Dye will be poured into the water off Exmouth on Wednesday [24 April] as South West Water carries out a series of tests.
The water company is reassuring local people that the substance is harmless and will not affect any wildlife in the area.
The dye is being used as part of investigation work at Maer Rocks, following discussions with the local community. The aim is to confirm the location of pipework, and check that an outfall is operating as it should be.
A spokesman for the water company said: “Local residents may notice some yellow/green discolouration to the sea near Maer Rocks between 8am and 1pm. The dye is harmless and will have no negative impact to the water or wildlife.”
There will be a drop-in event at the Ocean building between 2pm and 5pm on Wednesday for customers to hear more about why South West Water (SWW) is carrying out the test.
SWW will also be offering information about investments it is making in Exmouth, which include a pledge to spend £38 million to reduce sewage spills.
In December sewage was released into the estuary because of a burst pipe.
Wednesday’s dye tests are not connected to the sewage spills project.
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