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Ammonium

Ammonium ions are not very toxic in themselves, but depending on pH and temperature part of them turns into ammonia (NH₃), which damages the gills and stresses fish. The higher the pH and temperature, the more ammonia there is (Emerson et al. 1975). The repealed Directive 2006/44/EC gave a guideline of at most 0.04 mg/l of ammonium for salmonid waters and 0.2 mg/l for cyprinid waters.

Nitrites

Nitrites are toxic to fish: they pass through the gills into the blood and turn hemoglobin into methemoglobin, which does not carry oxygen, so fish "suffocate" even in oxygenated water. Salmonids are the most sensitive; chlorides in the water reduce the toxicity (Lewis and Morris 1986). Directive 2006/44/EC gave a guideline of 0.01 mg/l for salmonid waters.

Nitrates

Nitrates do not harm adult fish immediately, but they feed algae and plants and so contribute to over-fertilization and nighttime oxygen crashes. For sensitive aquatic animals (salmonids, amphibians, some invertebrates) Camargo et al. (2005) recommend keeping nitrates below 2 mg of nitrogen per liter, which is about 9 mg/l of nitrates.

Oxygen

Fish, especially salmonids, need a lot of oxygen. Directive 2006/44/EC gave a guideline of 9 mg/l for salmonid waters (in at least half of the measurements) and never less than 7 mg/l. Groundwater from deeper layers often has less of it; in the stream it is re-oxygenated only after it emerges.

Water temperature

Groundwater has an almost constant temperature throughout the year. Inflow from springs can therefore cool a stream in summer and warm it in winter, and such sections often serve as refuges for cold-water fish (Hayashi and Rosenberry 2002).

pH

Fish tolerate a pH of roughly 6 to 9 (Directive 2006/44/EC). Outside this range the gills are damaged and the toxicity of ammonia and metals changes.

Water hardness

Harder water (more calcium and magnesium) is better for fish: calcium protects the gills and reduces the toxicity of metals such as copper and zinc (Pagenkopf 1983), and keeps the pH stable. Soft water is more sensitive to metals and to acidification.

Iron

Iron is dissolved in groundwater at depth. When the water reaches the air, it oxidizes and precipitates as brown flocs that clog gills, cover gravel with eggs and invertebrates, and destroy shelters (Vuori 1995).

Manganese

Manganese is natural in deeper groundwater. For fish it tends to matter less than iron; the standard applies mainly because of the taste and color of drinking water.

Chlorides

Chlorides come from road salt, fertilizers and waste. The US EPA (1988) set 230 mg/l for long-term protection of aquatic life and 860 mg/l for short-term.

Sulphates

Sulfates are common in groundwater; they come from rocks (for example gypsum), fertilizers and mines. Together with chlorides they raise the mineralization of the water; the standard of 250 mg/l is set for drinking water.

Conductivity

Conductivity shows the total amount of dissolved salts. A higher value in groundwater is often natural (limestone, gypsum), but a sharp increase in a stream points to an inflow of salts from roads, mines or waste.

Metals and metalloids

Copper, zinc, nickel, cadmium and mercury are toxic to fish even at low concentrations, more so in soft water than in hard water (Pagenkopf 1983). The groundwater standard is set for drinking water, not for fish.

PFAS (“forever chemicals”)

PFAS are “forever chemicals”: they do not break down and accumulate in the tissues of fish and other animals (Giesy and Kannan 2001). The standard of 0.1 µg/l (sum of 20 substances) is set for drinking water.

Sources

1. Camargo, J. A., Alonso, Á., Salamanca, A. (2005): Nitrate toxicity to aquatic animals: a review with new data for freshwater invertebrates. Chemosphere 58, 1255–1267.
2. Lewis, W. M., Morris, D. P. (1986): Toxicity of nitrite to fish: a review. Transactions of the American Fisheries Society 115, 183–195.
3. Emerson, K., Russo, R. C., Lund, R. E., Thurston, R. V. (1975): Aqueous ammonia equilibrium calculations: effect of pH and temperature. Journal of the Fisheries Research Board of Canada 32, 2379–2383.
4. Directive 2006/44/EC on the quality of fresh waters needing protection or improvement in order to support fish life (repealed in 2013; we give its values here only as an expert guideline).
5. Vuori, K.-M. (1995): Direct and indirect effects of iron on river ecosystems. Annales Zoologici Fennici 32, 317–329.
6. Pagenkopf, G. K. (1983): Gill surface interaction model for trace-metal toxicity to fishes: role of complexation, pH, and water hardness. Environmental Science & Technology 17, 342–347.
7. U.S. EPA (1988): Ambient Water Quality Criteria for Chloride. EPA 440/5-88-001.
8. Hayashi, M., Rosenberry, D. O. (2002): Effects of ground water exchange on the hydrology and ecology of surface water. Ground Water 40, 309–316.
9. Giesy, J. P., Kannan, K. (2001): Global distribution of perfluorooctane sulfonate in wildlife. Environmental Science & Technology 35, 1339–1342.