The main sources of municipal tap water are surface water and groundwater – water from lakes, streams, rivers and deep wells. Almost all municipal water sources undergo a disinfection process using chlorine or chloramine (chlorine & ammonia) to kill bacteria and parasites. Although the EPA regulates water contaminants, there are many that still find their way into the tap water we use every day. 
Although the chlorine or chloramine used by municipalities are meant to be disinfectants, these chemicals themselves are considered contaminants and can pose health risks. Other contaminants commonly found in tap water include lead, copper, sulfate, nitrates, pesticides, herbicides, arsenic, industrial solvents and trihalomethanes, a chemical known to cause colon and bladder cancer in large concentrations. Contrary to popular belief, municipal water treatment facilities were not designed to take out contaminants such as synthetic organic chemicals and toxic heavy metals like lead.
Well water sources are typically not regulated at all, making it the responsibility of the well owner to test and evaluate the safeness of their well water. Well water sources are susceptible to contamination from septic tanks, farms, pesticides, and herbicides used in the surrounding area and can also have high concentrations of iron, calcium, and magnesium which cause hard water. Hard water and scaling can damage home appliances and plumbing, as well as leave stains and spots on clothing, dishes, and surfaces in your home.
Regardless of whether you use well water or get it from your local municipality, using a water filter can help ensure everyone in your home has access to healthy, great-tasting water.