2,4-D

CAS RN:94-75-7

Environmental Fate

TERRESTRIAL FATE: Based on a classification scheme(1), a Koc range of 20-136(2-4) indicates that 2,4-D is expected to have very high to high mobility in soil(SRC). The pKa of 2,4-D is 2.73(5), indicating that this compound will exist almost entirely in the anion form in the environment and anions generally do not adsorb more strongly to soils containing organic carbon and clay than their neutral counterparts(6). Volatilization of 2,4-D from moist soil surfaces is not expected to be an important fate process(SRC) given a Henry's Law constant of 9.75X10-8 atm-cu m/mole(5). 2,4-D is not expected to volatilize from dry soil surfaces(SRC) based upon a vapor pressure of 1.40X10-7 mm Hg at 25 deg C(5). Biodegradation is an important environmental fate process for 2,4-D in most soils, leading to various hydroxylic aromatic products(7-9). The rate of degradation is affected by the conditions, especially the concentrations of 2,4-D and water content temperature and the organic content of soil and the status of pre-exposure of the soil to 2,4-D or its salts or esters(10,11).

TERRESTRIAL FATE: Laboratory studies were conducted to determine the adsorption, desorption, hydrolysis, and breakdown of commercially formulated isooctyl ester and dimethylamine salt of 2,4-D in a Naff silt loam soil(1). More 2,4-D was adsorbed to the surface soil than to soil at lower depths, and the percentage of 2,4-D adsorbed decreased as the total amount of 2,4-D present increased(1). Adsorbed 2,4-D was gradually desorbed from soil by successively exchanging the solution in equilibrium with soil with distilled water(1). Formulated 2,4-D isooctyl ester applied to moist soil underwent hydrolysis to the anionic form at a rapid rate, with 80% of the ester hydrolyzed in 72 hr(1). High amounts of 2,4-D in runoff (sediment and water) retarded the active degradation of carboxyl 14C 2,4-D when 2,4-D was incubated in runoff from a wheat field treated with various formulations and rates of 2,4-D(1). At the end of the 10 wk of incubation in runoff or in soil, only 1% of the 14C 2,4-D originally applied to the soil could be identified as 2,4-D(1). In another study, the degradation kinetics of 14C-labeled 2,4-D were studied in a number of soils(2). Degradation rates in soils were not simple first order but generally increased until approx 20% of chemical remained, after which they declined(2). Average 50% decomposition time of 4.0 days was observed for 2,4-D(2). It was observed in a Riverside, CA study that 2,4-D soil half-lives were 20 times longer (half-life = 30.7 days) in soil under trees than in soil plated with turf grass (half-life = 1.6 days). The soil was a Hanford fine sandy loam; plants included Bradford pear (Pyrus calleryana), fescue grass (Fetuca arundinacea), mulches, and spring cinquefoil (Potentilla tabernaemontani). Mulch and groundcover soil surface half-lives were 3.7 and 3.9 days, respectively; 32.3 and 9.2 days, respectively, for subsurface (10-30 cm)(3).

FIELD STUDY: Soil persistence and lateral movement of 2,4-D (2,4-dichlorophenoxy acetic acid) and picloram (4-amino-3,5,6-trichloropicolinic acid) were examined following their application as a stem-foliage spray for brush control on two power line rights-of-way. Ditches to collect runoff water were located 3, 10, 20, and 30 m downslope from the treated areas. Runoff water and soil samples were collected after 0.14, 0.43, 0.57, 1, 2, 4, 7, 8, 11, 15, 16, and 48 weeks and were analyzed for picloram and 2,4-D residues. Only 3 of 85 soil samples downslope from the target areas contained residues of 2,4-D, and only 1 of 85 down slope samples contained a detectable residue of picloram. Of 56 runoff water samples, only 11 contained 2,4-D residues and only 1 contained residues of picloram. The greatest distances down-slope at which residues were detected in runoff water were 20 and 10 m for 2,4-D and picloram, respectively. No residues of either herbicide were recovered in soil or water at 15 weeks or 48 weeks after spraying. Despite normal rainfall frequency and amounts in the first several weeks after spraying in mid-June, significant runoff of either herbicide was not evident at either study site.

FIELD STUDY: In long-term degradation studies of massive quantities of 2,4-D and 2,4,5-T in test grids, field plots and herbicide storage areas were carried out. The method of herbicide application had significant impact on the amount applied per unit area and hence on residue persistence: spills > or = soil incorporation > aerial application. 2,4,5-T was more persistent in the soil than 2,4-D. The formulation of the herbicide also had significant impact on its persistence: isooctyl ester > butyl ester > acid. The addition of coconut charcoal increases persistence of the phenoxy herbicide residues, especially residues of 2,4,5-T. The appearance of dichlorophenol and trichlorophenol in soils treated with 2,4-D and 2,4,5-T suggests that they are degradation products of the herbicides. A massive concentration of herbicides does not sterilize the soils. Apparently, microbial populations respond both quantitatively and qualitatively to the presence of high concentrations of herbicides and may play an important role in their degradation.

AQUATIC FATE: Based on a classification scheme(1), Koc values ranging from 20 to 136(2-4) indicate that 2,4-D is not expected to adsorb to suspended solids and sediment(SRC). Volatilization from water surfaces is not expected(5) based upon a Henry's Law constant of 9.75X10-8 atm-cu m/mole(6). According to a classification scheme(7), an estimated BCF of 3(SRC), from its log Kow of 2.81(8) and a regression-derived equation(9), suggests the potential for bioconcentration in aquatic organisms is low(SRC). Half-lives of 2-4 days were reported for 2,4-D photolysis in water solution irradiated at 356 nm(10). When released to water, 2,4-D will tend to biodegrade with the rate dependent upon level of nutrients present, temperature, availability of oxygen, and whether or not the water has a prior history of contamination by 2,4-D or other phenoxyacetic acids(11). Typical half-lives of 10 to >50 days have been reported with longer half-lives expected in oligotrophic waters and where a high concentration of 2,4-D is present(11,12).

AQUATIC FATE: Persistence in aquatic systems depends on the water type, organic particulate matter, rain, sunlight, temperature, microbial degradation, volatilization, and oxygen content of the water. Accumulation in bottom sediments may also be a factor, but in general, not for the phenoxys. Microbial activity is the major means for detoxification of the phenoxys in soils, but is relatively unimportant in natural waters, but dominates in bottom mud sediments and in sludge.

FIELD STUDY: Herbicide products containing 2,4-D had been applied to catchments of two wetlands within the Manitoba, Canada Zero Till Research Farm at recommend label rates for seven years prior to a study in 2008. The half-life in the water column was 12 days in the ephemeral wetland and 13 days in the semi-permanent wetland(1). 2,4-D was detected in wetland sediments up to 77 days following wetland treatment in both systems(1). Samples from seven TVA (Tennessee Valley Authority) reservoirs were all positive following herbicide application to control European watermilfoil (Myriophyllum specatum L.); the concentration was 0.95-56 ppm measured 96 hr after application and 0.24-58 ppm measured 10 months after application of the butoxyethanol ester in the Watts Bar Reservoir; the concentration of the control was 0.14 ppm (control), 0.14-33.6 ppm (42 days later), 0.3-0.49 (9 months later) - Guntersville Reservoir(2). The concentration of 2,4-D residues in sediments of Florida and Georgia ponds were 0.6 ppm one day after treatment and ranged from trace to non-detectable 56 days after treatment(3). At the Loxahatchee National Wildlife Refuge, FL, the peak level of 2,4-D was 0.005 ppm which occurred 3-15 days after surface water application in 1971 to over 7000 of 4.48 kg/ha acid equivalent(4). Concentrations of 2,4-D in Guntersville Reservoir (TVA) at 1, 14, 28, 60, 120 and 180 days after application of 20-40 lb/acre acid equivalent were 0.11, 0.25, 0.11, 0.30, 0.25 and less than 0.1 mg/kg respectively(5).

ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), 2,4-D, which has a vapor pressure of 1.40X10-7 mm Hg at 25 deg C(2), will exist in both the vapor and particulate phases in the ambient atmosphere. Vapor-phase 2,4-D is degraded in the atmosphere by reaction with photochemically-produced hydroxyl radicals(SRC); the half-life for this reaction in air is estimated to be 19 hrs(SRC), calculated from its rate constant of 6.6X10-12 cu cm/molecule-sec at 25 deg C(SRC) that was derived using a structure estimation method(3). Particulate-phase 2,4-D may be removed from the air by wet and dry deposition(SRC). No data were available concerning direct photolysis of 2,4-D in the atmosphere(SRC). In aqueous media, phenoxy herbicides have ultraviolet maxima in the 280-290 nm range, suggesting that 2,4-D may be susceptible to direct photolysis(4).

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