Test results show that a black substance found earlier this month in a part of the Anacostia River is a rare algal bloom, the U.S. Environmental Protection Agency said.
Officials initially believed the substance was oil, but ruled that out fairly early.
The algae, known as dinoflagellate, is rare in fresh water like the Anacostia but conditions including temperature and nutrients in the water likely contributed to the rapid spread of the algae, according to the EPA.
At least 75 percent of the bloom has dissipated and rain and cooler temperatures likely will cause the algae to completely die off, according to the EPA.
Copyright The Associated Press
