Half-life In Soil Research Articles

Soil photolysis of herbicides in a moisture- and temperature-controlled environment.

The problem of maintaining the moisture content of samples throughout the course of a soil photolysis study is addressed. The photolytic degradations of asulam, triclopyr, acifluorfen, and atrazine were independently compared in air-dried soils and in moist (75% field moisture capacity at 0.33 bar) soils maintained at initial conditions through the use of a specially designed soil photolysis apparatus. Each pesticide was applied at 5 microg/g. The exposure phase extended from 144 to 360 h, depending on the half-life of the compound. A dark control study, also using moist and air-dried soils, was performed concurrently at 25 degrees C. The results showed significant differences in half-life. The dissipations generally demonstrated a strong dependence on moisture. In most cases, photolytic degradation on air-dried soil was longer than in the moist dark control soils. Half-lives in dry soil were 2-7 times longer, and in the case of atrazine, the absence of moisture precluded significant degradation. Moist soil experiments also tended to correlate more strongly with linear first-order degradations. The dark control experiments also demonstrated shorter half-lives in moist soil. Moisture was also observed to affect the amount of degradate formed in the soils.

Laboratory studies on formation of bound residues and degradation of propiconazole in soils.

Laboratory studies on the formation of bound residues and on the degradation of the triazole fungicide propiconazole were conducted in two different soils. Soils treated with 14C-propiconazole were incubated at 22 degrees C and extracted exhaustively with a solvent at each sampling date until no further propiconazole was extracted. The solvent-extractable residues were used to measure propiconazole remaining in the soil, and the extracted soils were used to investigate bound residues of propiconazole. Mineralization of propiconazole was investigated by measuring [14C]carbon dioxide evolved from the soil samples. Formation of bound residues of propiconazole was higher in silty clay loam soil than in sandy loam soil, giving approximately 38 and 23% of the applied 14C, respectively. In contrast, the rates of degradation and mineralization of propiconazole were lower in silty clay loam soil than in sandy loam soil. Decreased extractability of the 14C residues with incubation time was observed with increased formation of bound residues. When the propiconazole remaining in the solvent-extractable residues was quantitatively measured by high-pressure liquid chromatographic analysis, the half-life value in sandy loam soil was about 315 days, while the half-life in silty clay loam soil exceeded the duration of the 1 year experimental period. Increased formation of bound residues was observed as propiconazole degraded with incubation time, suggesting that degradation products are involved in the formation of bound residues. Our study suggests that the formation of bound residues of propiconazole contributes to the persistence of this fungicide in soil.

Dissipation of the herbicide dithiopyr in soil and residues in wheat (Triticum aestivum L) grain under Indian tropical conditions.

Dissipation of dithiopyr in soil was monitored after application to wheat crop as pre- or post-emergence applications at two rates, viz 100 and 200 g AI ha(-1). The level of dithiopyr in the soil was assessed by gas chromatography, and its disappearence was found to follow a first-order decay curve irrespective of rate or method of application. The half-life in soil ranged between 17.3 and 25.0 days and residues at harvest (150 days after application) ranged between 4.0 and 8.8% of amounts applied. Investigation of microbial degradation of dithiopyr was conducted in minimal salt and Czapek Dox media in which 80% of the compound degraded within 15 days. Residues were not detected in wheat grain at harvest.

Effects of fertilizer and soil components on pesticide photolysis.

An environmental fate study was performed analyzing the effects of soil composition on the soil photolysis of a chemical. The study was conducted in two phases in which both moist and air-dried soils were fortified with either the common fertilizer sodium nitrate or the natural soil components iron or humic acid and dosed with niclosamide. The soils were photolyzed under a xenon lamp for 7 days. Increasing concentration of sodium nitrate did not affect the degradation pattern but did produce a lower concentration of aminoniclosamide. Soils fortified with iron displayed an unknown, which was not observed in other experiments, and the degradation of niclosamide from these soils was slower than from the sodium nitrate-fortified soils. There were no extractable degradates from any of the soils fortified with humic acid. In irradiated moist soils, the half-life of niclosamide increased when sodium nitrate was present at 20 ppm, and the half-lives of niclosamide in iron- and humic acid-fortified soil were increased slightly over that in unfortified soil. The effect of the nitrate and iron on the half-lives in dark control moist soils was minimal, but humic acid increased the dark control half-life from 420 to 611 h. No transformation of niclosamide was observed in the dark control air-dried soils. Soils with higher organic or iron contents or exposed to fertilizers do not affect as dramatically the half-life of pesticides as does the presence of moisture in the soil. Soil photolysis samples that were not maintained with moisture exhibited differences in half-life and degradation pattern. The maintenance of moisture was found to be more crucial to the reliability of soil photolysis studies than soil composition.

Determination of isoxaflutole (balance) and its metabolites in water using solid phase extraction followed by high-performance liquid chromatography with ultraviolet or mass spectrometry.

Balance (isoxaflutole, IXF) belongs to a new family of herbicides referred to as isoxazoles. IXF has a very short soil half-life (<24 h), degrading to a biologically active diketonitrile (DKN) metabolite that is more polar and considerably more stable. Further degradation of the DKN metabolite produces a nonbiologically active benzoic acid (BA) metabolite. Analytical methods using solid phase extraction followed by high-performance liquid chromatography-UV (HPLC-UV) or high-performance liquid chromatography-mass spectrometry (HPLC-MS) were developed for the analysis of IXF and its metabolites in distilled deionized water and ground water samples. To successfully detect and quantify the BA metabolite by HPLC-UV from ground water samples, a sequential elution scheme was necessary. Using HPLC-UV, the mean recoveries from sequential elution of the parent and its two metabolites from fortified ground water samples ranged from 68.6 to 101.4%. For HPLC-MS, solid phase extraction of ground water samples was performed using a polystyrene divinylbenzene polymer resin. The mean HPLC-MS recoveries of the three compounds from ground water samples spiked at 0.05-2 microg/L ranged from 100.9 to 110.3%. The limits of quantitation for HPLC-UV are approximately 150 ng/L for IXF, 100 ng/L for DKN, and 250 ng/L for BA. The limit of quantitation by HPLC-MS is 50 ng/L for each compound. The methods developed in this work can be applied to determine the transport and fate of Balance in the environment.

A Multimedia Assessment of the Environmental Fate of Bisphenol A

A comprehensive multimedia assessment of the environmental fate of bisphenol A (BPA) is presented. Components of the assessment include an evaluation of relevant partitioning and reactive properties, estimation of discharge quantities in the U.S. and the European Union (E.U.) resulting in conservative and realistic emission scenarios, and a review of monitoring data. Evaluative assessments of chemical fate using the Equilibrium Criterion (EQC) model are described from which it is concluded that the low volatility of BPA will result in negligible presence in the atmosphere. It is relatively rapidly degraded in the environment with half-lives in water and soil of about 4.5 days and less than 1 day in air, and with an overall half-life of 4.5 to 4.7 days, depending on the medium of release. The degradation rate in water is such that it may be transported some hundreds of kilometres in rivers, but long-range transport potential in air is negligible. Its low bioconcentration factor is consistent with rapid metabolism in fish (half-life less than 1 day). The estimated concentrations were generally consistent with the monitoring data, with the exception of sediment-water concentration ratios. Several hypotheses for the apparent nonequilibrium sediment-water partitioning are presented.

Dissipation of the defoliant tribufos in cotton-producing soils.

Soil dissipation of the cotton defoliant tribufos was measured in laboratory incubations and on 0.2-ha research plots. Computed 50% dissipation time (DT(50)) using nonlinear and linear kinetic models ranged from 1 to 19 days. Data indicated that exchangeable soil aluminum inhibited tribufos-degrading soil organisms. Nevertheless, measured DT(50) values were 40 to 700 times less than the aerobic soil half-life (t(1/2)) values used in recent tribufos risk assessments. DT(50) values suggest that risk estimates were overstated. However, edge-of-field runoff concentrations measured on research plots exceeded invertebrate LOECs, thus some aquatic risk is indicated. Field data also suggested that volatilization may be a significant soil dissipation pathway. From this result, we conclude that volatilization should be included in simulation models used for pesticide registration. This will likely improve the accuracy of model outputs for products such as tribufos. Potential volatilization losses indicate a need to evaluate the atmospheric behavior of tribufos.

Soil metabolism of isoxaflutole in corn.

The herbicide isoxaflutole 1 (5-cyclopropyl-4-isoxazolyl)[2-(methylsulfonyl)-4-(trifluoromethyl)phenyl]-methanone) has been applied preemergence at the rate of 125 g ha(-1) on corn crops grown on fields located in regions different as to their soil textures. Its metabolite diketonitrile 2 (2-cyano-3-cyclopropyl-1-(2-methylsulfonyl-4-trifluoromethylphenyl)propane-1,3-dione)-which is the herbicide's active compound-and its nonherbicide metabolite 3 (2-methylsulfonyl-4-trifluoromethylbenzoic acid) were measured in the 0-10 cm surface soil layer of the corn crops after the treatment and until the harvest. At the opposite of what occurred in plant shoots, the transformation of isoxaflutole 1 into diketonitrile 2 was not immediate in soil. In the 0-10 cm surface soil layer, this transformation occurred progressively according to an apparent second-order kinetics, and the soil half-lives of isoxaflutole 1 self were comprised between 9 and 18 days. The adsorption of isoxaflutole 1 onto the solid phase of the soil and its organic matter should explain the stabilization effect of soil, increased by the application of fresh organic fertilizer. The sum of the concentrations of isoxaflutole 1 and diketonitrile 2 disappeared in the 0-10 cm surface soil layer according to an apparent first-order kinetics, and the soil half-lives of this sum were comprised between 45 and 65 days. The sum of the concentrations of isoxaflutole 1 and of its metabolites diketonitrile 2 and acid 3 did not account for the amount of isoxaflutole 1 applied. The discrepancy increased with the delay after the application, showing that the acid 3 was further metabolized in soil into common nontoxic products, and ultimately into CO2. The conjunction of the adsorption of isoxaflutole and its metabolites (which reduced their mobilities) onto the soil and its organic matter, and their further metabolism should explain why isoxaflutole and its metabolites were not detected in the 10-15 and 15-20 cm surface soil layers during the crops.

Distribution coefficient Kd of transuranics in soils : Experimental determination and consequences for dose assessment

The distribution coefficient (or Kd) of 241 Am between soil and soil solution was estimated on the basis of experiments carried out under controlled conditions for soils displaying pH and organic matter content (which are the relevant parameters for transuranic nuclides sorption ability) found in cultivated soils. In these experiments, designed with five experimental units (batches) per studied contact time, 10 g of dry soil are contaminated with 50 mL of osmosed water containing proper concentrations of the radionuclide. The tracer, initially in nitric form, is neutralized prior to addition with the water, to ensure a constant ionic strength and avoid disturbance of the soil chemistry. After shaking and sedimentation, for each batch, the supernatant is sampled and analyzed by either liquid scintillation or alpha spectrometry. The sorption-desorption kinetic time necessary to reach apparent equilibrium is evaluated (> 10 days). Fixation curves, e.g. activity of the soil vs activity of the solution are established, after measurements, for a single delay representative of the equilibrium, and for a range of several orders of magnitude for radionuclide concentrations. On the basis of several assumptions, Kd values are evaluated by simple linear adjustment. A numerical application to retention half lives in soil is made. Finally, the weight of using site-specific Kd values instead of default parameters, on the doses due to ingestion of terrestrial foodstuffs is discussed for different scenarios.

Persistence and Movement of the Herbicide Propoxycarbazone-Sodium in Winter Wheat Crops

The new herbicide propoxycarbazone (sodium methyl 2-[[[(4,5-dihydro-4-methyl-5-oxo-3-propoxy-1H-1,2,4-triazole-1-yl)carbonyl]amino]sulfonyl]benzoate, 1 ) has been measured in the soil of winter wheat crops. In the soil extract, propoxycarbazone was separated from its potential soil metabolites by repeated TLC. Propoxycarbazone was methylated with diazomethane. In the GC and GC-MS apparatus, the N -methylpropoxycarbazone 2 (methyl 2-[[[(4,5-dihydro-4-methyl-5-oxo-3-propoxy-1H-1,2,4-triazole-1-yl)carbonyl]methylamino]sulfonyl]benzoate) generated N -methylsaccharin 4 (1-dioxy-2- N -methyl-3-keto-1,2-benzisothiazol) which was measured. Propoxycarbazone has been applied in the spring at the rate of 70 g ha m 1 post-emergence on winter wheat crops grown in several sites different as to their soil texture and composition. In the 0-10 cm surface soil layer of the winter wheat crops grown on sandy loam (Melle) or on clay loam (Zevekote) soils, the half-life of propoxycarbazone was 54 days. In the winter wheat crop grown on loam soil (Cortil-Noirmont), the propoxycarbazone soil half-life was 31 days. The difference between the propoxycarbazone soil half-lives at the different sites was related to the organic fertilizer treatments applied in the past. After the winter wheat harvest at the end of August, the concentration of propoxycarbazone in soil was very low at Cortil-Noirmont; at the end of September, propoxycarbazone was no more detected. At Melle and Zevekote, the concentration of propoxycarbazone in soil was very low in September, and disappeared completely at the end of October. Since the treatment and until the end of October, propoxycarbazone was not detected in the 10-15 and 15-20 cm surface soil layers of the three trials.

Confirmation of an Enzyme-Linked Immunosorbent Assay to Detect Fluometuron in Soil1

Research was conducted to compare the results of an enzyme-linked immunosorbent assay (ELISA) to high-performance liquid chromatography (HPLC) for detecting fluometuron in the environment. A linear relationship for HPLC (R2 > 0.90) and ELISA (R2 > 0.66) analysis was observed between the natural logarithm of the detected fluometuron concentrations regressed against time in soil collected from a cropped area, a grass filter strip, and a riparian forest. Both methods detected the same initial fluometuron concentration (y-intercept) for two of the three soils evaluated. The ELISA and HPLC measurements of fluometuron concentrations compared favorably with r values from 0.83 to 0.98. Predicted fluometuron half-lives determined from HPLC and ELISA measurements were: 110 and 112 d in the cropped watershed, 28 and 29 d in the riparian area, and 11 and 11 d in the grass filter strip, respectively. Results from both techniques indicated shorter half-lives in soil from the grass filter strip and riparian area than in...

Atrazine and metolachlor degradation in subsoils

Degradation of atrazine [2-chloro-4-etylamino-6-isopropylamino-1,3,5-triazine] and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)-acetamide] in sterile and non-sterile soil samples collected at two different soil depths (0–20 and 80–110 cm) and incubated under aerobic and anaerobic conditions was studied. Under aerobic conditions, the half-life of atrazine in non-sterile surface soil was 49 days. In non-sterile subsoil, the half-life of atrazine (119 days) was increased by 2.5 times compared in surface soils and was not statistically different from half-lives in sterile soils (115 and 110 days in surface soil and subsoil, respectively). Metolachlor degradation occurred only in non-sterile surface soil, with a half-life of 37 days. Under anaerobic conditions, atrazine degradation was markedly slower than under aerobic conditions, with a half-life of 124 and 407 days in non-sterile surface soil and non-sterile subsoil, respectively. No significant difference was found in atrazine degradation in both sterile surface soil (693 days) and subsoil (770 days). Under anaerobic conditions, degradation of metolachlor was observed only in non-sterile surface soil. Results suggest that atrazine degraded both chemically and biologically, while metolachlor degraded only biologically. In addition, observed Eh values of soil samples incubated under anaerobic conditions suggest a significant involvement of soil microorganisms in the overall degradation process of atrazine under anaerobic conditions.

Accumulation and decay of chlorothalonil and selected metabolites in surface soil following foliar application to peanuts.

One of the principal uses of the fungicide, chlorothalonil, is control of foliar peanut diseases. Recent assessments indicate its runoff from treated fields in southeastern states presents risks to aquatic life. Two factors that control its runoff are how much reaches soil surfaces and degradation rates. To address these questions and to evaluate accumulation and decay of key metabolites, soil samples (0-2 cm) were collected after seven chlorothalonil applications on experimental peanut plots in south central Georgia during the 1999 growing season. At the start of and during laboratory incubations, samples were analyzed for the parent and degradates by HPLC-PDA-APCI-MS. The maximum observed residue levels were after the second application, after which canopy closure reduced soil deposition from later applications to 5-10% of applied amounts. After the last spray, < 5% of the cumulative chlorothalonil applied was detected in the soil. Foliar interception and dissipation and rapid soil degradation contributed to low residue levels. Soil half-lives were < 1-3.5 days for chlorothalonil and 10-22 days for its principal degradate, 4-hydroxychlorothalonil. Other daughter and granddaughter products were detected, some of which accumulated during the growing season. Results emphasize the plant canopy role in controlling the amount of fungicide sprays that reach soil surfaces and suggest concentration-dependent chlorothalonil degradation with degradation rates increasing as soil loading decreases. The study indicates that the 30-day field half-life often used for risk assessments of this pesticide is too long for one of its most important agronomic uses, i.e., in southeastern peanut production. It also indicates that the principal metabolites are more persistent than the parent, and more study is needed to identify and quantify their fate pathways.

Biodegradation kinetics for pesticide exposure assessment.

Understanding pesticide risks requires characterizing pesticide exposure within the environment in a manner that can be broadly generalized across widely varied conditions of use. The coupled processes of sorption and soil degradation are especially important for understanding the potential environmental exposure of pesticides. The data obtained from degradation studies are inherently variable and, when limited in extent, lend uncertainty to exposure characterization and risk assessment. Pesticide decline in soils reflects dynamically coupled processes of sorption and degradation that add complexity to the treatment of soil biodegradation data from a kinetic perspective. Additional complexity arises from study design limitations that may not fully account for the decline in microbial activity of test systems, or that may be inadequate for considerations of all potential dissipation routes for a given pesticide. Accordingly, kinetic treatment of data must accommodate a variety of differing approaches starting with very simple assumptions as to reaction dynamics and extending to more involved treatments if warranted by the available experimental data. Selection of the appropriate kinetic model to describe pesticide degradation should rely on statistical evaluation of the data fit to ensure that the models used are not overparameterized. Recognizing the effects of experimental conditions and methods for kinetic treatment of degradation data is critical for making appropriate comparisons among pesticide biodegradation data sets. Assessment of variability in soil half-life among soils is uncertain because for many pesticides the data on soil degradation rate are limited to one or two soils. Reasonable upper-bound estimates of soil half-life are necessary in risk assessment so that estimated environmental concentrations can be developed from exposure models. Thus, an understanding of the variable and uncertain distribution of soil half-lives in the environment is necessary to estimate bounding values. Statistical evaluation of measures of central tendency for multisoil kinetic studies shows that geometric means better represent the distribution in soil half-lives than do the arithmetic or harmonic means. Estimates of upper-bound soil half-life values based on the upper 90% confidence bound on the geometric mean tend to accurately represent the upper bound when pesticide degradation rate is biologically driven but appear to overestimate the upper bound when there is extensive coupling of biodegradation with sorptive processes. The limited data available comparing distribution in pesticide soil half-lives between multisoil laboratory studies and multilocation field studies suggest that the probability density functions are similar. Thus, upper-bound estimates of pesticide half-life determined from laboratory studies conservatively represent pesticide biodegradation in the field environment for the purposes of exposure and risk assessment. International guidelines and approaches used for interpretations of soil biodegradation reflect many common elements, but differ in how the source and nature of variability in soil kinetic data are considered. Harmonization of approaches for the use of soil biodegradation data will improve the interpretative power of these data for the purposes of exposure and risk assessment.

Soil dissipation of diuron, chlorotoluron, simazine, propyzamide, and diflufenican herbicides after repeated applications in fruit tree orchards.

In a pear tree orchard planted on loam soil, each plot was treated in April 1998 with either one of the ureas diuron or chlorotoluron, or triazine simazine herbicides applied at 3, 4, and 2 kg AI ha(-1), respectively. Some plots had not been previously treated with one of these herbicides. Other plots had been treated annually during the past 12 years with the same herbicide. One herbicide, and always the same, was thus applied to each plot. In the plots treated for the first time with either diuron, chlorotoluron, or simazine, the soil half-lives of these herbicides in the 0-10 cm surface soil layer were 81, 64, and 59 days, respectively. In the plots treated with the same herbicide for 12 years, the corresponding soil half-lives were 37, 11, and 46 days. Diuron thus produced a moderately enhanced biodegradation, chlorotoluron a high one, and simazine a low but significant one. In another pear tree orchard planted on sandy loam soil, each plot was treated in April 1998 with one of the amide propyzamide (1.25 or 1.0 AI kg ha(-1)) or diflufenican (250 g AI ha(-1)) herbicides. In the plots not previously treated with propyzamide, the propyzamide soil half-life was the same for both doses, i.e., about 30 days. In the plots treated annually for 3 or 14 years with propyzamide, the soil half-life was 12 and 10 days, respectively. In the plots treated for the first time with diflufenican and in those treated annually with diflufenican for 3 years, the diflufenican soil half-life was the same, i.e., 65 days. Propyzamide thus already showed a highly accelerated biodegradation after 3 years of repeated annual applications. Diflufenican, however, did not show enhanced biodegradation after 3 years of repeated annual applications.

Dissipation behavior of malathion and dimethoate residues from the soil and their uptake by garden pea (Pisum sativum).

The contamination of soil by pesticide chemicals can occur through direct application to control some inhabiting insect pests of a variety of economic plants while indirect avenues can occur through aerial applications, spray drifts, wash-off from the atmosphere, treated plants through precipitation, erosion, and run-off from agricultural and forest lands (Mulla et al. 1981). It is obvious, therefore, why a great deal of attention must be paid to studying the many complex interactions that occur between pesticides and the soil (Harvey 1983). The fate of pesticides and their behavior in soil is influenced by several factors including adsorption, movement and decomposition (Shamahat 1980). The breakdown of pesticides in soil is brought about by a variety of biological and nonbiological mechanisms; frequently total decay is due to a combination of events. The principal biological route is the microbial consumption of the pesticides as energy, carbon and/or nitrogen source (Bums and Edwards 1980). Data has been collected suggesting that organic molecules in soil slowly become sequestered within the soil matrix (Alexander 1995). The patterns of disappearance of persistent compounds in the field and laboratory studies show a declining bioavailability to microorganisms with time of residence in soil. If a chemical persists and thus remains in contact with particulate matter for some time, it becomes more and more resistant to extraction by solvents (Kelsey et al. 1997). This persistence leads to different behaviors: the pesticide may be stressed to a faster degradation (biological or chemical) on the active sites of particulate matter, or on the contrary, the pesticide may be protected and exhibit a longer half-life in soil (Lartiges and Garrigues 1995).

Screening Chemicals for Persistence in the Environment

A method is suggested for rapid screening of chemicals for persistence in the environment. Physical−chemical equilibrium partitioning information between air, water, and octanol are used as a first screen to identify the media for which degradation half-lives are required and those for which half-lives may be unnecessary. An overall persistence under equilibrium conditions is then estimated using half-lives in air, water, soil, and sediment using a steady-state mass balance model. A graphical technique to identify the key half-lives is demonstrated using 233 chemicals. For chemicals of more extreme partitioning properties, some half-lives may not be needed.

Hazard/Risk Assessment of Pyridaben: I. Aquatic Toxicity and Environmental Chemistry

The toxicity and environmental fate of the insecticide-miticide, pyridaben were investigated using both standardized laboratory procedures and outdoor studies with natural water. Outdoor studies provide a more realistic exposure scenario to aquatic organisms and any toxicity is a response to actual exposure concentrations resulting from the natural degradation and dissipation of the chemical. This paper describes the environmental chemistry/fate and aquatic toxicity of pyridaben. The subsequent paper describes the results of the outdoor aquatic toxicity studies and the use of the water-effect ratio in hazard/risk assessment. Environmental fate studies indicate that pyridaben has a low water solubility and high Kd and Koc values, which favors partitioning from water onto soil and sediment. Pyridaben is stable to hydrolysis but has a short photolysis half-life in water (<30 min) and soil (∼11 d). Furthermore, pyridaben has a short half-life in soil (12 to 14 d) when applied in the field to citrus crops. Laboratory studies with constant 48- to 96-h exposures to pyridaben show it is acutely toxic to fish (Lepomis macrochirus, Pimephales promelas, Oncorhynchus mykiss, Cyprinodon variegatus) and invertebrates (Daphnia magna, Mysidopsis bahia). Invertebrates are more sensitive (lower LC50s) than fish to pyridaben, and most mortalities occur <24 h for fish and <72 h for invertebrates. Chronic laboratory studies indicate that the MATCs for pyridaben and D. magna, M. bahia and P. promelas were 0.12, 0.15 and 0.39 μg/L, respectively. Acute-to-chronic ratios for pyridaben are low for fish and invertebrates, indicating a low potential for residual activity. Chronic toxicity to aquatic organisms is not an issue after application in the field because exposures tend to be brief.

Pattern and Rate of Dissipation of Pretilachlor and Mefenacet in Plow Layer and Paddy Water under Lowland Field Conditions: A Three-year Study

Field trials were conducted at the National Institute of Agro-Environmental Sciences experiment station from 1995 to 1997. Each trial was carried out from mid-May to early July every year. Determining the pattern and rate of dissipation of pretilachlor and mefenacet in paddy soil at three different depths (0-1, 1-5 and 5-10 cm) and in paddy water was the primary objective of the experiments. Half-lives (DT 50 ) of the herbicides were calculated using both the simple first order kinetics (SFOK) and the biphasic first order kinetics (BFOK). The pattern and rate of dissipation of pretilachlor in both soil and water were quite different from that of mefenacet. At the 0-1 cm soil layer (oxidative), dissipation of pretilachlor was quite rapid during the first 3 weeks but slowed down afterwards. In case of mefenacet, dissipation was a steady decline until the last sampling day. The DT 50 in this layer was between 9 to1 1 days and 7 tolO days for mefenacet and pretilachlor, respectively. At the lower soil layers (1-5 and 5-10 cm; reductive) pretilachlor appeared to leach from the upper layer within the first two weeks and then quickly disappeared after wards. Faster degradation of pretilachlor was noted in these reductive soil layers. On the other hand, mefenacet dissipation declined steadily at these layers. Also, leaching of mefenacet was not as evident as in pretilachlor. Mefenacet concentration in paddy water peaked over the second to third day and its dissipation was rapid until the fourth week. Dissipation pattern of pretilachlor was a steady curvilinear decline. In paddy water, DT 50 was about 3.3 to 4.1 1 days for mefenacet and 3.0 to 3.6 days for pretilachlor. Calculating the half-life In soil (0-1 cm layer) using SFOK appears sufficient for mefenacet while a more complex model such as the BFOK describes that of pretilachlor better. The half-life and dissipation pattern of both herbicides did not vary considerably across years. There was no evidence of residue build-up due to continuous application of the herbicides in the same plots.

Effects of Conventional and Mulch Tillage on Dicamba Transport1

Abstract: Leaching and runoff losses of the postemergence-applied herbicide dicamba were evaluated over a 3-yr period (1989 to 1991). Dicamba was applied at the recommended rate (0.56 kg ai/ha) to conventional and mulch tillage planted corn fields on Hagerstown silty clay loam (fine, mixed, mesic Typic Hapludalf). Mulch tillage followed several years of no-tillage corn. Root zone leachates were collected utilizing pan lysimeters placed 1.2 m below the soil surface. Surface runoff was monitored and collected with an HS-flume and automated sampling equipment. Leaching was greatest during 1989, and runoff events were recorded only during this season. Leachate samples containing measurable levels of dicamba were obtained within 21 d of herbicide application or within slightly more than one soil half-life of this chemical. More dicamba leached under mulch tillage than conventional tillage management. Tillage rotation (no tillage to mulch tillage) did not alter the leaching loss potential of dicamba beneath the...