Analysis of Groundwater Contamination by Terbuthylazine and Desethyl-terbuthylazine in the Plana Sur Aquifer of Valencia (Spain)

In this study, groundwater contamination in the Plana Sur Aquifer of Valencia was analyzed using the PRZM (Pesticide Root Zone Model). The analysis included modeling the concentrations of two pesticides, terbuthylazine and desethyl-terbuthylazine, based on observed concentrations in six monitoring wells belonging to the groundwater quality control network of the Júcar River Basin. These wells were specifically chosen as they contain data on the pesticides in question.

The modeling was based on data recorded from the quality network wells of the Júcar River Basin Authority. In the Plana Sur of Valencia, six wells were analyzed, with five of them showing desethyl-terbuthylazine concentrations exceeding permissible limits (with a maximum value of 0.3 µg/L recorded on April 14, 2015). In contrast, the detected concentrations of terbuthylazine were consistently below the legal threshold. For the region's piezometry, results from a previously developed flow model using MODFLOW were utilized. Climatic variables such as precipitation and evapotranspiration were derived from databases associated with the closest climatological monitoring stations to each well.

Crop typologies were determined based on land-use information from the SIOSE database. Application doses were inferred from interviews with farmers, supplemented by data from the Ministry of Agriculture’s "Annual Statistics on Pesticide Use" and "Five-Year Statistics on Pesticide Use in Agriculture." However, the information provided in these documents was insufficient to precisely determine the applied doses. Therefore, calibration was performed using proposed scenarios, with terbuthylazine application rates set at 0.5, 0.6, and 0.8 kg/ha.

A sensitivity analysis of parameters allowed for the formulation of 54 simulation scenarios for each of the six analyzed wells, varying the year pesticide application ceased, the applied dose, and the characteristics of the soil column. Results indicate that desethyl-terbuthylazine concentrations are typically higher than those of terbuthylazine, as it is a metabolite of the latter. In some simulated wells and specific scenarios, these concentrations exceeded the reference value of 0.1 µg/L established by current legislation.

This study underscores the need for detailed scenario-based analysis and calibration to accurately assess the contamination risks and inform effective water quality management.