Black colored sandy sediments caused by bacterial action, and the mechanism for arsenic enrichment of groundwater in Inner Mongolia

High As concentrations in groundwater were detected in the village of Ershe, Jianshe but were not detected in the village of Qishe, Fengle in Inner Mongolia. In this study, core samples from two comparable regions were mineralogically examined. Results of X-ray diffractometry (XRD) showed that the principal minerals present in the drill core samples are quartz, micas, feldspars, calcite and clay minerals. Variation of these mineral species does not correlate with depth but to lithofacies; minerals were As-rich in muddy sediments and As-poor in sandy sediments. The results of X-ray fluorescence analysis (XRF) are consistent with those of XRD. The As content shows a positive correlation with total Fe₂O₃, V and loss of ignition (LOI). In order to identify the chemical forms of As present, a selective sequential extraction (SSE) method was carried out. The results of the SSE experiment suggested that in highly reducing environments, As that is adsorbed on Fe oxyhydroxides is released. Extremely black sand was found in deep drill core samples. To investigate the cause of black sands, inorganic and bacterial culture experiments were carried out. The black color in sands is due to the Fe sulfide greigite evolved from the Fe oxyhydroxide film coating the sand grains. This change in color occurred upon reaction with H₂S gas most probably associated with bacterial activity. Transmission electron microscope (TEM) observations clarified that the sulfide structure of black sand grains is that of greigite. In summary, based on water chemistry, geology, mineralogical characteristics, and geomicrobiological features, the As release mechanisms in Ershe and Qishe are discussed. Then, a possible model was proposed that includes formation of Fe sulfides around sand grains from SO₄ ²⁻ under strongly reducing conditions.