The Ground Water Rule (GWR) was developed to reduce the risk of illness caused by microbial contamination in public ground water systems (GWSs). The GWR establishes a risk-targeted approach to identify GWSs susceptible to fecal contamination and requires corrective action to correct significant deficiencies and source water fecal contamination in all public GWSs. The GWR applies to all PWSS that use ground water, including consecutive systems, except that it does not apply to PWSS that combine all of their ground water with surface water or with ground water under the direct influence of surface water prior to treatment.
After a total coliform-positive result occurs in a routine distribution system monthly TCR sample, the GWR requires that applicable systems which are not providing, conducting compliance monitoring, and reporting at least 4-log treatment through inactivation (99.99% removal) of bacteria and viruses be tested for the presence of Escherichia coli (E. coli). Unlike the general coliform group, E coli are almost exclusively of fecal origin and their presence is thus an effective confirmation of fecal contamination. Most strains of E coli are harmless, but some can cause serious illness in humans.