Concentration, load and toxicity of spraydrift-borne azinphos-methyl at the inlet and outlet of a constructed wetland

Constructed wetlands have been widely used to treat different types of agri cultural point and nonpoint source pollution including nutrients, sediments and some herbicides. However, very little is known about the effectiveness of constructed wetlands in controlling nonpoint source insecticide polluti on. Here we describe the concentrations of total suspended solids (TSS), nu trients and spraydrift-borne azinphos-methyl (AZP) at the inlet and outlet of a 0.44-ha vegetated pond. One tributary, which receives contaminants fro m a 15-ha orchard area, flows directly through the wetland, before it enter s the Lourens River, South Africa. A tracer experiment with Rhodamin B was undertaken to find the optimal timing for inlet and outlet sampling perform ance. The retentions of TSS, ortho -phosphate and nitrate during dry weathe r conditions were 15, 54 and 70%. Concentration levels of AZP were reduced below the wetland by 90.8 +/- 0.7%, from 0.65 +/- 0.08 to 0.06 +/- 0.01 mug /l (n = 5). The reduction of AZP load was 54.1 +/- 3.8%. A 24-h in situ bio assay employing midge larvae (Chironomus spec.) revealed a significant (ANO VA, Fisher's PLSD; P less than or equal to 0.01) reduction of toxicity. Mor tality in two separate trials was 41.3 +/- 2.4% and 46.3 +/- 1.2% at the in let station and 2.5 +/- 1.4 and 3.8 +/- 1.2% at the outlet station during d ays with spraydrift deposition into the tributary upstream of the wetland. During time intervals without any spraying in the catchment, mortality of t he midge larvae in both trials was 1.2 +/- 1.2% at the inlet station and ze ro at the outlet station (n = 4 for each trial). We conclude that construct ion of small vegetated ponds is a suitable risk reduction strategy for agri cultural nonpoint source insecticide input into surface waters. (C) 2001 El sevier Science B.V. All rights reserved.