Environment Agency officers had to take action to stop an unknown chemical from polluting the River Arun after it was found pouring into the waterway from a surface drain.
Now they are trying to find the source of the discharge and prevent it from harming the environment.
It was first noticed on Thursday near Horsham and officers acted on the same day, launching an investigation into the source with the intention of taking action against those responsible.
A spokesman for the government agency said: “Our investigation has revealed that the pollutant discharged into the river from a surface water drain.
"Although the discharge has now been stopped, we continue to work closely with Southern Water to reduce environmental impacts.
Anyone with information or concerns about pollution in any waterway can call the agency's 24-hour incident hotline on 0800 807060.
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