Guernsey Water has found the pesticide ‘metamitron’ in the island’s largest reservoir – with that water supply now being treated to ensure it is safe to drink, amid warnings of water shortages if we get any more lengthy dry spells.
Longue Hougue Reservoir; the island’s largest raw water storage reservoir, has been isolated from Guernsey Water’s main supply network since the chemical was detected during routine sample testings at the end of February.
While the source of the contamination still hasn’t been identified, Guernsey Water believes it came from a single application within the central island water catchment area.
This is the first time metamitron has been detected in significant concentrations locally, and as a result the States have banned the pesticide.
Since the contamination was confirmed, water from Longue Hougue has been used in reduced volumes and diluted with water from other reservoirs.
Guernsey Water said the latest tests on drinking water have shown that this approach, along with the introduction of carbon dosing at the island’s northern treatment works, has worked to eradicate traces of metamitron.
The metamitron is believed to have entered the reservoir sometime after 27 January.
It was detected on 27 February during routine tests, suggesting a routine test a month earlier was clear.
Guernsey Water thinks the pesticide entered the water network after a single application within the catchment area. Although the exact source is unknown, the utility said this highlights the impact of pesticide use on our water resources.
The amount of metamitron detected that has caused this significant restriction and work to clear the water supply is equivalent to only about half a litre of the chemical within the 1,165 million litres of water the reservoir holds when full.
Over 25,000 routine water quality samples are tested annually for nearly 180 different chemicals, meaning Guernsey Water managed to detect the metamitron early.
Daffyd Griffiths leads a six-strong Water Quality Team, responsible for this monitoring our drinking water at every stage of the supply process, from source to tap.
He said they routinely monitor 18 streams and conduct around 15,000 tests on stream and stored waters checking for substances like pesticides.
Metamitron is one of more than 130 pesticides that Guernsey Water routinely tests for, but this is the first time it has been detected in any significant concentration.
“The majority of operational monitoring sampling is of raw water and goes to the UK for pesticide and PFAS analysis. This is all part of the ‘source selection’ methodology to ensure we collect water of the best quality from the sources that are available.
“This requires careful consideration as it is also vital to ensure that we have enough water to supply the island through the warmer, drier times of the year.
“Safe drinking water plays a vital role in protecting the health and wellbeing of our island. Ensuring a reliable, resilient, and high-quality supply of drinking water is Guernsey Water’s top priority – not only a core part of our business plan but a commitment that shapes the daily work of every one of our colleagues.”
Because the use of water from Longue Hougue – which holds 27% of the island’s stored water – has been restricted, Mr Griffiths warned that could impact our water supplies as we head into the summer.
With a dry spring coming to an end, we could see water use restrictions if the rainfall seen today is followed by another lengthy dry spell.
Any more pollution within the water supply network could also see the introduction of restrictions.
“We continue to urge islanders to follow guidelines when using any pesticide, that includes herbicides, fungicides and insecticides,” said Mr Griffiths. “Do not spray if rain is forecast, do not apply on or near streams, on hard surfaces or near drains. This includes road drains which often discharge into streams, and always ensure that any left over chemicals or containers are disposed of correctly.”
Despite the issue with Longue Hougue Reservoir, Guernsey Water said our drinking water currently has no detectable metamitron due to the steps taken above to ensure its safety.