Sources/Usage: Public Domain. View Media Details

Groundwater supplies 35 percent of drinking water in the United States. Mapping the quantity and quality of groundwater at the depths used for potable supplies requires an understanding of locational variation in drinking-water well depth and open-interval lengths. The depths to the top and bottom of the zones from which drinking water is withdrawn are important features for understanding the sources and quality of groundwater supplies, but maps of the depths to drinking-water-supply sources have not been available in much of the United States. To address this need, a set of digital maps (grids) was created from about 7.6 million well records, compiled from several sources, from across the conterminous United States. The grids provide approximate common depth to the bottom of open intervals of wells (see figure to the right) and length of open intervals in the wells from which water is withdrawn for domestic- and public-drinking-water supply. The data and results are summarized in a recently published U.S. Geological Survey Scientific Investigations Report (Degnan and others, 2021) and the data are in a data release (Kauffman and others, 2021).

In much of the eastern half of the country, public-supply wells generally tap much deeper groundwater than domestic-supply wells. In New England, however, many domestic wells are deeper than public-supply wells. Typical depths of groundwater withdrawn for domestic- and public-water supply in New England vary with water-table and aquifer depth and water-capacity requirements.

Sources/Usage: Public Domain. View Media Details

Well depths mapped as part of the study can be examined by using the online interactive maps available with the published report. The maps allow the user to easily explore and zoom into the grids. For example, in the interactive map shown above, users can see where domestic wells are deeper than public-supply wells in parts of central New England. Zooming into the summary results available in the data release, it is apparent that public-supply wells drilled in terrain with crystalline rock that is overlain by coarse glacial deposits have a median depth of less than half the median depth of domestic-supply wells. In central New England (New Hampshire, Vermont, Massachusetts, and western Maine), domestic drinking-water supplies rely on many deep wells with open boreholes in fractured bedrock and few shallow wells completed in thin, discontinuous till or stratified sediments. Conversely, public drinking-water supplies throughout the region commonly make use of thick deposits of permeable stratified sediments but also include deep bedrock wells where productive glacial deposits are not present.