Evaluation of the risk to groundwater after treating logs with cypermethrin

In order to protect conifer logs against attacks from the striped ambrosia beetle (Trypodendron lineatum ) during spring in Swiss forests, logs are treated with the insecticide cypermethrin. Rainfall can cause the insecticide to leach into the ground, potentially threatening the groundwater quality. Forest groundwater is widely used for drinking water, which means that any contaminants within it should be avoided. This study assesses the risk of groundwater contamination in field conditions. The two study areas are located on unconsolidated sediments (Censieres, or CS) and on karstic rocks (Grand Bochat, or GB). An analytical method was developed to determine the concentration of cypermethrin and its degradation products 3-PBA and DCVA in water samples. Intensive rainfall was simulated in order to mimic a situation that threatens groundwater. The study’s results show that, when treated according to the manufacturer’s instructions, a certain amount of insecticide was leached during the first rainfall event (2.2 g or 4.4 % of the applied cypermethrin). This leaching threatens groundwater quality, but can be avoided by decreasing the pesticide amount applied while maintaining a satisfactory protection. The insecticide amount that reached the groundwater was very low and was related to simulated rainfall, not natural rainfall. In Censieres, only one groundwater sample presented a cypermethrin concentration (4 l g/l). In Grand Bochat, after a simulated rainfall of 36 mm, 3.3 % of the insecticide (5 g) was leached and 0.05 % of the total applied insecticide amount (corresponding to 1.5 % of the leached insecticide) reached the groundwater under the epikarst layer.