Rainfall-induced sediment and pesticide input from orchards into the lourens river, western cape, south africa: importance of a single event

Rainfall-induced runoff transported sediments and pesticides into the Lourens River and its tributaries during a 28.8-mm rainstorm in mid-December 1998. Average 1-h peak levels of current-use insecticides applied to adjacent orchard plots were 1.5 μg l−1 azinphos-methyl, 0.2 μg l−1 chlorpyrifos and 2.9 μg l−1 total endosulfan (α, β, S) in the river itself. Respective average 1-h pesticide levels associated with suspended particles were 1247, 924 and 12082 μg kg−1, along with 980 μg kg−1 of prothiofos. Total suspended solids increased during runoff from 32 to 520 mg l−1. The contaminated edge-of-field runoff entered the river via the tributaries directly bordering the orchard-growing areas. Increased concentrations of azinphos-methyl and prothiofos associated with suspended sediments were demonstrated to persist for about 3.5 months without any further input in one of the tributaries. This illustrates that the short-term exposure has the potential to result in long-term contamination of surface waters. In terms of chemical load during the 1-h peak discharge period, the single rainfall event caused a loss of 173 g h−1 azinphos-methyl, 55 g h−1 chlorpyrifos, 740 g h−1 total endosulfan (α, β, S) and 41 g h−1 prothiofos. Levels of contamination were extremely high; they exceed the national water quality standards and those established by the US EPA. A comparison with standard toxicity data and 24-h LC50 s for the local amphipod species Paramelita nigroculus, obtained during this study, indicates that the concentrations found in the river may result in acute toxic effects on aquatic invertebrates and fishes. A probability analysis of 10-y rainfall data revealed that the frequency of a similar storm event occurring within the main spraying season is 1.7 y−1.