Almost 90% of MCPA loss to water occurs through surface runoff, a study by Ulster University and the Agri-Food and Biosciences Institute has found.
Speaking at a Teagasc sustainable agriculture webinar on Friday morning, Ulster University Professor Philip Jordan said that 87% of MCPA loss to water occurs through surface runoff, while 13% takes place through leaching to base flow.
The research team conducted a catchment scale study as part of the 'Source to Tap' project. The River Derg catchment was assessed, rising in the Blue Stack Mountains and flowing beyond Castlederg.
Trends show mean source water concentration of MCPA has increased nationally in the past 10 years, with the herbicide accounting for over 60% of threshold surpasses most recently recorded in source water.
Regulated standards allow for just 0.1ug/l or less than one billionth of a gram of MCPA to be present in treated drinking water.
The source water in the River Derg catchment had an average MCPA concentration of 0.4ug/l, which is costly to remove. Over 85kg of MCPA was found to have travelled down through the catchment during the seven-month period the monitoring was conducted.
The researchers also found that MCPA concentration in source water increases during storm events and often exceeds threshold values.
The study concludes that the more improved grassland in a sub-catchment, the higher the MCPA concentration is likely to be, while the more rough grazing in a sub-catchment, the lower average MCPA concentration will be.
Jordan said: “While it is known that MCPA degrades under oxygen-rich conditions, reduced breakdown can occur in oxygen-poor environments, potentially creating a legacy in saturated soils.
“Fast runoff pathways post-application are likely transport routes, but the relative contribution from the mobilisation of legacy MCPA from oxygen-poor zones has yet to be quantified.”