Reactivity of vadose-zone solids to S-metolachlor and its two main metabolites: case of a glaciofluvial aquifer

Degradation potential of MCPA, metolachlor and propiconazole in the hyporheic sediments of an agriculturally impacted river

Hyporheic sediments are influenced by physical, biological, and chemical processes due to the interactions with river water and has been shown to play an important role in the environmental fate of pesticides. Therefore, this study evaluated the bacterial degradation potential of MCPA, metolachlor and propiconazole in hyporheic sediments sampled along a 20 km long stretch of an agriculturally impacted river dominated primarily by water losing conditions. Water physicochemical parameters in the river and nearby groundwater wells were assessed along with pesticide sorption to sediments and bacterial community composition. Degradation and mineralisation batch experiments were set up from six locations (five water losing, one water gaining) using environmentally relevant concentrations of pesticides (10 μg kg^-1). Highly variable DT₅₀ values from 11 to 44 days for MCPA, 11-27 days for metolachlor (MTC) and 60-147 days for propiconazole were calculated based on ~140 day studies. Degradation of MTC led to accumulation of the transformation products MOA and MESA in batch experiments. Noteworthy, MESA was detected in the groundwater wells adjacent to the part of the river impacted by losing conditions suggesting that degradation processes in hyporheic sediments may lead to the formation of transformation products (TP) leaching towards groundwater. Further, from propiconazole was identified a persistent transformation product being different from 1,2,4-triazole. Specific calculated DT₅₀ values could not the linked to bacterial diversity. However, generally all sediment samples were characterised by high bacterial diversity, where approximately 80% of the relative sequence abundances were < 1%, which may increase the likelihood of finding contaminant-degrading genes, thereby explaining the general high contaminant-degrading activity. The studied sediments revealed a high potential to degrade pesticides despite only being exposed to low diffuse pollutant concentrations that is similar to calculated DT₅₀ values in agricultural soils.

Leaching and degradation of S-Metolachlor in undisturbed soil cores amended with organic wastes

Organic waste (OW) reuse in agriculture is a common practice fostered by benefits in terms of waste recycling and crop production. However, OW amendments potentially affect the fate of pesticide spread on fields to protect the crops from pests and weeds. The influence of OW on the sorption, degradation, and leaching of pesticides is generally studied for each mechanism separately under artificial laboratory conditions. Our study aims at evaluating the balance of these mechanisms under more realistic conditions to clarify the influence of three common OW amendments on the fate, in soil, of the widely used herbicide S-Metolachlor. We performed leaching experiments in large undisturbed soil cores amended with raw sewage sludge, composted sludge, and digested pig slurry (digestate), respectively. We monitored S-Metolachlor and its two main metabolites MET-OA and MET-ESA in the leachates during a succession of 10 rainfall events over 126 days. We also quantified the remaining S-Metolachlor and metabolites in the soil at the end of the experiments. S-Metolachlor leaching didn't exceed 0.1% of the applied dose with or without OW amendment. Despite a soil organic carbon increase of 3 to 32%, OW amendments did not significantly affect the amount of S-Metolachlor that leached through the soil (0.01 to 0.1%) nor its transformation rate (6.0 to 8.6%). However, it affected the degradation pathways with an increase of MET-OA relative to MET-ESA formed after OW amendment (28 to 54%) compared to the controls (8%). Concentration of S-Metolachlor and metabolites in the leachates of all treatments greatly exceeded the regulatory limit for groundwater intended for human consumption in Europe. These high concentrations were probably the consequence of preferential macropore flow. Colloids had comparable levels in the leachates after S-Metolachlor application. Dissolved organic carbon was also comparable in the controls, digestate, and sludge treatments but was 65% higher in the compost-amended cores. These results, along with a great variability among replicates inherent to experiments performed under realistic conditions, partly explain the limited impact of OW on the transport of S-Metolachlor.

Pesticides in groundwater at a national scale (France): Impact of regulations, molecular properties, uses, hydrogeology and climatic conditions

Contaminants in groundwater are a major issue worldwide. Temporal trends of such occurrences in French groundwaters were evaluated for several active substances of pesticides belonging to different chemical classes, to identify key factors explaining groundwater contamination. Our study relied on exploitation of a French national database (ADES, created in the mid-1990s and remarkable for the available data, including over 88 million analyses). Temporal changes in the frequency of exceeding a reference value of 0.1 μg/L for several substances were determined at yearly and monthly scales. Such trends were examined by distinguishing different periods according to changes in regulations (new approval, withdrawal, or dose reduction), and were combined with data on effective rainfall as a proxy for groundwater recharge, on aquifer lithology, and on sales of active substances as a proxy for actual applications. A review of monthly data shows that a rapid transfer of pesticides with contrasting physico-chemical properties can occur after application in many aquifers, regardless of their lithology. For substances such as metolachlor, showing a sharp increase in sales, a clear relationship exists between quantities sold and frequency of exceeding the reference value. For other active substances, such as isoproturon or chlortoluron, frequencies of exceedance are governed by both sales and effective rainfall. Finally, the occurrence of active substances in groundwater several years after their withdrawal from the market is explained by at least three major mechanisms: the transfer time from soil into groundwater, processes of remobilization from soil and/or unsaturated zone, and no or low degradation in the saturated zone. While these processes are well documented for atrazine and different types of aquifers, they can be virtually unknown for other active substances.

The role of two organic amendments to modify the environmental fate of S-metolachlor in agricultural soils

S-metolachlor is a widely used herbicide that may contaminate groundwater when applied to irrigated crops, especially when the soil has a low organic carbon (OC) content. The objective here was to assess the capacity of two organic wastes, namely, green compost (GC) and pelletised organo-mineral manure fertilizer (PM), applied to two soils (S) with different textures at a rate of 10% dry weight to modify the fate of S-metolachlor. The herbicide's Freundlich adsorption coefficient (Kf) increased within a range of 3.2-8.2 times in S + GC and 3.8-6.8 times in S + PM. A positive correlation between adsorption and OC and the coefficient of variation of the OC normalised adsorption coefficients (Kf_oc) higher than 20% indicated the evident influence on this process of soil OC content and its nature. The increase in adsorption did not prevent the dissipation of S-metolachlor in the amended soils, although the degradation rate decreased up to ~2 times or was not significantly modified across the different soil types. The S-metolachlor metabolites, metolachlor ethane sulfonic acid and metolachlor oxanilic acid, were detected in the herbicide's dissipation in the unamended soils, but they were not detected in the amended soils. The mobility experiments indicated leached amounts of S-metolachlor higher than 50% in unamended soil. The amounts decreased 1.1-1.7 times and 1.7-1.8 times in the S + GC and S + PM when a saturated flow was applied. Moreover, breakthrough curves indicated a slow leaching kinetics of herbicide in amended soils, with low concentrations continuously detected in the leachates together with a decrease in the maximum peak concentration. The results show the effect of the application of organic wastes especially in sandy soils to promote the immobilisation and/or degradation of S-metolachlor, avoiding its transfer to other environmental compartments.