ABSTRACT
Constructed wetlands are built because they can act as sinks for many pollutants, thereby protecting the water quality of downstream ecosystems. The treatment performance is generally assessed using mass balance calculations. Along with the mass balance approach, we compared the metal content of populations 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 particulate and dissolved metals were taken every 3 days for the duration of the experiment. Overall, the wetland retained most dissolved metals, including Fe, Mn, Cu, Zn, Ni, and Pb, but released dissolved As. However, the wetland released particulate Fe and Mn. With the exception of Zn, the metal concentrations of the downstream snails were on average higher than those measured in the upstream population. The higher metal content of downstream snails 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 determine the treatment efficiency and toxicological risk associated with constructed wetlands.
Subject(s)
Environmental Monitoring/methods , Helix, Snails/physiology , Metals, Heavy/analysis , Waste Disposal, Fluid , Animals , Facility Design and Construction , Risk Assessment , Tissue Distribution , Water Pollution/prevention & controlABSTRACT
Diel changes in Fe concentrations were examined from spring to late fall at two surface-flow wetlands. The highest concentrations of ferrous, dissolved and total Fe were measured at night in the littoral zone, when oxygen and pH were low. The lowest Fe concentrations were measured during the day when oxygen and pH were highest. The amplitude of change over the day-night cycle was greatest in July and lowest in May and October. These diel changes were also observed at the outlet of both wetlands. Overall O2 and pH explained 60% of the observed seasonal and diel variation in water Fe (R2 = 0.60, p = 0.004). The treatment performance of wetlands can be overestimated when based on samples collected during the day.
