ASSESSMENT
OF EXISTING NATURAL WETLANDS AFFECTED BY LOW pH, METAL CONTAMINATED
SEEPAGES (ACID MINE DRAINAGE)
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Project
3.12.1
May 1990
EXECUTIVE
SUMMARY
Acid mine drainage
(AMD) occurs when pyritic mine tailings are exposed to oxygen and
water. Surface runoff and groundwater seepage through waste piles
are typically very acidic and contain high concentration of dissolved
metals including iron, aluminum, manganese, copper, lead, nickel
and zinc. Conventional treatment of AMD usually involves liming
the runoff to neutralize the water and chemically precipitate the
metals. Liming is expensive and may be required long after the mine
has ceased operating. Recently, attempts have been made to develop
low maintenance, low-cost AMD treatment systems that can restore
water quality. One method of passive treatment utilizes the vegetation
and sediment microbial communities found in natural wetlands to
reduce acidity and precipitate the metals.
This study
examined natural weds near six base metal mines in eastern Canada
for evidence of biological AMD mitigation. Potentially suitable
wetlands were identified by Environment Canada in consultation with
mining companies and provincial resource departments. A field program
was conducted to describe and compare these sites in terms of water
and sediment chemistry, hydrology, and vegetation. Two sampling
periods, summer and late fall, were originally planned to enable
within-site comparisons between presumed biologically-active"
and "inactive periods. The project was started too late to
conduct a summer campaign. This report presents the results of the
early fall sampling period.
There were
large differences among wetlands in metal loadings, groundwater
inputs. water retention times, metal concentrations in wetlands
sediments, and vegetation communities. None of the sites, however,
appeared to significantly reduce downstream loadings of metals or
acidity levels during the fall. Prolonged drought at one site, Nickel
Rim, severely reduced flows and prevented calculation of influent
and effluent metal loadings. The apparent lack of biologically-related
improvement of AMD was most likely a consequence of (1) low temperatures,
ca. 4 °C, (2) channeled flow patterns within the wetlands, and (3)
limited contact of AMD with anoxic sediments. Inputs of other seeps
and groundwaters to the wetlands were significant complicating factors,
especially at the Kam-Kotia site. High metal concentrations in wetlands
sediments indicates that the wetlands had removed metals in the
recent past.
Français
| Contact Us
| Help | Search
| Canada Site
Home | What's
New | CANMET-MMSL
| MMS Site
| NRCan Site
|