An investigation into the cleanliness of rivers feeding Washington's Potomac River has revealed the presence of sex-changing chemicals.
The Potomac runs through the US capital, Washington DC
Pollutants which contain the chemicals, known as endocrine disrupters, were found in several tributaries and in the smallmouth bass fish living within.
The US Geological Survey (USGS) study followed the discovery of high numbers of intersex fish in the Potomac basin.
Endocrine disrupters can mimic or block hormones in the body.
Either naturally occurring or man-made, they can interfere with the endocrine system causing birth defects and reproductive irregularities.
The Potomac River is fed by rivers and streams in Maryland, West Virginia and Virginia.
The USGS investigators found pesticides, flame retardants, and personal-care products containing known or suspected endocrine disruptors in all eight sites that were tested.
The chemicals were also found in all of the smallmouth bass examined by the team.
"We analysed samples of 30 smallmouth bass from six sites, including male and female fish without intersex and male fish with intersex," lead scientist Douglas Chambers said.
"All samples contained detectable levels of at least one known endocrine-disrupting compound, including samples from fish without intersex."
Looking for cause
In an effort to pinpoint the source of the pollution the scientists studied wastewater and run-off from several sites.
They discovered that wastewater effluent - both treated and untreated - agricultural and pest control activities and industrial wastewater all contributed to the problem.
Of particular concern was municipal effluent, which contained a cocktail of at least seven compounds containing endocrine disruptors.
"Antibiotics were detected in municipal wastewater, aquaculture, and poultry-processing effluent, with the highest number of antibiotics and the greatest concentrations found in municipal effluent," the USGS wrote in its report.
The discovery that there were high numbers of intersex fish present in the Potomac basin was made by accident in 2003, when scientists began investigating unusually high numbers of fish deaths.