Performance of an agricultural wetland-reservoir-irrigation management system

Abstract

Constructed wetlands (CW) have gained recognition as a management option for the treatment of various agricultural wastewaters. This study involved the design, construction, and initial evaluation of a wetland-reservoir-irrigation (WRI) system. The system was established in Truro, Nova Scotia, Canada, with the goal to capture, treat, and re-use agricultural sub-surface drainage water. It consisted of a 1.8-ha area of a cropped field that was systematically tile drained. Drainage water was directed through a 2-cell CW and then into a reservoir-irrigation pond. Flow rate hydraulics, residence time distributions, and treatment efficiencies for nitrate-nitrogen (NO3--N) and Escherichia coli (E. coli) were monitored for 14 months. Mass reductions of NO3--N and E. coli from the CW were 67.6% and 63.3%, respectively. However, average E. coli concentrations increased to 178 CFU 100 mL-1 in the reservoir during the warm season. It may therefore be best to use reservoir water for irrigation of crops that are not consumed raw. To aid in the future design of similar systems, mean first-order rate constants (ks) for NO3--N and E. coli were calculated to be 8.0 and 6.4 m y-1, respectively. The volume of water collected in the reservoir exceeded typical irrigation requirements of the drained land and could therefore provide irrigation to additional land beyond the drainage area.