Short CommunicationConcentration, load and toxicity of spraydrift-borne azinphos-methyl at the inlet and outlet of a constructed wetland
Introduction
Constructed wetlands have been suggested and used as a potential risk reduction strategy for nonpoint source pollution (Mitsch, 1992). While the fate and retention of nutrients and sediments in wetlands are understood quite well (Brix, 1994), the same cannot be said of agrochemicals (Baker, 1993). Only very few studies refer to the potential of wetlands for the removal of herbicides and some other organic chemicals (Wolverton, 1987, Kadlec and Hey, 1994, Lewis et al., 1999). Since wetlands have a high ability to retain and process material, it seems reasonable that constructed wetlands, as buffer strips between agricultural areas and receiving surface waters, could mitigate the impact of pesticides (Rodgers et al., 1999).
The organophosphate insecticide azinphos-methyl (AZP) is applied against moths and mites in apple, pear and plum orchards in the Western Cape of South Africa and is transported via spraydrift (Schulz et al., 2001) and runoff (Schulz, 2001) to surface waters. With a total application rate of 770 kg per season on fruit trees in the 400-ha catchment of the Lourens River, AZP is the predominant insecticide in use here. It is also one of the most important insecticides with regard to total amount of application in the United States (USEPA, 1994). AZP is highly toxic to fishes and aquatic invertebrates and has a relatively high water solubility of 28 mg/l (USDA ARS database).
The aim of the present study is to test the hypothesis that a small constructed wetland can be used for retaining spraydrift-related input of the insecticide AZP into the Lourens River, South Africa. Retention is assessed for aqueous-phase AZP input following drift during application in orchards by applying a composite-sample design at the inlet and the outlet of the wetland. Furthermore, the effects of the wetland in terms of toxicity reduction are addressed using an in situ exposure bioassay with Chironomus spec. at the same sampling stations during periods with and without spraydrift. Knowledge about the retention capabilities for insecticides, which are of high ecotoxicological relevance, would help to assess the use of wetlands in the management of nonpoint source pollution of surface waters.
Section snippets
Study area
The Lourens River originates at an altitude of 1080 m in a naturally vegetated fynbos area (Tharme et al., 1997) and flows in a southwesterly direction for approximately 20 km before discharging into False Bay at Strand (S34°06′; E18°48′). The catchment region is characterised by intensive farming, with orchards and vineyards in its middle reaches, for details see Schulz (2001).
The constructed wetland studied in the present investigation is located along one of the tributaries of the Lourens
Standard water quality parameters
Results obtained for inlet–outlet measurements of standard water quality parameters are summarised for low-discharge conditions in Table 1. TSS, ortho-phosphate and nitrate levels were reduced by the constructed wetland by 15, 54, and 70% based on concentrations. Discharge, water temperature and pH were not altered by the wetland.
A TSS retention of 15% seems to be relatively low in comparison with other studies reporting values of 78% (Cooper and Knight, 1991, Kadlec and Hey, 1994). However,
Conclusions
Vegetated ponds can be very helpful in reducing nonpoint source insecticide pollution in surface waters. Our results show that the reduction of azinphos-methyl entering the wetland following spraydrift averages 91% in terms of concentration. Most likely, those reduced concentration levels are responsible for the reduction in toxicity we found at the wetland outlet in comparison to the inlet toxicity levels. In terms of insecticide load, the wetland retained 54% of azinphos-methyl, illustrating
Acknowledgements
This study is part of a larger project, including the collaboration of the University of Stellenbosch and the Technical University Braunschweig (Germany). Financial support was obtained from the National Research Foundation (NRF), Pretoria and the Lourens River Conservation Society, Somerset West. The farmers gave permission to enter their property and provided information on the spraying programmes. Beate Helling, Christina Hahn and Ann Thorson gave helpful comments on the manuscript.
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