Fate of glyphosate and its metabolite aminomethyl phosphonic acid (AMPA) in the urban environment

Abstract

Glyphosate is a popular, easily accessible and cheap herbicide that is widely used and in agricultural and urban applications. Once applied, glyphosate has strong persistence and adsorption characteristics with limited mobility, degrading to AMPA. Private use and commercial use in urban areas are increasing rapidly yet there is no information available about its occurrence and fate in urban environments. This thesis aims to quantify glyphosate and AMPA in urban soils, stormwater sediment and estuarine sediment and their biodegradation (soil and estuarine sediment) and sorption (soil) characteristics in Auckland, New Zealand. Soil and stormwater sediment samples were taken over a six week period following a routine glyphosate spray on road berms. Marine samples were taken at one point at time. Glyphosate biodegradation experiments were run over a 3 week period and total organic carbon was measured each week as a proxy for glyphosate use by microbes. In sorption experiments, soil was spiked with a range of pH adjusted glyphosate concentrations in NaCl. Rapid degradation was observed in soil samples as glyphosate increased rapidly then decreased over time while AMPA increased over time and was present in control samples, suggesting that spray drift and over spraying is occurring. Biodegradation results also support good degradation in soils. Sorption data was evaluated in linear regression, Freundlich and Langmuir models with linear regression and Freundlich models providing the best fit and showing that most sorption occurs in subsoil which correlates with a low soil pH. Biodegradation was poor in stormwater sediment, with only slight changes in glyphosate and AMPA concentrations over time and was likely affected by spray drift and over spraying in gutters. Concentrations in marine sediment were much lower than that found terrestrially and were comparable to previously published data with slow biodegradation. However it is expected that the estuary can adapt to glyphosate loads. All aims were achieved in this study with a full pathway of glyphosate application to its final sink being described.