Superfund Research Program
Molecular, Population, and Community Responses of Aquatic Insects to Heavy Metal Contamination
Located about 120 miles west of Denver, Colorado, the Leadville Mining District has been one of the state's most productive mining regions. Since the discovery of gold there in 1859, this historic mining district produced over 2.5 million ounces of gold, 240 million ounces of silver, 2 billion pounds of lead, as well as large quantities of zinc and copper. Mining continued in the Leadville District until the mid-1980s, which is about the time it was added to the Superfund program's National Priority List.
Although Leadville's mines produced large quantities of valuable metals for the country, tons of slag, tailings, and waste rock were left behind in the district's network of tunnels and drainage channels. Decades of drawing metal rich ores out of the earth also left behind a legacy of environmental problems.
One of the consequences of mining in the Leadville District is the contamination of the upper Arkansas River with toxic heavy metals. Over time, natural oxidative processes released large quantities of lead, arsenic, cadmium, and zinc from the district's numerous piles of accumulated mining wastes. Snowmelt and thunderstorm runoff subsequently carried these untreated metals into the upper Arkansas River, which has been polluted with heavy metals for over 120 years.
This environmental problem is not unique to the Leadville Mining District. Thousands of miles of streams in the western United States have been contaminated with heavy metals as a result of past mining activities. While chronic metal contamination of waterways is a problem at many mining sites, the effects of metal pollution on aquatic ecosystems are still poorly understood.
To fill in some of these knowledge gaps, scientists in the Colorado State University (CSU) Superfund Basic Research Program are conducting studies to better understand the ecological effects of heavy metal contamination in the upper Arkansas River. The researchers are focused on understanding the genetic, population, and community changes that have occurred in Arkansas River aquatic insects after decades of exposure to mixtures of heavy metals. An important part of this effort includes studies examining how long-term exposure to heavy metals affects the ability of aquatic insects to tolerate other forms of environmental stress.
These studies are utilizing benthic aquatic insects collected from a metal contaminated section of the Arkansas River that is located downstream from the Leadville Mining District. To serve as a comparison, experiments are also being conducted with organisms collected from unpolluted Colorado streams.
Benthic aquatic insects offer several advantages for understanding the long-term effects of metal contamination in freshwater systems. They are abundant, tend to remain in their original habitat, and respond quickly to changes in water quality.
Initially, the CSU scientists tested the "metal tolerance" of mayflies (Baetis tricaudatus) collected from the Arkansas River and an unpolluted stream (the Cache la Poudre River). In these experiments, mayflies were exposed to a mixture of metals composed of lead, arsenic, cadmium, and zinc. The researchers found that organisms from the Arkansas River were more tolerant to metals than those from the Cache la Poudre River. Results of DNA testing showed that the Arkansas River mayflies also had reduced genetic diversity in comparison to mayflies collected from the unpolluted stream. The loss of metal sensitive genotypes within the Arkansas River mayfly populations is a concern because these genotypes may be necessary for protection against other types of environmental stressors.
In subsequent experiments, the CSU researchers tested the hypothesis that reduced genetic variability may increase the vulnerability of metal tolerant aquatic insects to other types of environmental stress.
The CSU researchers recently completed field and laboratory experiments investigating whether metal tolerant populations and communities of aquatic insects have an increased sensitivity to the effects of UV-beta radiation and acidic conditions. These particular environmental stressors were tested because metal contamination, acid deposition, and increasing levels of UV radiation can potentially co-exist in freshwater ecosystems of the western United States. Metal contamination from historic and contemporary mining operations is well documented. Acidification of streams can result from acid precipitation caused by the burning of fossil fuels and from acid mine drainage caused by mining activities. Evidence of increasing UV radiation at temperate latitudes has also been reported.
What the scientists found is that the metal tolerant organisms from the Arkansas River were more sensitive than control insects to the effects of UV-beta radiation and acidic conditions. Although benthic aquatic insects living downstream from the Leadville Mining District have developed a tolerance for heavy metals, the acquisition of metal resistance appears to have some costs.
In addition to elucidating some of the ecological effects of Leadville's past mining activities, these studies are significant for increasing our understanding of the potential interactions between metal contamination and other environmental stressors that affect freshwater streams. The results suggest that streams contaminated by metals may be especially sensitive to increased UV radiation and acidic stream conditions. This understanding points to the need for a comprehensive approach when developing strategies to restore and protect the thousands of miles of western U.S. streams affected by mining activities.
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To learn more about this research, please refer to the following sources:
- Courtney LA, Clements WH. 1998. Effects of acidic pH on benthic macroinvertebrate communities in stream microcosms. Hydrobiologia 379:135-145.
- Kiffney PM, Clements WH, Cady TA. 1997. Influence of ultraviolet radiation on the colonization dynamics of a Rocky mountain stream benthic community. Journal of the North American Benthological Society 16:520-530.
- Kiffney PM, Little EE, Clements WH. 1997. Influence of ultraviolet-B radiation on the drift response of stream invertebrates. Freshwater Biology 37:485-492.
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