This study reported the efficiency of a free water surface flow constructed wetland (CW) system that receives runoff impacted stream water from a forested and agricultural watershed. Investigations were conducted to examine the potential effect of hydraulic fluctuations on the CW as a result of storm events and the changes in water quality along the flow path of the CW. Based on the results, the incoming pollutant concentrations were increased during storm events and greater at the near end of the storm than at the initial time of storm. A similar trend was observed to the concentrations exiting the CW due to the wetland being a relatively small percentage of the watershed (<0.1%) that allowed delays in runoff time during storm events. The concentrations of most pollutants were significantly reduced (p < 0.05) except for nitrate (p = 0.5). Overall, this study suggests that the design of the system could feasibly function for the retention of most pollutants during storm events as the actual water quality of the outflow was significantly better by 21–71% than the inflow and the levels of pollutants were reduced to appreciable levels.

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