Simply diverting contaminated mine water to a wetland may do more harm than good. For instance, Kalin and co-workers (1991) reported that acidic bogs in Nova Scotia, Canada, receiving AMD were severely injured, despite their alleged tolerance of acidic water. Similarly, Carlson and Carlson (1994) documented the severe impact of coal pile leachate (very similar to AMD) on a forested wetland. Interestingly, their analysis indicated that low pH and high salinity were primarily responsible for plant dieback, whereas high aluminum and manganese concentrations in the leachate played a less important role. However, other natural wetlands receiving contaminated mine water have been unimpacted by it (Dollhopf et al., 1988; Hambley, 1996). So the issue is not a simple one.
Assuming that a wetland treatment system has been properly designed, what evidence is there that it lessens the overall impact on the environment?
Since treatment wetlands receive contaminated mine drainage, it would be expected that animals living in them will be adversely impacted. Remarkably, this doesn't usually appear to be the case. Albers and Camardese (1993) demonstrated that, in a constructed wetland treating AMD, metal concentrations in wetland invertebrates were independent of water chemistry (metal concentrations or pH), even when the water pH was artificially acidified. Pascoe and co-workers (1994) found that neither plants nor small mammals living in a wetland overlying mining waste (tailings) accumulated toxic elements (As, Cd, Cu, Pb, and Zn). Lacki and co-workers (1992) found that wetlands treating mine drainage provided valuable habitat to reptiles and amphibians, without any deleterious impact on their populations. On the other hand, a report indicated that the caddisfly Limnephilus indivisus experienced greater mortality in a wetland impacted by AMD from a strip mine (Usis and Foote, 1991). Another report suggested that crabs in a freshwater wetland bioaccumulated metals, but the metal sources included mine, industrial and municipal wastewater, confounding the significance of these data (van Eeden and Schoonbee, 1991).
Metals uptake by wetland plants is one potential point of entry into the food chain. In the Carbonate Mountain study (Dollhopf et al., 1988), aluminum, arsenic, cadmium, copper, iron, lead, manganese, nickel, and zinc concentration in the sedge Carex rostrata fell within the ranges reported by Hutchinson, 1975. However, lead in plant leaves was reported to exceed these concentrations.
In my investigations, I found that, typically, wetland plants do not take up metals which are accumulated in sediments. For example, the table below compares metal concentrations in two natural wetlands receiving metal-contaminated water.
| S. McQuesten wetland | No Cash wetland | Galkeno wetland | Non-impacted sites | |
| Metal | ||||
| Cadmium | ||||
| Copper | ||||
| Lead | ||||
| Zinc |
South McQuesten wetland provides background metal concentrations in the area. It has the same vegetation as the No Cash and Galkeno wetlands, but does not receive mine water. Ranges and mean metal concentrations in aquatic grasses and forbs from non-impacted sites, are from Hutchinson, 1975.
Notice that the No Cash and Galkeno wetlands have accumulated metals in their sediments, indicating they removed metals from mine water. Yet metal concentrations in plant tissues from these wetlands are comparable with those from the South McQuesten wetland, and fall within the range of those from unimpacted site. Thus, there is no significant metal uptake by plants in these wetlands.
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Last Updated: Tuesday, November 12, 1996 6:15:28 PM