Source to Tap are monitoring water quality in the River Derg to gauge the impact of grants given to farmers to help them act to reduce the level of pesticides in drinking water. This monitoring will help establish if the financial incentives for farmers provides a more cost-effective way of reducing pesticide concentrations in drinking water than treatment at the works does.
Ensuring the health of our rivers and lakes is not just a priority for the ecosystems they support, but also for the supply of clean drinking water. 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. These limits refer to treated drinking water, with the levels of contaminants reduced during the treatment process. Nonetheless, high pesticide concentrations in raw water are costly to remove and exceedances of the standards in treated water are observed on occasions.
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.
In order to better understand the sources of MCPA within the tributaries of the River Derg, we conducted a water quality survey across the catchment between April and June 2018. Water samples, taken manually from the river, were collected at 11 locations throughout the catchment every Tuesday for 13 weeks, and analysed in AFBI’s laboratories to produce weekly snapshot maps of MCPA concentration. To capture the daily and seasonal variability in MCPA concentrations, we also established a high-resolution bankside sampling system just upstream of the intake for the Derg water treatment works (Figure 1). A 24-bottle refrigerated autosampler was programmed to take a sample once every seven hours, giving us coverage day and night over the course of a week and ensuring that storm events – which are frequently the means by which pollutants are washed off land and into the river – are captured. Once a week, the samples are collected for analysis at AFBI and we now have a full year of high frequency results between April 2018 and April 2019.
The weekly samples from across the catchment showed that concentrations increased as we moved downstream from the peatcovered and forested areas around the headwaters, through rough grazing land and into the increasingly more intensively farmed land around Castlederg. The average concentration of MCPA in the spatial samples across all tributaries over the 13-week period was 0.14 μg/L. Notably, the tributary with the highest concentration of MCPA (8.97 μg/L) also had the highest average concentration (0.84 μg/L), suggesting that this tributary is frequently having the greatest negative effect on the water quality in terms of pesticide release. What was more concerning was that the average annual MCPA concentration for the high frequency river water samples, taken just upstream of the NI Water drinking water abstraction point, was 0.15 μg/L. Additionally, over 25% of the samples contained more than 0.1 μg/L of MCPA (Figure 2). This means that both the organisms in the river and the water treatment works’ filters are subject to high levels of MCPA for prolonged periods. Data on the toxicity of MCPA to insects, fish and other animals is sparse and mostly inconclusive and therefore such prolonged high concentrations in the river water may contribute to a loss of biodiversity and a reduction in the number of some species. The frequency of high MCPA concentrations in the river water also increases the costs of treating it for drinking.
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.
Comparison of our high-resolution data with the statutory monitoring of raw water at the water treatment works highlights that the statutory monitoring, which is taken less frequently, is underestimating the extent of the problem with herbicide contamination. Our monitoring is therefore of benefit to the water companies because it gives them a better idea of the scale of the problem and can help them target resources and measures towards removing pesticides and improving drinking water. AFBI and UU will continue to monitor MCPA concentrations in the River Derg for the next 2 years. The data will be used to determine whether the financial incentive scheme provides a more costeffective way of reducing pesticide concentrations in drinking water than treatment at the works does.
Source – catchments.ie