Small local rivers originating in the Siwalik Hills, including the Turia, Jharia and Bhaluhi, dissect the floodplain in a North-South orientation (Pathak and Rao, 1998). The plain is situated on the Rapti–Gandaki interfan region and is mainly comprised of Holocene alluvium (NASC, 2004). Unlike other regions of Terai, where finer of sediments typically increase toward the south, fines predominate in the north and sand and gravels are found near the Nepal–India border (Shrestha et

al., 2004). In the areas with fine grained sediments, elevated concentrations of As are typically recorded (Brikowski et al., 2004 and Brikowski et al., 2013). All glass and plasticware used during sampling and laboratory analysis were soaked in 5% HCl for 24 h and then rinsed with deionized Selleckchem HSP inhibitor water (Milli-Q) for at least 24 h. All reagent solutions were prepared with Milli-Q water having a resistivity of 18.2 MΩ/cm. In October 2012, tubewell water samples were collected along the floodplain LBH589 of the Bhaluhi River, Nawalparasi district

(Fig. 1). The sampling area was divided into three topo-gradient regions along the flow path of the Bhaluhi River referred herein as (i) the Upper region, (ii) the Middle region and (iii) the Lower region. The upper region lies on the edge of the Bhabar zone, while the middle and lower region are situated on the Terai plain. This division was based on the recognition that geomorphic and landform features can exert a vital control on aquifer stratigraphy and corresponding geochemistry and As concentrations (McArthur et al., 2011, Nath, 2012, Shamsudduha et al., 2008, Weinman et al., 2008 and Winkel et al., 2008) and the distribution of As concentrations in the aquifer derived from prior testing of the region. Seventy-three water samples from tube wells

and eight samples from the Bhaluhi River were collected for detailed aqueous phase characterization (Fig. 1). Most of the investigated tube wells were currently used by local people for domestic drinking or irrigation purposes. Information such as depth, age, screen interval, method of drilling and construction of tube well were collected via interview with the owner or nearest CYTH4 household user of the well. Each tube well was subjected to 5–10 min of continuous pumping, during which time the redox potential, pH, temperature, dissolved oxygen (DO) (luminescence probe) and electrical conductivity of the water was measured with HACH multimeters (HQd) and freshly calibrated probes. After 5–10 min of pumping and stabilization of physico-chemical parameters, water samples were collected into a clean high-density polyethylene (HDPE) bottle flushed with sample water three times and filled without any headspace.