Thallium

Thallium is a metallic element which turns bluish-gray when exposed to air. In the past, thallium was used in rodenticide and ant killer. Thallium is also used in photocells, glass manufacturing, the electronics industry, pharmaceutical products, and to treat skin infections. Potential sources of contamination in drinking water include leaching from ore-processing sites, discharge from electronics, glass, and drug factories.
The USEPA has set a drinking water standard of 0.002 mg/L for Thallium in public drinking water supplies. These regulations do not apply to private water supplies but the health implications are the same for private well owners.
With long term exposure at concentrations above 0.002 mg/L, possible health effects include hair loss, changes in blood chemistry; kidney, intestinal, testicular or liver problems.
Treatment of water for thallium can be accomplished with activated alumina or ion exchange systems.