What is Selenium, and how does it enter the water supply? (Source: US EPA)
"Selenium is a metal found in natural deposits such as ores containing other elements. The major sources of selenium in drinking water are discharge from petroleum and metal refineries; erosion of natural deposits; and discharge from mines."
Discharge from mines is how selenium can enter tap water systems.
How can Selenium affect my health? (Source: US EPA)
"Some people who drink water containing selenium well in excess of the maximum contaminant level (MCL) for many years could experience hair or fingernail losses, numbness in fingers or toes, or problems with their circulation."
Can Aquagear remove Selenium from my drinking water? (Source: Los Angeles County Agricultural Commissioner)
Yes, Aquagear water filters remove up to 97.50% of Selenium.