Fate and Behavior

Degradation Pathway
Fate in Soil
Fate in Aquatic Environments
Plant Fate and Metabolism
Aquatic Organism Bioaccumulation and Metabolism
Terrestrial Animal Fate and Metabolism

Degradation Pathway

Chlorpyrifos is a degradable compound, and a number of environmental forces may be active in its breakdown. In all systems (soil, water, plants and animals), the major pathway of degradation begins with cleavage of the phosphorus ester bond to yield 3,5,6-trichloro-2-pyridinol (TCP). This first step is a detoxification, as TCP has no insecticidal activity and is considered toxicologically insignificant by regulatory authorities. In soil and water, TCP is further degraded via microbial activity and photolysis to carbon dioxide and organic matter. In animals, TCP may be excreted directly or following conjugation; in plants TCP conjugates are sequestered.

Fate in Soil

Chlorpyrifos may enter the soil environment by direct application or through spray drift/foliar washoff. Laboratory soil studies demonstrate that chlorpyrifos is only moderately persistent and binds strongly to soil particles, preventing leaching to ground water.

When applied at normal agricultural rates, typical aerobic soil degradation half-lives are on the order of one to two months. Both microbial degradation and abiotic degradation (i.e., hydrolysis) are important factors in its dissipation from soil, with the latter being especially predominant in alkaline soils.

Chlorpyrifos has a soil adsorption coefficient (K_oc) of greater than 5000, and so
exhibits a strong tendency to be adsorbed by soil and soil organic matter. This places chlorpyrifos in the "immobile" leaching category and field-testing has confirmed the negligible downward mobility of chlorpyrifos. The strong adsorption to soil, together with the rapid degradation, results in limited surface runoff potential in agricultural settings. Large-scale field runoff studies have confirmed that even under relatively severe conditions (heavy rainstorms closely following application), generally less than 1 percent of the applied chlorpyrifos can move off the edge of treated fields through runoff water and eroding soil particles.

Fate in Aquatic Environments

Laboratory studies on the fate of chlorpyrifos in pure water indicate that hydrolysis and photolysis occur at moderate rates under neutral conditions. Hydrolytic and photolytic half-lives are both around a month, at neutral pH 25° C. Under more alkaline conditions, hydrolysis proceeds more rapidly, with half-lives of around two weeks observed at pH 9. In natural water samples, however, degradation often proceeds significantly faster; a 16-fold enhancement of hydrolysis rate has been observed in pond and canal water samples. Results of field-testing conducted in aquatic ecosystems corroborate the rapid dissipation of chlorpyrifos from natural waters. Half-lives in the water column of less than one day are typical, due to a combination of degradation, volatilization and partitioning into sediments. Dissipation rates in sediment are similar to those observed in soil.

Plant Fate and Metabolism

Results of greenhouse and field research studies show no tendency for chlorpyrifos soil residues to be taken up into plants through growing plant roots; chlorpyrifos is non-systemic in nature. On the surface of plant foliage, rapid dissipation of residue occurs. The most important route of dissipation is volatility, with photodegradation being somewhat less important. Typical foliar dissipation half-lives of 1 to 7 days have been observed. Dissipation rates from turfgrass and thatch are often slightly longer, with 7 to 10 day half-lives most common. Following foliar application, small quantities of chlorpyrifos are absorbed into plant tissue and are readily metabolized/detoxified into TCP conjugates.

From the standpoint of biological availability, dislodgeable foliar residues of chlorpyrifos comprise a rather small proportion of the total residue present and decline even more rapidly than total residues. Dislodgeable residues typically represent less than 10 percent of total residues, and half-lives of 0.5 to 3 days are common.

Aquatic Organism Bioaccumulation and Metabolism

Trace amounts of chlorpyrifos can be absorbed from surrounding water by fish and other aquatic organisms. Reported aquatic bioconcentration factors (BCF) for fish have ranged from 100 to 5,100, which reflects a propensity for chlorpyrifos to partition from water into tissues. Importantly, the residues absorbed by fish and other aquatic organisms are rapidly detoxified and excreted and, as a result, chlorpyrifos is not a bioaccumulative compound and does not increase in concentration by transfer up the food chain. Elimination half-lives of 0.6 to 3.4 days have been observed in fish and of 1.6 to 2.2 days for oysters. Fish biotransform chlorpyrifos to a variety of metabolites, including TCP and glucuronide conjugates of TCP. The TCP and conjugates formed are then excreted into the surrounding water.

Terrestrial Animal Fate and Metabolism

Chlorpyrifos is readily taken up from food, and absorption efficiencies of 41 to 72 percent have been observed. Dermal uptake of chlorpyrifos, however, has been found to vary significantly across species. Absorption efficiencies in mammals via the dermal route have been observed as low as 1 percent for man and as high as 60 percent for rats. In contrast, cuticular uptake by target insects may approach 90 percent efficiency. Mammals, birds and insects readily transform and metabolize chlorpyrifos. The primary metabolite, TCP, is readily excreted itself or in its conjugated form. Chlorpyrifos metabolism in animals is a dynamic process, and this is exemplified by the observed elimination half-life of 17 hours in the rat following an oral dose.

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