Evaluation of the cost-risk relationship of groundwater contamination in weed control of soybean

The relationship between weed control cost in soybean and the risk of groundwater contamination was evaluated for five typical pedo-floristic situations of the Po Valley. For the weed control treatments, a 'GroundWater Danger Index' (GWDI) was calculated as the proportion of the active ingredient applied which leaches downwards, relative to the guidelines for drinking water calculated according to the World Health Organisation methodology. The fraction of the active ingredient which can leach was estimated with the Pesticide Root Zone Model (PRZM) and by means of the Attenuation Factor (AF) and the Groundwater Ubiquity Score (GUS). For each chemical option a total cost was calculated adding the treatment cost to the value of the yield loss caused by residual weeds. The optimal treatments were very different depending on the pedo-climatic and floristic situations. A large number of options had higher values of GWDI than those of atrazine under the same conditions, indicating a potential risk of groundwater contamination in soybean weed control. The GWDI calculated with the PRZM were then compared to those obtained with the other simplified indices, AF and GUS. Using the AF gave a good correlation with the values obtained on the basis of the PRZM, while with GUS the correlation, even where significant, was poor.