Wetlands, whether natural or created, are used as water treatment systems around the world. While extensive research has been conducted in warm climates, there is limited information on the mechanisms affecting the retention of pollutants by surface-flow wetlands in northern climates. In particular, there are serious concerns about the efficiency of metal retention by wetlands built in Canada and other temperate countries. The mechanisms affecting metal retention by surface-flow wetlands were investigated by relating variations in metal retention to variables such as hydraulic residence time, temperature (i.e. seasonal effects), phytoplankton biomass, photosynthesis and the presence of emergent vegetation.

The importance of hydraulic residence time was investigated at the Monahan surface-flow wetland in Kanata, Ontario. This wetland retained metals best during summer and fall whereas during winter, the metal retention was significantly lower. The first-order removal model predicted Fe and Mn retention in the spring and Zn retention from spring to fall in both years of the study. Hydraulic residence times, greater than 7 days, provided the maximum retention of Fe, Mn, and Zn. However, first-order removal models failed to fit summer, fall and winter data for almost every metal under investigation (Fe, Mn, Cu, As) suggesting that hydraulic residence time (<1-25 days) do not regulate metal retention during these seasons.

The Monahan wetland also affected the partitioning of metals between particulate and dissolved phases thus potentially affecting the bioavailability of metals to downstream systems. On a yearly basis, the wetland showed significant retention of the dissolved phase, but the retention of total Fe and Mn was poor. The wetland transformed dissolved into particulate metals from spring to fall whereas during the winter, dissolved metals were released. Changes in pH, alkalinity and temperature could explain 11% and 40% of the variation in the ratio of dissolved to total Fe and Mn respectively. Furthermore, from spring to late summer, planktonic algal biomass was negatively related to the ratio of dissolved to total Fe and Mn, which suggests the importance of phytoplankton in affecting the partitioning of metal in young plankton-dominated wetlands.

In the Monahan wetland and in an acid mine drainage wetland (Falconbridge) near Sudbury, Ontario, diel changes in metal concentrations followed diel changes in pH and oxygen induced by photosynthesis. During the day, metal concentrations in the water column were low because high pH and oxygen favored the precipitation of Fe and Mn oxides at the sediment-water interface. At night, Fe and Mn oxides were reduced and released to the water column because intense biological respiration decreased oxygen and pH. Diel changes in metal concentrations have to be considered when evaluating the retention performance by surface-flow wetlands.

Finally, the effect of emergent vegetation on the concentration and partitioning of metals in surface sediments of four wetlands was investigated. Surface sediments with and without cattails (Typha latifolia L.) had similar total metal concentrations (Fe, Mn, Zn and Cu). The partitioning of metals in shallow vegetated sediments differed from deep unvegetated sediments at the Monahan and Falconbridge wetlands. In contrast, there was no significant difference in the partitioning of metals in surface sediments with or without cattails at the Panel and Riverwalk wetlands. Generally, half of the total metal concentration in sediments was dissolved in the pore water or associated with metal oxides, monosulfides and adsorbed onto organic matter. Zn was an exception as half of the total Zn was associated with the persistent organic fraction. The effect of cattails on the bulk concentration and partitioning of metals appears to be minimal in surface-flow constructed wetlands.