Predicted and measured levels of azinphosmethyl in the Lourens River, South Africa: Comparison of runoff and spray drift

Runoff and spray drift are important sources of nonpoint pesticide pollution in surface waters, but few studies have directly compared these routes of input in an exposure assessment scenario. To this end, a runoff formula suggested by the Organization for Economic and Cooperative Development (Paris, France) and basic drift values (95th percentiles) were integrated into a geographical information system (GIS) to predict runoff and spray drift-related loading of azinphosmethyl (AZP) in the Lourens River (LR), South Africa. The GIS-integrated calculations were first validated in the tributaries of the river, where measured loads were well predicted for both runoff (r(2) = 0.95; p < 0.0001; n = 9) and spray drift (r(2) = 0.96; p = 0.0006; n = 8). Through extrapolation to the catchment scale containing 400 ha of orchards, the GIS-integrated calculations predicted similar loads of AZP as measured in the Lourens River mainstream for six runoff (between a factor of 1.03 and 1.86 lower) and six spray drift (between a factor of 1.1 and 2.4 higher) events. Mean measured loads per event were significantly (p = 0.004) higher for runoff (27.8 +/- 19.1 g) than for spray drift (0.69 +/- 0.32 g). Based on long-term meteorological data and average application regimes, runoff leads to a higher annual load (47.6 g) than spray drift (5.5 g) in the Lourens River. Runoff is clearly a more important source of nonpoint pollution in the studied catchment, and mitigation strategies should focus first on addressing this aspect on a catchment scale and second on addressing problem areas on a subcatchment scale.