Monitoring pesticides in the Erne catchment

In general, pesticide analysis on water samples within the group water sector is largely limited to compliance monitoring completed once or twice per annum as part of the drinking water audit sampling regime. It was this sampling programme that alerted Stranooden GWS to an exceedance in the MCPA herbicide concentration in their supply back in 2018 and to the inclusion of its White Lough source in the NFGWS Phase II Source Protection Project.

As part of this project, a more extensive and catchment wide sampling programme was developed across the catchment. It soon became apparent that elevated MCPA concentrations were far more frequent that had been indicated by infrequent drinking water sampling and that a season-long sampling programme provides invaluable information about what is happening in a catchment.

The extent of pesticide contamination in surface water supplies was confirmed during 2020, when a baseline pesticides study was completed on a further eight GWS sources in the wider Upper Lough Erne catchment.

Sampling was completed at each lake on a weekly or bi-weekly basis from April to December (the traditional spraying season), with an average of 20 samples being analysed per source. Six acid herbicides – MCPA, MCPB, MCPP, 2,4-D, 2-4-DB and dichlorprop – were analysed.


The results of this analysis were compared against the pesticide ‘Guide Value’ developed by the NFGWS and contained within its ‘Framework for Drinking Water Source Protection’. This gives a value of.075 μg/l for MCPA and 0.375 μg/l for total pesticides in a source/ untreated.

MCPA exceedances were recorded at least once on 6 of the eight sources. The exceptions were on the sources of both Corduff GWS and Magheracloone GWS, both of which have been involved in large scale source protection initiatives in recent years. 2,4-D was detected at trace levels on occasion, but not exceeding the ‘Guide Value’. This was an encouraging finding as national trends would indicate that 2,4-D concentrations in drinking water sources are on the rise.

None of the remaining four herbicides were detected in the course of the programme of sampling and analysis. As to when the MCPA exceedances occurred, concentrations above the ‘Guide Value’ were occasional in some sources and regular in others.

While an exceedance was recorded in May on one source, no elevations were detected thereafter. Spikes in concentrations occurred later in the season on other sources, with most exceedances detected in August, September and October.

These monitoring results will be useful for the group water schemes involved in highlighting the period when their sources may be most at risk. Additionally, further sampling of these sources in 2021 would help strengthen the understanding of this risk.


The results of this monitoring programme also provided information on how persistent the problem can be, should herbicide contamination occur. For lakes with lower flushing rates (e.g. small stream outflows), a pesticide pollutant event, if sufficiently high, can contribute to elevations in the ‘Guide Value’ a number of months later. This is particularly true in the event of subsequent herbicide contributions to the lough.

Such a phenomenon was observed in one source during 2020; exceedances were detected consistently throughout the summer and early autumn before concentrations below the ‘Guide Value’ were recorded in October.

Elevated concentrations in another lough during September/early October also reduced significantly towards the end of October, signifying that a similar process had occurred. In both these instances it is suggested that a reduction in MCPA application in late autumn, in combination with an increase in the lake’s turnover, contributed to late October reduction in MCPA levels. The study highlights how a season-long sampling programme of a drinking water source can provide invaluable information on what is happening in the catchment and the associated consequences.

Origins of contaminant

The next logical step is to identify the origins of the contaminant (i.e. the Critical Source Areas). A number of the schemes that detected exceedances in 2020 have plans to expand their sampling programme out into the catchment this year in an effort to do exactly that.