Study Highlights the Complexity of Chemical Mixtures in United States Streams

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A new study highlights the complexity of chemical mixtures in streams and advances the understanding of wildlife and human exposure to complex chemical mixtures.

Chemical contaminants do not occur alone in our water resources and detections of complex chemical mixtures are commonly documented in rivers and streams across the United States. Fish and other wildlife are exposed to these complex mixtures of chemicals. There is also the potential for human exposure through direct skin absorption, ingestion of contaminated drinking water, or consumption of contaminated wildlife.

U.S. Geological Survey (USGS) scientist collecting water-quality samples from the Enoree River, South Carolina

USGS scientist collecting water-quality samples from the Enoree River, SC

(Credit: Celeste A. Journey, USGS - South Atlantic WSC. Public domain.)

This study builds on previous work that identified mixtures of contaminants in streams across the United States. The U.S. Geological Survey (USGS) and the U.S. Environmental Protection Agency (EPA) collaborated to assess more than 700 organic chemicals as well as indicators of biological effects in 38 streams within the United States, which represented one of the most comprehensive set of contaminants ever monitored at individual stream sites. This study was an opportunity to assess the factors related to site-to-site differences in stream-contaminant complexity and to put the chemical data into context with potential effects on aquatic animal health.

USGS scientist processing water-quality samples collected from the Enoree River, South Carolina

USGS scientist processing water-quality samples. She is surrounded by lots of equipment

(Credit: Paul M. Bradley, USGS - South Atlantic Water Science Center. Public domain.)

The results indicate that the mixtures of chemicals in streams reflect combined contaminant contributions originating from a combination of agricultural, industrial, municipal, and residential sources in individual watersheds. Complex chemical mixtures (a range of 4 to 161 chemicals per stream) were detected in streams across a range of urban and agricultural areas in the United States. As expected, fewer chemicals were detected in the 4 streams that had low population density compared to the 34 urban/agricultural-influenced locations.

Indicators of biological activity, such as estrogen receptor activation, were present in all stream samples except for one reference site. Estrogenicity activity values have yet to be firmly established; however, multiple sites exceeded activity levels estimated to signal potential adverse effects on aquatic organisms. The indicators were related to target chemicals in the water samples to different degrees, which advanced our understanding of exposure and relative risk of specific chemicals and mixtures.

The results of this overall effort yield insights into the complexity of chemical mixtures present in a wide range of surface waters across the United States and their potential for eliciting adverse ecological health consequences. These results are important to answer questions about minimizing health risks and economic costs, if any, associated with mixed-contaminant exposures.

The USGS Toxic Substances Hydrology Program, the EPA Safe and Sustainable Water Resources, and the Chemical Safety for Sustainability Research Programs funded this study.

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