Engineered Wetlands

The removal of dissolved metals in wetlands (land in which the soil is saturated) may be mediated through a variety of processes, including direct uptake by plants, ion exchange or adsorption on clays or organic macromelecules in the root zone, update by micro-organisms, and preciptation with metabolically-produced sulphies in anaerobic sediments. The first three provide for short-term, temporary immobilation with the wetland, whereas the precipitation of metal sulphides represents a potentially long-term dsposition process. The anaerobic environment in sediments generated by a high level of microbial activity is critical for the promotion of sulphate-reducing conditions and the production of sulphides.

At Noranda’s Bell mine in B.C., which is being decomissioned, experimental engineered wetlands are being used to investigate metals removal, specifically copper in concentrations between 0.10 to 10.0 mg/L (dissolved), from contaminated seepage out of the dailings dams. A full-scale wetlands may be incorporated into the closure design. The mechanisms of treatment and its operating limitations will also be researched.

Required Ponds: small, large and batch mixing ponds

Dimensions at soil surface: large pond – 5 x 60 m; small pond – 2.5 x 30 m

Depth: gravel and filter – 300 mm; soil – 300 mm; water – 200 mm; freeboard – 500 mm; Slide slopes: 1.5 H:1V; Liner: 30 mil HDPE; Inflow: 8 L/min (large pond) 2 L/min (small pond)

Contaminated water is pumped to the batch (mixing) pond from a nearby seepage collection pond. To obtain a copper concentration of 1 to 10 ppm, lake water is added to the mixing pond to attain the desired contaminant level. From the mixing pond, the batch is pumped into the two ponds utilizing metered pumps. Piezometers have been installed to allow for sampling along the flow path. The water level can be varied within the two ponds. Sampling is facilitated by daily grab samples and composites from automated flow proportionate sampling equipment.

The vegetation, mainly sedges and cattail plants, were transplanted from a nearby lake during the summer of 1990. All transplanted vegetation has successfully over-wintered and growth has been equal to that of the donor site.

Anytime we attempt to engineer a biological system, nature likes to remind us of how little we know. The creation of these wetland ponds established a niche for a small animal called a vole, a stocky, mouse-like creature. The vole population increased and over-grazed the vegetation. The addition of the contaminated water had to be delayed until the spring of 1991 to allow the plants to recover from the grazing stress.

After resolving some pump and pipeline changes, contaminated feed water was finally added in July 1991. Typical feed water has a pH of 7.2, with a dissolved copper concentration of 0.49 ppm and a total copper concentration of 1.21 ppm. The wetland ponds discharge has a corresponding pH of 6.2, with dissolved and total copper concentration of 0.022 ppm. Copper retained by this treatment technology is in excess of 95%, making the resultant effluent suitable for discharge to the environment. The system continues to operate in 1992, and copper continues to be effectively removed to concentrations below 0.05 ppm.

Vegetation and sediment monitoring will continue to help identify the processes responsible for treatment and to resolve project objectives.

Preliminary results indicate this wetland technology effectively removes copper from Bell mine effluent. The project requires up to two more years to determine the long-term success of wetlands treatment.

(The preceding is a condensed version of a paper prepared by Ross Gallinger, Environmental Co-ordinator, and Lisa Williamson, Environmental Assistant, Bell Mine.)