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TECHNICAL REPORT
Organic Compounds in the Environment

Degradation of Soil Fumigants as Affected by Initial Concentration and Temperature

^a USDA-ARS, Soil Physics and Pesticides Research Unit, George E. Brown Jr. Salinity Lab., Riverside, CA 92507
^b Univ. of California, Riverside, CA 92521

* Corresponding author (jgan{at}UCRAC1.UCR.EDU)

Received for publication June 26, 2000. Soil fumigation using shank injection creates high fumigant concentration gradients in soil from the injection point to the soil surface. A temperature gradient also exists along the soil profile. We studied the degradation of methyl isothiocyanate (MITC) and 1,3-dichloropropene (1,3-D) in an Arlington sandy loam (coarse-loamy, mixed, thermic Haplic Durixeralf) at four temperatures and four initial concentrations. We then tested the applicability of first-order, half-order, and second-order kinetics, and the Michaelis–Menten model for describing fumigant degradation as affected by temperature and initial concentration. Overall, none of the models adequately described the degradation of MITC and 1,3-D isomers over the range of the initial concentrations. First-order and half-order kinetics adequately described the degradation of MITC and 1,3-D isomers at each initial concentration, with the correlation coefficients greater than 0.78 (r² > 0.78). However, the derived rate constant was dependent on the initial concentration. The first-order rate constants varied between 6 and 10x for MITC for the concentration range of 3 to 140 mg kg^-1, and between 1.5 and 4x for 1,3-D isomers for the concentration range of 0.6 to 60 mg kg^-1, depending on temperature. For the same initial concentration range, the variation in the half-order rate constants was between 1.4 and 1.7x for MITC and between 3.1 and 6.1x for 1,3-D isomers, depending on temperature. Second-order kinetics and the Michaelis–Menten model did not satisfactorily describe the degradation at all initial concentrations. The degradation of MITC and 1,3-D was primarily biodegradation, which was affected by temperature between 20 and 40°C, following the Arrhenius equation (r² > 0.74).

Abbreviations: 1,3-D, 1,3-dichloropropene • GC, gas chromatography • MITC, methyl isothiocyanate

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