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February 9, 2022

Contaminants known as per- and polyfluorinated alkyl substances, or PFAS, were found to be widely distributed in untreated well water throughout Delaware, according to a new report by the U.S. Geological Survey.

Contaminants known as per- and polyfluorinated alkyl substances, or PFAS, were found to be widely distributed in untreated well water throughout Delaware, according to a new report by the U.S. Geological Survey.

The wells sampled by the USGS all receive groundwater from the Columbia aquifer, which supplies water for more than 90% of the state. The researchers found a variety of types of PFAS in water from 16 of the 30 wells sampled in Delaware; one sample contained eight different types. Those that contained PFAS generally had low concentrations.

The findings are representative of groundwater and not necessarily drinking water, as groundwater can be filtered or treated prior to becoming drinking water.

“Even though use of some PFAS have been banned, we found that the chemicals are still present in the environment,” said USGS scientist Betzaida Reyes, the author of the new study. “The Columbia aquifer is an important drinking water source for hundreds of thousands of people, and our research will help decision-makers and communities understand how PFAS are impacting the quality of some area groundwater.”

PFAS are a group of more than 4,000 manmade chemicals. The EPA's PFAS website highlights that scientific research indicates high concentrations of some PFAS may lead to adverse health outcomes in people, and that research is still ongoing to better understand the potential health effects of PFAS exposure over long periods of time, especially in children.

The wells sampled in Delaware with the highest number of types of PFAS detected were in New Castle and Sussex counties, with five to eight compounds found in each well. Two wells, one in each county, had concentrations of two types of PFAS, perfluorooctanoic acid, known as PFOA, and perfluorooctane sulfonate, known as PFOS, above the Environmental Protection Agency’s advisory level for lifetime consumption.

The four most frequently detected PFAS in this study were PFOA; perfluorohexanoic acid, known PFHxA; PFOS; and perfluorohexane sulfonate, known as PFHxS.

“This USGS study is the first of its kind for Delaware, providing a comprehensive, baseline understanding of which PFAS are present and at what concentrations in well water throughout most of the state,” Reyes said. “We could not have done this work without the support of our partners and the property owners and water managers who gave us access to their property so we could sample wells.”

PFAS are or have been used in a range of consumer and industrial applications such as firefighting products, cookware, pizza boxes, carpets and stain repellants. There are a variety of ways people can be exposed.

PFAS were used starting in the 1940s and although some have since been prohibited, they are a continued concern because they are long-lasting and have been widely used across the country. Sometimes called “forever chemicals,” PFAS are not readily biodegradable and break down very slowly, and because of that can build up in people, animals and the environment over time.

PFOA and PFOS have been the most widely produced, used and studied chemicals in the PFAS group. They are also two types of PFAS banned in recent years.

This is the third time since 2000 the USGS has sampled these specific wells – or a well in close proximity if the original well was no longer available – but it’s the first time they were tested for the presence of PFAS. With PFAS being an emerging concern and focus for the state, 18 types of PFAS identified by the EPA as some of those commonly detected in drinking water were added to the list of select chemicals included in the most recent USGS study. Sampling was done between August and November 2018.

The USGS conducted this study in cooperation with the Delaware Department of Natural Resources and Environmental Control and the Delaware Geological Survey. Both the DNREC and the Delaware Department of Health and Social Services continue their ongoing work to address PFAS in the state’s drinking water. More information from the state, including how private well owners can get their water tested, is available at

Although this study focuses on Delaware, the Columbia aquifer also supplies water to portions of Maryland, New Jersey, New York, North Carolina and Virginia.

Read the USGS strategy that outlines the agency’s scientific role in the study of PFAS.

See the White House fact sheet on the plan to combat PFAS pollution.

PFAS Detected in Samples of Untreated Well Water in Delaware
USGS map showing the number of PFAS detected in samples of untreated well water in Delaware that are supplied by the Columbia aquifer. The aquifer supplies groundwater to areas within the Atlantic Coastal Plain, which includes most of Delaware and portions of other states. The area of Delaware that receives groundwater from the aquifer is the Delaware Atlantic Coastal Plain, which is below the map's purple line known as a "fall line.”


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