Council teams have continued to monitor the pollution at the lake in Brooklands Park after a diesel spill nearby reached the water earlier this year.
Diesel spilled from a tanker at Lancing Business Park at the end of May and was carried through surface water drains to the lake at Brooklands, where it was contained in a small section of the lake.
Specialist teams scraped and mopped the diesel off the surface of the lake to reduce the risk to wildlife, though some fish were unable to escape the flow and were photographed floating lifelessly on the surface.
Now, more than a month later, containment measures remain in place at the lake and the Environment Agency continues to investigate, the council said.
In a statement on twitter, Adur and Worthing Councils said: "After a diesel spill at Lancing Business Park reached Brooklands Park in May, our teams are still monitoring the pollution.
"Containment measures will stay in place for the next few months and will gradually be reduced.
"This remains under investigation by the Environment Agency."