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

Abstract

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).

Propanil in a Manitoba soil: an interactive spreadsheet model based on conventional chemical kinetics

“…Sparks has emphasized the importance of the kinetics and mechanisms. 7 Consistent with these other reports, [6][7][8][9][10][11][12][13][14][15][16][17][18] the experimental curves in Fig. 1 show that sorption, desorption, and chemical reaction were all strongly dynamic.…”

“…7 Several authors have reported the dynamic nature of pesticide behaviour in soils. [8][9][10][11][12][13][14][15][16][17][18] They all reported that the pesticide interactions with soils changed with time during their experiments, so that equilibrium calculations would not have been valid. Because independent studies with different combinations of pesticides and soils were used, the phenomenon is evidently quite general.…”

Impact of Basic Soil Parameters on Pesticide Disappearance Investigated by Multivariate Partial Least Square Regression and Statistics

“…For example, variability of herbicide dissipation for the pesticide isoproturon has been related to sorption to clay (Alletto et al, 2006) and extractability (Charnay et al, 2005), and several studies have demonstrated interactive effects between organic matter content, microbial degradation, and sorption processes (Bollag et al, 1992; Di et al, 1998; Madsen et al, 2000). Also, several publications provide a descriptive approach to the effect on pesticide degradation, i.e., bioavailability (Shelton and Doherty, 1997; Reid et al, 2000; Guo et al, 2000; Delle Site, 2001; Saffih‐Hdadi et al, 2006). In contrast, it has recently been suggested that the decrease in pesticide biodegradation with soil depth may be caused by an increase in the lag phase of the microbial degradation process rather than a decrease in bioavailability (Bending and Rodriguez‐Cruz, 2007).…”

Ability of the MACRO Model to Predict Long-Term Leaching of Metribuzin and Diketometribuzin

“…As pesticides and metabolites are increasingly detected in groundwater (4), it is imperative to evaluate the ability of these environmental fate models to predict the long-term leaching of pesticides and especially of their metabolites. However, such studies have not hitherto been reported in the literature (5).…”