EPA developed the Ground Water Rule (GWR) to provide for increased public health protection for consumers of water from public water systems that use ground water. A key element of the GWR is to identify public ground water sources that are susceptible to fecal contamination. The GWR also ensures that these systems take corrective action to eliminate the source of contamination or to remove or inactivate pathogens in the drinking water they provide to the public. Fecal contamination is a broad term that refers to microbial contaminants from human or animal feces. It is a likely source of microbial pathogens in drinking water. These microbial pathogens are a significant threat to public health because they can cause serious illness and even death when consumed. Fecal contamination may be introduced into finished ground water via inadequately treated or inadequately protected source water or from problems in the distribution system. Common sources of ground water contamination include septic systems, leaking sewer pipes, landfills, sewage lagoons, and improperly abandoned wells. Microbial contamination in an aquifer can be localized or may be transported as water moves through the aquifer. The GWR requires ground water systems (GWSs) that provide less than 4-log removal, inactivation, or State-approved combination of these or that do not perform compliance monitoring of treatment to sample their source water for the presence of a fecal indicator when total coliform bacteria are detected in the distribution system. This monitoring requirement is triggered by the results of routine coliform sampling performed for compliance with the Total Coliform Rule (TCR). The triggered monitoring requirement is designed to allow systems and States to identify and to correct public health risks from fecal contamination found at the source.