New York (CNN Business) A conservancy group denounced Fox News host Tucker Carlson on Monday for claiming that immigrants make the Potomac River "dirtier."

"There's a time to speak truth against ignorance and to stand up for our values -- and that time is now," Hedrick Belin, president of the Potomac Conservancy, said in a statement.

Belin added, "We at Potomac Conservancy, a nonprofit land trust and leading clean water advocate in the region, feel compelled to respond to this groundless accusation not just because it's factually incorrect, but because it's racist plain and simple."

Carlson, whose rhetoric on immigration has generated outrage before, told The Atlantic for a profile published Sunday that immigrants are "almost exclusively" responsible for the littering in the Potomac River. Carlson did not provide any data to back up his claim, only telling the magazine that he knows, "Because I'm there. I watch it."

Belin said he hoped to "dispel" Carlson's "false and bigoted statements," saying that the "thinly veiled racist beliefs have no place in any serious public discourse."

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