Analysing agricultural plant protection product concentrations in groundwater in Germany: Nationwide database with site and compound insights.

Pesticides from agricultural practices are among the most pressing reasons why groundwater sources do not reach the good chemical status standards as required by the European Water Framework directive. Complementary to previous federal pesticide reports, we analysed groundwater-monitoring data from 13 German Länder assembled in a database consisting of 26.192 groundwater measuring sites sampled between 1973 and 2021 of in total 521 parent compounds and metabolites. This study focuses on agricultural plant protection products. The monitored substance spectrum and site density developed over time and differs between Länder. More than 95 % of all samples lie below the respective (multiple) limits of quantification (LOQ). We thus report the frequency of exceedance above concentration thresholds, which allows to compare measurements temporally and spatially. Pesticide detections were found in all aquifer types, land uses and well screen depths. Most detections of higher concentrations were found in agricultural areas, at shallow screen depth in porous aquifers. Karst aquifers showed also a higher percentage of samples in higher concentration classes. Metabolites with high mobility and persistence were found in higher concentration ranges. Herbicides and metabolites thereof dominate the top 20 of pesticides that most frequently exceed 0.1 µg L-1. The ranking for 2010-2019 includes both authorised and banned compounds and their occurrence is discussed in the context of their mobility, persistence and underlying monitoring density. Yearly exceedance frequencies above 0.05, 0.1 µg L-1 and higher thresholds of metazachlor and its esa-metabolite, and national sales data of the parent compound did not show a temporal correlation in subsequent years. This study stresses the need for the harmonisation of heterogeneous pesticide data. Further, a characterisation of the groundwater data used to analyse pesticide occurrence in selected concentration ranges for relevant site factors and compound properties and provides a pesticide ranking based on exceedance frequencies is provided.

Gain and retain - On the efficiency of modified agricultural drainage ponds for pesticide retention.

Drainage ponds have the potential to serve as long-term interface measures primarily for flood control, and mass retention. They are often considered as promising supplements for the mitigation of drainage pipe loads to improve the water quality in agricultural landscapes. In this study, a highly dynamic drainage pond system with non-steady inflows and groundwater interaction was modified and investigated regarding its potential for pesticide and transformation product (TP) retention. For this purpose, two 104-day monitoring campaigns were conducted before and after pond modification. Field experiments with fluorescent tracers, Uranine and Sulforhodamine-B, proved that structural modifications improved the hydraulic functionality of the ponds. The effective volume (Ɛ) increased from 20% to almost 100% in the modified pond and the mean hydraulic residence time (τ) was ten times longer. After a dry period, pesticide retention was high during slow refilling of the ponds, still TP loads posed a risk by infiltration into shallow groundwater due to the permeable ground. During wet periods, short nominal detention times together with high inflows led to rare high retention rates through peak attenuation. Moderate inflows resulted in extremely variable retention values, owing to the small pond storage capacity. Along with this, the total retention efficiency after modification reached up to 38% for mobile, 29% for sorptive pesticides, and 32% for mobile TPs. To achieve the best performances for ponds as natural landscape elements, they should be analysed for their hydrological functionality as a prerequisite and then modified for delayed pesticide and TP transport. Then, dynamic drainage ponds can utilize their full potential regarding mitigation of pesticide and TP loads in agricultural catchments.

Field insights into leaching and transformation of pesticides and fluorescent tracers in agricultural soil.

The frequent detection of residues from pesticides in various natural water types has raised public awareness. This study investigated the pesticide transformation in soil and their loss to shallow groundwater in a small agricultural catchment in Northern Germany. The pesticide Flufenacet and its transformation product Flufenacet ESA were examined in Luvisol and Colluvic Gleyosol under field conditions during two consecutive years. In the second year, a fluorescent tracer experiment applying Uranine and Sulforhodamine - B was carried out to gain additional insights into leaching and formation of transformation products in soil during and after a drought. We found preferential flow in response to low precipitation as an important transport pathway for Flufenacet in dry soil, as a Flufenacet concentration (1.57 µg L-1) was detected in shallow groundwater within 10 days after application. Leaching of Flufenacet to shallow groundwater by preferential flow posed greater risks during the dry than during the wet period. In contrast, Flufenacet ESA was detected in all groundwater samples. During the dry period, we detected no formation of TP510 (tracer transformation product) in the immediate topsoil. A fraction of both tracers remained there, suggesting also long-term residues of pesticides in the topsoil caused by limited living conditions for microorganisms under dry conditions. Newly formed transformation products of Uranine and Flufenacet were mainly trapped in upper soil if capillary flow was marginal. Formation of TP510 could be related to a soil water optimum and a soil temperature threshold. The occurrence of increased TP510 amounts in soil after drought was concurrent with the main peak of Flufenacet ESA in shallow groundwater. This suggested similar retention and transformation processes of fluorescent tracers and organic pesticides inside the soil. This study contributed to an extended understanding of the leaching and transformation of organic pollutants in agricultural soil under real field conditions.