Water is not either contaminated or not contaminated. For one, different types of contamination require different types of redmediation. Also, the target remediation level is important. Consider lead. It's not "lead or no lead". It's "what concentration of lead is acceptable and what kind of technology is required to meet the acceptable concentration for safety and quality?". If you look hard enough with good enough equipment, you will find probably find lead atoms in your water - but is it enough to cause harm? Home purification systems may be very good at significantly reducing iron loads from well water so that it tastes acceptable, but taking lead from xxx ppb to yyy ppb to meet a safety standard may be beyond their capability. Such remediation may require more expensive hardware and technical expertise beyond the reach of typical consumers, or may require completely replacing public water infrastructure.
If you are concerned about the quality and safety of your municipal water, you should contact your local government to have it tested and discuss remediation strategies - contamination could occur at your house (which you would have to pay to resolve) or it could be a more systemic problem (the city's problem). If the water is truly unsafe, then you will probably need to purchase bottled water until the problem is addressed. Filtering at your faucet does not address the problem's cause, and only is a temporary solution because safety assurance would require constant testing.