The purpose of the Groundwater Rule (GWR) is to provide for increased protection against microbial pathogens in public water systems that use ground water sources. The GWR was written with particular concern about those ground water systems that are susceptible to fecal contamination since disease-causing pathogens may be found in fecal contamination.
The GWR applies to public water systems that serve ground water. The rule also applies to any system that mixes surface and ground water if the ground water is added directly to the distribution system and provided to consumers without treatment.
The Groundwater Rule (U.S. EPA)