That was the assessment of Guernsey Water’s water quality risk manager Margaret McGuinness, as a ban on retail sales approaches.
The decision not to retain the pesticide on the list of approved retail products has been taken by the Health & Safety Executive, rather than directly by Guernsey Water, but the analysis work of Mrs McGuinness’ small team over several years has informed the decision.
It means, when the approval ceases at the end of the month, that gardening supply stores will no longer be able to sell the popular herbicide to their customers, though the island’s 100 or so commercial users will still be allowed to spray with it.
It has been determined that pollution of water catchments has occurred through amateur use, rather than professional.
‘We typically see sharp increases in pesticide levels after a downpour, especially following a dry spell, as the rain brings it off the land and into our supply,’ Mrs McGuinness said.
‘We monitor continually and we’re able – through selective sourcing – to ensure the water supplied to customers meets the required standards.’
Her team of five – two water quality scientists, their assistant, a catchment inspector and a compliance monitor – carry out this monitoring alongside other duties and determine which water will be supplied around the network according to the information they obtain.
If levels exceed safety limits, Guernsey Water has an infrastructure of pipes and valves that can be used to switch supplies from one source to another and to divert streams to different water storage facilities than those they have traditionally fed.
‘We’re very fortunate that glyphosate is the only pesticide to be problematic,’ Mrs McGuinness said.
‘If we limit its use, we won’t need expensive treatment processes.’
Levels of the chemical have exceeded the safety limit in half of Guernsey’s catchment areas but only during April, in which readings have been double the next highest month.
Gardeners typically use the product in the spring, as it kills plants by absorption through new leaves into the sap, where it transfers to the root and prevents the plant taking in any further nutrients.
The selective sourcing has meant the effective unavailability of up to 250m litres of water at certain times – about 5% of the total required.
This has so far been manageable but the utility fears any further increases in stored water will necessitate a different treatment process.
‘If we couldn’t source sufficient water through this selectivity, we would need to invest in the next level of water treatment – Granulated Activated Carbon,’ Mrs McGuinness said.
‘I can’t give an exact figure but it would cost in the tens of millions of pounds and there would be very significant ongoing costs because the carbon has to be renewed regularly.’