Regional Controls on Arsenic Cycling in Alluvial Aquifers of Upper Indus Basin, Punjab Pakistan

Abstract

The large scale geogenic contamination of groundwater with arsenic (As) in deltas and floodplains of South and Southeast Asia (Bangladesh, India, Vietnam, Cambodia, and Pakistan) have received a lot of attention over the last two decades with comprehensive regional studies being done in these areas. In Pakistan, on village scale studies have been done but the the detailed hydro-geochemistry and hydrogeological framework of the area remains unknown and no regional scale studies to date have been carried out. For this purpose, a detailed field survey was carried out between 2015-2017 and approximately, 20,000 wells were tested using field kits for As, SO4, NO3, and Fe in 186 villages of Punjab Plain.. An additional 1576 tests were also conducted across 35 villages present in Bari doab and towards the main Indus River in Thal doab during mid of 2018, bringing the total number of test conducted near to 21000. Villages identified with As> 100 µg/L were also tested on site for As speciation using anion-exchange resins. For laboratory measurements, a subset of 345 groundwater samples were collected during field survey for As and other redox sensitive elements analysis in groundwater. Field results indicate groundwater of the Ravi River floodplain to contain particularly elevated concentrations of arsenic (As). Keeping this in mind, a detailed health risk assessment was also carried out particularly focused on the floodplain of the Ravi River. Furthermore, to understand the origin of this pattern and generally lower groundwater As concentrations elsewhere in the Punjab plain, 14 new sites were drilled to 12-30 m depth across river floodplains and doabs of Pakistan. The field testing and laboratory measurements indicated elevated concentrations of EC and SO4 in villages present in Chaj and Rachna doabs while in contrast, villages with a higher proportion of wells having elevated concentrations of Fe were located primarily in floodplains of the Chenab and Ravi rivers. Unlike Fe, wells elevated in As were limited to the Ravi floodplain. The village classification based on field and lab measurements of EC and Fe, divides the whole area into four different quadrants. The proportion of wells with >10 and >50 μg/L As was the highest in the hiEC-hiFe quadrant (44 and 23%), followed by the loEC-hiFe (36 and 15%), loEC-loFe (19 and 8%), and hiEC-loFe (13 and 5%) quadrants. This confirms that saltier, less reduced aquifers prevail in the doabs and fresher, more reduced aquifers in the floodplains., with high groundwater As associated with both oxidizing and reducing conditions prevailing in the aquifers. Radiocarbon dating of clay cuttings from the Ravi floodplain indicates deposition of the underlying aquifer sands during the Pleistocene age. Little if any Holocene accumulation is consistent with floodplain incision inferred from high-resolution elevation. A majority of the 132 samples of sand cuttings across the region were gray in color, indicating partial reduction to Fe(II) oxide coatings. However, as much as a third of the sand cuttings were orange in color, suggesting the prevalence of Fe(III) oxides and less reducing conditions. Associations between groundwater electrical conductivity, dissolved Fe, sulfate, and nitrate across tested villages in the region suggest that some wells are elevated (>10 μg/L) in As due to reductive dissolution of Fe oxides and/or alkali desorption. In the Ravi floodplain, 47% of 6,445 wells tested were elevated in As relative to this threshold, however, compared to only 9% of 14,165 tested wells in doabs and floodplains other than the Ravi. We hypothesize that the oxidation of sulfides, possibly by nitrate, is responsible for elevated As in groundwater in the Ravi floodplain. The As content of aquifer sands outside the Ravi floodplain averages 2±2 mg/kg (n=51) and increases to 4±4 mg/kg (n=66) in the portion of the Ravi floodplain within Pakistan, and 6±7 mg/kg (n=99) upstream in India. Synchrotron spectroscopy and column-based speciation suggests that much of this more abundant As is present in sulfide minerals, and that the As that is evolved is arsenate. Regardless of the mechanisms at play, extreme lateral and vertical heterogeneity of As in well water could be taken advantage of to lower the exposure across the region with more testing and targeting of low As aquifers. Key words: Groundwater, arsenic, Indus plain, iron reduction, alkali desorption, sulfide oxidation, Pleistocene