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Characterizing surface water concentrations of hundreds of organic chemicals in United States for environmental risk prioritization



Thousands of chemicals are observed in freshwater, typically at trace levels. Measurements are collected for different purposes, so sample characteristics vary. Due to inconsistent data availability for exposure and hazard, it is complex to prioritize which chemicals may pose risks.


We evaluated the influence of data curation and statistical practices aggregating surface water measurements of organic chemicals into exposure distributions intended for prioritizing based on nation-scale potential risk.


The Water Quality Portal includes millions of observations describing over 1700 chemicals in 93% of hydrologic subbasins across the United States. After filtering to maintain quality and applicability while including all possible samples, we compared concentrations across sample types. We evaluated statistical methods to estimate per-chemical distributions for chosen samples. Overlaps between resulting exposure ranges and distributions representing no-effect concentrations for multiple freshwater species were used to rank estimated chemical risks for further assessment.


When we apply explicit data quality and statistical assumptions, we find that there are 186 organic chemicals for which we can make screening-level estimates of surface water chemical concentration. Of the original 1700 observed chemicals, this number decreased primarily due to a predominance of censored values (that is, observations indicating concentrations too low to be measured). We further identify 423 chemicals where all measurements were censored but, through consideration of detection limits, risk might still be prioritized based on the detection limits themselves. In the final set of 1.5 million samples, the median environmental concentration of one chemical (acetic acid) exceeded the 5th percentile of no-effect concentrations for the most delicate freshwater species (the highest priority risk condition identified here), and a further 29 chemicals were identified for possible further evaluation based on a small margin between occurrence and toxicity values.


This method shows the broad range of chemical concentrations seen for organic chemicals across the country and identifies methods of determining their central tendency, allowing for researchers to characterize higher-than-normal or lower-than-normal surface water conditions as well as providing an overall indication of the presence of organic chemicals in the United States. The highest chemical concentrations did not always indicate the highest-risk conditions. Even when accounting for the high level of uncertainty in these data due to differences in data collection and reporting across the set, some chemicals may still be categorized as higher environmental risk than others using this method, providing value to chemical safety decision makers and researchers by suggesting avenues for more focused investigation.

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Fig. 1: Visual representation of criteria used to filter samples and arrive at the final evaluation set.
Fig. 2: Limit values often exceed result values.
Fig. 3: The amount of seasonal concentration variation was similar for pesticides and non-pesticides.
Fig. 4: Most records are below a limit value.
Fig. 5: Statistical methods agree when data are less censored.
Fig. 6: Risk prioritization based on bioactivity exposure range overlap.

Data availability

The initial analysis set was downloaded from using queries described in the file, hosted at The representative concentration ranges and bioactivity:exposure ratio results are available at the same GitHub repo in the file all_chem_res.csv.


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The authors thank Dr. Jon Arnot, Ms. Lindsay Eddy, Ms. Colleen Elonen, and Dr. Peter Fantke for their helpful reviews of the manuscript.


The United States Environmental Protection Agency (EPA) through its Office of Research and Development (ORD) funded the research described here. This project was supported in part by an appointment to the Research Participation Program at the Center for Computational Toxicology and Exposure, U.S. Environmental Protection Agency, administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and EPA.

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The original concept for this analysis is based on work by RWS. All authors here were working together on another project; the idea for making this a stand-alone manuscript was identified by JFW and RRS. RRS wrote the text of the article, along with JFW. RRS wrote the extraction script. A first idea for the analysis script was written by RWS (although the current work uses different methods). RRS conceptualized and wrote the current analysis script, with assistance from JFW and RWS. MLS and RWS provided guidance on statistical methods. All authors reviewed and approved both the work as it progressed and the final form of the manuscript.

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Correspondence to Risa R. Sayre.

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Sayre, R.R., Setzer, R.W., Serre, M.L. et al. Characterizing surface water concentrations of hundreds of organic chemicals in United States for environmental risk prioritization. J Expo Sci Environ Epidemiol (2022).

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  • Environmental statistics
  • Water
  • Chemicals


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