Metamitron banned as Guernsey Water manages pesticide presence at Longue Hougue Reservoir
The pesticide metamitron is now banned locally, following its detection in the island's largest raw water storage reservoir.
The chemical was found in a routine water quality sample taken at Longue Hougue Reservoir on the 27th February, and the reservoir was immediately isolated from supply whilst further investigations were carried out.
Samples taken from catchment streams and other storage reservoirs have not shown presence of metamitron, except at Juas Reservoir, where water from Longue Hougue is transferred for treatment. However the concentrations there were much lower and remained well within strict drinking water standards in both the reservoir and treated water supplied to customers.
The source has not been identified, but it is believed it may be the result of a single application somewhere within the water catchment area that collects water from the centre of the island. It is suspected this occurred sometime after 27 January, because the chemical was not detected in routine samples taken from Longue Hougue on that date.
Daffyd Griffiths, Water Quality Risk Manager at Guernsey Water said: "Although this situation is far from ideal, with the island's largest water resource now only able to provide a very limited supply of water, thanks to our rigorous testing schedules, metamitron was detected early enough to protect the island's drinking water.
"This situation offers a very real picture of the importance of using pesticides responsibly. Longue Hougue reservoir holds a total of around 1,165 million litres of water when full, and the level of metamitron that has resulted in the reservoir being severely restricted for use is equivalent to around half a litre.
"We continue to urge islanders to follow guidelines when using any pesticide, that includes herbicides, fungicides and insecticides. 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."
Water from Longue Hougue is now having to be used in reduced volumes, and diluted with water from other reservoirs. 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 resulted in metamitron being completely removed.
"Despite this approach now eliminating the presence of metamitron in drinking water, the restricted use of Longue Hougue reservoir does impact our water resources position.
"With an exceptionally dry spring and the restricted use of 27% of the Island's stored water, if the current dry spell of weather continues or another pollution incident occurs we could be facing a water shortage, and a requirement for usage restrictions as time goes on. We would like to take this opportunity to remind islanders to always use water wisely", said Mr Griffiths.
The law on the ban can be viewed here.