Arsenic in Water
This element is found in small amounts in nature. Mostly, it occurs as the arsenides of true metals or as pyrites.
Arsenic may have valences of +3, +4 and +5. In its elemental form, it is not poisonous. Although many arsenic compounds are extremely toxic and have been used as herbicides and insecticides.
Arsenic is notorious for its toxicity to humans. Ingestion of as little as 130 mg has proved fatal.
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.