A seamless map of the major groundwater areas used by community public supply wells in the United States was needed in order to describe and compute the number of equivalent people using public supply water. This goal was met by the delineation of hydrogeologic mapping units (HMUs). An HMU is a mapped polygon, within which, all public supply wells have a common source of water. The source of water can be either a national Principal Aquifer (PA) as defined in USGS (2003) or a Secondary Hydrogeologic Region (SHR) as defined in Belitz et al. (2018). Collectively, both PAs and SHRs are referred to as Hydrogeologic Regions (HRs). The common source of water can be a single HR or multiple HRs, as HRs can overlap one another. The proportion of the wells assigned to an HR within an HMU determined the proportion that an HR was assigned to the HMU. In the case of the glacial HR, pumping rates were also used. The boundaries of the HMUs are derived from established PA and SHR boundaries. Likewise, the name of the HMU is derived from the PA or SHR from which it was based. Although an HMU may have been named and derived its boundary from an HR, the public supply wells within it may be getting water from a completely different (overlapping or underlaying) HR. In total, 177 unique HMUs were created.
The HMU polygons have several beneficial properties. There are no overlapping polygons. The HMUs are internally consistent with the original source HRs and their buried extents. At least 95% of the wells located within an HMU source their water from HRs within the HMU. The HMU boundary only contains areas where an HR can exist. See the related publication information for more descriptive information about this dataset and the process to create it.