ESTIMATING THE POLLUTION POTENTIAL OF PESTICIDES TO GROUND-WATER

A screening model available in the literature has been used to evaluat e the ground water pollution potential of a number of commonly used pe sticides under irrigated horticulture in Bassendean sand of the Swan C oastal Plain of Western Australia. The original model assumed a decrea sing rate of pesticide degradation but a constant organic matter conte nt with depth in the soil profile. A modified version of the model was developed to take into account the generally decreasing organic matte r 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 base d 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 app reciable levels of the residue (>0.1% of applied mass). The number inc reased to 21 and was accompanied by a decrease in the travel times req uired for the pesticides to reach ground water when the decreasing org anic matter contents of the profile with depth were taken into conside ration. To assess the validity of using sorption and degradation data from the literature for the local soil, comparisons of model calculati ons were made for five pesticides whose sorption coefficients and degr adation half-lives were measured on the local soil. For some pesticide s, the predictions based on literature values were significantly diffe rent 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 re lative pollution potentials of different pesticides.