Detection of the herbicide asulam in groundwater: translocation to roots of treated docks (Rumex) and exudation to subsoil as a potential input pathway

Abstract

BACKGROUND

The herbicide asulam is a weakly adsorbing and, thus, mobile substance. However, due to its rapid degradation, leaching to groundwater is not expected based on simulations with soil leaching models used to predict pesticide concentrations in groundwater as part of the authorization process.

RESULTS

Asulam was, nevertheless, frequently detected in samples from 112 groundwater monitoring sites in Switzerland, albeit at low concentrations up to 0.017 μg/L. A potential input pathway for the systemic herbicide includes the processes uptake by leaves of treated weeds, translocation to roots, exudation to subsoil, and leaching to groundwater. In a field trial, we first studied formation and decline of residues in leaves and roots of Rumex alpinus. Residues in leaves declined rapidly from ≈100 to ≈3 mg/kg within 3 weeks after treatment. Residues in roots reached a maximum of 6.5 mg/kg after 2 weeks and then declined to similar levels as in leaves. Maximum residues in roots corresponded to ≈2.6% of the applied amount, that is, translocation was rapid and significant. In a greenhouse experiment with Rumex obtusifolius, it was shown that asulam is also exuded from roots and found in soil leachate at concentrations of a few μg/L.

CONCLUSION

Transport of asulam to groundwater via root exudation may thus be a plausible pathway, in particular, if exudates from deep roots are released into subsoils, where degradation and sorption are minimal. This pathway is currently not considered in leaching models used in the authorization process. © 2025 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.