ESTIMATING THE POLLUTION POTENTIAL OF PESTICIDES TO GROUND-WATER

A screening model available in the literature has been used to evaluate the ground water pollution potential of a number of commonly used pesticides under irrigated horticulture in Bassendean sand of the Swan Coastal Plain of Western Australia. The original model assumed a decreasing rate of pesticide degradation but a constant organic matter content with depth in the soil profile. A modified version of the model was developed to take into account the generally decreasing organic matter contents with depth in the soil profile. Residual masses and travel times of 40 pesticides were calculated by the model using sorption and degradation data available from the literature. The calculations based on the constant OM mode predicted that for a recharge rate of 0-5 m/yr, some 14 of the pesticides were likely to reach ground water at appreciable levels of the residue (>0.1% of applied mass). The number increased to 21 and was accompanied by a decrease in the travel times required for the pesticides to reach ground water when the decreasing organic matter contents of the profile with depth were taken into consideration. To assess the validity of using sorption and degradation data from the literature for the local soil, comparisons of model calculations were made for five pesticides whose sorption coefficients and degradation half-lives were measured on the local soil. For some pesticides, the predictions based on literature values were significantly different from those based on measured parameters indicating, as expected, that overseas data may not always represent local conditions. However, they may still provide valuable first approximations of the likely relative pollution potentials of different pesticides.