Environmental fate of trifluralin.

Abstract

Trifluralin, a preemergence, soil-applied and soil-incorporated herbicide, has been in agricultural use since 1963. The environmental chemistry and fate of dinitroaniline herbicides, including trifluralin, has been studied extensively in agricultural soils. Probst et al. (1975) and Helling (1976) have summarized pre-1975 data on the mobility, persistence, and degradation or metabolism of dinitroaniline herbicides as a group. Since then, numerous studies have been carried out on the fate of dinitroanilines, especially trifluralin, in the environment to understand further their degradation in soil, potential for mobility and persistence, and environmental concentration in water and air. The present review, while summarizing briefly earlier data, concentrates primarily on the post-1975 data on degradation, mobility, and persistence of trifluralin in soils and its potential concentrations in water and air. Trifluralin is readily degraded under sunlight in all media, with half-lives (t1/2) of minutes to several months, depending on the substrate. In addition, other dissipation processes, such as microbial and chemical, are also operative in soils, water, and sediments. Several degradation products of trifluralin have been identified and characterized, both under photolysis and following aerobic and anaerobic metabolism in soils and water-sediment systems. The differences between various degradative pathways of trifluralin appear to be more quantitative than qualitative in nature, leading eventually to the same end products that are subject to binding or mineralization with time. The general lack of accumulation of the breakdown products of trifluralin suggests that these are also subject to the same degradative mechanisms as the parent compound. Trifluralin has low water solubility and is strongly bound to soil components; mean Koc values range from 4,000 to 13,000. Once applied and incorporated into the soil, trifluralin remains relatively immobile with minimal or no potential for contamination of groundwaters under or near the treated zones. Trifluralin residues in soil surface layers are subject to loss via transport in runoff water or volatilization into the air. Seasonal losses in surface runoff are consistently less than 0.5% of the amounts applied, with concentrations in edge-of-the-field run-off water typically < 1.0 microgram L-1. Consequently, trifluralin is infrequently detected in surface waters and, if present, usually occurs below levels of quantification. Seasonal trifluralin losses into the atmosphere can be as high as 25% of that applied. Maximum trifluralin residues in the air above treated fields are in the 2-3 micrograms m-3 range following application, decreasing to < 100 ng m-3 in ambient air of intensive use areas, indicating its rapid dissipation in air. Trifluralin residues at < 100 pg m-3 in the atmosphere of remote nonuse regions have been reported, suggesting its potential for long-range transport. However, there is a general lack of understanding of the mechanisms controlling its potential for long-distance transport, especially considering its rapid photodegradation in vapor and solution states. The persistence of trifluralin in agricultural soils following incorporation is highly variable, depending on several factors such as depth of incorporation, soil moisture, soil temperature, soil air, and soil organic matter content. Estimated half-lives under a variety of agronomic conditions range from 25 to > 201 d, thus categorizing its persistence from 'moderate' to 'persistent'. The estimated half-life data for trifluralin under agronomic conditions, however, cannot be extrapolated to other potential scenarios, such as its dissipation in nontarget areas where trifluralin residues, if any, are essentially deposited on surfaces. Surface deposits on nontarget areas, unlike soil-incorporated residues, would be subject to volatilization and photolysis and thus more short lived. (ABSTRACT TRUNCATED)