Groundwater Arsenic in Nepal: Occurrence and Temporal Variation
In Nepal, over two million people are exposed to excessive natural arsenic (10–1500 ppb) in groundwater. The majority of these people live in the agricultural Terai region, on the edge of Ganges floodplain at the base of the Himalayan foothills. The remai
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Groundwater Arsenic in Nepal: Occurrence and Temporal Variation T. H. Brikowski, L. S. Smith and A. Neku
Abstract In Nepal, over two million people are exposed to excessive natural arsenic (10–1500 ppb) in groundwater. The majority of these people live in the agricultural Terai region, on the edge of Ganges floodplain at the base of the Himalayan foothills. The remainder are exposed via deep wells in the Kathmandu Valley in a primarily urban setting. Tube wells down to 50 m in the Terai commonly exhibit cyclical, temporally correlated variation in dissolved arsenic, iron, and other species. In Nawalparasi, the most arsenic-affected district, these wells tap thin (2 m) gray sand aquifers embedded in a thick (>50 m) sequence of organic clays. Monsoon recharge refreshes these aquifers, temporarily minimizing dissolved arsenic concentrations. Post-monsoon, average groundwater compositions exhibit increasing water–rock interaction with time (increasing TDS and cation exchange, forming increasingly Na-HCO– waters) and increasing dissolved arsenic and iron. Collectively these observations strongly support a model of reductive mobilization of arsenic from adjacent clays into aquifers in the Terai, tempered by repeated flushing during periods of heavy precipitation. In Kathmandu Valley, moderately elevated arsenic (up to 150 ppb) may be leached from overlying silts and clays, but concentrations remain constant throughout the year. In the Terai, effective mitigation is challenging, depending primarily on well-switching (marking contaminated wells) and installation of household point-of-use filters. Mitigation in the urban setting will emphasize blending with clean surface water from mountain reservoirs. Keyword Arsenic
Nepal South Asia Hydrology
T. H. Brikowski (&) Geosciences Department, The University of Texas at Dallas, 800 W. Campbell Rd., Richardson, TX 75080-3021, USA e-mail: [email protected] L. S. Smith Filters for Families, Denver, CO, USA A. Neku Association of Professional Geoscientists of Ontario, Toronto, Canada © Springer Nature Singapore Pte Ltd. 2018 A. Mukherjee (ed.), Groundwater of South Asia, Springer Hydrogeology, https://doi.org/10.1007/978-981-10-3889-1_23
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Introduction
Groundwater arsenic poses a scattered but significant hazard in Nepal. This natural contamination was detected in the mid-1990s as concern spread following identification of severe contamination in West Bengal and Bangladesh in the 1980s (Mukherjee et al. 2006; Muehe and Kappler 2014). The earliest published reports of arsenicosis in Nepal appeared around 2000 (e.g., Tandukar et al. 2001), and initial testing indicated a sparse but clustered distribution of tube wells yielding arsenic well above the 50 ppb Nepalese standard (Neku and Tandukar 2003; Shrestha et al. 2004a). Two population groups are exposed to groundwater arsenic hazard in Nepal: (1) about 1 million people residing in the low-lying agricultural lands of the Terai near the border with India and (2) around 1 million peopl
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