Évaluation de la capacité de rétention des métaux dissous d'un marais artificiel en utilisant le périphyton et le gastéropode Helisoma trivolvis The use of periphyton and the freshwater gastropod Helisoma trivolvis to assess dissolved metal retention by a constructed wetland

Résumé

Le but de cette étude est de déterminer si le périphyton et le gastéropode pulmoné Helisoma trivolvis peuvent être utilisés pour évaluer la capacité de rétention des métaux dissous d'un marais construit. Cette étude a été menée dans un marais de la région d'Ottawa-Carleton (Ontario, Canada), qui a été construit en 1995 afin d'améliorer la qualité des eaux de ruissellement provenant d'un bassin versant à usage résidentiel et agricole. Au cours du mois de septembre 1999, des échantillons d'eau ont été prélevés, des escargots (H. trivolvis) ont été recoltés et un substrat artificiel a été utilisé pour faire croître du périphyton, à l'entrée et à la sortie d'eau du marais. En moyenne, les calculs de balance de masse indiquaient une rétention des formes dissoutes du Cu, Mn, Ni et Zn dans le marais. Cependant, les tendances observées pour les concentrations de métaux dissous et pour les concentrations dans les organismes différaient pour certains métaux. Les concentrations de Cd et Ni dans les tissus de H. trivolvis et le périphyton étaient significativement plus élevées à l'entrée qu'à la sortie. Cependant, les concentrations de Cr et Al dans les organismes n'étaient pas significativement différentes entre l'entrée et la sortie alors que celle de Mn était significativement plus élevée à la sortie du marais. Pour tous les métaux sauf le Cd et le Zn, les concentrations dans le périphyton étaient en moyenne plus elevées que celles dans les escargots. Le périphyton peut donc fournir une mesure plus conservatrice de la contamination du milieu pas les métaux. Cette étude montre l'importance de considérer, non-seulement les mesures chimiques, mais aussi les mesures biologiques, dans l'évaluation de l'efficacité d'un ouvrage de contrôle de pollution.

Mots-clés:

• Efficacité de traitement,
• gastropode pulmoné,
• helisoma trivolvis,
• marais artificiels,
• métaux,
• périphyton

Abstract

Urban and agricultural development has had a significant impact on the water quality of rivers and lakes around the world. In the last few decades, constructed wetlands have been designed as wastewater treatment systems to prevent water quality deterioration in natural receiving waters. Constructed wetlands are built because they are considered sinks for many pollutants thereby protecting the water quality of downstream ecosystems. The treatment performance of these wetlands is generally assessed using mass balance calculations. However, the retention of metals by constructed wetlands is highly variable and the factors involved are still poorly understood. If wetlands are sinks for metals, the metal content of organisms should be lower downstream than upstream. In this context, organisms can be useful to assess the retention or transformation of metals by wetlands.

The objective of this study was to determine whether periphyton and the gastropod Helisoma trivolvis could be used to evaluate the retention of dissolved metals in a constructed wetland. H. trivolvis is a freshwater pulmonate snail widespread in ponds across North America. It feeds mostly on periphyton and is more or less sedentary. Snails have been used as biomonitors because several species are metal tolerant. However, compared to snails, periphytic microorganisms may track more closely dissolved metal concentrations as they take up metals principally from the water column.

This study was conducted at the Monahan Pond in Kanata, Ontario (Canada). This wetland was built in 1995 to treat run-off from an agricultural and residential watershed. Water chemistry samples, snails and periphyton grown on artificial substrata were collected at both the inlet and the outlet of the wetland in the fall of 1999. Tissue samples were digested with concentrated nitric acid and metal analyses were done by ICP-MS (Inductively Coupled Plasma Mass Spectrometry). During the experiment several chemical parameters differed between the inlet and the outlet. Alkalinity was significantly higher at the inflow and all major cation concentrations were higher at the inflow. The temperature was on average 3 ºC higher at the outlet. Mass balance calculations showed that the wetland was a sink for most dissolved metals. Snails and periphyton tissue metal concentrations were higher at the inflow than at the outlet for Cd and Ni. However, no significant differences were observed between inflow and outlet tissue concentrations for Cr and Al, whereas Mn was actually significantly higher at the outlet. As a result, the metal content of the organisms did not consistently reflect the dissolved metal concentrations in water. For all metals except Cd and Zn, periphyton concentrations were on average higher than snail metal concentrations. Periphyton analyses can provide a more conservative measure of metal contamination and, when artificial substrates are used, correspond to defined and known periods of exposure.

This study demonstrates that constructed wetlands may lead to increased metal content of downstream organisms even if these wetlands appear to be overall sinks for dissolved metals based on mass balance calculations. It also shows the need to consider not only metal concentrations, but also biological data when assessing the performance of pollution control facilities.

Keywords:

• Constructed wetlands,
• treatment efficiency,
• metals,
• periphyton,
• pulmonate snail,
• helisoma trivolvis