CASE STUDY:

Using Chemcatcher® to help solve a pesticide puzzle

The acid herbicide MCPA (4‐chloro‐2‐methylphenoxy acetic acid) is a relatively common pesticide used in agriculture and horticulture to control broadleaved weeds and rush. It has been detected in surface water, groundwater and drinking water worldwide. In fact, it is one of the most frequently encountered herbicides in water quality monitoring the US, Canada, Australia and much of Europe.

Despite this, there are significant gaps in knowledge about the fate and impact of MCPA in the environment.

For instance, MCPA is known to degrade relatively quickly under aerobic conditions. But what happens in the anaerobic environment of saturated soils?  And if legacy MCPA from anaerobic zones can become remobilised, how is it possible to distinguish this from MCPA resulting from more recent pesticide applications?

An Irish research team has been using Chemcatcher® passive samplers over the past 18 months in catchment studies aimed at answering these and other questions.

Strategies to protect drinking water resources

Their work forms part of a Horizon 2020-funded European project known as WaterProtect, which involves collaboration with six other ‘ActionLabs’ in Belgium, Denmark, Spain, Poland, Romania and Italy.

The shared work programme aims to improve strategies for protecting drinking water sources from nitrate and pesticide pollution linked to agriculture.

Chemcatcher® is being used by Teagasc, the Irish ‘ActionLab’, as part of the pesticide component of the project. “In Ireland, MCPA is our key herbicide of concern. It accounts for over 80% pesticide exceedances of EU limits in drinking water,” explained Per-Erik Mellander, a research officer at Teagasc and Chief Scientist of Ireland’s agricultural catchment research programme.

Understanding pathways

Pesticides are known to enter water bodies through a number of pathways, including surface run-off, erosion, leaching, drain flow and spray drift. MCPA, in particular, can be highly mobile post-application due to its high water-solubility and low sorption with soil.

But precisely what happens to it in the environment can depend on a complex range of factors, including the subsoil geology and the soil type, structure, pH, microbiology and moisture levels, as well as the formulation and the time of application of the pesticide.

From the group at Teagasc’s perspective, key questions are: ‘Do we understand enough about the process of herbicide loss to water?’ and ‘Do current practices require modification to minimise this loss?’

Catchments

The MCPA study, which has been ongoing since November 2018, has focused on two river catchments in the southeast of Ireland. Both catchments are intensively farmed and exposed to the same weather conditions. However, one comprises mostly free-draining soils, planted with arable crops, while the other includes mostly poorly drained soils and is dominated by grassland production for beef and dairy cattle. Low levels of groundwater abstraction occur in both catchments, supplying individual farms and a small number of households.

On a field scale, the study is also focusing on MCPA and its behaviour and fate post-application for rush control treatment in a poorly draining, impermeable grassland field. Data from the monitoring will support the evaluation of the herbcide’s movement via subsurface drains, shallow groundwater, overland flow and surface water streams.

The catchments were chosen as they have already been well characterised as part of an existing decade-long monitoring programme, evaluating the effectiveness of regulations introduced in response to the Nitrates Directive.

Through this Agricultural Catchments Programme (ACP), a total of six river catchments, representative of Ireland’s geological and agricultural mix, have been sampled at 10-minute intervals and analysed for nutrients over a period of more than 10 years.

This has generated a wealth of high-frequency data, which, in turn, has greatly improved the team’s ability to model nutrient transport and behaviour in catchments, and they are now hoping to gain a similar understanding of the pathways and fate of MCPA in the environment.

Why Chemcatcher®

Hence the selection of passive sampling, Per-Erik explained. “Since there is no continuous monitoring method available for herbicides, we saw Chemcatcher® as the next-best approach available.”

“MCPA is very soluble. It moves very quickly in flashy conditions. So, if we rely on taking a grab sample, the likelihood is that we may miss the pulses,” he added.  “With Chemcatcher®, we are getting a time-weighted average, distributed over a 14-day period.”

The other benefit of Chemcatcher® is that it enables detection at lower levels than traditional sampling. This is important, he pointed out as “we are monitoring rivers that are representative areas as opposed to problem areas”.

One of the misconceptions the research is challenging is that MCPA currently being detected is due to current or very recent activity.

In aerobic environments, due to rapid oxidation, MCPA has a short half-life of approximately 20 days.  However, when MCPA gets down to an anaerobic environment – in groundwater or in sediments in lakes – its half-live can be up to 2,000 days, creating legacy problems on remobilisation.

Per-Erik likens the challenge of modelling herbicide pathways to solving a jigsaw puzzle. The data the team has been getting from Chemcatcher® over the past 18 months is adding to the pieces provided by conventional, high-frequency sampling and analysis, bringing the big picture more closely into focus. “With the analysis coming back, we are getting very powerful information,” he says.

Further work

The team hopes to continue the monitoring using Chemcatcher® beyond the life of the WaterProtect project, rolling it out to all six catchments in the ACP programme.

The researchers at Teagasc have also started working with the National University of Ireland – Galway (NUIG) on a Pesticide Management for Better Water Quality (PEST-MAN) project funded by the Irish Environmental Protection Agency and the Department of Agriculture Food & the Marine, which aims to build on the work of this project, again using Chemcatcher® for sampling.

The Teagasc team would like to ultimately gain more clarity on the ecological impact of MCPA. Although ecotoxicological effects have been examined, little is known about the interaction of MCPA and its degradation products with nutrients, colloids or other pesticides – and how these substances may combine with other environmental factors to produce multiple stressor effects for species.

Where to find more information

For further information about Chemcatcher®, visit https://chemcatcher.ie/

The WaterProtect project has received funding from Horizon 2020 Work under the theme ‘Food security, sustainable agriculture and forestry, marine and maritime and inland water research and the bioeconomy.’ The project is also aiming to identify bottlenecks in the water governance structure, which may hinder the progress in achieving good water quality. Project website: https://water-protect.eu/

Background reading: Morton, PA, Fennell, C, Cassidy, R, et al. A review of the pesticide MCPA in the land‐water environment and emerging research needs. WIREs Water. 2020; 7:e1402. https://doi.org/10.1002/wat2.1402

Information and videos about the Irish Agricultural Catchments Programme are available at https://www.teagasc.ie/environment/water-quality/agricultural-catchments/

For further information about the Pesticide Management for Better Water Quality (PEST-MAN) project, visit http://www.nuigalway.ie/gene/activeprojects/