Islanders are being advised against swimming at the southern end of Cobo Bay.
The States says it has received poor water quality results after taking samples from a nearby outfall.
"Seawater samples that were taken demonstrate that these concerns relate purely to the outfall."
Environmental Health has investigated upstream and in nearby roads to try to track the pollution, but hasn't been able to identify the source.
Landowners have been contacted and provided information about land use.
We're being reminded to maintain good hand hygiene after visiting the beach.
"If any members of the public have information on possible sources they are advised to contact the Office of Environmental Health and Pollution Regulation by emailing firstname.lastname@example.org or calling 01481 221161"
Last September Cobo beach-goers were advised not to touch or play in the water from the outfall between Route de Cobo and the slipway next to the kiosk when e.coli was found in water samples.