One drop of pesticide in Olympic pool enough to exceed limit in 30 km of stream

Pesticide contamination of drinking water sources quadrupled since 2014, and has been linked to farmers using MCPA herbicide to prevent the overgrowth of rushes which can render their land ineligible for EU payments.

Records of pesticide contamination in drinking water supplies between 2007 and 2016 show incidences shooting upwards, reaching nearly 80 in 2014, nearly 160 in 2015, and about 140 in 2016, compared to no more than 40 in each of the seven previous years.

Pesticide exceedances peaked in May and September, and about 70% of exceedances were for the MCPA herbicide, widely used in agriculture, but also in amenity use (on golf courses, parks, sports grounds, etc); and for amateur use (home gardens).

It is effective against rushes, and the Department of Agriculture has advised farmers to use physical or mechanical control of rushes as much as possible (grazing, cutting, topping), rather than use the MCPA herbicide.

Drainage, or liming the land, are also recommended where there are rush infestations.

Use chemical control only if necessary, and then favour spot treatments and/or weed licking, rather than broadcast spray treatments, farmers are advised.

They should avoid chemical control near water bodies used to supply drinking water.

Department officials say farmers should never fill sprayers directly from a water course; buffer zones must be respected; application rates must not be exceeded; and herbicide must be applied at the correct time.

The drinking water standard for individual pesticides is 0.1 micrograms per litre (not a health-based standard), which is 1 part in 10 billion.

Therefore, the equivalent of one drop of pesticide in an Olympic-sized swimming pool can breach the limit along 30 km of a typical stream.