Evaluation of soil temperature effect on herbicide leaching potential into groundwater in the Brazilian Cerrado Lourival Costa Para  ıba a, * , Antonio Luiz Cerdeira a , Enio Fraga da Silva b , Jo~ ao Souza Martins b , Heitor Luiz da Costa Coutinho b a Embrapa Meio Ambiente, Embrapa Environment, Jaguari  una, CP 69, 13820-000 SP, Brazil b Embrapa Solos, Embrapa Soils, Rua Jardim Bot ^ anico, 1024 Rio de Janeiro, 22460-000 RJ, Brazil Received 22 January 2003; received in revised form 4 June 2003; accepted 11 June 2003 Abstract The effect of annual variations in the daily average soil temperatures, at different depths, on the calculation of pesticide leaching potential indices is presented. This index can be applied to assess the risk of groundwater contam- ination by a pesticide. It considers the effects of water table depth, daily recharge net rate, pesticide sorption coefficient, and degradation rate of the pesticide in the soil. The leaching potential index is frequently used as a screening indicator in pesticide groundwater contamination studies, and the temperature effect involved in its calculation is usually not considered. It is well known that soil temperature affects pesticide degradation rates, air–water partition coefficient, and water–soil partition coefficient. These three parameters are components of the attenuation and retardation factors, as well as the leaching potential index, and contribute to determine pesticide behavior in the environment. The Arrhenius, vanÕt Hoff, and Clausius–Clapeyron equations were used in this work to estimate the soil temperature effect on pesticide degradation rate, air–water partition coefficient, and water–soil partition coefficient, respectively. The relationship between leaching potential index and soil temperature at different depths is presented and aids in the understanding of how potential pesticide groundwater contamination varies on different climatic conditions. Numerical results will be presented for 31 herbicides known to be used in corn and soybean crops grown on the municipality of S~ ao Gabriel do Oeste, Mato Grosso do Sul State, Brazil. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Herbicide; Groundwater; Leaching index; Temperature; Cerrado 1. Introduction The protection and conservation of the quality of groundwater resources are an essential part of a sus- tainable agricultural production systems. Thus, the im- pact of pesticides on groundwater quality has been the subject of scientific and public health concerns world- wide, especially in areas where groundwater is mainly used for human consumption. Only in the last decade has pesticide leaching to groundwater been considered an important nonpoint source of agricultural pollution. Over a decade ago, it was assumed that pesticides would not leach to underlying groundwater (Bouwer, 1990). However, groundwater quality surveys conducted during this decade have revealed contamination of some USA aquifers with both inorganic and organic compounds, several of them are pesticides (Williams et al., 1988). Chemosphere 53 (2003) 1087–1095 www.elsevier.com/locate/chemosphere * Corre sponding author. Tel.: +55-19-386-78767; fax: +55- 19-38678740. E-mail addresses: lourival@cnpma.embrapa.br (L.C. Para-  ıba), cerdeira@cnpma.embrapa.br (A.L. Cerdeira), enio@ cnps.embrapa.br (E.F. da Silva), martins@cnps.embrapa.br (J.S. Martins), heitor@cnps.embrapa.br (H.L. da Costa Cout- inho). 0045-6535/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0045-6535(03)00594-0