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 I-h peak levels of current-use insecticides applied to adjacent orchard plots were 1.5 mug l(-1) azinphos-methyl, 0.2 mug l(-1) chlorpyrifos and 2.9 mug l(-1) total endosulfan (alpha, beta, S) in the river itself. Respective average I-h pesticide levels associated with suspended particles were 1247, 924 and 12082 mug kg(-1) along with 980 mug 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) azinphosmethyl, 55 g h(-1) chlorpyrifos, 740 g h(-1) total endosulfan (alpha, beta, 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 round 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) (C) 2001 Elsevier Science Ltd. All rights reserved.
