Elevated arsenic concentrations in groundwater of the Upper Indus Plain of Pakistan across a range of redox conditions

Nisbah Mushtaq; Abida Farooqi; Junaid Ali Khattak; Ishtiaque Hussain; Brian Mailloux; Benjamin C Bostick; Athena Nghiem; Tyler Ellis; Alexander van Geen

doi:10.1016/j.scitotenv.2023.168574

Elevated arsenic concentrations in groundwater of the Upper Indus Plain of Pakistan across a range of redox conditions

Sci Total Environ. 2024 Feb 20:912:168574. doi: 10.1016/j.scitotenv.2023.168574. Epub 2023 Nov 22.

Authors

Nisbah Mushtaq¹, Abida Farooqi², Junaid Ali Khattak¹, Ishtiaque Hussain¹, Brian Mailloux³, Benjamin C Bostick⁴, Athena Nghiem⁴, Tyler Ellis⁴, Alexander van Geen⁵

Affiliations

¹ Environmental Hydrogeochemistry Laboratory, Department of Environmental Sciences, Quaid-i-Azam University, Islamabad 45320, Pakistan.
² Environmental Hydrogeochemistry Laboratory, Department of Environmental Sciences, Quaid-i-Azam University, Islamabad 45320, Pakistan. Electronic address: abida.farrukh@gmail.com.
³ Environmental Sciences Department, Barnard College, New York, USA.
⁴ Lamont-Doherty Earth Observatory, Columbia University, New York, USA.
⁵ Lamont-Doherty Earth Observatory, Columbia University, New York, USA. Electronic address: afv2@columbia.edu.

PMID: 38000757
DOI: 10.1016/j.scitotenv.2023.168574

Abstract

Groundwater of the Ravi River floodplain is particularly elevated in arsenic (As) on both sides of the Pakistan-India border. To understand this pattern, 14 sites were drilled to 12-30 m depth across floodplains and doabs of Pakistan after testing over 20,000 wells. Drill cuttings were collected at 1.5 m intervals, 132 of which were sand overlain by 77 intervals of clay and/or silt. Radiocarbon dating of clay indicates deposition of the aquifer sands tapped by wells 20-30 kyr ago. Most (85 %) of the sand samples were gray in color, indicating partial reduction to Fe(II) oxides, whereas most (92 %) of the clay and/or silt samples were orange. Associations between groundwater electrical conductivity, dissolved Fe, sulfate, and nitrate suggest that wells can be elevated (>10 μg/L) in As in the region due to either reductive dissolution of Fe oxides, evaporative concentration, or alkali desorption. In the Ravi floodplain, 47 % of 6445 wells tested contain >10 μg/L As compared to only 9 % of 14,165 tested wells in other floodplains and doabs. The As content of aquifer sands in the Ravi floodplain of Pakistan averages 4 ± 4 mg/kg (n = 66) and is higher than the average of 2 ± 2 mg/kg (n = 51) for aquifer sands outside the Ravi. Synchrotron spectroscopy and column-based speciation indicate predominance of As(V) over As(III) in both aquifer sands and groundwater. Whereas multiple processes may be responsible for elevated levels of As in groundwater across the region, spatial heterogeneity in groundwater As concentrations in the Ravi floodplain seems linked to variations in As concentrations in aquifer sands. Regulation by the solid phase may limit variations in groundwater As over time in response to natural and human-induced changes in hydrology. This means spatial heterogeneity could be taken advantage of to lower the exposure across the region with more testing and targeted drilling.

Keywords: Alkali desorption; Arsenic; Groundwater; Indus plain; Sulfide oxidation; iron reduction.