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Arsenic release mechanism in Indus river basin, Punjab, India

Student Name: Mr Anand Kumar
Guide: Dr Chander Kumar Singh
Year of completion: 2019

Abstract:

Elevated concentration of Arsenic (As) in groundwater exceeding WHO limits of 10 μg/L has become a serious health concern in South and South East Asia. Globally > 137 million of people in 70 countries consume high As groundwater. The densely populated floodplain of major rivers in South and South East Asia are severely impacted with high As in groundwater. The problem is enhanced further by the extensive use of groundwater and installation of new tubewells/ hand pumps.

This study has been conducted in a part of Indus River Basin; one of the most exploited aquifers system around the globe. The aim of this study was to understand the mechanism associated with the As enrichment of groundwater in the region. Around 13000 wells have been tested using field test kits for As, nitrate (NO3), fluoride (F), pH and electrical conductivity (EC) in the study area along three major tributaries of Indus basin in India i.e. Ravi, Sutlej, Beas. The results suggest around 25% of wells has high As, high NO3 and 8% of the samples has high F. The spatial distribution of all these contaminants are confined to specific region within the Punjab State. High As groundwater is found in shallow wells (<50 m) along the Ravi river floodplain, high NO3 is found along Beas river floodplain however high F is confined in southern part of the Punjab.

11 villages have been selected to understand the difference in geochemical characteristics of groundwater , these villages have been blanket tested for iron (Fe), sulfate (SO4), As and NO3 and to characterize the mineralogical and geochemical nature of the sediments 14 bore holes were at different locations in the region. The sediments were analyzed on the field using handheld X-Ray Fluorescence and reflectance meter at every 1.5 m depth and the leachable sediment As was also determined using field kit. The species of aqueous As is determined using ion exchange column. The sediment samples were collected and tested in lab using X-Ray diffraction, Scanning Electron Microscope, X-ray absorption near edge structure (XANES). The radiocarbon dating of the clay from certain depth was also done to define the accumulation of the sediment in the region. The grey to grey-brown colour of drill cuttings confirmed by reflectance measurements indicates the reduction of Fe oxides in the affected villages. Elevated NO3 was found in groundwater although aquifer condition was reducing enough to maintain >1 mg/L dissolved Fe in half of all the wells. Remarkably, high concentrations of leachable As (up to 40 mg/kg) in the drill cuttings determined with the field kit and by X-ray fluorescence correspond to depth intervals of high As in groundwater. Anion-exchange separation in the field and synchrotron-based X-ray spectroscopy of drill cuttings preserved in glycerol indicate speciation of As in both groundwater and aquifer sands dominated by As (V) in the enriched depth intervals. 30-100 mg/L groundwater SO4 concentrations throughout the region suggest that NO3 may have triggered the release of As by oxidizing sulphide-bound As supplied by erosion of black shale and slate in the Himalayas. Radiocarbon dating of sub-surface clay indicates that multiple episodes of inferred As sulphide input occurred over the past 30 kyr but were somehow limited to the Ravi floodplain. NO3 in groundwater, presumably from extensive fertilizer application (although a natural source cannot be excluded), probably also limits the accumulation of As in groundwater by oxidizing both Fe(II) and As(III) throughout the region.

Apart from As, high concentration of F in the southern part of the study area is found due to dissolution of fluorite minerals. As the southern part of the study area comes under semi-arid climatic condition with high Na and HCO3 in alkaline pH favors the dissolution of fluoride from fluorite bearing minerals. However anthropogenic activities i.e excessive use of fertilizers seems as the major source for high NO3 in groundwater in the study region. As distribution is spatially heterogeneous even within the village, wells currently with low As are not likely to became contaminated in recent future, so large scale testing and finding safe wells in close vicinity can be used as first measure to reduce exposure. In the Ravi floodplain, where groundwater and sediment As concentrations are correlated, the field kit can be used to analyse cuttings while drilling in order to target As-safe depths on the basis of this information. A new domestic well, high in As was reinstalled using drill cutting data for As at shallower depth which indicated low As in sediment and the well has remained low in As for more than 2 years. Combination of these approaches could lower the As poisoning of population and would be much less expensive than the interventions such as centralized water supply schemes and not so well maintained community RO systems provided by the government.

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