Pollution characteristics, migration and transformation of hexavalent chromium in groundwater of a chromium slag

The historical stockpile of chromium slag in China is large, and the hexavalent chromium in slag leachate is highly toxic and migratory. In order to investigate the migration and transformation pattern of Cr(Ⅵ) in groundwater under the combined effect of pollution source, site hydrogeological condition and hydrogeochemical process, a hexavalent chromium contaminated site was taken as an example in this study, the spatial distribution, hydrogeochemical characteristic, occurrence form and proportion of Cr(Ⅵ) in groundwater, and the main factors affecting migration and transformation of Cr(Ⅵ) are analyzed by sampling and testing groundwater samples, and the combination using of methods such as Kriging interpolation, factor analysis, hydrogeochemical calculation, Piper diagram and ion ratio. The results show that (1) the two aquifers below ground surface 40 m are polluted by Cr(Ⅵ), but the size and degree are different obviously. (2) The main forms of Cr(Ⅵ) in groundwater are \mathrmCrO_4^2- and \mathrmHCrO_4^- , \mathrmCr_2\mathrmO_7^2- content is extremely low, and the anions of samples with high Cr(Ⅵ) concentration are mainly \mathrmHCO_3^- and \mathrmSO_4^2- , the cations are mainly Na⁺ and Ca²⁺. (3) Precipitation leaching and seepage result in the leachate containing large amounts of Na⁺, \mathrmSO_4^2- and Cr(Ⅵ) entering groundwater. The increasing pH, high concentrations of \mathrmHCO_3^- and dissolution of iron oxides under low oxidizing environment in groundwater can facilitate the migration of Cr(Ⅵ). Manganese oxides and organic matter are able to change Cr(Ⅵ) content through redox reaction. Evaporation also plays an important role on the enrichment of Cr(Ⅵ) in groundwater. The results of this research can provide strong support for risk management and post remediation of chromium slag contaminated sites.