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February 18, 2021

Mixtures of organic chemicals are ubiquitous in small U.S. streams, reports a new study by the U.S. Geological Survey. In many streams the mixtures could be affecting stream organisms.

USGS scientists collected 3 to 12 water samples from 305 small streams across four major regions of the U.S. as part of the USGS Regional Stream Quality Assessment. The samples were analyzed for 389 organic chemicals comprising pharmaceuticals, pesticides, and wastewater-indicator compounds. Pesticides and pharmaceuticals are designed to have a biological effect.

Two or more organic compounds—that is, a mixture—were detected at least once at 303 of the 305 stream sites. Further, a chemical mixture was detected in at least half of the samples collected at 252 sites with some urban or agricultural development in the watershed. These results indicate that exposure to multiple organic contaminants is common for small streams in developed watersheds.

The chemical mixtures measured presented a moderate risk of acute toxicity—mortality—at 44% of sites for aquatic invertebrates, which include stoneflies and mini-crustaceans. For nonvascular plants, which include some algae and are a key part of the aquatic food chain, there was a moderate risk of acute toxicity at 20% of sites. The mixtures presented a moderate risk of sub-lethal effects to vertebrates—organisms, such as fish and salamanders, with a backbone—at 99% of sites.

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