Lead can enter drinking water when plumbing materials that contain lead corrode, especially where the water has high acidity or low mineral content that corrodes pipes and fixtures. The most common sources of lead in drinking water are lead pipes, faucets, and fixtures. In homes with lead pipes that connect the home to the water main, also known as lead services lines, these pipes are typically the most significant source of lead in the water. Among homes without lead service lines, the most common problem is with brass or chrome-plated brass faucets and plumbing with lead solder.
Source: EPA Basic Information about Lead in Drinking Water