Runoff-Related Endosulfan Contamination and Aquatic Macroinvertebrate Response in Rural Basins Near Buenos Aires, Argentina S. Jergentz, 1 H. Mugni, 2 C. Bonetto, 2 R. Schulz 1 1 Zoological Institute, Technical University, Fasanenstrasse 3, D-38092 Braunschweig, Germany 2 Instituto de Limnologı ´a Dr. Ringuelet, Av. Calchaqui km 23,5, 1888 Florencio Varela, Buenos Aires, Argentina Received: 15 January 2003 /Accepted: 31 August 2003 Abstract. Information in the open literature about the fate and effects of pesticides in small streams from agricultural areas of Argentina is very rare. The objective of the present work was to study the pesticide contamination and potential biological effects in basins that have undergone intense agricultural ac- tivity, mainly related to the cultivation of soybeans. Three streams (Maguire, Helves, and Horqueta) with a low-flow discharge (0.1 and 0.2 m 3 /s) in March close to the city of Arrecifes were studied during the period of maximum insecti- cide application, between February and April 2001. Various sampling devices were installed to trap suspended particles, runoff, and floodwater plus sediment throughout the study period. The suspended-particle samples were analyzed for the insecticides endosulfan (END), chlorpyrifos, and cyper- methrin. Water chemistry and the macroinvertebrate commu- nities were assessed on four occasions and the organismic drift was measured continuously. Following a 184-mm rainfall on March 1, 2001, -endosulfan concentrations up to 318 and 43 g/kg were measured from suspended-particle samples from Horqueta and Helves, respectively. No END contamination was detected in Maguire. Chlorpyrifos and cypermethrin were not detected in any of the streams. A significant decrease in the average macroinvertebrate species density was observed in Horqueta (from 12.8  0.5 to 9  0.7 species; ANOVA, p  0.05) and Helves (from 10.8  1.7 to 3.3  1.3 species; p  0.001) following the same rainfall event at the beginning of March, while the species density in Maguire remained constant at 7.9  0.3 species. The runoff primarily reduced species abundances of Odonata and Ephemeroptera significantly (p  0.01) in Horqueta and Helves but not in Maguire. A greater drift of Smicridae (Trichoptera) and Ephemeroptera occurred in Helves and Horqueta during this runoff event, while no changes in the macroinvertebrate drift were detectable in Maguire. This study highlights the potential pesticide effects on macroinvertebrate communities in Argentinian rural streams. It is suggested that a small wetland area formed by Maguire between the agriculturally used catchment and the sampling site contributes to the absence of contamination and effects at this site. Soybean production in the province of Buenos Aires has in- creased notably, from 2.8 million tons in 1991 to 3.9 million tons in 1998 (SAGPyA 2002). The rapid implementation of new technological developments, such as the utilization of a transgenic soybean resistant to glyphosate and the direct seed modality, has contributed to elaboration of soybean and related products (oil, flours, pellets) into the main Argentine export, earning about $5 billion U.S. (Cları ´n 2002). Increased soybean production was closely followed by increased pesticide con- sumption, which has grown from 39 million to 124 million kg between 1991 and 1997 (Pengue 2000). The study area for this investigation is located in the main region of soybean produc- tion in the province of Buenos Aires. Most of the insecticides currently in use are pyrethroids, which are recently preferred to organochlorine or organophosphate pesticides because of their comparatively low toxicity to humans and their shorter half- lives in the field. Fifty percent of the total amount of insecti- cides used in the soybean area of Buenos Aires is accounted for by the pyrethroid cypermethrin, followed by the organophos- phate chlorpyrifos, the pyrethroid deltamethrin, and the or- ganochlorine endosulfan. Pesticides are freely sold to farmers in rural areas, and management practices performed by the farmers are difficult to supervise. The comparatively large size of the fields makes airplane fumigation very common within the area, which might cause direct pesticide input into surface waters (Ernst et al. 1991). However, the study region is also characterized by short and heavy rainfall events in summer with 1- or 2-week dry periods in between. Thus, rainfall-induced surface runoff from agricul- tural fields may be another important nonpoint route of pesti- cides into surface waters (Wauchope 1978; Willis and McDow- ell 1982; Schulz et al. 1998). It has been reported in various studies that pesticides in surface runoff are often associated with suspended particles (House et al. 1992; Mian and Mulla 1992), and even single runoff events have been found to transport large amounts of insecticides into surface waters (Domagalski et al. 1997; Correspondence to: S. Jergentz, Technical University Braunschweig, Zoological Institut, Fasanenstrasse 3, D-38092 Braunschweig, Ger- many; email: s.jergentz@tu-bs.de Arch. Environ. Contam. Toxicol. 46, 345–352 (2004) DOI: 10.1007/s00244-003-2169-8 ARCHIVES OF Environmental Contamination and T oxicology © 2004 Springer-Verlag New York Inc.