Changes in groundwater bacterial community during cyclic groundwater‐table variations

Column experiments containing an aquifer sand were subjected to static and oscillating water tables to investigate the impact of natural fluctuations and rainfall infiltration on the groundwater bacterial community just below the phreatic surface, and its association with the geochemistry. Once the columns were established, the continuously saturated zone was anoxic in all three columns. The rate of soil organic matter (SOM) mineralization was higher when the water table varied cyclically than when it was static due to the greater availability of NO3‐ and SO42‐. Natural fluctuations in the water table resulted in a similar NO3‐ concentration to that observed with a static water table but the cyclic wetting of the intermittently saturated zone resulted in a higher SO42‐ concentration. Rainfall infiltration induced cyclic water‐table variations resulted in a higher NO3‐ concentration than those in the other two columns, and a SO42‐ concentration intermediate between those columns. As rainwater infiltration resulted in slow downward displacement of the groundwater, it is inferred that NO3‐ and SO42‐ were being mobilized from the vadose zone. NO3‐ was mainly released by SOM mineralization (which was enhanced by the infiltration of oxygenated rainwater), but the larger amount of SO42‐ release required a second mechanism (possibly desorption). Different groundwater bacterial communities evolved from initially similar populations due to the different groundwater histories.