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Organisms and heavy metal tolerance
Heavy metals can increase the toxicity of mine drainage and also act as metabolic poisons. Iron, aluminum, and manganese are the most common heavy metals which can compound the adverse effects of mine drainage. Heavy metals are generally less toxic at circumneutral pH. Trace metals such as zinc, cadmium, and copper, which may also be present in mine drainage, are toxic at extremely low concentrations and may act synergistically to suppress algal growth and affect fish and benthos (Hoehn and Sizemore, 1977). Some fish, such as brook trout, are tolerant of low pH, but addition of metals decreases that tolerance. In addition to dissolved metals, precipitated iron or aluminum hydroxide may form in streams receiving mine discharges with elevated metals concentrations. Ferric and aluminum hydroxides decrease oxygen availability as they form; the precipitate may coat gills and body surfaces, smother eggs, and cover the stream bottom, filling in crevices in rocks, and making the substrate unstable and unfit for habitation by benthic organisms (Hoehn and Sizemore, 1977). Scouring of iron flocculant increases turbidity and suspended solids and may inhibit fish feeding.

Aluminum rarely occurs naturally in water at concentrations greater than a few tenths of a milligram per liter; however, higher concentrations can occur as a result of drainage from coal mines, acid precipitation, and breakdown of clays (Hem, 1970). The chemistry of aluminum compounds in water is complex. Aluminum combines with organic and inorganic ions and can be present in several forms. Aluminum is least soluble at a pH between 5.7 and 6.2; above and below this range, aluminum tends to be in solution (Hem, 1970; Brown and Sadler, 1989).

Most information on the effects of low pH and aluminum on aquatic life is based on studies of acid precipitation, such as those summarized in Haines (1981), Morris et al. (1989), and Mason (1990). Of the three major metals present in mine drainage, aluminum has the most severe adverse effects on stream aquatic life. The addition of aluminum ions compounds the effect of low pH by interacting with hydrogen ions, further decreasing sodium uptake, and increasing sodium loss in blood and tissues. High calcium concentrations generally reduce mortality and sublethal effects of low pH and elevated aluminum by reducing the rate of influx of hydrogen ions into the blood. Streams most susceptible to degradation from elevated aluminum, however, normally have low concentrations of calcium.

Stream investigations by the author have indicated that a combination of pH less than 5.5 and dissolved aluminum concentration greater than 0.5 mg/L will generally eliminate all fish and many macroinvertebrates. Fishflies, alderflies, and several genera of stoneflies, caddisflies, and true flies (particularly within the family Chironomidae) are tolerant of low pH and high dissolved aluminum. Mayflies are the aquatic insects most affected by a combination of low pH and acidic water. Some exceptions do occur, for example, the mayflies Ameletus and Ephemerella funeralis are tolerant of slightly acidic water, especially at low aluminum concentrations (less than 0.2 mg/L). Aluminum is most toxic to fish at pH between 5.2 and 5.4 (Baker and Schofield, 1982).

Streams with precipitated aluminum usually have lower numbers and diversity of invertebrates than streams with low pH and high dissolved aluminum. Precipitated aluminum coats the stream substrate, causing slippery surfaces and difficulty for insects to maintain position in the current. Aluminum precipitate can also be directly toxic to macroinvertebrates and fish. Rosemond et al. (1992) stated that deposition of aluminum hydroxide particles on invertebrates blocks surfaces important for respiratory or osmoregulatory exchange. Aluminum precipitate also eliminates most of the filter feeders, such as Hydropsychid caddisflies, which normally comprise a major portion of total stream macroinvertebrates. Precipitated aluminum can also accumulate on fish gills and interfere with their breathing (Brown and Sadler, 1989).

Iron is a common component of mine drainage which can have a detrimental effect on aquatic life. Like aluminum, iron can be present in several forms and combines with a variety of other ions. The impact of mine drainage containing elevated iron on aquatic ecosystems is complex. Little animal life may be found in streams with the lowest pH (under 3.5) and elevated dissolved iron concentrations. Alderflies, fishflies, dipterans, and aquatic earthworms will be present if the pH rises slightly. With further increases in pH, a more diverse assemblage of macroinvertebrates may be present, although total numbers may be lower than in nondegraded streams (Table 4.2 and Figure 4.1). Wiederholm (1984), Letterman and Mitsch (1978), and Moon and Lucostic (1979) presented results of research on the effects of mine drainage and elevated iron on aquatic life.

Iron precipitates at a pH greater than 3.5 and does not reenter solution at higher pH. Because iron can form precipitates at a lower pH than aluminum and can be present in streams with pH less than 4.5, separating the effect of iron from the effect of low pH is difficult. Precipitation of ferric hydroxide may result in a complete blanketing of the stream bottom, adversely affecting both macroinvertebrates and fish (Hoehn and Sizemore, 1977). The severity is dependent on stream pH and the thickness of the precipitate. Generally, the effect of precipitated iron is less severe in alkaline conditions. Many fish and macroinvertebrates are tolerant of iron precipitate in alkaline water; however, total numbers and diversity are usually lower than in unaffected streams. Koryak et al. (1972) found that ferric hydroxide greatly diminished total biomass of benthic organisms and limited fish populations in streams with survivable pH levels. The caddisfly genus Hydropsyche, which is generally sensitive to low pH, can live in alkaline streams with iron precipitate. The Hydropsychid caddisfly Diplectrona, however, is tolerant of iron precipitate and pH less than 5.0. Mayflies are generally more tolerant of alkaline mine drainage than acid mine drainage. Mayflies such as Ephemerella, Baetis, Attenella, and Acentrella may be found in alkaline streams with iron precipitate. Acroneuria and Paragnetina stoneflies are tolerant of alkaline water with iron precipitate but are intolerant of acid (Table 4.2). Since iron precipitate particles often cover the bodies of macroinvertebrates that otherwise appear healthy, the iron precipitate itself appears to be less toxic than aluminum precipitate. Smallmouth bass, rock bass, creek chub, johnny and rainbow darter, white sucker, common shiner, and river chub are some fish species that can be found in alkaline water with iron precipitate.

A diversity of aquatic insect orders
Abundance of EPT taxa (mayflies, stoneflies,
caddisflies) and other orders
Variety of fish species, depending on habitat
pH <3.5 high dissolved iron

STREAM pH 3.5 - 4.5
Elimination of most EPT taxa
Dominance by midges (Chironomidae)
Also present: alderfly (Sialis), fishfly (Nigronia),
diving beetles (Dytiscidae), water bugs (Corixidae)
Algae: Euglena, Ulothrix, Pinularia, Eunotia
No Fish
STREAM pH 4.5 - 5.5
More insect orders represented
Stoneflies: Leuctra, Amphinemura
Caddisflies: Diplectrona, Lepidostomis, Polycentropus;
Blackflies (Simuliidae),
Craneflies (Tipulidae)

At higher pH range: Mayfly: Ameletus;
Fish: creek chub, white sucker, blacknose dace, brook trout
STREAM pH 5.7 - 6.0
Additional EPT taxa: Acroneuria, Stenonema, Ephemerella
funeralis; Elmid Beetles
Algae: Diatoms, Flagellates, Green algae, Oscillatoria
STREAM pH > 6.0
Variety of EPT taxa: Ephemerella, Baetis, Isonychia, Acentrella,
Attenella, Hydropsyche
Variety of fish species
STREAM pH > 6.0
Variety of EPT taxa may be present;
But usually low abundance
Ephemerella, Baetis, Acentrella, Paragnetina, Acroneuria,
Leuctra, Cheumatopsyche, Hydropsyche, Elmid beetles,
Variety of fish species, reduced numbers: creek chub, river chub,
white sucker, johnny darter, rainbow darter, rock bass,
smallmouth bass, pumpkinseed

Sources: Roback and Richardson, 1969; Parsons, 1968; Warner, 1971; Kimmel, 1983; and stream investigations by author.

Manganese is another metal that is widely distributed in mine drainage. It can be present in a variety of forms and compounds and complexes with organic compounds. Manganese is difficult to remove from discharges because the pH must be raised to above 10.0 before manganese will precipitate. Manganese, therefore, is persistent and can be carried for long distances downstream of a source of mine drainage. Less information is available on the effects of elevated manganese concentrations on aquatic life than the effects of iron and aluminum. Perhaps this is because manganese in mine drainage is usually associated with other metals which may have a more deleterious effect or mask the effect of the manganese. Manganese discharge limits have traditionally been based on the objectionable discoloration effects of manganese at concentrations as low as 0.2 mg/L in water supplies rather than effects on aquatic life.

Kleinmann and Watzlaf (1988) summarized the results of manganese toxicology tests on fish and invertebrates. They concluded that manganese tolerance limits for fish reported in the literature varied widely and that the lowest toxic concentrations were reported in watersheds with very low levels of hardness. They reported that several researchers found that hardness concentrations as low as 10 mg/L protected fish from adverse effects of manganese. Bioassay tests on invertebrates produced tolerance rates for manganese ranging from 15 to 50 mg/L, depending on the test organism.

The less common precipitated form of manganese may be more toxic than the dissolved form. Werner et al. (1982) noted the presence of precipitated manganese hydroxide which formed a black coating over the substrate of a Pennsylvania stream receiving mine drainage. They reported that the precipitate along with siltation significantly lowered macroinvertebrate species diversity and changed the stream community structure.

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