Arsenic behavior in different biogeochemical zonations approximately along the groundwater flow path in Datong Basin, northern China.

Studies have shown that arsenic is desorbed/released into groundwater as a result of bacterial reduction of As(V) and Fe(III). However, bacterial activities like sulfate reduction process can also reduce the content of arsenic in groundwater. In this study, we examined the effects of different biogeochemical processes (e.g. NO3 - and SO4 2- reduction) on arsenic, by investigating the chemical characteristics and bacterial community structure of groundwater in the Datong Basin, northern China. Along the groundwater flow path, arsenic concentration increased from <1 to 947.6μg/L with dominant bacteria change from aerobic (Fluviicola, Rhodococcus) to denitrifying bacteria (Thauera, Gallionella), and then to sulfate reducing bacteria (Desulfosporosinus). According to the groundwater redox sensitive indicators (Eh, NO3 - , SO4 2- /Cl- and Fe2+ ) concentrations (or ratios), the sampling points were approximately divided into three zones (I, I'' and II). Variation in features of these indicators suggested that the groundwater evolved from a weakly oxidizing environment (Zone I, Eh average 93.3mV, respectively) to strong reducing environment (Zone II, Eh average -101.8mV). In Zone I, bacteria mainly consuming O2 or NO3 - were found which inhibits Fe(III) and As(V) reduction reaction, resulting in a low As zone (<1 to 3.3μg/L). However, in Zone II, where O2 and NO3 - have been depleted, SO4 2- reduction appears to be the dominant process, and the Fe(III) and As(V) reduction processes are also occurring and hence, enrichment of As in the groundwater (2.8 to 947.6μg/L, average 285.6μg/L). Besides, bacterial Fe(III) reduction process was retarded due to the weakly alkaline conditions (pH7.60-8.11, average 7.83), but abiotic Fe(III) reduction by HS- may be continued. Therefore, we conclude that the Fe(III) and As(V) reduction processes contributed to arsenic enrichment in the groundwater, and the reductive desorption of arsenate is the main occurring process especially in the weakly alkaline environment. Moreover, NO3 - reduction process can significantly restrain the release of arsenic, but the process of SO4 2- reduction is insignificant for arsenic concentration decline in natural groundwater.