Redox Control on Groundwater Arsenic Variability in Groundwater of Punjab, Pakistan

Abstract:

The floodplain of the Ravi River contains particularly elevated levels of arsenic (As) on both sides of the Pakistan-India border. To try to understand the origin of this pattern and generally lower groundwater As concentration elsewhere in the Punjab plains, a total of 14 wells were drilled in 11 villages in floodplains of the Ravi (8 sites) and Chenab (3) rivers and intervening Rachna (2) and Chaj (1) doabs. One striking difference relative to observations during previous drilling of the Punjab plains conducted in Indian side of border, where most aquifer sands encountered were grey to brownish-grey in color, was the frequent occurrence of brown-orange to grey colored sands in Pakistan. As reported in above mentioned study, elevated concentrations of As in grey to brownish-grey aquifer of the upper portion of the Ravi River were linked to elevated As levels in aquifer sands. This relationship, however, no longer held further downstream and in other floodplains or doabs. Here, instead, aquifers with brown-orange to grey colored sediments were encountered both in high (>10 µg/L) and low (<10 µg/L) arsenic villages and with little to no dependence on solid phase As levels in aquifer sands. The depth to brown-orange colored sands varied greatly in floodplains and doabs. On average, it was usually encountered at depth intervals of 9-21 m in Ravi floodplain while in other floodplain and doabs it was found at more shallower depths of 5-12 m. These brown-orange colored sands were mostly associated with both low sand As and low dissolved levels of iron (Fe) measured with kit. Such findings suggest that the redox state of aquifers played a dominant role in these regions. It is presently unclear why so much more orange sands was found outside the northern reaches of the Ravi River. The depositional history of the various states should be constrained by radiocarbon dating of organic carbon in clay layers from the various sites.