Impact process of the aquitard to regional arsenic accumulation of the underlying aquifer in Central Yangtze River Basin.

The clayey aquitard has the potential to release geogenic poisonous chemicals such as arsenic (As) to the adjacent aquifer owing to complex hydrologic or biogeochemical processes. However, it remains unclear whether the aquitard has effect on As enrichment in the underlying aquifer in regions without extensive groundwater pumping, and the related processes have been poorly known. Based on piezometer water chemistry, stable water isotopes, sediment chemistry and reactive-transport model, this study aims to reveal the impact process of the aquitard to As accumulation of underlying aquifer from central Yangtze River Basin, a As-affected area without extensive groundwater pumping. On the whole, As migrated from top to bottom of the aquitard (especially the depth over 10 m) and significantly influenced the As accumulation in the underlying aquifer. Nonetheless, the results of three topical boreholes showed two different hydrogeological conditions affected As release in the aquitard and enrichment in the underlying aquifer. Different hydrogeological conditions could result in the input of different species organic carbon and then impact As concentrations in the aquifer. When the aquitard was near surface water bodies, the reductive dissolution of iron oxides was the main driver for As release and the aquitard had a significant influence on the enrichment of arsenic in the aquifer. At areas without surface water bodies nearby, the desorption of As(V) from minerals was the main source of As and the concentrations of As in pore water were quite low; this pattern had little effect on the enrichment of arsenic in the aquifer.