**1. Introduction**

Arsenic concentrations found in the groundwater in quaternary alluvial sediments in the lowland Terai region of Nepal and other countries of South Asia (Bangladesh, India, Nepal, Myanmar, China, Vietnam, Cambodia, and China) often exceed the World Health Organization (WHO) drinking water guideline (10 μg/L). The oral intake of arsenic causes various detrimental health issues: skin lesions including pigmentation changes, mainly on the upper chest, arms, and legs, keratosis of the palms of the hands and soles of the feet, and as the most severe effect, cancer of the skin and internal organs [1–3]. The origin of the arsenic contamination is clearly geogenic, and its elevated concentrations in natural ground waters are considered to be due to natural weathering of the Himalayan belt [4–9]. These quaternary alluvial sediments are carried by the Ganga-Brahmaputra river system and build up the Himalayan foreland basin and the Bengal fan—one of the largest modern fluvial deltas of the world [10, 11]. Arsenic is not known as an

element with a high abundance in the Earth's continental crust. Sulfide-bearing mineral deposits are the most common sources of As which also has a strong affinity for pyrite. Weathering of pyrite leads to formation of hydrous iron oxides containing As, but clay minerals are important hosts of As as well. Sediments usually containing 1–20 mg/kg (near crustal abundance) of arsenic can already be a cause of high dissolved arsenic (>50 μg/L) in groundwater if one or both of two potential "triggers"—an increase in pH above 8.5 or the begin of reductive iron dissolution are initiated [12]. Pyrite represents the largest reservoir of As. Besides this iron sulfide, As is mainly concentrated in hydrous iron oxides and clay minerals. Arsenic can be easily solubilized in groundwaters depending on pH, redox conditions, temperature, and solution composition. The sediments of the Terai plain in Nepal are commonly reducing with a pH between 7.0 and 7.8. Beyond that, As in ground water is only weakly negatively or not correlated with Fe (decoupling of As) but positively correlated with lithophile elements like Na and K. Decoupling between aqueous As and Fe has also been described by [9, 13–15]. The mentioned elements are specifically derived from alumosilicates such as clay minerals during weathering. More immobile elements such as Fe and Al will therefore be concentrated in the remnants. Clay minerals that are extremely fine-grained are noted to be compatible of adsorbing arsenic to a high degree [16–18]. So far, there is basically a small number of source materials recognized as significant contributors to arsenic in the aquifer: for example, organic-rich or black shales, Holocene alluvial sediments with slow flushing rates, mineralized and mined areas (most often gold deposits), volcanogenic sources, and thermal springs. As there is no correlation found between As and Fe concentrations in groundwater in the Terai of Nepal, an obvious correlation between lithophile element concentration and As the initial source rocks is of felsic and not mafic composition. These trace elements in groundwater reflect the origin of the arsenic in the high Himalayas of Nepal.

### **2. Geology**

All of the four major Himalayan tectonic units are exposed in the Terai (lowland of Nepal): (1) the Tethys Himalaya, delimited at the base by the South Tibetan Detachment system (STDS), (2) the Higher Himalayan Crystallines (HHC) delimited at the base by the Main Central Thrust I (MCT I), (3) the Lesser Himalaya (LH) divided into upper and lower Lesser Himalaya is delimited at the base by the Main Boundary Thrust (MBT), and (4) the Siwaliks, delimitated at its base by the Main Frontal Thrust (MFT) as much as the quaternary foreland basin. In the Terai once, east-west flowing rivers built up the Archean crystalline formations deep beneath the Alluvium of the Terai as well as the marine sedimentary deposits forming the high Himalayas and the Siwalik formation. The debris of these formations can be found within confined space [19].

These four units include a wide range of diverse rocks of metamorphic, sedimentary, and igneous in origin. Their differential erosion is accounting for some of the groundwater arsenic heterogeneity seen in the foreland and delta (e.g., [5, 7, 20, 21]). In the realm of provenance of the Terai sediments, the Tethys Himalaya is made up of 10 km of various metasedimentary rocks (limestones, calc-schists, shales, and quartzites) ranging from Cambrian to Jurassic. Leucogranites like the Manaslu leucogranite are also found emplaced within the Tethyan rocks (e.g., [22]).

The Terai region of Nepal itself is the direct prolongation of the Bengal Delta Plain (BDP), and it is the elongation of Indo-Gangetic trough. The Terai plain is composed of quaternary sediments including molasse units consisting of gravel,

**53**

examination.

*The Provenance of Arsenic in Southeast Asia Discovered by Trace Elements in Groundwater…*

sand, silt, and clay and represents an active foreland basin. The rivers in the Terai generally flow from north to south. All major rivers have their origin in the high Himalayas, while minor rivers also emerge from the proximate Siwalik Hills, and hence sediments are deposited in the form of a fan along the flanks of the Terai basin. Organic material is widespread in the fine sediments which were once depos-

The district of Nawalparasi is the most acute studied Terai province related to arsenic-contaminated groundwater in Nepal. The lithology of the Nawalparasi province sedimentary basin belongs to Holocene alluvium including the presentday alluvial deposits, channel sand, and gravel deposits as well as outwash deposits [26]. The major river, the Narayani/Gandaki, which has its source in the Higher Himalaya, flows along the eastern boundary of the Nawalparasi district and exerted a major influence on the underlying unconsolidated Holocene fluvial deposits that include the floodplain aquifer system. Atypical for the Terai, where finer sediments typically increase toward the south, in Nawalparasi, fines predominate in the north, and sand and gravels are found near the Nepal-India border [27]. In the areas with fine-grained sediments, elevated concentrations of As are

Around 20 years ago, co-workers from CAWST (Centre for Affordable Water Sanitation Technology) Calgary, Canada, in cooperation with ENPHO (Environment and Public Health Organization) Kathmandu, Nepal, began to install iron-assisted bio-sand filters built on the basis of arsenic removal from water using zero-valent iron (ZVI) media. The modified model now used in Nepal is known as Kanchan filter [30, 31]. Due to growing concerns about the malperformance of some of these filters, a groundwater sampling campaign was initiated by CAWST together with Eawag (Swiss Federal Institute for Environmental Science and Technology), Dübendorf, Switzerland. As reported in [32] and CAWST, the Kanchan filter efficiency under field conditions operating for a long period has scarcely been observed. Since a part of the Kanchan filters still had effluent arsenic concentrations exceeding the Nepal drinking water quality standard value (50 μg/l), groundwater, intermediately filtered, and effluent water for trace element analyses were sampled in October 2015 (post-monsoon). A second field campaign was arranged in pre-monsoon time (April 2017) in order to detect for differences in arsenic concentration in groundwater between the two seasons. Filters were also inspected at household levels. Measurements to improve the efficiency of the filters

Thirty-five water samples from around Ramgram, the capital of the district

Nawalparasi, were collected from hand pumps in October 2015 and again in April 2017. All pumps were thoroughly flushed before sample collection. Household for sample collections was selected referring to a register established by ENPHO including all groundwater samples exceeding the Nepal drinking water quality standard value (50 μg/L). Sampling sites consisted of groundwater from private tube wells located within the municipalities of Ramgram (former name, Parasi, the capital of the district Nawalparasi), Manari, Panchanagar, Sukauli, and Tilakpur (within proximity of Ramgram). Water samples were acidified with HNO3 and sent to the laboratory in Switzerland for further

ited in inter-fan lowlands, in wetlands, and in swamps [23–25].

*DOI: http://dx.doi.org/10.5772/intechopen.83014*

typically recorded [15, 28, 29].

**3. Material and methods**

**3.1 Sample collection**

are under progress.

*The Provenance of Arsenic in Southeast Asia Discovered by Trace Elements in Groundwater… DOI: http://dx.doi.org/10.5772/intechopen.83014*

sand, silt, and clay and represents an active foreland basin. The rivers in the Terai generally flow from north to south. All major rivers have their origin in the high Himalayas, while minor rivers also emerge from the proximate Siwalik Hills, and hence sediments are deposited in the form of a fan along the flanks of the Terai basin. Organic material is widespread in the fine sediments which were once deposited in inter-fan lowlands, in wetlands, and in swamps [23–25].

The district of Nawalparasi is the most acute studied Terai province related to arsenic-contaminated groundwater in Nepal. The lithology of the Nawalparasi province sedimentary basin belongs to Holocene alluvium including the presentday alluvial deposits, channel sand, and gravel deposits as well as outwash deposits [26]. The major river, the Narayani/Gandaki, which has its source in the Higher Himalaya, flows along the eastern boundary of the Nawalparasi district and exerted a major influence on the underlying unconsolidated Holocene fluvial deposits that include the floodplain aquifer system. Atypical for the Terai, where finer sediments typically increase toward the south, in Nawalparasi, fines predominate in the north, and sand and gravels are found near the Nepal-India border [27]. In the areas with fine-grained sediments, elevated concentrations of As are typically recorded [15, 28, 29].
