Characterization of pesticide fate in the unsaturated zone using numerical transport modeling and stable carbon isotopes

Agrochemicals including nitrate and pesticides pose threats to ecosystems and groundwater. A field study was conducted at two lysimeters located in Wielenbach, Germany, with different soil textures within the soil cores (sandy gravel and clayey sandy silt). The lysimeters were vegetated with maize, and four different herbicides were applied according to common agricultural practice. Over a period of 4.5 years, concentrations of the herbicides and some of their metabolites were monitored in lysimeter discharge. Stable carbon isotopes (δ¹³C) were analyzed for investigating biodegradation influences. Numerical modeling with HYDRUS-1D was carried out for describing unsaturated flow and reactive transport of the herbicides, and simulations were interpreted in combination with measured δ¹³C. 

At the end of the observations, 0.9 to 15.9% of the applied herbicides (up to 20.9% for herbicides plus metabolites) were recovered in lysimeter drainage. Metabolites were observed to accumulate, and biodegradation was indicated by small changes in δ¹³C between applied herbicide solution and leached herbicides. Results point towards a higher biodegradation activity when infiltrated (herbicide-contaminated) water was residing a longer time in the unsaturated zone. Herbicide dynamics in lysimeter discharge could be covered well by modeling. Compound-specific isotope analysis using δ¹³C provided valuable hints for microbial degradation of herbicides in the unsaturated zone.