Maintenance versus Maturation: Constructed Storm-Water Wetland's Fifth-Year Water Quality and Hydrologic Assessment

Abstract

Constructed storm-water wetlands (CSWs) have become popular storm water control measures (SCMs) in low-lying coastal environments, offering a hybrid between larger detention practices (wet ponds) and newer green infrastructure technologies. However, it is relatively rare that CSWs are monitored once the practice has aged 4 years, causing some concern among stakeholders about the long-term performance of CSWs. The specific objective of this research was to investigate the effects that wetland maturation and lack of maintenance have on the ability of a 5-year-old CSW to mitigate hydrology and improve water quality. A CSW was monitored from 2012–2013 that had not been maintained since construction in the spring of 2007. Since then, it has established as a diverse and healthy ecosystem. The monitoring results were compared with the results of a study conducted at the same site for a 1-year period immediately following construction (2007–2008). In 2007–2008, runoff volumes were reduced 54% and peak discharges 80%; whereas in 2012–2013, no change was observed in volume, and peak discharges were reduced by 64%. The stark difference in hydrologic mitigation was attributed to less availability of design volume in 2012–2013 because of (1) organic detritus accumulation during the intervening 5 years measured to be 7 cm; (2) modest siltation; and (3) an upward water-table adjustment owing to differing average antecedent dry periods. The 2012–2013 hydrologic performance of this CSW was similar to other CSWs (that were 2 years of age) studied in North Carolina, indicating that ecological systems should be monitored for performance after the point of establishment (2–3 years) for regulatory purposes. Overall nitrogen event mean concentration (EMCs) were reduced as the wetland matured, but not for total phosphorus (TP), as the effluent concentrations increased from 2007–2008 to 2012–2013. CSW maturation seemed to outweigh the lack of maintenance for both nitrogen treatment and stabilization of internal sediment. Maintenance (removal/replacement of forebay sediments and detritus material) need was apparent, however, for TP treatment and hydrologic mitigation. This appeared to be the only form of maintenance that would have been needed in the last 6 years for an SCM treating a 47-ha watershed. In comparison to the other practices, namely bioretention and permeable pavement, similar CSWs may have much lower operation and maintenance costs. This may make CSWs a viable option in areas where little resources exist for SCM maintenance.