Galvanized or Lead water main
Modern water mains are typically copper or a type of plastic. Older installations, say about 60 years ago or more may be lead or galvanized steel.
Lead pipe was very common in the 1920's and 30's. The issue with lead is one of health. The following is an excerpt from Health Canada:
Inorganic lead compounds have been classified as probably carcinogenic to humans, based on findings in experimental animals. However, the cancer effects are not the main health effects of concern in humans.
The toxicity of lead has been extensively documented in humans, based on blood lead levels (BLLs). Effects that have been studied include reduced cognition, increased blood pressure and renal dysfunction in adults, as well as adverse neurodevelopmental and behavioural effects in children. The strongest association observed to date is between increased BLLs in children and reductions in intelligence quotient (IQ) scores. The threshold below which lead is no longer associated with adverse neurodevelopmental effects cannot be identified.
Galvanized steel is not very common but it can still be found in some older homes. The problem with galvanized is that it builds up with sediment on the inside, making the inside diameter of the pipe smaller and smaller over time, eventually to the point where water flow is unusable. These pipes also corrode at the joints, which can lead to leaks. Further when the pipe is run under ground as with a water main it also becomes subject to corrosion from the outside of the pipe.
Weather your main is lead or galvanized I will always recommend immediate replacement of the pipe by a licensed plumbing contractor. The cost will be several thousands of dollars.