Arsenic in groundwater in the southern lowlands of Nepal and its mitigation options: A review

As in several other countries of south east Asia (namely Bangladesh, India, Myanmar, China, Vietnam, Cambodia) arsenic concentrations in the groundwater of the lowlands of Nepal (the so called Terai) can reach concentrations which are unsafe to humans using the groundwater as drinking water. Whereas Bangladesh has received much international attention concerning the arsenic crisis, Nepal was more or less neglected. The first report about arsenic contamination of the groundwater above toxic levels in Nepal was published in 1999. Twenty-four percent of samples analyzed (n = 18,635) from the Terai Basin exceeded the WHO limit of 10 μg/L. Since the first overall survey from 2001, only sporadic information on the situation has been published. The geological and geochemical conditions favour the release of the contaminant as arsenic can be easily solubilized in groundwaters depending on pH, redox conditions, temperature, and solution composition. The thin alluvial aquifers of the Teri are some of the most severely As contaminated. These sediments constituting a hugh proportion of the Terai aquifers are derived from two main sources, (i) sediments deposited by large rivers that erode the upper Himalayan crystalline rocks, (ii) weathered meta-sediments carried by smaller rivers originating in the Siwalik forehills. The generally low redox potential, low SO42−, high DOC, PO43− and HCO3− concentrations in groundwater signify ongoing microbial mediated redox processes favoring As mobilization in the aquifer. Other geochemical processes, e.g., Fe-oxyhydroxides reduction and carbonate dissolution are also responsible for high As occurrence in groundwaters. Originally, Gagri filters (two filter system with chemical powder ) and later iron-assisted bio-sand filters were commonly used to remove arsenic and iron from well water in Nepal – these two options were believed to be the best treatment option at household levels. This review will focus on the description of the overall situation (geogenic issues, occurrence of arsenic in the sediments of the Terai, mechanisms of the release of arsenic to the groundwater, mitigation options).The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author