Adventus Americas Inc.

Mining Related Sediment Impacts: Causes, Significance and Solutions1

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Courtesy of Adventus Americas Inc.

The Problem
As with many production activities related to the use of natural resources, initial legacies in the form of negative impacts
are often characterized by acute pollution of surface waters. In the case of the mining industry, long after the original
mining-related activity ceases, residues such as tailings, spoil piles, sediments in settling ponds or sediments reaching natural water courses can become long-term sources for the continual, or periodic, introduction of contaminants into the environment. The typically recognized sources of such contaminants come from by-products of intentional mining processes such as heap-leaching, or occur as a result of precipitation leaching through uncontrolled spoil piles. The mining industry has recognized these problems, and new processes address the majority of these concerns, for example, the application of process changes, and the provision of appropriate control technology for run-off. However,
other significant areas of contamination resulting from past practices have yet to be completely addressed. In other instances, as a result of mining activities such as the excavation of overburden and ores, and attendant lowering of water tables by pumping, changes to oxygen and pH levels in surface and ground waters can create immediate reaction such as fish kills. Continued discharges from mining operations or permanently exposed formations, while perhaps containing lower level contaminants (if in conjunction with permanent changes to pH levels), can prevent full stream recovery, as relatively low chronic levels of contaminants or changes, for example, to pH, conductivity, or O2 levels can impinge on the reproductivity of species. Of a more insidious nature are the longer- term impacts of such activities that result in continued impairment of the ecosystem, long after surface water quality issues have been addressed. Even if water quality issues associated with contaminated groundwater discharges to a stream or direct surface run-off are fully addressed (and samples of overlying surface waters test clean), the underlying sediments can be a source of chronic toxicity that also impinge on the reproductivity of native species. Sediment contamination, even at relatively low apparent levels that might not acutely, or even chronically, impact lower order species such as macroinvertebrates, can in fact introduce contaminants that are bioaccumulative into the food chain. This has been seen to result in chronic or acute toxicity to higher orders (including man). As noted above, another route of ecological impairment is the exposure of rock strata to oxygen as a result of pit mining. This can often result in changes to groundwater pH levels, and subsequent increases in the solubility and mobility of minerals including heavy metals. Such elements as lead, copper, zinc, nickel, mercury and others have an affinity to fine soil particles, particularly easily erodable clay minerals, through the cation exchange capacity of the clay materials. These particles bring the contaminants into direct oral and dermal
contact with macroinvertebrates residing in the sediment. Metals and organic contaminants may also be absorbed by organic carbon present in the sediments, which is a food source for lower order forms residing in the sediment. In addition, these finegrained particles are quite easily transported by surface water flows, often traveling significant
distances prior to reaching low-energy areas and settling out of the water column to form sediment deposits. Subsequent plant uptake of contaminants from sediments could become an additional route for introduction into the food chain, if such materials are ingested by fauna.

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