4-chloro-2-methylphenoxyacetic acid (MCPA) in Drinking Water

MCPA accounts for 75pc of all exceedances in water

Tradenames: Mortone, Agritox, M50, Lupo

The four most commonly detected compounds accounting for pesticide exceedances in drinking-water in Ireland are MCPA, 2,4-D, Mecoprop and Clopyralid

Irish Water pointed out the detection of 81 pesticide and herbicides exceedances in public drinking water supplies in Ireland in 2020, an increase of 5 from 2019.

MCPA, the active substance in many herbicide products used to control thistles, docks and rushes, is still the most commonly detected pesticide in drinking water.

Where MCPA enters a stream it can be detected in surface water (drinking water abstraction point) a long way from where it was applied, >30km.

MCPA is the most popular grassland spray by far, and the fact it is highly soluble means it is prone to run-off and leaching

Under the Drinking Water Directive, the limit for a single pesticide in drinking water is 0.1 μg/L and for the sum of all pesticides is 0.5 μg/L.

The River Derg flows east from Donegal across the border into Country Tyrone, where it supplies 16 million litres per day to nearly 40,000 people in Northern Ireland. However, concentrations of the pesticide MCPA have been detected up to 4.33 μg/L at the NI Water drinking water abstraction point and up to 8.97 μg/L in River Derg tributaries.

These concentrations were detected by the Agri-Food and Biosciences Institute (AFBI) and Ulster University (UU) as part of the Source to Tap project.

Source to Tap is monitoring water quality in the River Derg to assess the impact of a financial incentive scheme for agriculture that aims to improve drinking water quality in the catchment.

The Source to Tap monitoring program is focused on MCPA, which is primarily used in Ireland to reduce the cover of rushes (Juncus species) on grazing land.

MCPA is preferred over other herbicides as it does not kill grass and can therefore be sprayed, whereas other herbicides such as glyphosate must be wiped, requiring additional equipment.

The majority of the high concentrations occurred in late spring/early summer and late summer/early autumn, which coincides with peak periods for MCPA application.

Additionally, MCPA is highly soluble and the majority of high concentrations coincide with storm events when rainfall would have washed pesticides into the river.

However, peaks occur that are not associated with rainfall, which could be caused by farming activities such as spraying too close to water courses or accidental spills.

Interestingly, MCPA was present in river water over the entire year, albeit at low concentrations over winter, indicating a persistent source to the river system, either from water stored in the soil or from groundwater.


Note: there are no plans to use predictive modelling for substances with high usage in Ireland but for which little or no monitoring data is available (Source: National Aquatic Environmental Chemistry Group (NAECG) minutes 18/01/2021)

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