The abiotic hydrolysis of the organophosphorus insecticide chlorpyrifos was examined in 37 different soils, which were chosen to represent a wide variety of physicochemical characteristics (e.g., pH 3.8-8.5). Samples of soil were sterilized via γ-irradiation, treated with [ 14 C]chlorpyrifos at 10 µg/ g, and incubated under standardized conditions (25 °C, field moisture capacity, darkness) for up to 4 months. Chlorpyrifos hydrolysis proceeded at a slow rate (<0.008 day -1 ) in acidic soils (pH e 7). In alkaline soils, however, hydrolytic rate constants varied greatly (0.004-0.063 day -1 ). Corresponding hydrolytic half-lives for acidic and alkaline soils ranged from 92 to 341 and 11 to 200 days, respectively. Correlation analyses indicated that soil pH was the independent variable displaying the strongest association with hydrolytic rate constant (r ) 0.55), but multiple regression models based on combinations of this parameter with other soil properties, including phosphatase enzyme activities, did not offer strongly predictive models for explaining the variability in kinetics observed (best fit r 2 ) 0.59). Incubation of chlorpyrifos with both sterile and nonsterile soils revealed that although both microbial and hydrolytic mechanisms contributed to chlorpyrifos degradation in all soils, there were clearly soils in which hydrolysis constituted the major route of degradation. Chlorpyrifos hydrolysis was greatly accelerated under low moisture conditions, both in acidic and alkaline soils. Additional experiments in several soils that displayed rapid chlorpyrifos hydrolysis at 10 µg/g provided evidence that the hydrolytic reaction was inhibited at higher concentration (1000 µg/g). Results highlight the importance but also the complex nature of the hydrolytic breakdown of chlorpyrifos in soil. Under certain conditions (e.g., some alkaline soils, air-dry soils) hydrolysis may be the driving factor modulating chlorpyrifos persistence.

Chlorpyrifos [O,O‐diethyl O‐(3,5,6‐trichloro‐2‐pyridyl) phosphorothioate] is an organophosphorus insecticide applied to soil to control pests both in agricultural and in urban developments. Typical agricultural soil applications (0.56 to 5.6 kg ha−1) result in initial soil surface residues of 0.3 to 32 μg g−1. In contrast, termiticidal soil barrier treatments, a common urban use pattern, often result in initial soil residues of 1000 μg g−1 or greater. The purpose of the present investigation was to understand better the degradation of chlorpyrifos in soil at termiticidal application rates and factors affecting its behaviour. Therefore, studies with [14C]chlorpyrifos were conducted under a variety of conditions in the laboratory. Initially, the degradation of chlorpyrifos at 1000 μg g−1 initial concentration was examined in five different soils from termite‐infested regions (Arizona, Florida, Hawaii, Texas) under standard conditions (25°C, field moisture capacity, darkness). Degradation half‐lives in these soils ranged from 175 to 1576 days. The major metabolite formed in chlorpyrifos‐treated soils was 3,5,6‐trichloro‐2‐pyrid‐inol, which represented up to 61% of applied radiocarbon after 13 months of incubation. Minor quantities of [14C]carbon dioxide (< 5%) and soil‐bound residues (⩽ 12%) were also present at that time. Subsequently, a factorial experiment examining chlorpyrifos degradation as affected by initial concentration (10, 100, 1000 μg g−1), soil moisture (field moisture capacity, 1.5 MPa, air dry), and temperature 15, 25, 35°C) was conducted in the two soils which had displayed the most (Texas) and least (Florida) rapid rates of degradation. Chlorpyrifos degradation was significantly retarded at the 1000 μg g−1 rate as compared to the 10 μg g−1 rate. Temperature also had a dramatic effect on degradation rate, which approximately doubled with each 10°C increase in temperature. Results suggest that the extended (3–24 + years) termiticidal efficacy of chlorpyrifos observed in the field may be due both to the high initial concentrations employed (termite LC 50 = 0.2– 2 μg g−1) and the extended persistence which results from employment of these rates. The study also highlights the importance of investigating the behaviour of a pesticide under the diversity of agricultural and urban use scenarios in which it is employed.

Degradation of the sulfonanilide herbicide diclosulam was studied on nine soils from three countries to determine the rates and products of aerobic metabolism. Diclosulam was applied to four agricultural soils from the United States, three from Argentina, and two from Brazil at a rate of 0.1 ppm, equivalent to approximately twice the maximum field application rate of 52 g of active ingredient/ha. U.S. and Brazilian soils were incubated in the dark at 25 degrees C at 75% 0.3 bar moisture; Argentinean soils were incubated in the dark at 20 degrees C and 45% moisture holding capacity. Samples were analyzed up to one year after treatment. Two-compartment DT(50) and DT(90) values averaged 28 +/- 12 and 190 +/- 91 days, respectively. Three soil metabolites reached levels of >10% of applied in at least one soil and were identified as the 5-hydroxy analogue of diclosulam (5-OH-diclosulam), aminosulfonyl triazolopyrimidine (ASTP), and the 8-chloro-5-hydroxy analogue of diclosulam (8-Cl-diclosulam). The terminal products of diclosulam soil metabolism were mineralization to CO(2) and bound soil residues. Apparent sorption coefficients (K(d)) were determined on a subset of samples by extraction with a 0. 01 M CaCl(2) solution followed by an acidified acetone extraction. Initial sorption coefficients were similar to those obtained in a batch equilibrium study and averaged 1.1 L/kg for the six soils tested. K(d) coefficients for the metabolites, when available, tended to be slightly lower than that for diclosulam. Sorptivity of diclosulam and degradates increased with time.

The rate and pathway of degradation in the presence of light for the triazolopyrimidine herbicide florasulam was determined on soil and in aqueous systems. Florasulam was exposed to natural sunlight for up to 32 days; solar irradiance was measured with either chemical actinometers or by radiometry. The quantum yield for direct photodegradation in a sterile, buffered aqueous solution was determined to be 0.096; an analogous quantum yield for the sum of direct and indirect photodegradation on soil was 0.245. The quantum yields were used to estimate half-lives due to photodegradation as a function of season and temperature. Estimated half-lives due to photodegradation in summer at 40 degrees N latitude were 14 days on soil and 36 days in sterile, buffered water. Photodegradation was much faster in a natural water system, with a measured half-life of 3.3 days in summer at 51.5 degrees N latitude, indicating that indirect photolytic processes will be important contributors to photodegradation of florasulam in aqueous environments.

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