Chemical occurrence of pesticides and transformation products in two small lentic waterbodies at the head of agricultural watersheds and biological responses in caged Gasterosteus aculeatus.

Recent monitoring campaigns have revealed the presence of mixtures of pesticides and their transformation products (TP) in headwater streams situated within agricultural catchments. These observations were attributed to the use of various agrochemicals in surrounding regions. The aim of this work was to compare the application of chemical and ecotoxicological tools for assessing environmental quality in relation to pesticide and TP contamination. It was achieved by deploying these methodologies in two small lentic water bodies located at the top of two agricultural catchments, each characterized by distinct agricultural practices (ALT: organic, CHA: conventional). Additionally, the results make it possible to assess the impact of contamination on fish caged in situ. Pesticides and TP were measured in water using active and passive samplers and suspended solid particles. Eighteen biomarkers (innate immune responses, oxidative stress, biotransformation, neurotoxicity, genotoxicity, and endocrine disruption) were measured in Gasterosteus aculeatus encaged in situ. More contaminants were detected in CHA, totaling 25 compared to 14 in ALT. Despite the absence of pesticide application in the ALT watershed for the past 14 years, 7 contaminants were quantified in 100 % of the water samples. Among these contaminants, 6 were TPs (notably atrazine-2-hydroxy, present at a concentration exceeding 300 ng·L-1), and 1 was a current pesticide, prosulfocarb, whose mobility should prompt more caution and new regulations to protect adjacent ecosystems and crops. Regarding the integrated biomarker response (IBRv2), caged fish was similarly impacted in ALT and CHA. Variations in biomarker responses were highlighted depending on the site, but the results did not reveal whether one site is of better quality than the other. This outcome was likely attributed to the occurrence of contaminant mixtures in both sites. The main conclusions revealed that chemical and biological tools complement each other to better assess the environmental quality of wetlands such as ponds.

Occurrence of pesticides and their transformation products in headwater streams: Contamination status and effect of ponds on contaminant concentrations.

In France, more than 90% of monitored watercourses are contaminated with pesticides. This high contamination level increases at the head of agricultural watersheds, where dilution capacities are low and transport from treated lands is direct. Ponds, numerous around headwater streams, could provide additional protection against pesticide pollution. Because of their long hydraulic residence time and large water volumes, they mitigate pesticide concentrations between upstream and downstream rivers. However, pesticide transformation products may also be responsible for the degradation of environments, owing to their presence at high concentrations and their persistence, but related data are scarce, particularly because of their high level of molecular diversity. We first reported on the state of water contamination in agricultural headwater streams, based on high frequency water sampling. Analysis of 67 molecules (HPLC-ESI-MS/MS) showed pesticides and pesticide transformation product mixtures of up to 29 different compounds in one sample. Regardless of the sampling location, transformation products represented at least 50% of the detected compounds. Then, we demonstrated the capacity of a pond to reduce contaminant concentrations in downstream rivers for 90% of the detected compounds. Upstream from this pond, environmental quality or ecotoxicological standards were exceeded during sampling, with pesticide and transformation product sum concentrations of up to 27 µg/L. Downstream from the study pond, few exceedances were observed, with a maximum total concentration of 2.2 µg/L, reflecting significant water quality improvement.

Barrage fishponds: Reduction of pesticide concentration peaks and associated risk of adverse ecological effects in headwater streams.

Constructed wetlands have been suggested as pesticide risk mitigation measures. Yet, in many agricultural areas, ponds or shallow lakes are already present and may contribute to the control of non-point source contamination by pesticides. In order to test this hypothesis, we investigated the influence of extensively managed barrage fishponds (n = 3) on the dissolved concentrations of 100 pesticides in headwater streams over the course of a year. Among the 100 pesticides, 50 different substances were detected upstream and 48 downstream. Highest measured concentration upstream was 26.5 µg/L (2-methyl-4-chlorophenoxyacetic acid, MCPA) and 5.19 µg/L (isoproturon) downstream. Fishponds were found to reduce peak exposure levels as high pesticide concentrations (defined here as ≥ 1 µg/L) generally decreased by more than 90% between upstream and downstream sampling sites. The measured concentrations in the investigated streams were compared to laboratory toxicity data for standard test organisms (algae, invertebrates and fish) using the toxic unit approach. When considering the threshold levels set by the European Union within the first tier risk assessment procedure for pesticide registration (commission regulation (EU) N° 546/2011), regulatory threshold exceedances were observed for 22 pesticides upstream from fishponds and for 9 pesticides downstream. Therefore, the investigated barrage fishponds contributed to the reduction of pesticide peak concentrations and potential risk of adverse effects for downstream ecosystems.