Water contaminants can enter your supply system after it has passed through your household water meter. The main culprits can be your roof-top Tinaco or underground Cisterna where municipal or trucked-in water is stored: copper pipes and fittings can sometimes cause serious problems.
The US EPA issues tables of specific, regulated, contaminants in their Drinking Water Regulations (www.epa.gov/ogwdw/mcl.html). There are tables for Primary Regulations and Secondary Regulations.
The Primary Regulations of 86 named contaminants list the MCLG (Maximum Contaminant Level Goal) that in time the EPA hopes can be enforceable, and the current MCL (Maximum Contaminant Level) for each regulated contaminant. These are generally expressed as Milligrams per Liter (Mg/L) which is the same as parts per million (ppm). The Secondary Regulations just issue a standard level for another 15 contaminants.