Environmental scientists have combed canals that feed into Donna Lake for nearly 20 years, searching for polychlorinated biphenyl's known as PCB's.
"This water is actually the drinking water for Donna and it TMs also being used for agricultural purposes," environmental scientist Rafael Casanova, Environmental Protection Agency, said.
Even though PCB's are known to cause cancer, the EPA say's the water is safe to drink, but eating the fish caught in the lake and canals that feed into it is bad for your health.
It's been a long process but now scientists believe they are one step closer to finding the source of the pollution.
"We installed seven of what we call passive sediment samplers," Casanova said. "The canister has a membrane inside that we throw into the water an leave them for a month. Then we come back and collect and analyze them."
He said each trip to the site is crucial to pinpointing the exact spot where the pollution started.
Scientists have narrowed down the location from eight to two miles.
The EPA is working closely with Donna city officials who will eventually play a part in the clean-up process and have plans to turn Donna lake into an outdoor tourism destination.
"We are trying to make sure that it's safe for people to go there," Donna city manager Oscar Ramirez said. "One of the long-term goals for the City of Donna is to try and create an eco-tourism site."
Officials want to add parks and trails and hope bird watchers will come.
But the top priority is safety and finding the source of the pollution is the key.
EPA officials hope to locate the source by this spring.