**3. Material and methods**

#### **3.1 Sample collection**

*Trace Metals in the Environment - New Approaches and Recent Advances*

of the arsenic in the high Himalayas of Nepal.

found within confined space [19].

Tethyan rocks (e.g., [22]).

**2. Geology**

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

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

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

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,

**52**

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 are under progress.

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 examination.

#### **3.2 Trace element analysis**

All trace elements in the groundwater samples were determined by ICP-MS (Agilent Technology, 7500 Series, Agilent Technologies, Waldbronn, Germany) at Eawag, Dübendorf, Switzerland, after 1:2 dilution with 0.5 M HNO3. Each measurement was conducted in triplicate. All ICP-MS determinations agreed to within 3–5% standard deviation [33].
