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Alluvial aquifers are the main source of groundwater worldwide. In Hyderabad area of Sindh province, aquifers are naturally polluted by arsenic (As) like other alluvial aquifers of the world. Present study was carried out to decipher the mobilization mechanism of arsenic in Holocene aquifers of Indus river basin, where a large population is at the risk of arsenic ingested diseases. Fifty groundwater samples were collected from Hyderabad and its surrounding areas to examine their physical, chemical and microbiological characteristics. In 80% of the groundwater samples, TDS is above the WHO limit. Dominant (40%) hydrofacies in groundwater of study area is NaCl, which shows water-rock interaction and cation exchange mechanism. In order to investigate the source of arsenic, eleven near-surface soil samples were also collected and analyzed for elemental and mineral composition using XRD and AES techniques. Correlation Coefficient, Principal Component Analysis (PCA) and multivariate statistical analyses were used to interpret the data. Arsenic ranges between 10-150 μg/L in groundwater, while in soil samples it ranges from 77 and 137μg/kg. Findings showed that arsenic is mobilized in the alluvial aquifers of Indus river through dissolution/ leaching of iron oxyhydroxides under anoxic conditions. Arsenic is mainly leached from mica and phlogopite under reducing conditions. Meandering of Indus river through different historical time periods is an important factor for the distribution of redox zones created by mirco-biodegradation of organic matter rich with clayey sediments. Irrigation return flow, infiltration of sewerage in groundwater and unlined sanitation are also important anthropogenic factors for creating local anoxic conditions to mobilize arsenic in groundwater.

Arsenic Contamination in Indus Alluvial Plain Sediments and Groundwater of Hyderabad and its Surroundings, Pakistan