State scientists found high levels of E. coli, phosphorus and other pollutants at nearly every test point in the tributaries that feed the Scioto River northwest of Columbus.
The Ohio Environmental Protection Agency found that E. coli levels at sites used by boaters and swimmers were higher than federal safety thresholds.
Dina Pierce, a spokeswoman for the Ohio EPA, said the results are not surprising and that the overall health of the Upper Scioto Watershed has improved since the mid-1990s.
But the report shows that this mostly rural watershed is still troubled and worse off than others in the state.
“There’s some work that we need to do here, and we need local people to be on board, because there’s only so much we can do as a regulatory agency,” Pierce said.
E. coli in the Upper Scioto Watershed, which stretches across Hardin, Marion and Union counties, likely came from manure spread on area farms and from wastewater-treatment plants, according to the report.
About 80 percent of the land around the watershed is farmland, and treatment plants line the rivers and streams there.
The Upper Scioto is a small part of the larger Scioto River Watershed, which includes creeks, streams and rivers in more than 6,500 square miles ending at the Ohio River near Portsmouth.