Exploration geologists have known for many years that metals, most commonly copper, iron, manganese, uranium, and zinc, frequently accumulate in swamps and bogs located in mineralized areas (Levinson, 1980; Brooks et al., 1995). I've spoken to a few old-timers, such as Lew Green and Bob Boyle [retired geologists with the Canadian Geological Survey], who told me that they would regularly come upon bogs high in arsenic, copper, pyrite, or uranium.
Some of these sites have been investigated, and the results have been published in the literature or as theses (e.g., Cannon, 1955; Horsnail et al., 1969; Lett, 1978). I will discuss two examples of such reports on this page.
This 2.5 acres (1 hectare) wetland was staked out in 1950, and though it is estimated to contain 300 tons of copper (Fraser, 1961), an economic method to recover the copper has not been found to date. The wetland sediments contain 3 to 6% copper (dry weight). Fraser reports that the wetland is largely devoid of vegetation, except for mosses growing by seepages of water. The moss Pohlia nutans dominates areas of high copper concentrations, with liverworts growing in areas where copper falls to 0.1-1%. The moss reportedly contains up to 12% copper (this is not a typo!) (Boyle, 1977).
The other wetland is much less well studied. It was only found in the late 1950's through a copper anomaly in sediments of a creek draining its southerly end (Smith, 1960).
Fraser (op. cit.) indicates that the pH in sediments of the northern wetland varies from 5.5 to 8.1, depending on the time of sampling. This is unlike native copper bogs, which are normally acidic (pH 4 to 6). The Eh in the wetland sediments varies from 0.08 to 0.23.
Copper enters the wetland through the bottom, emerging as distinct seeps containing 0.005 to 1.0 mg/L. Its source appears to be copper mineralization associated with the Boss Point Formation (Boyle, 1977). The copper is retained in wetland sediments soon after the seepage emerges. Fraser (1961b) demonstrated that it is attenuated predominantly through association with organic matter, which in the wetland sediments reaches 10-20% (Boyle, 1977).
Fraser determined that accumulating 300 tons of copper in the wetland would require approximately 4,000 years. He notes that the wetland can't be older than that, because sea water retreated from the area 4,000 years ago. Therefore, he concludes that this wetland has been removing copper from solution since the time it was formed.
This example shows that wetlands have an extraordinary capacity to accumulate copper. In this wetland, the copper is retained by forming extremely stable complexes with organic matter (prolonged acid attack or boiling in aqua regia for three hours does not remove all the copper). Its most remarkable aspect is that it has apparently been accumulating copper for 4,000 years, longer than any mine abandonment plan I've seen!
Uranium has a strong affinity toward organic matter (Szalay, 1964), suggesting that it should easily be retained in the organic-rich sediments of wetlands. Another process in wetland sediments, the microbial reduction of uranyl ions [U(VI)] to the less mobile U(IV) (Lovley et al., 1991), may also be important for its retention. Given this, it would be expected that uranium should be commonly retained by wetlands. Indeed, uraniferous "bogs" are reported from Scandinavia, Canada, the former Soviet Union, and the western United States Zielinski et al. 1987).
Beginning in 1982, the U.S. Geological Survey conducted reconnaissance work in mountain wetlands located in the Western United States to determine whether they accumulate uranium (Owen and Otton, 1995). Among other things, it was found that of the 145 wetlands investigated in the Front Range of Colorado, approximately half were enriched to some degree for uranium, with 15% being highly enriched (100-1,000 ppm U, dry weight) and 1% being very highly enriched (over 1,000 ppm U, dry weight). Based on this work, several wetlands were investigated to understand in detail their underlying geology and geochemistry.
The north fork of Flodelle Creek, in northeastern Washington, U.S.A., harbours uraniferous wetlands which were investigated. The creek is fed by water draining a highland area underlain by uraniferous granite (See map here).
Wetlands on the Flodelle occupy two areas. In the upper part of the creek, a five to thirty (5-30) metre wide valley floor supports wetlands. They are fed by seeps and spring highly enriched in uranium (50-318 ppb; Zielinski et al. 1987). Uranium accumulates in their organic-rich sediments, reaching up to 8,960 ppm (average 1,243 ppm in one stratigraphic unit; Johnson et al. 1987).
Other wetlands in the broader, central part of the drainage basin were formed after beavers occupied the valley 5,000 years ago. Organic-rich sediments accumulated behind the dams they built, eventually giving rise to the wetlands. The wetlands occupied nearly 4 acres when investigated in 1984, and they contained on average 300 ppm uranium (they were stripped mined at the time of the investigation, which may have skewed downward their average uranium content).
Leach tests were conducted to determine how strongly the uranium is retained in wetland sediment. These tests revealed that uranium will be retained under normal environmental conditions. It's release required leaching with sulfuric acid or with a concentrated bicarbonate solution (Zielinski and Meier, 1988). The latter information supports the idea that wetlands could be used to remove uranium from contaminated mine drainage. Indeed, there are examples of both natural and constructed wetlands used for this purpose (e.g., Cluff lake, described in this table, and ERA Ranger Mine (Shinners, 1996).
To me, these case studies indicate that processes exist in nature which can be used to help solve current environmental problems. With good science, such as the body of work described above, natural treatment systems can be designed rationally and predictably. The challenge for us is to put all these pieces together.
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Last Updated: Sunday, March 16, 1997