Swimmers have been advised to avoid Clevedon Marine Lake after E.coli, Coliform bacteria and enterococci bacteria were found in the water.
Results from a sample taken from the popular swimming spot earlier this month have shown that the water quality in the lake has decreased.
The lake will now be drained on July 31 so that it can refresh with water from the Severn Estuary - and repeatedly during the next sequence of overtops.
"Swimming in Clevedon Marine Lake is not advised until further notice," a spokesperson for Marine Lake Enthusiasts Society (MARLENS) said.
"A sample was taken from Clevedon Marine Lake on Monday (July 22) and the results indicate that the water quality in the lake has decreased.
"The lake will be drained on Wednesday (July 31) so that it can refresh with water from the Severn Estuary - and repeatedly during the next sequence of overtops between July 31 and August 5."
The news comes a year after swimmers were advised to stay out of the lake after E.coli was found in the water twice last summer.
It was believed at the time that the contamination came from water which had run off into the Bristol Channel which contained faecal matter from farm animals and other pollutants.
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