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TECHNICAL REPORTS

Organic Compounds in the Environment

A Model for Linking the Effects of Parathion in Soil to its Degradation and Bioavailability Kinetics

^a Institut National de la Recherche Agronomique (INRA), CSE Bat. Sol, Site Agroparc, 84914 Avignon cedex 09, France
^b Département "Environnement et Agronomie", Institut National de la Recherche Agronomique (INRA), Bat. CSE-Sol, Site Agroparc, 84914 Avignon cedex 09, France
^c Institut National Agronomique de Paris-Grignon, EGC-sol, BP: 01, 78850 Thiverval-Grignon, France

^* Corresponding author (Laurent.Bruckler{at}avignon.inra.fr)

Received for publication February 17, 2005. Parathion is an insecticide of a group of highly toxic organophosphorus compounds. To investigate the dissipation and toxicological impact of parathion [O,O-diethyl O-(4-nitrophenyl) phosphorothioate] and its highly toxic metabolite, paraoxon, soil laboratory experiments were conducted in columns during a 19-d experiment under variably saturated conditions. Water and pesticide transport, sorption, and biodegradation of parathion were measured in three soil pools (soluble phase, weakly and strongly sorbed phases) using C-labeled pesticide. The effects of parathion and its metabolite on the mobility of soil nematodes were observed and then modeled with an effective variable, which combined pesticide concentration and time of application. Results showed that parathion was highly sorbed and slowly degraded to a mixture of metabolites. The parent compound and its metabolites remained located in the top 0.06-m soil layer. A kinetic model describing the sorption, biodegradation, and allocation into different soil pools of parathion and its metabolites was coupled with heat and water transport equations to predict the fate of parathion in soil. Simulated results were in agreement with experimental data, showing that the products remained in the upper soil layers even in the case of long-term (11-mo) simulation. The strongly sorbed fraction may be regarded as a pesticide reservoir that regularly provides pesticide to the weakly sorbed phase, and then, liquid phase, respectively. From both modeling and observations, no major toxicological damage of parathion and paraoxon to soil nematodes was found, although some effects on nematodes were possible, but at the soil surface only (0.01- and 0.02-m depth).

Abbreviations: EC₅₀, effective concentration that inhibits the mobility of 50% nematodes • HPLC, high performance liquid chromatography • PASTIS, Prediction of Agricultural Solute Transfer in Soil

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