Arsenic in Water
Arsenic is found to a small extent in nature in elemental form.
It occurs mostly in the form of arsenites of true metals or as pyrites. Other sources of arsenic are from mining debris and wastes and certain insecticides and herbicides.
Arsenic may have valences of +3, +4 and +5. In its elemental form, it is not poisonous.
Arsenic is cumulative in the tissues of many organisms and eventually exerts its effects even though the natural level is low. Arsenic is notorious for its toxicity to humans. Ingestion of as little as 130 mg of arsenic has proved fatal.
U.S. Public Health Service drinking water standards recommend an upper limit of 0.05 mg/L of arsenic.
Aquatic organisms seem to be able to tolerate levels up to 2 mg/L. with respect to lower forms of aquatic life, arsenic concentrations of 3 to 14 mg/L have not harmed may fly nymphs and 10 to 20 mg/L have been harmless toward dragon and damsel flies.
Concentrations of 2 to 4 mg/L of arsenic are reported not to interfere with self purification of streams. Bacteria grow even in the presence of 10,000 mg/L of potassium arsenate and algae are not killed at 1,000 mg/L of arsenate.
Some aquatic organisms may concentrate arsenic although it does not appear to concentrate throughout the food chain. Concentrations of 1 mg/L of arsenic have been present in drinking water that was used for short periods of time producing no ill effects, but long term use of water containing 0.21 mg/L of arsenic has been reported to have poison effects.
With respect to lower forms of aquatic life, arsenic concentrations of 3 to 14 mg/L have not harmed may fly nymphs, and 10 to 20 mg/L have been harmless towards dragon and damsel flies.