The evaluation of metal retention by a constructed wetland using the pulmonate gastropod Helisoma trivolvis (Say)

Constructed wetlands are built because they can act as sinks fur many pollu tants, thereby protecting the water quality of downstream ecosystems. The t reatment performance is generally assessed using mass balance calculations. Along with the mass balance approach, we compared the metal content of pop ulations of a common pond snail (Helisoma trivolvis Say) collected upstream and downstream of a 3-year-old constructed wetland. Snails were collected in early May, June, and August 1998. At the same time, water samples for pa rticulate and dissolved metals were taken every 3 days for the duration of the experiment. Overall, the wetland retained most dissolved metals, includ ing Fe, Mn, Cu, Zn, Ni, and Pb, but released dissolved As. However, the wet land released particulate Fe and Mn. With the exception of Zn, the metal co ncentrations of the downstream snails were on average higher than those mea sured in the upstream population. The higher metal content of downstream sn ails was likely related to the significant export of particulate metals by the wetland, despite the overall retention of dissolved metals. This study points to the need for biological as well as chemical monitoring to determi ne the treatment efficiency and toxicological risk associated with construc ted wetlands.