Selenium
Source of Selenium
Selenium (Se) is essential for human nutrition, with the majority coming from food. The concentration found in drinking water is usually low, and comes from natural minerals. Selenium is also a by-product of copper mining / smelting. It is used in photoelectric devises because it's electrical conductivity varies with light. Naturally occurring selenium compounds have not been shown to be carcinogenic in animals. However, acute toxicity caused by high selenium intake has been observed in laboratory animals and in animals grazing in areas where high selenium levels exist in the soil. The US EPA has established the MCL for selenium at 0.05 mg/l.
Treatment of Selenium
Anion exchange can reduce the amount of selenium in drinking water by 60 - 95%. Reverse osmosis is excellent at reduction of selenium.