PROVENANCE OF GEOGENIC ARSENIC IN THE HIMALAYAN RIVERS SYSTEM, PAKISTAN

Abstract

Arsenic (As) in natural hydrological systems is known as a worldwide major environmental concern. Over the past few decades, many studies have highlighted that groundwater in various parts of the Punjab floodplains is highly contaminated with As, particularly evident along the major river basins of the Himalayas. Although its magnitude of contamination and release mechanism has been largely studied and understood, there remain numerous uncertainties concerning its primary source and a possible source region. Therefore, this study aimed to identify the role of the Himalayan River in As contamination, its potential sources, and the provenance of As in the upper Indus Basin. A geochemical and sedimentological study was done to ascertain the origin and geographical distribution of As in surface water and fluvial sediment collected from the floodplains/basins of the River Indus and its four major tributaries that drain through northern Pakistan. The hydrogeochemical compositions of all five rivers were similar with calcium (Ca2+) and bicarbonate (HCO3-) being the dominating ions. The concentration of As ranged from 0.62-24.2 μg/L (average = 3.7 ± 3.5 μg/L, n = 61) in river water, 2.32-23.0 mg/kg (average = 5.7 ± 3.9 mg/kg) in suspended sediment, and form 2.0-13.0 mg/kg (average = 4.13 ± 2.3 mg/kg) in fluvial sediment. Various approaches were used, including mineralogical, sequential extraction, geochemical, and statistical analyses to investigate the distribution, behavior, and potential sources of As in the sediment. The effect of geological factors on the distribution of As was also studied. No noticeable changes in the relative abundance of minerals were observed in the sediment at different locations along the river. Both mineralogical and statistical analyses showed that silicate minerals like muscovite, biotite, and chloride are the main As reservoir while oxides minerals like hematite and magnetite, which are minor components in the sediment are second largest As carrier. A geochemical baseline, using the normalization and cumulative frequency distribution curves was constructed for As. The estimated mean human contribution of As was 3.4% in the Indus River, 1.9% in the Jhelum River, 13.7% in the Chenab River, 17.5% in the Ravi River and 5.4% in the Sutlej River, indicating relatively higher effects of anthropogenic activity along the Chenab River and Ravi River. The almost negligible effects of anthropogenic activities were additionally confirmed through the pollution assessment parameters estimated from the geo-enrichment, contamination factors, and i Provenance of Geogenic arsenic in the Himalayan rivers system, Pakistan Abstract accumulation index analyses. Furthermore, in this study, borehole cores up to the depth of 30 m were collected from the As hot spot regions of the central Indus basin, in order to study sedimentological and mineralogical composition as well as zircon U-Pb ages dating to elucidate the source and provenance of As-enriched aquifer. Two different lithofacies associations have been observed within the vertical lithofacies sequences which include the fine-grained overbank layer and the sandy channel fill layer. The concentration of As in the aquifer sediment of the study area varies from up to 16 mg/kg. The mineralogy of aquifer sediment is predominantly composed of quartz (30 57%), albite and anorthite (4.5-25%), muscovite and biotite (3-10%), and trace amount of pyrite and hematite. According to EDS analysis, biotite, chlorite, amphibole, garnet, and magnetite are principal As host minerals containing high amounts of Fe and As in their mineral composition. U-Pb ages were determined from zircon grains extracted from the selected boreholes along the floodplain of the Ravi River, yielding age values ranging from 51 to 1784 Ma and dominated by 307-929 Ma (50% of total), which indicates mostly eroded from the Tethyan and Greater Himalaya. Similarly, U-Pb ages obtained from the zircons of the Chenab river floodplain range from 12.9 to 1902 Ma and are mostly dominated by the grains which have ages between 800 to 2000 Ma (70% of total), which indicates mostly sediment of Chenab River eroded from Lesser Himalaya and Sawalik. Trace element data from all the analyzed zircon samples were plotted on the on discriminatory diagrams which indicate mainly crustal origin, suggesting their derivation from magmatic rocks that commonly occur in the Lesser and Higher Himalayan rock sequences, the magmatic island arc (the Kohistan Island Arc), and partly from the Karakoram batholith. The association of the As-bearing sediment of the aquifer to a particular and known magmatic unit within the higher Himalayan litho-tectonic units is complex because the sediments could have been derived from a vast area within the Himalayan region and may contain a mixed parental source. At least, we could identify the maximum age of magmatism (ca. 1902 Ma) and the minimum age of sediment deposition (12 Ma) of the studied samples from the boreholes. Keywords: Arsenic, Geochemical baseline, Aquifer sediment, Provenance, Zircon U Pb age, Indus Basin