This guide is designed for owners and operators of public water systems (PWSs) serving 10,000 or fewer persons that are required to comply with the Ground Water Rule (GWR). The GWR applies to all public water systems that are supplied by a ground water source, including wholesale and consecutive water systems, and public water systems that are supplied by both surface water and ground water. Consecutive systems include all systems that buy or otherwise receive some or all of their finished water from another public water system on a regular basis. The GWR does not apply to PWSs that combine all their ground water with surface water or with ground water under the direct influence of surface water (GWUDI) prior to treatment under subpart H of Part 141 CFR (Filtration and Disinfection). Systems that will typically find this guide useful include: small towns; rural water districts; tribal systems; manufactured housing parks; home owner associations; small private systems; and factories, schools, and religious institutions that have their own water supplies.