A study of atrazine transport through a soil profile on the Gnangara Mound, Western Australia, using LEACHP and Monte Carlo techniques

Abstract

A preliminary risk assessment of atrazine leaching at a strawberry farm on the Gnangara Mound, Western Australia, was performed using LEACHP in conjunction with Monte Carlo techniques. Monte Carlo techniques were also used to implement a sensitivity analysis of LEACHP to determine influential parameters. As an extension to the sensitivity analysis, a model comparison was performed with similar models, in order to quantify differences arising from the use of different models. The Monte Carlo risk assessment involved simultaneous variation of LEACHP's soil parameters and selected chemical parameters to assess the risk of atrazine leaching past the root-zone at the experimental site. This Monte Carlo risk assessment predicted that of the 100 mg/m2 of atrazine applied at the experimental site, the mean cumulative mass per unit area of atrazine to leach below the root-zone after 263 days was 49 mg/m2, with a standard deviation of 4.1 and a range from 37 to 62 mg/m2. The Monte Carlo sensitivity analysis of LEACHP tested selected soil and chemical parameters, one at a time, to determine the influence of each parameter on the models predictions of cumulative mass per unit area of atrazine to leach below the root-zone. The chemical parameters organic carbon partition coefficient (Koc) and degradation rate constants were found to be more influential than the soil parameters, with Koc being the most influential parameter. The most influential of the soil parameters was organic carbon content, followed by bulk density. The model comparison involved 4 separate simulations: LEACHP using Richards' equation (LEACHP (Richards)), LEACHP using Addiscott's model of water and solute movement (LEACHP (Addiscott), MACRODB, and GLEAMS. The results from this exercise showed that LEACHP (Richards) and MACRODB made similar predictions for the cumulative mass per unit area of atrazine to leach below the root-zone (49 and 50 mg/m2, respectively), while GLEAMS and LEACHP (Addiscott) made significantly lower predictions (35 and 38 mg/m2, respectively). The results from this study indicate there is a significant risk of atrazine leaching at the experimental site; however, more detailed field measurements and further modelling would enable more accurate predictions.