Characteristics of nutrient retention in a stormwater wetland during dry and wet days.

A 0.23-ha pilot wetland was constructed to examine its effectiveness for the control of nonpoint source pollution (NPS) from a 7.42-ha agricultural watershed in Korea. The wetland was operated during base flow and storm flow conditions and monitored for four years (2006-2009). The wetland was operated under high hydraulic loading rates (HLRs, 0.78 m/day on average) during rainy days. The average influent nitrogen concentration was 9.8 mg/L during wet days, but around 11.6 mg/L during dry days. Due to the short hydraulic retention time (HRT) on wet days, nitrogen retention at the wetland was poor, only 6.1%; whereas, during dry days the efficiency was higher, around 17.9%. On rainy days, the wetland presented high retention for total phosphorus (TP), with an average efficiency of 68.3%, accompanied with a total suspended solids (TSS) removal of 84.7%. However, on dry days, the wetland showed positive soluble reactive phosphorus (SRP) retention other than TP. The interior of the wetland was observed to have different nutrients levels. Furthermore, suggestions are addressed in terms of improved design for better performance.