Elsevier

Ecological Engineering

Volume 18, Issue 2, December 2001, Pages 239-245
Ecological Engineering

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

https://doi.org/10.1016/S0925-8574(01)00073-8Get rights and content

Abstract

Constructed wetlands have been widely used to treat different types of agricultural 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 pollution. Here we describe the concentrations of total suspended solids (TSS), nutrients and spraydrift-borne azinphos-methyl (AZP) at the inlet and outlet of a 0.44-ha vegetated pond. One tributary, which receives contaminants from a 15-ha orchard area, flows directly through the wetland, before it enters the Lourens River, South Africa. A tracer experiment with Rhodamin B was undertaken to find the optimal timing for inlet and outlet sampling performance. The retentions of TSS, ortho-phosphate and nitrate during dry weather 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 μg/l (n=5). The reduction of AZP load was 54.1±3.8%. A 24-h in situ bioassay employing midge larvae (Chironomus spec.) revealed a significant (ANOVA, Fisher's PLSD; P≤0.01) reduction of toxicity. Mortality in two separate trials was 41.3±2.4% and 46.3±1.2% at the inlet station and 2.5±1.4 and 3.8±1.2% at the outlet station during days with spraydrift deposition into the tributary upstream of the wetland. During time intervals without any spraying in the catchment, mortality of the midge larvae in both trials was 1.2±1.2% at the inlet station and zero at the outlet station (n=4 for each trial). We conclude that construction of small vegetated ponds is a suitable risk reduction strategy for agricultural nonpoint source insecticide input into surface waters.

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.

References (22)

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