Skip to main content
U.S. flag

An official website of the United States government

Man-Made Chemicals Found in Drinking Water at Low Levels

Right-click and save to download

Detailed Description

Safe drinking water supplies are critical to maintaining and preserving public health. But how healthy is that resource? A recent USGS study found low levels of man-made chemicals in water entering and leaving drinking water treatment plants. USGS lead scientist Greg Delzer explains the findings of this study.




Public Domain.



[Starting Theme Music] 


Jennifer LaVista: Thanks for tuning in to this episode of CoreCast. I’m Jennifer LaVista. 

Safe drinking water supplies are critical to maintaining and preserving public health, but how healthy is that resource.

A recent US Geological Survey study found low-levels of man-made chemicals in water entering and leaving drinking water treatment plants. Joining me now on the phone is the National Coordinator of this city, Greg Delzer.

Greg thanks for joining us.


Greg Delzer: Thank you Jen, it’s my pleasure.

Jennifer LaVista: What were the major findings of this study?


Greg Delzer: Well, there were really three major findings. But before mentioning them, I think it’s important to describe some of the most important features of this study, just in general, to help put these major findings in perspective.


Jennifer LaVista: Ok.


Greg Delzer: For example, this study is currently in progress so the findings represent initial results based on monitoring about 260 man-made, primarily unregulated contaminants in source water, which is untreated water. And finished water, which is treated water of nine different community water systems located around the nation.


 The water systems that we studied use rivers as our source or primary source of drinking water supply. At each of these sites, samples are generally collected on a monthly basis for about one year. So, with that and a little bit of a background, our first major finding was that, a diverse group of contaminants were detected in source water. 

However, the concentrations were low, this indicates that there is a variety of different sources and pathways for these contaminants to reached our drinking water supply. But when the concentrations were low, about 95% of the concentrations were less than one-part per billion, and in all cases, none of the time-weighted averages for those compounds were greater than a drinking water benchmark. 


Jennifer LaVista: Ok.


Greg Delzer: Our second major finding was that almost 2/3 of the most commonly detected contaminants in source water were generally detected in finished water at about the same frequency and concentration. However, about 25% of these contaminants were detected much less frequently in finished water than in the source water.

And our third major finding is that individual contaminants seldom occurred alone. They occurred in chemical mixtures. We found that there was an association between the numbers of contaminants detected in both source water and finished water and the amount of urban and agricultural land within the water shed. That is, as the amount of urban and agri-lands increased within the water shed, the numbers of contaminants in an individual sample also increased and vice-versa.  


 So for example, about 80% of samples had five contaminants present, but about half of these samples contained mixtures containing 14 or 15 different contaminants.


Jennifer LaVista: Tell me about the chemicals that you tested for.


Greg Delzer: Ok. The chemicals we’ve monitored again are primarily man-made organic contaminants, most of which are unregulated. Only about 40 of the 260 are regulated that is they have maximum contaminant level and the rest are unregulated.

For our purposes, we’ve broken them into 13 different categories based on the predominant use of those compounds. And these categories include things such as pesticides, solvents, gasoline-related compounds, and personal care and domestic use compounds, as well as others.


Jennifer LaVista: Where did these studies took place?


Greg Delzer: The studies were conducted at nine individual community water systems located in Colorado, Georgia, Indiana, Maryland, Massachusetts, Nevada, North Carolina, Oregon, and Texas. The rivers monitored include the Clakamas, the Truckee, Cache La Poudre, Elm Fork Trinity, White, Chattahoochee, Neuse, Potomac, and Running Gutter Brook.


Jennifer LaVista: What are some of the chemicals that your study found most frequently?


Greg Delzer: Many compounds present in the 13 categories I described before were detected at least in some of the samples. However, those that were detected most frequently include the disinfection byproduct chloroform as well as herbicides - Zymosine, Atrazine, the degradation product of Atrazine, and also a compound common in fragrances which goes by the acronym of HHCB.  


Jennifer LaVista: Are these findings surprising?


Greg Delzer: Well, it’s been pretty well established for many years that rivers receiving municipal and industrial discharge, as well as discharges from other point and non-point sources will be impacted. So the occurrence of low level concentrations of these types of contaminants in river water at an intake is not necessarily a surprising finding.


In addition, because most of the community water systems that we studied, employed conventional water treatment. They were not specifically designed to remove the types of contaminants we monitored.

So, it’s probably not so surprising that 2/3 of the most commonly occurring contaminants in source water were also detected in finished water at similar concentrations.

What is more surprising is that a quarter of those most commonly occurring contaminants in source water were not detected in finished water. It is presumed that this is primarily due to chlorine oxidation or other aspects of water treatment that removed, or at least transformed the contaminants into something different. However, additional research is really needed to better understand those processes.


It is also a surprising that individual contaminants seldom occurred alone.  Again, about half of the source water and finished water samples contained mixtures of 14 or 15 different individual contaminants. Drinking water standards like MCL and other human health benchmarks typically are based on toxicity data for individual contaminants and the effects of mixtures of contaminants at low levels are not well understood.

The frequent occurrence of mixtures in both source and finished water samples, really points to a need for identifying those mixtures that are most common and most likely to have potential human health effects.


Jennifer LaVista: So, is it safe to drink water from the tap?


Greg Delzer: Well, the finished water or the treated water data that we are reporting were collected following all of the treatment steps. But prior to the water entering the distribution system and ultimately to the tap, the locations that we monitored were the locations where compliance monitoring is done by the water utility to ensure compliance with all States and Federal Regulations.

Although our data provides significant insight into the quality of our drinking water, many things can happen in the water and distribution. For example, it’s known that free chlorine can continue to degrade certain organic compounds in the finished water we monitored, did in fact, of course, contain free chlorine.


However, the best way to characterize tap water is actually to sample the tap water, and this is not something we are doing at this time as part of these studies.


Jennifer LaVista: Are there any more studies planned for the future?


Greg Delzer: Yes. In addition to the nine studies included in this report, we hope to complete about 20 more by the year 2013 and we’re about 2/3 of the way there right now.


Jennifer LaVista: Is there anything else you’d like to add?


Greg Delzer: I just like to emphasize that these results represent initial findings of studies currently in progress and as results become available for other studies of source water and finished water from community water systems around the country.


We will continue to work and better understand not only the results but also the implication of those findings and to continue working with other Federal, States, and local agencies in the process.


Jennifer LaVista: Greg, thank you so much for your time.


Greg Delzer: Thank you Jen, it was my pleasure.


Jennifer LaVista: That was Greg Delzer, National Coordinator for the recent US Geological Survey Study on source water quality assessment. To learn more, visit

Thanks for tuning in to CoreCast. I’m Jennifer LaVista. CoreCast is a product of the US Geological Survey, Department of the Interior. 

Mentioned in this segment:

Thank you for listening to a USGS CoreCast. For complete transcripts or more information, visit CoreCast is a product of the U.S. Geological Survey, Department of the Interior.

Show Transcript