The most common cause is corrosion, a reaction between the water and the lead pipes or solder. Any electric current traveling through the ground wire will accelerate the corrosion of lead in the pipes. Wires shouldn't be removed from pipes unless a qualified electrician installs an adequate alternative grounding system. Lead contaminated drinking water is most often a problem in houses that are either very old or very new. Plumbing installed before 1930 is most likely to contain lead.
Scientific data indicate that the newer the home, the greater the risk of lead contamination.
Lead levels decrease as a building ages. This is because as time passes, mineral deposits form a coating on the inside of the pipes. This coating insulates the water from the solder. You should have your water tested for lead. Testing is especially important in high rise buildings where flushing might not work.