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 envhealth@gov.gg 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.
'Major housing development' near Guernsey Airport granted outline permission
Alderney enjoys a Royal Visit for the 80th Homecoming anniversary
Exhibitors finalised for first Guernsey travel show
WATCH: Last Blue Islands plane leaves Jersey
New woodland for Alderney's 80th Homecoming
Guernsey States 'in talks' with easyJet
Guernsey's airline Aurigny takes on six new First Officers
Call for more transparency over Guernsey Electricity's charges
