Synergetic effects of the hydrogeochemical and sedimentological factors on arsenic mobility in a sand-gravel deep confined aquifer

Both groundwater and sediment were sampled to investigate mechanisms of groundwater arsenic (As) mobility in sand-gravel confined aquifers from the Songnen Basin, China. In the study area, geogenic high-As groundwater occurred in both shallow Late-Pleistocene sandy unconfined aquifers and deep Early-Pleistocene sand-gravel confined aquifers. In relative to shallow groundwater, higher dissolved As and Mn concentrations, proportion of HCO3- to equivalent anion concentrations, and BDOC/DOC in deep groundwater were observed, being accompanied by relatively constant dissolved Fe(II) and DOC concentrations along the depth profile. The deep confined aquifer also had considerably higher As concentrations of sediment porewater than those in the shallow one. Furthermore, sand-gravel sediments in deep aquifers had remarkably higher contents of extracted Fe(III) oxides and the bound As, HCl-extractable Fe(II) minerals, and adsorbed As, but considerably lower contents of Mn oxides and Mn(II) minerals than the shallow sandy sediments. Together the study shows that high dissolved groundwater and porewater As concentrations in the deep confined aquifer were caused by arsenic-bearing Fe(III) oxide reduction, being facilitated by high contents of Fe(III) oxide-bound arsenic and negligible Mn oxides in sediments.