Nearly one-fifth of U.S. households are not connected to a public sewer.

More than 21 million households in the United States use septic systems — not a public sewer — to trap and filter their toilet waste.

The underground tanks are most common in rural areas, especially in New England and the Deep South. They are an often overlooked source of water pollution and disease transmission.

In the graphic below, the top map shows the portion of homes in each state that use septic systems. Darker colors indicate a higher percentage. Vermont tops the chart, with 55 percent of homes on septic. The data is from 1990, which is the most recent state-level septic system assessment. The U.S. Census Bureau, which collected the data, stopped doing so for states and counties after 1990 because no federal agency regulates septic systems. A national estimate, from a much smaller sample size, is completed every two years as part of the American Housing Survey.

Many homes are still being built with septic systems. The bottom map shows the portion of homes built in 2013 that came with septic systems. The trend follows historic geographical patterns: more than half of new homes in New England did not connect to a public sewer.

Graphic © Kaye LaFond / Circle of Blue The share of households that use septic systems varies by state. The highest concentrations are in New England, where 55 percent of Vermonters use septic, and the Southeast. The same regions have the highest share of new homes built with septic systems. Click image to enlarge.

This graphic was made to accompany the article America’s Spreading Septic Threat by Circle of Blue reporter Brett Walton. Contact Brett Walton or by @waltonwater on Twitter.