According to the U.s. Environmental Protection Agency (EPA), emerging contaminats are defined as following:

Chemicals are being discovered in water that previously had not been detected or are being detected at levels that may be significantly different than expected.  These are often generally referred to as “contaminants of emerging concern” (CECs) because the risk to human health and the environment associated with their presence, frequency of occurrence, or source may not be known.  EPA is working to improve its understanding of a number of CECs, particularly pharmaceuticals and personal care products (PPCPs) and perfluorinated compounds among others.

There is an interesting article about EC’s on