubilization of arsenic into ground waters rigorously depends on pH, redox conditions, temperature, and solution composition. In the ground water of the Terai, arsenite, the reduced trivalent form [As (III)], is principally present in ground water [14,15]. The ground water in the Terai seems mostly to be of near-neutral to alkaline state within a pH range of 6.1-8.1 [16,15]. Variations of redox potential (Eh) between -0.20 to -0.11 V suggest fairly reduced conditions in the aquifers.

There is an ongoing debate about the most probable mechanisms leading to the dissolution of As from alluvial sediments as the iron concentration in ground water is much lower than e.g. in Bangladesh or Vietnam. Beyond that, As concentration in the ground water of the Terai is weakly negatively or not correlated at al with the Fe concentration. This decoupling of As is in contrast with the positively correlated As concentration with the lithophile elements like Na, and K Mueller and Hug, [15]. Decoupling between aqueous As and Fe has also been described by [17,18,14]. Typically, Na and K are lost from alumosilicates such as clay minerals (including micas) during weathering, leading to an enrichment of immobile elements such as Fe and Al in the remnants. It is widely described that arsenic can be adsorbed efficiently by pure clay minerals [19-21] and are capable to release a substantial amount of arsenic depending on local and climatic conditions. The present article deals with geological issues leading to arsenic contaminated ground water in Nepal.


Being a landlocked country in South-East Asia, Nepal’s topography is delineated by a diverging rugged and undulating topography and geology, a common cold climate and it is presented predominantly as mountainous. As many as 6,000 rivers and rivulets, with a total drainage area of about 194,471 km2, flow through the country, whereof 76% of this drainage area is contained within Nepal. The surface variations in Nepal are largely controlled by geology [22,2]. The Southern and Northern parts of Nepal are extremely distinct from each other due to the discrete geological history. So said, the marine sedimentary deposits forming the high Himalayas, the Siwalik hills built by the then east-west flowing rivers and the Archan crystalline formations deep beneath the Alluvium of the Terai can be found within comparatively short distances [23]. The Terai Plain in the southern part of Nepal close to the Indian border is built up by Quaternary sediments that include molasse units along with gravel, sand, silt, and clay and represents an active foreland basin. All major rivers originate in the high Himalayas whilst smaller rivers also arise from the nearby Siwalik Hills. Fine sediments as well as organic material are deposited in inter-fan lowlands, in wetlands and swamps [24].

Extreme monsoon precipitation (1,800-2,000 mm) and yearround snow-fed river systems replenish the Terai sediments, leading to a high potential for ground water resources. The majority of shallow aquifers (‹50 m) are unconfined or semi-confined, while the deep aquifers (>50 m) are generally confined by impermeable clay layers. The aquifer system is markedly sensitive to precipitation [6].

Nawalparasi is so far the most acute studied Terai province concerning local geology and ground water contaminated by arsenic. The district of Nawalparasi extends into the Terai plain as the continuation of Indo-Gangetic plain (Figure 1). From the Indian border, this district spreads northward across the Narayani River (one of the major rivers of Nepal) alluvium, later across the low gradient fan of locally derived alluvium and finally into the Himalayan foothill (also known as Churia hills) [25]. The Narayani River originating in the Higher Himalaya, flows along the eastern boundary of the Nawalparasi district and exerts a essential influence on the underlying unconsolidated Holocene fluvial deposits. Characteristic geomorphic features of the area include small natural ponds and meandering rivers [26]. In the areas with fine-grained sediments, elevated concentrations of As are typically recorded [27,28,14].