Towards a Sustainable Solution: Factors and Prerequisites of Improving the Kanchan Arsenic Filters used in the Terai of Nepal. A Review

Unfortunately, Nepal's current arsenic issueconcerning ground water was recognized much later than in other countries of South-Asia (e.g.West Bengal (India), Bangladesh, Cambodia, Vietnam, China).Nepal did not seem to be much affected by As poisoning ground water hosted by quaternary alluvial sediments as the landlocked country is dominated by the mountain chain of the high Himalayas and only features a very narrow band of flat land (the so called Terai, the Indo-Gangetic Plain of southern Nepal) builtup by those quaternary alluvial sediments.The actual drinking water guideline (10 µg/l) for As imposed by the World Health Organization (WHO) is exceed in several districts (namely Nawalparasi, Bara, Parsa, Rautahat, Rupandehi, and Kapalivastu (Shrestha et al., 2014).As soon concentration of arsenic exceeds the guideline, detrimental health effects are likely to occur.Characteristic skin lesions including pigmentation changes (melanosis, keratosis); various reproductive, neurological, respiratory, cardiovascular, gastrointestinal and diabetic effects as well as cancers of almost all inner organsare among the most prominent health impacts caused by the longterm intake of As (Smith et al., 2000;Adhikari and Ghimire, 2009;Smith and Steinmaus, 2009;Abdul et al., 2015).Sharma (1999) in a first report ever published mentioned arsenic contamination above toxic levels in ground water in the Terai Basin of Nepal.Later, Nakano et al. (2014) stated correctly that the population of the southern Indo-Gangetic Plain of Nepal (Terai) struggle with the same arsenic issue as do the inhabitants of Bangladesh, for example.But it was not before 2004 when Shrestha and Shrestha (2004)mentioned that twenty-four percent of ground water samples analyzed (n = 18,635) from the Terai Basin exceeded the WHO limit of 10 μg/L.According Panthi et al. (2006) 25,058 tube wells in the Terai region had been tested for As, of which 5,686 tube wells (22.7%) exceeded the WHO (World Health Organization) As guideline (As = 0.01 mg/L) and 1,916 tube wells (7.6%) exceeded the Nepal Interim As Standard (As = 0.05 mg/L).In a first report compiled by NRCS-Abstract:The issue concerning the high arsenic concentrations in ground water used as drinking water in the lowland of Nepal was neglected for a long time.Whereas mainly Bangladesh received much international attention and support to develop appropriate filters to remove As, in Nepal the installation of the so called Kanchan filters only begun in the early nineties when adverse health effects were already observed.Arsenic itself can be readily released into ground water depending on pH, redox conditions, temperature, and solution composition.Moreover, the widely spread hypothesis asserts that reductive dissolution of Fe-bearing minerals releases As-oxyanions.However, there is an obvious de-coupling of As and Fe concentrations in ground water resulting in loss of correlation concerning these two elements.Beyond that, As is positively correlated with Na and K, the molar ration of Fe/As in the ground water is very low and in conclusion therefore clay minerals (containing a minor amount of Fe in their structure) haveto be regardedas a substantial host of As.In this regard, the partial low removal efficiency of the installed Kanchan filters can tentatively be explained by the ratio of main and trace elements (particularly Fe and As), pH, flow rates, contact time with the nails, and filter maintenance.This review summarizes the identified geological background, origin of the As, the established mitigation option and future improvements of the filters.