Fate and Effects of Azinphos-Methyl in a Flow-Through Wetland in South
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Environ. Sci. Technol. 2003, 37, 2139-2144 preventing it from entering downstream aquatic habitats (1, Fate and Effects of Azinphos-Methyl 2). The implementation of retention ponds in agricultural in a Flow-Through Wetland in South watersheds was mentioned by Scott et al. (3) as one strategy to reduce the amount and toxicity of runoff-related insecticide Africa pollution discharging into estuaries. The usefulness of aquatic plants for removal of insecticides from water has been shown ,² ² in an indoor microcosm study (4), and the effects of the RALF SCHULZ,* CHRISTINA HAHN, organophosphate phorate have been assessed using littoral ERIN R. BENNETT,² mesocosms in South Dakota wetlands (5). However, infor- JAMES M. DABROWSKI,² mation about the fate or effects of spray drift-borne GERALDINE THIERE,² AND SUE K. C. PEALL³ insecticide input in constructed wetlands is limited. Processes important for removal of nonpoint-source Department of Zoology, Private Bag X1, University of pesticide pollution in wetlands may include adsorption, Stellenbosch, Matieland 7602, South Africa, and Forensic Chemistry Laboratory, Department of Health, decomposition, hydrolysis, microbial metabolism, photolysis, Cape Town 8000, South Africa and volatilization (6). The macrophytes present in the wetland may play an important role in providing an increased surface area for sorption as well as for microbial activity (7). Further- more, they may contribute directly to metabolism (8). Our knowledge about the effectiveness of constructed Spray drift is an important route for nonpoint-source wetlands in retaining agricultural nonpoint-source pesticide pesticide pollution of aquatic habitats (9, 10). Specifically, pollution is limited. A 0.44-ha vegetated wetland built orchard applications result in a large amount of drift due to small droplet size and the trajectory of release (11). Spray along a tributary of the Lourens River, Western Cape, South drift has been shown to be a significant route of insecticide Africa, was studied to ascertain the retention, fate, and entry into tributaries of the Lourens River in South Africa effects of spray drift-borne azinphos-methyl (AZP). Composite (12). water samples taken at the inlet and outlet during five The organophosphate pesticide azinphos-methyl (AZP) spray drift trials in summer 2000 and 2001 revealed an overall [O,O-dimethyl-s-[(4-oxo-1,2,3-benzotriazine-3(4H)-yl)meth- reduction of AZP levels by 90 ( 1% and a retention of yl)]phosphorodithioate], in comparison to other insecticides, AZP mass by 61 ( 5%. Samples were collected at the inlet, has a relatively low KOC of 1000 L/kg; a high water solubility outlet, and four platforms within the wetland to determine of 29 mg/L at 25 °C(13); and a Henry's law constant of 9.5 × -11 3 -1 the fate and effect of AZP in the wetland after direct 10 atm m mol (14). It has been shown to persist in spray drift deposition in the tributary 200 m upstream of pond water with a half-life of about 2.4 d (15). AZP is frequently applied to apple, pear, and plum orchards in the the inlet. Peak concentrations of AZP decreased, and the catchment and has been regularly detected following runoff duration of exposure increased from inlet (0.73 µg/L; 9 and spray drift activity in the Lourens River and its tributaries h) via platforms 1 and 4 to outlet (0.08 µg/L; 16 h). AZP sorbed (12, 16). The estimated total application in fruit orchards of to plants or plant surfaces, leading to a peak concentration the Western Cape is 52 000 kg of active ingredient per year. of 6.8 µg/kg dw. The living plant biomass accounted for It is also one of the most heavily applied pesticides in the 10.5% of the AZP mass initially retained in the wetland, United States, and in 1997 almost 950 000 kg of active indicating processes such as volatilization, photolysis, ingredient was applied throughout the entire country (17). hydrolysis, or metabolic degradation as being very important. To minimize the input of sediment into the Lourens River, AZP was not detected in sediments. Water samples a 0.44-ha vegetated siltation pond was constructed in 1991 taken along two 10-m transects situated perpendicular to along one of the tributaries. Its effectiveness in retaining the shore indicated a homogeneous horizontal distribution runoff-related insecticide input was the subject of an earlier ( ( ) study based on inlet and outlet measurements (1). The of the pesticide: 0.23 0.02 and 0.14 0.04 µg/L (n present study describes the retention, the fate, and the ) 5), respectively. Both Copepoda (p 0.019) and Cladocera biological effects of spray drift-borne aqueous-phase AZP in (p ) 0.027) decreased significantly 6 h postdeposition this flow-through wetland. and remained at reduced densities for at least 7 d. In parallel, the chlorophyll a concentration showed an increase, Materials and Methods although not significant, within6hofspray deposition. Study Region and Study Period. The study area is located The study highlights the potential of constructed wetlands in the catchment of the Lourens River, which originates at as a risk-mitigation strategy for spray drift-related pesticide an altitude of 1080 m in fynbos, a naturally sclerophyllous pollution. vegetation, and flows in a southwesterly direction for approximately 20 km before discharging into False Bay at the Strand (34°06′ S, 18°48′ E). The Lourens River has a total catchment area of 92 km2 and receives an annual mean Introduction rainfall of 915 mm. Approximately 87% of its 35 × 106 m3 It has recently been shown that constructed wetlands have mean annual discharge occurs during the winter months the potential to retain runoff-related insecticide pollution, between April and October (18), as is characteristic of the region's Mediterranean climate. * Corresponding author present address: Syngenta Crop Protec- The 400-ha orchard area in the middle reaches of the tion AG, Ecological Sciences, Jealott's Hill International Research Lourens River is mainly covered by pears, plums, and apples. Centre, Bracknell, Berkshire, U.K.; phone: +44 1344 41 4437; fax: +44 1344 41 4124: e-mail: [email protected]. The pesticide application period in the study area's orchards ² University of Stellenbosch. begins in early August and continues until the end of March. ³ Department of Health. Organophosphorus (OP) insecticides, such as AZP and 10.1021/es026029f CCC: $25.00 2003 American Chemical Society VOL. 37, NO. 10, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2139 Published on Web 04/12/2003 FIGURE 1. Schematic view of the wetland showing size, vegetation coverage, and six main sampling sites (large circles) as well as sites for transect sampling at platforms 2 and 3 (small circles). The inset (not to scale) shows the orientation of the wetland to the Lourens River and to the orchard areas as well as the additional site (spray deposition site, circle) about 200 m upstream of the wetland. The arrow indicates the flow direction. chlorpyrifos, are applied quite frequently to pears and plums TABLE 1. Mean (( SE; n ) 14) of Various Parameters between October and February. Endosulfan is applied mainly Measured at the Wetland Inlet between January and May in apple orchards (19). The constructed wetland studied in the present investi- parameter mean (( SE) gation is located along one of the tributaries 15 m before its discharge (m3/s) 0.041 ( 0.004 entry into the Lourens River (1). This tributary has an average ( × total suspended solids (mg/L) 43.7 5.2 width and depth of 0.89 m 0.30 m, has a current velocity temperature (°C) 19.3 ( 0.8 of approximately 0.1 m/s in summer, and approximately 50% dissolved oxygen (mg/L) 8.6 ( 0.4 of its surface is covered with emergent vegetation. It has a pH 7.1 ( 0.1 total length of approximately 1.8 km and flows from a dam orthophosphate (mg/L) 0.16 ( 0.02 through a forest area for 800 m and then into pasture land nitrate (mg/L) 3.6 ( 0.4 for 400 m before flowing through the orchard area for a further 600 m. Average discharge in the tributary is 0.03 m3/s in January and 0.32 m3/s in July. The discharge during the study 45, 85, and 110 m from the inlet were used as sampling period was 0.043 ( 0.002 m3/s; all studies were done during stations in the wetland to ensure that sampling would not the dry summer season. cause unnecessary damage to the macrophytes and/or The retention of spray drift-borne AZP in the wetland sediments (Figure 1). An additional seventh site, representing was derived from measurements undertaken in January and the spray drift deposition site, was located in the tributary February 2000 and 2001 during the dry summer periods with about 200 m upstream of the wetland inlet. average monthly rainfall <25 mm. A detailed study of the Discharge of the tributary was calculated, on the basis of fate and transport of AZP was done on February 5, 2001, standard formulas (20), from velocity measurements along during the last pesticide application of the spraying season cross-sectional profiles. Total suspended solids (TSS; (0.1 2000/2001, and the resulting biological effects were monitored mg/L) were measured using a turbidity meter (Dr. Lange, until May 17, 2001. All studies were done while AZP was Duesseldorf, Germany). Turbidity readings were calibrated being applied in the conventional manner to fruit orchards as described by Gippel (22). Temperature ((0.1 °C), dissolved bordering the tributary, which flows through the wetland. oxygen ((0.1 mg/L), and pH ((0.1) were measured with Such application resulted in spray deposition into the portable electronic meters (WTW, Weilheim, Germany).