The quality and source of drinking water are fundamental public health concerns in the United States, which relies on two primary sources: surface water and groundwater. Surface water comes from lakes, rivers, and reservoirs. Groundwater is accessed through wells drilled into underground geological formations. Groundwater is a significant component of the nation’s water supply, serving millions of people daily. Understanding this reliance requires separating the population served by municipal systems from those who supply their own water from private sources. The distribution and management of this resource vary widely, influencing its availability and protection.
The National Reliance Statistic
Approximately 115 million people, or more than one-third of the nation’s population, depend on groundwater for their daily drinking water needs. This figure is broken down into two distinct categories. The first category involves citizens served by public water supply systems. According to the U.S. Geological Survey (USGS), about 107 million people in this system receive groundwater, while 162 million people are supplied by surface water.
The second category is the self-supplied population, consisting of homeowners who rely on private wells. An estimated 43 million Americans draw their water from private domestic wells, making this segment nearly 100% reliant on groundwater. While the majority of the total population receives surface water from public utilities, roughly 80% of individual community water systems utilize groundwater.
Defining Groundwater Versus Surface Water
Groundwater and surface water are two different hydrological sources that require unique approaches to access and treatment. Surface water is the more visible source, collecting in rivers, lakes, and reservoirs. This water is relatively easy to access and is often the main supply for large metropolitan areas.
Groundwater, by contrast, is stored underground in saturated zones called aquifers, which are formations of permeable rock, soil, or gravel. It is accessed by drilling wells that penetrate the aquifer. The movement of groundwater is significantly slower than surface water, as it filters through geological material. This natural filtration often gives groundwater a higher initial purity from surface-borne contaminants. However, this slow movement makes it susceptible to accumulating dissolved minerals and persistent pollutants, which can be stored within the aquifer for long periods.
Geographic and Population Distribution
Groundwater reliance is not evenly distributed across the United States; it is influenced by population density and local hydrology. A difference exists between urban and rural areas, with nearly all rural populations depending on groundwater. Urban centers, with their large water demands, generally rely on complex infrastructure to deliver surface water from distant reservoirs or rivers.
Rural and smaller communities find it more economical and simpler to drill wells into a nearby aquifer. Certain regions demonstrate near-total dependence on groundwater for their public supply due to limited surface water availability or favorable geology. For example, San Antonio, Texas, relies exclusively on groundwater for its municipal needs. Large-scale aquifers, such as the Glacial Aquifer System, serve enormous populations, supplying over 17 million people in the central and northeastern U.S. States like Florida and California also have large populations served by groundwater, underscoring how reliance is concentrated where groundwater is the most practical resource.
Source Monitoring and Regulation
The safety framework for drinking water in the U.S. is sharply divided based on whether the source is a public utility or a private well. Public water systems, which serve the majority of the population, are subject to the strict requirements of the Safe Drinking Water Act (SDWA). This federal law mandates regular testing, treatment, and monitoring of the water supply for a wide range of contaminants, with oversight provided by federal, state, and local authorities.
The regulatory landscape for the 43 million Americans using private wells is entirely different, as these systems are not subject to federal regulation under the SDWA. The responsibility for testing and treating the water falls solely on the homeowner. While the federal government provides water quality information and testing guidance, state and local health departments play a significant role in managing the overall health of groundwater resources.
Monitoring efforts by agencies like the USGS track aquifer health by assessing water levels and the presence of both natural and manmade contaminants, such as arsenic or pesticides. State governments often manage groundwater withdrawals to prevent depletion and saltwater intrusion, which is essential for maintaining the long-term sustainability of the resource.