Temporal variation of groundwater level and arsenic concentration at Jianghan Plain, central China

•Groundwater tables fluctuated during the sampling period.•Temporal variation of groundwater As content had three different patterns.•The rise in As content was attributed to reductive dissolution of Fe oxyhydroxides.•The decline in As content was attributed to scavenging onto fresh Fe oxyhydroxides.•Labile organic carbon may play a role in regulating groundwater As content.

Elevated arsenic in groundwater affects some 60 thousand people in the Jianghan Plain, yet mitigation efforts are hindered by persistent uncertainty about the proximal source of As and mechanisms for its mobilization. Samples were collected every month from May 2012 to April 2014 in a set of 39 monitoring wells and two rivers in the field monitoring site, Jianghan Plain, to evaluate the temporal variation of groundwater level and chemical composition for As and other constituents. Results showed that groundwater tables fluctuated during the sampling period, with higher water levels during the rainy season and lower water levels during the dry season. Wells at 25 m and 50 m deep had lower water level than those of the wells at 10 m deep. Water levels of the wells located in the south of the field monitoring site were always 1 m higher than the wells in the north. Only five wells showed no significant changes in As concentrations during the monitoring period; 23 wells exhibited seasonal variations in groundwater As concentration that were consistent from year to year; the rest of the 11 wells not only showed seasonal variations but also exhibited increasing trends over the monitoring period. Seasonal As concentration changes in groundwater were positively correlated to groundwater level changes, with lower concentration corresponding to lower water level during dry season and vice versa during rainy season. The rise in As concentration during the rainy season could be attributed to enhanced reductive dissolution of iron oxyhydroxides and/or reductive desorption of As(V) as the conditions turn to be more reducing, while during the dry season more As could be scavenged onto fresh iron oxyhydroxides. Meanwhile, temporal variations showed compatible trends between As and Fe, Fe(II), S2−, indicating that As variation would be related to Fe and S cycling in the aquifer systems. Additionally, the availability of labile organic carbon as a driver of microbial reduction may play an important role in controlling the spatial and temporal variation of groundwater As concentration in the Jianghan Plain.