The erratic problem of pesticide pollution of the Drentsche Aa river as resource for drinking water

Contamination of surface water with pesticides is a well-recognised, notorious problem for decades. The Drentsche Aa river in the northern Netherlands is used as drinking water resource for the city of Groningen and its surroundings. Its quality is thus relevant with respect to both ecological and human water quality. Pesticide pollution in the Drentsche Aa catchment has been recognised for years. Environmental management programs have been implemented that aim to improve the water quality and in which a broad group of stakeholders has been involved. Last year, a committee evaluated the current status and suggested measures to improve the water quality. The waterboard ‘’Hunze en Aa’s’’ helpfully provided the data for the evaluation.

Water quality with respect to pesticides has been intensively studied for the period 2011-2022 (except 2014). The monitoring changed somewhat in the course of time but is summarised as follows. Time proportional sampling of river water was done on a weekly basis at different locations in the catchment and up to 141 pesticides and metabolites get analysed. Targeted grab sampling campaigns were additionally performed. 

Non-compliance at the inlet for drinking water production was most frequently observed for MCPA followed by dimethenamid, MCPP and metamitron and to a lesser degree 13 other pesticides. MCPA and MCPP are widely used. Dimethenamid and metamitron are used for flower bulbs, the former also for field vegetables and the latter for sugar beets. In the catchment, non-compliance to the drinking water standard is typically observed during May-August for the pesticides. Metabolites were observed year-round but do not show non-compliance.

Targeted grab sampling shows that pesticides reach surface water during heavy rain storms in the summer by surface runoff and drainflow. This is a well recognised leaching mechanism for pesticides from agricultural fields. However, pesticides also became detected in river water in weeks with little or no rain. This may be attributed to drift despite mitigating measures to reduce drift to surface water. There is also evidence that irrigation of drained parcels leads to surface runoff and drainflow of pesticides to surface water. The erratic occurrence of several pesticides is thus explained by a combination of reasons: 1. heterogeneous application of pesticides in space and time, 2. non-uniform leaching partly due to surface runoff that shows catastrophic event behaviour and 3. limited success of the implementation of measures being implemented on a voluntarily basis and not to the full extent. The data also show that pesticides not only originate from agricultural activities but also from urbanised areas. Here, pesticides were found that were forbidden for non-agricultural purposes at the time of observation.

The monitoring results show an erratic spatio-temporal pattern and no trend-based improvement as required by the EU Water Framework Directive. The voluntarily measures are thus not sufficiently effective. Climate change with more frequent heavy rain storms and greater need for irrigation may worsen the situation. Thus, more stringent measures are required which implementation may be forced by new legislation under the Environmental Law that came into force at 1 January 2024.