Unveiling Distribution, Hydrogeochemical Behavior and Environmental Risk of Chromium in Tannery Wastewater

Chromium (Cr)-contaminated tannery wastewater is a major environmental concern, especially in developing countries, such as Pakistan, due to its use for crop irrigation, resulting in food-chain contamination and health issues. In this study, we explored the distribution, speciation, hydrogeochemical behavior and environmental risks of Cr in tannery wastewater collected from various tanneries of Kasur district in Punjab, Pakistan. Tannery wastewater samples were taken during the summer (TWW-summer; n = 82) and winter (TWW-winter; n = 82) seasons. The results showed that high Cr concentration was observed in TWW-winter (mean: 49 ± 32 mg L⁻¹) compared to TWW-summer (mean: 15 ± 21 mg L⁻¹). In TWW-summer and TWW-winter samples, the Cr concentration exceeded the National Environmental Quality Standard (1 mg L⁻¹), with the total Cr ranging from 2.8 to 125 mg L⁻¹. Hexavalent Cr (Cr(VI)) and trivalent Cr (Cr(III)) concentrations spanned 2.7 to 2.9 and 12.4 to 46 mg L⁻¹, respectively. The Piper plot showed that hydrogeochemistry of wastewater was dominated by Ca-Mg-SO₄ and Ca-Mg-Cl type water, and geochemical modeling indicated that the presence of Cr-iron (Fe)-bearing mineral phases—notably, FeCr₂O_4, MgCr₂O₄ and Cr(OH)₃) may control the fate of Cr in the tannery wastewater. Environmental risk assessment modeling categorized the tannery wastewater as the ‘worst quality’, which is not fit for use in crop irrigation without treatment. This study highlights that immediate monitoring, remediation and mitigation strategies are required to reduce the risk of Cr exposure from tannery wastewater in many areas of Pakistan.