Investigating herbicide transport and fate in vegetated lysimeters with numerical modeling and stable carbon isotopes

The application of pesticides can induce severe impacts to the vadose zone, groundwater, and their ecosystems. A study was carried out on two lysimeters located in Wielenbach, Germany. Different soil textures were considered within the soil cores, consisting of 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 selected metabolites were monitored in the lysimeter drainage. In addition, stable carbon isotopes (δ¹³C) were analyzed for investigating biodegradation influences of two of the applied herbicides.

In a first step, we characterized unsaturated flow in the lysimeters based on stable water isotope measurements (δ²H and δ¹⁸O) combined with modeling. Different setups within the numerical model HYDRUS-1D were compared, including single and dual porosity approaches. Then, the unsaturated flow models were extended for describing reactive transport of the herbicides, and simulations were interpreted in combination with measured δ¹³C values. 

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. Some metabolites were observed to accumulate in drainage, and biodegradation was indicated by small isotopic shifts in δ¹³C to less negative values in the leached herbicides. In the later sampling campaign (7.5 months after herbicide application), a higher increase in δ¹³C (less negative values) compared to earlier sampling (19 days after application) points towards stronger biodegradation. This can be explained by a higher biodegradation potential when the infiltrated water and the herbicides were affected by longer mean transit times in the unsaturated zone.

Observations were reproduced by modeling, where the overall dynamics of herbicide concentration in the lysimeter drainage could be covered well by the model setups. The concentration peaks were partly associated with heavy precipitation, which in turn indicates that the transport was influenced by preferential flow. Limitations were found for describing preferential flow events by using single and dual porosity models, as some concentration peaks were over- or underestimated. The use of δ¹³C for compound-specific isotope analysis allowed obtaining some evidence on biodegradation of the two herbicides in the unsaturated zone, which was also validated with the model results.