EVALUATION OF THE COST-RISK RELATIONSHIP OF GROUNDWATER CONTAMINATIONIN WEED-CONTROL OF SOYBEAN

The relationship between weed control cost in soybean and the risk of groundwater contamination was evaluated for five typical pedo-floristi c situations of the Po Valley. For the weed control treatments, a 'Gro undWater Danger Index' (GWDI) was calculated as the proportion of the active ingredient applied which leaches downwards, relative to the gui delines for drinking water calculated according to the World Health Or ganisation methodology. The fraction of the active ingredient which ca n leach was estimated with the Pesticide Root Zone Model (PRZM) and by means of the Attenuation Factor (AF) and the Groundwater Ubiquity Sco re (GUS). For each chemical option a total cost was calculated adding the treatment cost to the value of the yield loss caused by residual w eeds. The optimal treatments were very different depending on the pedo -climatic and floristic situations. A large number of options had high er values of GWDI than those of atrazine under the same conditions, in dicating a potential risk of groundwater contamination in soybean weed control. The GWDI calculated with the PRZM were then compared to thos e 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 .