Constructed treatment wetlands (CTWs) are unique ecotechnologies that can sustainably treat a range of wastewaters. This study focused on a 0.23 ha vegetated surface-flow CTW polishing nitrate-rich (3–6 mg-N/L) tertiary effluent from a municipal wastewater treatment plant. Water quality was monitored longitudinally in the fall of 2009 and 2010. The CTW cooled water by from around 20 W C to <15 W C in both years. Longitudinal temperature profiles were successfully modeled using an energy balance approach (2009 R2 0.69; 2010 R2 0.92). The magnitude of key model fitting parameters, including albedo (0.1–0.2) and convective transfer coefficient (0.1–0.9 MJ/m2-d-W-C), were within ranges reported in the literature. In both years, dissolved oxygen decreased through the wetland from 6–7 mg/L to 3–4 mg/L, yielding an oxygen mass consumption rate of 0.08–0.09 g/m2-d. Longitudinal nitrate profiles were well represented by the P-k-C* model (2009 R2 0.88; 2010 R2 0.92). First order removal rates were 20.2 m/yr in 2009 and 29.0 m/yr in 2010 at a P value of 6.0. Levels of ammonia and total phosphorus increased negligibly through the wetland, remaining below 0.25 mg/L. This study shows that vegetated surface-flow CTWs are well suited to cool and polish low-BOD nitrate-dominated tertiary effluents with little degradation of other water quality parameters of concern, including phosphorus and ammonia.