Use of a constucted wetland to reduce nonpoint-source pesticide contamination of the Lourens River, South AmericaThis article is part of a larger document. View the larger document here.
The Lourens River, Western Cape, South Africa, and its tributaries situated in an intensively cultivated orchard area receive pesticide contamination during rainfall-induced runoff and during spraydrift. A 0.44-ha constructed wetland, built in 1991 in one of the tributaries (summer flow 0.03 m3 per second), was studied in order to assess its effectiveness in reducing nonpoint-source agricultural pesticide contamination. Even high levels of particle-associated azinphos-methyl (43 µg per kilogram), chlorpyrifos (31 µg per kilogram) and prothiofos (6 µg per kilogram) introduced via runoff were not detectable in the outlet suspended-particle samples. Recovery of water-diluted azinphos-methyl contamination at inlet concentrations of about 0.55 µg per liter during spraydrift indicated much less retention, approximately 51 percent. Comparison of in situ bioassays of bloodworms (Chironomus spec.) above and below the wetland revealed a reduction of contamination in terms of toxicity from 41 to 2.5 percent. TSS, ortho-phosphate, and nitrate were retained in the wetland with trapping efficiencies of 78, 75, and 84 percent, respectively.