Nickel is a metal used in stainless steel and batteries. It enters water from industrial waste and corrosion of plumbing. Nickel exposure can cause skin allergies and lung issues.

Nickel occurs naturally in soil and rock, but human activities significantly increase its presence in water. Besides stainless steel and batteries, it's used in electroplating and alloy production. Nickel enters water through natural erosion, industrial discharge, and corrosion of pipes. It's typically measured using atomic absorption spectroscopy. While skin allergies and respiratory issues are common concerns, some studies suggest potential carcinogenic effects at high exposure levels. The EPA has set a drinking water health advisory level for nickel, though it's not currently regulated with an enforceable standard.