Lead can contaminant drinking water when plumbing materials that contain lead corrode. We see this happen especially when water is high in acidity or have low mineral content that corrodes pipes and fixtures.
The most common sources of lead in drinking water are lead pipes, faucets, and fixtures.
More about Lead contamination
Basic information about Lead in Drinking Water, United States Environmental Protection Agency (EPA)
Lead In Your Drinking Water (pdf) - Oregon State University Extension Service - Be well Project