Effectiveness of a constructed wetland for retention of nonpoint-source pesticide pollution in the Lourens River catchment, South Africa.

Abstract

Constructed wetlands have been widely used to control both point- and nonpoint-source pollution in surface waters. However, our knowledge about their effectiveness in retaining agricultural pesticide pollution is limited. A 0.44-ha vegetated wetland built along a tributary of the Lourens River, Western Cape, South Africa, was studied to ascertain retention of runoff-related agricultural pollution. Total suspended solids, orthophosphate, and nitrate were retained in the wetland in the proportions 15, 54, and 70%, respectively, during dry weather conditions (with rainfall less than 2 mm/d) and 78, 75, and 84% during wet conditions (with rainfall between 2 and 35 mm/d). Retention of water-diluted azinphos-methyl introduced via runoff at a level of 0.85 microg/L was between 77 and 93%. Chlorpyrifos and endosulfan were measured during runoff in inlet water at 0.02 and 0.2 microg/L, respectively. However, both pesticides were undetectable in the outlet water samples. During a period of 5 months, an increased concentration of various insecticides was detected in the suspended particles at the wetland inlet: azinphos-methyl, 43 microg/kg; chlorpyrifos, 31 microg/kg; and prothiofos, 6 microg/kg. No organophosphorus pesticides were found in the outlet suspended-particle samples, highlighting the retention capability of the wetland. A toxicological evaluation employing a Chironomus bioassay in situ at the wetland inlet and outlet revealed an 89% reduction in toxicity below the wetland during runoff.