Investigating pesticide contamination in arable soils: An advanced monitoring study from Northern Germany

Pesticide contamination in agricultural soils remains a critical environmental issue due to the persistence and mobility of parent compounds and transformation products (TPs). Most studies have focused on surface soils and short-term dynamics, overlooking the long-term transport of pesticides in the subsurface. This study investigates spatiotemporal contamination patterns of applied and non-applied pesticides and their TPs across soil depths at two agricultural sites adjacent to lentic small water bodies (LSWBs) in Northern Germany. Over a 20-month period, monthly composite soil samples were collected at four depth intervals (0-90 cm) and analyzed for 25 pesticides and TPs. Two extraction solutions were applied to separate aliquots of the same soil sample: CaCl₂ to assess the mobile, bioavailable fraction, and MeOH to evaluate the sorbed fraction. The results revealed distinct contamination patterns among applied pesticides, TPs, and non-applied residues. Sorptive compounds such as diflufenican and tebuconazole accumulated in upper soil layers, while mobile TPs like metazachlor-ESA and flufenacet-ESA were detected in deeper horizons. Seasonal trends reflected application timing and precipitation dynamics, particularly in CaCl₂ extracts. Residues not applied during the monitoring period, including bixafen and metrafenone, were persistently detected, indicating legacy contamination and potential off-site inputs. The study underscores the importance of long-term and multi-depth monitoring approaches to capture the full extent of soil contamination. Differences between extractions highlight the need to assess immediate bioavailability and long-term storage in soils. These findings support the inclusion of TPs and deeper soil layers in monitoring frameworks to improve pesticide risk management and protect soil and groundwater.