Test of the first-order removal model for metal retention in a young constructed wetland

The first-order removal model is widely used in constructed wetland design. The suitability of this model was tested to predict metal retention in a y oung constructed wetland receiving agricultural and urban runoff. During tw o years, water samples for total and dissolved metal analyses were collecte d every third day at both the inlet and the outlet. The wetland retained me tals best during summer and fall whereas during winter the retention of met als was significantly lower. The first-order removal model predicted Fe and Mn retention in the spring and dissolved Zn retention from spring to fall in both years. During those periods, hydraulic retention times: (HRTs) grea ter than 7 days provided maximum retention for Fe, Mn, and dissolved Zn. Ho wever, first-order removal models failed to fit summer, fall and winter dat a fur almost every metal under investigation (Fe, Mn, dissolved Cu, dissolv ed As) suggesting that HRTs (< 1-25 days) did not affect metal retention du ring these seasons. The metal loading to the wetland was low and the input of metals through internal loading may be more significant consequently dec reasing the metal retention. Therefore, the first-order removal model is in adequate to predict metal retention on a seasonal basis. Models used to des ign constructed wetlands under cold climates must consider seasonal changes that affect biological as well as hydrological variables. (C) 2001 Elsevie r Science B.V. All rights reserved.