Well water and drill cuttings from north Punjab, Pakistan

Groundwater of the Ravi River floodplain is particularly elevated in arsenic (As) on both sides of the Pakistan-India border. 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 in a subset of the 215 villages containing 20,348 wells that were tested across the region. Radiocarbon dating of clay cuttings from the Ravi floodplain indicates deposition of the underlying aquifer sands during the Pleistocene. 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 are 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.