Manganese mobilization from sediment to groundwater in alluvial-lacustrine aquifer system along the lower reaches of Han River

Geogenic contaminants in groundwater limit the drinking water safety and sustainable water supply for domestic purposes throughout the world. Elevated concentrations of manganese (Mn) in groundwater have been one of the major concerns since its adverse health effects to humans, whereas the underlying enrichment mechanism within aquifers is poorly understood. This research aims to connect the spatial distributions of groundwater Mn in different units of the aquifer and the variations of sediment geochemistry to provide comprehensive under-standing of Mn provenance and its mobilization through sediment pore-networks into groundwater. Along the lower reaches of Han River within the central Yangtze River Basin, significant heterogenous Mn distribution has been observed with over 60 % of groundwater exceeding the World Health Organization drinking water guideline. Groundwater chemistry and sequential extraction results indicated that the oxidation of organic matter and reduction of Mn oxides and Fe (hydr)oxides in sediments could drive groundwater Mn enrichment. Herein, we propose a burial-dissolution type as the major hydrogeochemical process of Mn mobilization in the region. These findings are of great importance in understanding the geogenic sources of Mn and the controlling processes for its spatial heterogeneity in alluvial-lacustrine aquifer system, which might be applicable to similar Mn-affected geological settings.