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TECHNICAL REPORT
Ecological Risk Assessment

Spray Deposition of Two Insecticides into Surface Waters in a South African Orchard Area

^a Dep. of Zoology, Private Bag X1, Univ. of Stellenbosch, Matieland 7602, South Africa
^b Forensic Chemistry Lab., Dep. of Health, Cape Town 8000, South Africa

Corresponding author (R.Schulz{at}tu-bs.de)

Received for publication May 26, 2000. Drift from pesticide spray application can result in contamination of nontarget environments such as surface waters. Azinphos-methyl (AZI) and endosulfan (END) deposition in containers of water was studied in fruit orchards in the Western Cape, South Africa. Additionally, attention was given to the contamination in farm streams, as well as to the resulting contamination of the subsequent main channel (Lourens River) approx. 2.5 km downstream of the tributary stream inlets. Spray deposit decreased with increasing distance downwind and ranged from 4.7 mg m^-2 within the target area to 0.2 mg m^-2 at 15 m downwind (AZI). Measured in-stream concentrations of both pesticides compared well with theoretical values calculated from deposition data for the respective distances. Furthermore, they were in the range of values predicted by an exposure assessment based on 95th-percentile values for basic drift deposition (German Federal Biological Research Centre for Agriculture and Forestry [BBA] and USEPA). Pesticide deposition in the tributaries was followed by a measurable increase of contamination in the Lourens River. Mortality of midges (Chironomus spp.) exposed for 24 h to samples obtained from the AZI trials decreased with decreasing concentrations (estimated LC₅₀ from field samples = 10 µg L^-1 AZI; lethal distance: LD₅₀ = 13 m). Mortality in the tributary samples averaged 11% (0.5–1.7 µg L^-1 AZI), while no mortality was discernible in the Lourens River samples (0.041 µg L^-1). The sublethal endpoint failure to form tubes from the glass beads provided was significantly increased at all sites in comparison with the control (analysis of variance [ANOVA], Fisher's protected least significant difference [PLSD], p < 0.01).

Abbreviations: AZI, azinphos-methyl • BBA, Federal Biological Research Centre for Agriculture and Forestry, Germany • END, endosulfan • SDTF, Spray Drift Task Force

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