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 micrograms l-1 azinphos-methyl, 0.2 microgram l-1 chlorpyrifos and 2.9 micrograms l-1 total endosulfan (alpha, beta, S) in the river itself. Respective average 1-h pesticide levels associated with suspended particles were 1247, 924 and 12,082 micrograms kg-1, along with 980 micrograms 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 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.