Increased levels of pesticide have been found in Guernsey's raw water supply.
High levels of glyphosate, a weed killer, have been detected in recent samples, Guernsey Water said.
The company said it was in 19 out of 20 samples but the quality of the island's drinking water "was not compromised".
Guernsey Water said levels of the pesticide had been much lower in the past and it had not ruled out calling for a ban on the substance at some point in the future.
Margaret McGuinness, water quality and risk manager for Guernsey Water, said: "I would like to reassure our customers that we have an effective way of controlling this particular compound during the water treatment process.
"Our sample results prove that the quality of our drinking water is not compromised in any way.
"We have seen elevated levels of glyphosate in the island's raw water supplies and it is our duty to highlight its prevalence and the potential impact."