Diel changes in iron concentrations in surface-flow constructed wetlands

Diel changes in Fe concentrations were examined from spring to late fall at two surface-flow wetlands. The highest concentrations of ferrous, dissolved and total Fe were measured at night in the littoral zone, when oxygen and pH were low. The lowest Fe concentrations were measured during the day when oxygen and pH were highest. The amplitude of change over the day-night cycle was greatest in July and lowest in May and October. These diel changes were also observed at the outlet of both wetlands. Overall O2 and pH explained 60% of the observed seasonal and diel variation in water Fe (R2 = 0.60, p = 0.004). The treatment performance of wetlands can be overestimated when based on samples collected during the day.