Mine drainage is a complex of elements that interact to
cause a variety of effects on aquatic life that are difficult to separate
into individual components. Toxicity
is dependent on discharge volume, pH, total acidity, and concentration
of dissolved metals. pH is the most critical
component, since the lower the pH, the more severe the potential effects
of mine drainage on aquatic life. The overall effect of mine drainage
is also dependent on the flow (dilution rate), pH, and alkalinity
or buffering capacity of the receiving stream. The higher the concentration
of bicarbonate and carbonate ions in the receiving stream, the higher
the buffering capacity and the greater the protection of aquatic life
from adverse effects of acid mine drainage (Kimmel, 1983). Alkaline
mine drainage with low concentrations of metals may have little discernible
effect on receiving streams. Acid mine drainage with elevated metals
concentrations discharging into headwater streams or lightly buffered
streams can have a devastating effect on the aquatic life. Secondary
effects such as increased carbon dioxide tensions, oxygen reduction
by the oxidation of metals, increased osmotic pressure from high concentrations
of mineral salts, and synergistic effects of metal ions also contribute
to toxicity (Parsons, 1957). In addition to chemical effects of mine
drainage, physical effects such as increased turbidity
from soil erosion, accumulation of coal fines, and smothering of the
stream substrate from precipitated metal compounds may also occur (Parsons,
1968; Warner, 1971).
Benthic (bottom-dwelling) macroinvertebrates are often used as indicators
of water quality because of their limited mobility, relatively long
residence times, and varying degrees of sensitivity to pollutants. Unaffected
streams generally have a variety of species with representatives of
all insect orders, including a high diversity of insects classed in
the taxonomic orders of Ephemeroptera (mayflies), Plecoptera (stoneflies),
and Trichoptera (caddisflies) (EPT taxa). Like many other potential
pollutants, mine drainage can cause a reduction in the diversity and
total numbers, or abundance, of macroinvertebrates and changes in community
structure, such as a lower percentage of EPT taxa. Moderate pollution
eliminates the more sensitive species (Weed and Rutschky, 1971). Severely
degraded conditions are characterized by dominance of certain taxonomic
representatives of pollution-tolerant organisms, such as earthworms
(Tubificidae), midge larvae (Chironomidae), alderfly larvae (Sialis),
fishfly larvae (Nigronia), cranefly larvae (Tipula), caddisfly larvae
(Ptilostomis), and non-benthic insects like predaceous diving beetles
(Dytiscidae) and water boatmen (Corixidae) (Nichols and Bulow, 1973;
Roback and Richardson, 1969; Parsons, 1968). While these tolerant organisms
may also be present in unpolluted streams, they dominate in impacted
stream sections. Mayflies are generally sensitive to acid mine drainage;
however, some stoneflies and caddisflies are tolerant of dilute acid
mine drainage.
Fish are often used as indicators of pollution; however, they are not
as useful as macroinvertebrates because
of their greater mobility. Fish may temporarily swim through a non-lethal
impacted area or away from a discharge of intermittent duration.
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