The Provenance of Arsenic in Southeast Asia Discovered by Trace Elements in Groundwater from the Lowlands of Nepal

Abstract

Arsenic concentrations in groundwater extracted from quaternary alluvial sediments pose a serious health issue for inhabitants living in several countries in Southeast Asia. A widely approved hypothesis states that reductive dissolution of Fe-bearing minerals releases As oxyanions to ground water and the original source of As has to be located in mafic rocks occurring across the entire Himalayan belt. Yet, recent trace element analyses of ground water from the lowlands (Terai) of Nepal show a clear decoupling of As and Fe. The positive correlation of K, Na, and trace elements like Li, B, and Mo with arsenic points out to clay minerals hosting the toxic element. This pattern of trace elements found in the ground water of the Terai also advocates against an original source of As in mafic rocks. The lithophile elements like Li, B, P, Br, Sr, and U reflect trace element composition typical for felsic rocks as an origin of As. All the mentioned elements are components of clay minerals found ubiquitously in some of the most characteristic felsic rocks of the Nepal Himalaya: metapelites and leucogranites—all these rocks exhibiting a high abundance of especially B, P, and As besides Cd and Pb.

Unique Geochemistry of Arsenic-Contaminated Groundwater and Corresponding Mitigation Efforts in Southern Nepal

“…The Manaslu area represents a major catchment realm of the Terai sediments. 28,29 Fine-grained sediments interspersed with organic material were deposited in the interfan lowlands, wetlands, and swamps and are recharged by intense monsoon precipitation (1,800− 2,000 mm) and year-round snow-fed river systems, resulting in a high potential for groundwater resources. The entire aquifer system is highly sensitive to precipitation.…”

“…45,46 Several groundwater samples from Nawalparasi show similar detectable concentrations of Cd. 28,29 The protoliths of the so-called peraluminous granites often consist of low-grade metapelites, 47,48 hosting significant amounts of As, Sb, Be, B, Ba, and Rb, enriched by a factor of 5 to 10, compared to the average crustal abundances (2−5 ppm 49 ). The leucogranites found in the Himalayas of Nepal are unequivocally peraluminous in composition; 50 therefore, a comparison with the findings of previous research 44 is clearly justified.…”

“…Moreover, felsic rocks, such as the leucogranites found, for example, in the Manaslu area, are embedded within the Tethyan rocks. The Manaslu area represents a major catchment realm of the Terai sediments. , …”

“…Compared to the groundwater from the Hanoi area of Vietnam and Bangladesh, with a molar ratio between 60 and 68 in As-contaminated groundwater and between 14.88 and 90.49, respectively, the average molar Fe/As ratio from the samples in Nepal is only up to 5.3. This ratio, together with a low concentration of Fe, is a prominent indicator of sediments representing a heterogeneous mixture of parent rocks of felsic origin. ,,,, …”

“…Compared with the mean continental crust values, As and other incompatible trace elements (e.g., Be, B, Rb, Sn, Sb, and Ta) exhibit enrichments by factors of 10 to 100 and Li, Cd, and Cs, enrichments by factors of 100 to 200. Apart from As, up to 300 ppm of Cd was also found in the quartz-hosted fluid and melt inclusions of hydrous peraluminous systems (pegmatites and leucogranites). , Several groundwater samples from Nawalparasi show similar detectable concentrations of Cd. , The protoliths of the so-called peraluminous granites often consist of low-grade metapelites, , hosting significant amounts of As, Sb, Be, B, Ba, and Rb, enriched by a factor of 5 to 10, compared to the average crustal abundances (2–5 ppm). The leucogranites found in the Himalayas of Nepal are unequivocally peraluminous in composition; therefore, a comparison with the findings of previous research is clearly justified.…”