14C-labeled Chemicals Research Articles

Biodegradation of Volatile Chemicals in Soil: Separating Volatilization and Degradation in an Improved Test Setup (OECD307).

During environmental risk assessments of chemicals, higher-tier biodegradation tests in soil, sediment, and surface-water systems are required using OECD standards 307, 308, and 309 guidelines, respectively. These guidelines are not suitable for testing highly volatile chemicals, and a biometer closed-incubation setup is recommended for testing slightly volatile chemicals. In this setup, the degradation kinetics of highly volatile chemicals can largely be influenced by volatilization. Additionally, guidelines lack sufficient information on test-system geometry and guidance on how to measure and maintain aerobic conditions during the test. Our objectives were (1) to design a closed test setup for biodegradation tests in soil in which the maintaining and measuring of aerobic conditions was possible without the loss of volatile test chemicals and (2) to suggest data-treatment measures for evaluating the degradation kinetics of volatile test chemicals. With the new setup, full-scale OECD 307 tests were performed using the volatile 14C-labeled chemicals decane and tetralin. For both test chemicals, reproducible complete mass balances were observed, and the new setup ensured that the volatilization losses were kept below the mineralized fraction. Based on the obtained data, an extended model was developed that enabled consideration of the volatilization in the modeling of degradation kinetics.

Nondestructive real-time radioisotope imaging system for visualizing 14C-labeled chemicals supplied as CO2 in plants using Arabidopsis thaliana

We have developed a real-time radioisotope imaging system (RRIS) that can nondestructively trace 14C-labeled chemicals in plants. In an experiment, after feeding 14CO2 to a plant, the plant was fixed inside a box where lighting was regulated, and beta rays emitted from the 14C in the plant were intermittently imaged using the developed system. As a first step, using a series of standard sources of 14C, the data depth and detection limits of the 14C images captured by the RRIS were evaluated for various integral times. As a result, the linearity between the 14C activity and signal intensity was determined for the range 103. Next, the linearity was validated using plant (Arabidopsis thaliana) organs, resulting that the linearity was shown in the case of young leaf, but was not maintained in the thick organs, such as a flower, mature leaf, siliques, and stem. Considering the good correlation between the intensity by RRIS and the PSL value by an imaging plate (IP) as well as the relative low energy of beta rays emitted from 14C, the thickness of the organs would easily affect the quantitativity of the RRIS as well as an IP. Our findings prove that sequential images of 14C in a living plant sample in a regulated light and air environment can be nondestructively analyzed using the developed system, whose quantitativity is similar to that of an IP.

A radioactive assay allowing the quantitative measurement of cuticular permeability of intact Arabidopsis thaliana leaves

Cuticular penetration of five different ¹⁴C-labeled chemicals (benzoic acid, bitertanole, carbaryl, epoxiconazole and 4-nitrophenol) into Arabidopsis thaliana leaves was measured and permeances P (ms⁻¹) were calculated. Thus, cuticular barrier properties of A. thaliana leaves have been characterized quantitatively. Epoxiconazole permeance of A. thaliana was 2.79 × 10⁻⁸ ms⁻¹. When compared with cuticular permeances measured with intact stomatous and astomatous leaf sides of Prunus laurocerasus, frequently used in the past as a model species studying cuticular permeability, A. thaliana has a 48- to 66-fold higher permeance. When compared with epoxiconazole permeability of isolated cuticles of different species (Citrus aurantium, Hedera helix and P. laurocerasus) A. thaliana permeability is between 17- to 199-fold higher. Co-permeability experiments, simultaneously measuring ¹⁴C-epoxiconazole and ³H₂O permeability of isolated cuticles of three species (C. aurantium, H. helix and P. laurocerasus) showed that ³H₂O permeability was highly correlated with epoxiconazole permeability. The regression equation of this correlation can be used predicting cuticular transpiration of intact stomatous leaves of A. thaliana, where a direct measurement of cuticular permeation using ³H₂O is impossible. Water permeance estimated for A. thaliana was 4.55 × 10⁻⁸ m⁻¹, which is between 12- and 91-fold higher than water permeances measured with isolated cuticles of C. aurantium, H. helix and P. laurocerasus. This indicates that cuticular water permeability of the intact stomatous leaves of the annual species A. thaliana is fairly high and in the upper range compared with most P values of perennial species published in the past.

Test system to establish mass balances for 14C-labeled substances in soil-plant-atmosphere systems under field conditions.

A test system has been developed that allows the formation of mass balances for 14C-labeled organic compounds in soil-plant-atmosphere systems under field conditions. The main focus was the quantification of all different 14C-labeled gaseous losses from soil and plant surfaces after the application of 14C-labeled chemicals; therefore, a two-chamber system with specially designed trapping facilities was designed. Volatile 14C-labeled compounds and 14CO2 resulting from the total degradation of the applied 14C-labeled chemical could be trapped separately, and 14C gaseous losses from plant and soil surfaces could be determined separately as well. With this test system, it was feasible for the first time to distinguish between 14C volatile and 14CO2 losses from soil surfaces and from plant surfaces in soil monolith systems under real environmental conditions. The system itself was established on surfaces of soil monoliths (lysimeters) to study the above-mentioned processes along with the transport and leaching behavior of the chemicals in soil cores. With the new system, the behavior of organic chemicals was followed up for a whole vegetation period, and a mass balance for the applied chemical was established. Therefore, a better prediction of the long-term behavior of organic chemicals under real environmental climatic conditions was achieved. The first results for the fate of the model herbicide isoproturon in two different types of agricultural soils are presented and compared with results from an extensive laboratory study.

The degradation of 14C-labelled drilling chemicals in a simulated seabed study

In an experiment, which lasted nine months, real sea bed sections and naturally occurring sediment dwelling organisms have been used to study the decomposition of today’s “green” chemicals, i.e. drilling mud chemicals comprising α-olefins or esters made from fatty acid extracted from fish. Both types have been shown to be degraded at aerobic conditions at sea bed. However, the fate of the alcoholic part of the ester has been unknown, as was the rate of degradation of the olefin. Both olefin and alcohol were labelled with 14C at the C1 atom. The results show that the ester hydrolyses quickly and after a lag phase the alcohol is oxidised, while the olefin degrades more slowly. 133Ba-labelled baryte was used as a bioturbidity marker. The measurements, i.e. scanned sediment columns, show very little bioturbation in the boxes where oil-contaminated sediment was present whereas the control boxes showed more activity from the sediment dwelling organisms down to the depth of the contaminated layer.

QUANTIFICATION OF 14C-LABELED HYDROPHOBIC ORGANIC COMPOUNDS IN SOIL SAMPLES BY A SCINTILLATION FLUID EXTRACTION METHOD

Hydrophobic organic chemicals labeled with 14C are often used as tracers to study chemical fate and transport in soil. Wet oxidation is the recognized but labor-demanding method used most often to measure the concentration of radioactive organic tracers in the soil. In this study, we test an alternative, simpler method for quantification of 14C-labeled hydrophobic organic chemicals in soil samples. The soil samples were extracted directly with scintillation fluid in glass scintillation vials (Scintillation Fluid Extraction, SFE method) and were subsequently analyzed by liquid scintillation counting. Application of internal standards showed no significant quenching or reduced counts of the 14C-labeled chemical caused by the presence of settled soil particles in the scintillation vials. Hence, the scintillation fluid could be used simultaneously as both scintillation and extraction media. Decreasing the amount of soil containing 14C-labeled chemicals added to the scintillation vial was shown to increase the extraction efficiency of the SFE method. Aging (contact time) and concentration were shown to affect the results of the SFE method. However, in a naphthalene diffusion experiment, where aging varied between almost no contact time and 20 days and concentration of naphthalene varied between ∼0 to 40 μg g−1, the estimated naphthalene diffusion coefficient was shown to be influenced only minimally when using the SFE method compared with the wet oxidation method. We emphasize that the SFE method is applicable only to sterile systems with no degradation or assimilation of 14C-labeled compounds. If this condition is met and there is appropriate consideration of the effects of chemical aging and concentration, the SFE method seems to be a useful and labor-saving alternative to the traditional wet oxidation method for determination of the concentration of 14C-labeled organic chemicals in soil samples.

Metabolism of the herbicide glufosinate-ammonium in plant cell cultures of transgenic (rhizomania-resistant) and non-transgenic sugarbeet (Beta vulgaris), carrot (Daucus carota), purple foxglove (Digitalis purpurea) and thorn apple (Datura stramonium).

The metabolism of the herbicide glufosinate-ammonium was investigated in heterotrophic cell suspension and callus cultures of transgenic (bar-gene) and non-transgenic sugarbeet (Beta vulgaris). Similar studies were performed with suspensions of carrot (Daucus carota), purple foxglove (Digitalis purpurea) and thorn apple (Datura stramonium). 14C-labelled chemicals were the (racemic) glufosinate, L-glufosinate, and D-glufosinate, as well as the metabolites N-acetyl L-glufosinate and 3-(hydroxymethylphosphinyl)propionic acid (MPP). Cellular absorption was generally low, but depended noticeably on plant species, substance and enantiomer. Portions of non-extractable residues ranged from 0.1% to 1.2% of applied 14C. Amounts of soluble metabolites resulting from glufosinate or L-glufosinate were between 0.0% and 26.7% of absorbed 14C in non-transgenic cultures and 28.2% and 59.9% in transgenic sugarbeet. D-Glufosinate, MPP and N-acetyl L-glufosinate proved to be stable. The main metabolite in transgenic sugarbeet was N-acetyl L-glufosinate, besides traces of MPP and 4-(hydroxymethylphosphinyl)butanoic acid (MPB). In non-transgenic sugarbeet, glufosinate was transformed to a limited extent to MPP and trace amounts of MPB. In carrot, D stramonium and D purpurea, MPP was also the main product; MPB was identified as a further trace metabolite in D stramonium and D purpurea.

Importance of the test volume on the lag phase in biodegradation studies

Increasing the total volume of test medium resulted in decreased lag times (T L) in biodegradability shake flask batch tests conducted with either surface water or with synthetic mineral medium inoculated with supernatant from settled activated sludge. Experiments were performed with test volumes ranging from 1.8 ml to 100 L using two 14C-labeled model chemicals, 2,4-dichlorophenoxyacetic acid (2,4-D) and p-nitrophenol (PNP), both of which are known to be readily degradable after variable lag phases. The TL ranged from 2.1 to 30.4 d for PNP and from 16 to 37 d for 2,4-D. Decreasing the test volume tended to increase the lag time, even when a single test batch was redistributed into smaller flasks. With 5 ml supernatant added to different volumes of mineral medium, lag times for PNP were independent of the test volume in a range from 10 to 1,000 ml. At small volumes of 10 ml or less, degradation failed randomly. Our findings are partly explained by the hypotheses that a sufficient total amount as well as a sufficient concentration of specifically degrading microorganisms or consortia of bacteria must be present initially for biodegradation to get started, from which follows that with too small inoculations or with too small test volumes, biodegradation may fail randomly. A straightforward practical implication of the findings is that the test volume in biodegradability tests can significantly influence the lag time and thus sometimes be decisive for the outcome in biodegradation studies.

Shake-Flask Test for Determination of Biodegradation Rates of 14C-Labeled Chemicals at Low Concentrations in Surface Water Systems

A simple shake-flask surface water biodegradability die away test with 14C-labeled chemicals added to microgram per liter concentrations (usually 1–100 μg/L) is described and evaluated. The aim was to provide information on biodegradation behavior and kinetic rates at environmental (low) concentrations in surface water systems. The basic principle of measurement was to determine evolved CO2 indirectly from measurements of total organic activity in subsamples after stripping off their content of CO2. Used with surface water alone the test simulates a pelagic environment and amended with sediments (0.1–1 dry weight/L) the test is intended to simulate a water environment with suspended solids (e.g., resuspended sediments). A protocol of the test used with the 14C technique or with specific chemical analysis was recently developed by the International Organization for Standardization. Practical experience with the method is presented for a set of reference substances. These substances could be ranked in five groups of decreasing biodegradability: aniline>p-nitrophenol, 2,4-dichlorophenoxyacetic acid>4-chloroaniline>maleic hydrazide, pentachlorophenol>atrazine. It was found that degradation rates and lag periods varied considerably among sampling sites and sometimes also among samples from the same site. No significant correlation could be established between degradation rates and microbial biomass estimates. Even small portions of added sediments greatly enhanced biodegradation of the absorbable compound pentachlorophenol, probably by providing sites for microbial attachment. Repeated tests indicated consistent degradation behavior for the readily degradable substances, whereas degradation sometimes stopped or failed with the more recalcitrant substances. A preadaptation step involving regular reinoculation with freshly collected surface water could, however, overcome the problems of false-negative results.

ENGINEERING HUMIFICATION PROCESSES TO IMMOBILIZE PHENOLIC CONTAMINANTS IN SOILS

ENGINEERING HUMIFICATION PROCESSES TO IMMOBILIZE PHENOLIC CONTAMINANTS IN SOILSSorption-desorption behaviors of several phenolic chemicals (phenol, 1-methylphenol, 2,4- dichlorophenol and 1-naphthol) were studied on two sandy loam soils collected from field and forested sites. 14C-labelled chemicals were used to better track the distribution of the contaminant in the soil matrix. The effectiveness of engineered humification processes catalyzed by horseradish peroxidase...Author(s)Alok BhandariInkwon ChoFangxiang XuSourceProceedings of the Water Environment FederationSubjectSession 27 - Remediation of Soil and Groundwater Symposium: Contaminant Behavior in Soil SystemsDocument typeConference PaperPublisherWater Environment FederationPrint publication date Jan, 2000ISSN1938-6478SICI1938-6478(20000101)2000:12L.616;1-DOI10.2175/193864700784608487Volume / Issue2000 / 12Content sourceWEFTECFirst / last page(s)616 - 634Copyright2000Word count176

Volatilization and mineralization of 14C-labelled pesticides on lysimeter surfaces

The volatilization and mineralization of 14C-labelled chemicals applied on soil surfaces in lysimeters was determined continuously with a new developed test-system. 14C-Terbuthylazine and 14C-Pendimethalin were applied on two different types of soil, one typical for a sandy agricultural soil and the second one simulating a soil of an adjoining forest ecosystem. Volatilization of terbuthylazine within 32 – 36 days was 0.26 – 0.65% of the amount initially applied, that of pendimethalin 1.24 – 2.78 within 28 days. Mineralization was 1.02 – 7.41% for terbuthylazine and 2.79 – 5.39% for pendimethalin within 28 – 36 days.

Mobilities of organic compounds in reconstituted cuticular wax of barley leaves: Determination of diffusion coefficients

AbstractIsolated cuticular wax, obtained from barley leaves, was mixed with 14Clabelled organic chemicals including aromatic pesticides and long‐chain linear alkanes, alcohols and acids. These mixtures were reconstituted from the melt and labelled chemicals were desorbed from the wax by immersing the wax samples in aqueous phospholipid suspensions. Diffusion coefficients (D) of radiolabelled test compounds in the wax were derived from desorption kinetics. Diffusion coefficients ranged from 10−17 to 10−22 m2 s−1 and decreased rapidly with increasing molar volumes of solutes. However, size selectivity of D was much more pronounced with the linear, long‐chain molecules than with the aromatic compounds. It is argued that the two different groups of chemicals (compounds occurring naturally in cuticular waxes vs pesticide molecules) were trapped in different fractions of wax during crystallization from the melt. The normal long‐chain aliphatic compounds appear to be incorporated into the crystalline fraction of the wax, while the cyclic pesticide molecules are confined to the solid amorphous regions. Our data indicate that constituents of cuticular waxes are not immobile. In fact, relatively small linear aliphatic molecules have mobilities that do not differ too much from those of cyclic pesticides. However, the pronounced size selectivity of diffusivities of long‐chain aliphatic compounds causes a rapid decrease in D with increasing chain length. The value of D of hexadecanoic acid was 3.81 × 10−18 m2 s−1 while that of dotriacontane was only 4.07 × 10−22 m2 s−1. Thus, increasing the carbon number by a factor of two resulted in a decrease in mobility by almost four orders of magnitude. Diffusivities of selected pesticide molecules in reconstituted wax were compared with permeances measured using intact barley leaves and were found to be in satisfactory agreement.

Biological and chemical interactions of pesticides with soil organic matter

There is little doubt that organic matter plays a major role in the binding of pesticides in soil, and that this phenomenon is usually the most important cause for interaction of pesticides in the soil environment. Fulvic or humic acids are the chemicals most commonly involved in the binding interactions. Binding can occur with the original pesticide or a transformation product, the reaction being caused by abiotic agents or biotic agents (microbial or plant enzymes). The reactions or processes involved appear to be the same as those responsible for the formation of humic substances, i.e. for the humification process. Binding of pesticides to organic matter can occur by sorption (Van der Waal's forces, hydrogen bonding, hydrophobic bonding), electrostatic interactions (charge transfer, ion exchange or ligand exchange), covalent bonding or combinations of these reactions. Our investigation focused primarily on the binding of substituted phenols and aromatic amines to humus monomers and humic substances. In model reactions, we demonstrated the formation of covalent linkages between pesticides and humus constituents and fulvic or humic acids in the presence of phenol oxidases or clay minerals. With chlorinated phenols and carboxylic acids, it was possible to isolate and identify cross-coupling products and to elucidate the site and type of binding. The binding of chlorinated phenols to humic substances was determined by using 14C-labelled chemicals and by measuring the uptake of radioactivity by the humic material. These experiments provide a base for explaining the formation of bound residues in certain cases and for assuming the toxic potential of the immobilized pollutants.

A comparative study of test methods for assessment of the biodegradability of chemicals in seawater—Screening tests and simulation tests

A comparative study has been performed on test methods for assessing the biodegradability of chemicals in seawater environments. A simple shake flask die-away test with natural seawater and 14C-labeled chemicals added in μg/liter concentrations is proposed as a “simulation” test. The analytical parameter used in this test is residual dissolved 14C activity. The performance of the simulation test has been compared with the performance of similar screening tests with dissolved organic carbon analysis and test compounds added in mg/liter concentrations to nutrient-enriched seawater. All chemicals investigated that passed the screening tests were also degradable in the simulation test and some results with simulation tests were positive; even screening tests were negative, while some compounds, including maleinhydrazide, known to be degradable in soil, remained undegraded in either type of test. Disappearance times after the ended lag time were smaller in screening tests than in simulation tests, but the rates of biodegradation cannot be meaningfully compared, as zero-order kinetics in combination with an exponentially growing population of degraders prevail in screening tests, while first-order kinetics and frequently a constant activity of degraders (cooxidation) prevail in simulation tests where the test material is a secondary substrate only. In screening tests, lag times are sometimes excessively long and highly variable. Whether the lag times could be decreased and their variability narrowed by supplementation with a cosubstrate (yeast extract) or by inoculation with seawater that had been preadapted to the test material was investigated. In most experiments such test modifications had no significant effect but in one experiment with 4-nitrophenol, inoculation with 1% preadapted seawater decreased the lag phase from >35 to 9 days.

Evaluation of a Flow-ThroughIn VitroSkin Penetration Chamber Method Using Acetone-Deposited Organic Solids

AbstractA systematic evaluation of a flow-through skin penetration chamber, obtained from Oak Ridge National Laboratory, was conducted by measuring in vitro cutaneous penetration with five organic chemicals: urea, caffeine, benzoic acid (BA), salicylic acid (SA), and acetylsalicylic acid (ASA). These hydrophilic test materials were selected for evaluation since it has been established by several investigators that the highly lipophilic compounds are difficult to study in any in vitro absorption system. They were also selected because of their known penetration of the skin of several species, including humans. Each chemical was applied in 5–10 µg/cm2 amounts as 14C-labeled chemicals in 10 µl volumes of acetone. The extent and rate of cutaneous penetration were compared using full-thickness skin from both HRS/J hairless mice and Fischer 344 rats. A rank order for the extent of penetration (percentage of dose absorbed over a uniform time period) was established using Fischer rat skin: caffeine > BA > SA > ur...

On-column radiometric detector for capillary isotachophoresis and its use in the analysis of 14C-labelled constituents

A radiometric detector suitable for capillary isotachophoresis (ITP) is described. This detector, based on solid scintillation counting, consists of three subunits, viz., an on-column detection module, measurement electronics and a microcomputer system. A key part of the detector, a small volume detection cell, has a sensing part made of a plastic scintillator. Counting efficiencies of 10–15% were typical for these cells of effective volumes of 70 or 210 nl (0.3 mm I.D.) for 14C-labelled constituents in a coincidence mode of measurement. By using the cell of effective volume of 210 nl, e.g., a detection limit of ca. 16 Bq was achieved for [U- 14C]acetate under typical ITP working conditions. It is shown by practical examples (cytidine 5′-tri- and diphosphates) that the detector can be very useful in the purity control of 14C-labelled chemicals by ITP. A practical utility of the detector in monitoring a biotransformation of 14C-labelled Cytostasane (a cytostatic drug) by ITP indicates its potential, e.g., in pharmaceutical research. From the ITP analysis of an [U- 14C]protein hydrolysate it can be deduced that the radiometric detector in conjunction with suitable spacing constituents is convenient also for analytical work with complex mixtures of radiolabelled constituents.

Mechanisms of Nitrosourea-Induced β-Cell Damage: Activation of Poly (ADP-Ribose) Synthetase and Cellular Distribution

It has been hypothesized that the critical step in streptozocin (STZ)-induced beta-cell toxicity is the overactivation of the nuclear enzyme poly(ADP-ribose) synthetase resulting from DNA strand breaks. Overactivation of this enzyme leads to a lethal depletion of its substrate, NAD, in the beta-cell. However, recently it has been shown that a lethal concentration of STZ and a nontoxic concentration of its nitrosoamide moiety methylnitrosourea (MNU) damage beta-cell DNA to the same extent and cause comparable amounts of DNA strand breaks. This study was performed to determine whether STZ and MNU activate poly(ADP-ribose) synthetase to the same extent. Monolayer cultures of islet cells from neonatal rats were exposed to concentrations of MNU and STZ of 10(-3) to 10(-2) M. The results show that both chemicals caused comparable activation of the enzyme at all concentrations tested. These data demonstrate that activation of poly(ADP-ribose) synthetase alone is not the critical step in STZ-induced beta-cell toxicity. Based on this finding, it appeared possible that STZ may be selectively sequestered into some critical site in the beta-cell other than the nucleus. Therefore, studies were initiated with 14C-labeled STZ and MNU to determine whether STZ might be distributed in the beta-cell differently than MNU. Total cellular DNA and protein from both RINr (clone 38) and islet cell monolayers were separated on hydroxylapatite columns after exposure to 14C-labeled chemicals. The amount of label incorporated into each fraction was determined by liquid scintillation spectrometry, and the ratio of label incorporated in protein to that in DNA was determined.(ABSTRACT TRUNCATED AT 250 WORDS)

Biomineralization rates of 14C-labelled organic chemicals in aerobic and anaerobic suspended soil

The formation of 14CO 2 from 13 14C-labelled organic chemicals in aerobic and anaerobic suspended soil was determined. After 5 days at 35°C, 14CO 2 was between 70.1% (urea, anaerobic) and < 0.1% (2,6-dichlorobenzonitrile and hexachlorobenzene, aerobic and anaerobic) of the 14C initially applied.

Factors affecting the uptake of 14C-labeled organic chemicals by plants from soil

The uptake of 14C from various 14C-labeled organic chemicals from different chemical classes by barley and cress seedlings from soil was studied for 7 days in a closed aerated laboratory apparatus. Uptake by roots and by leaves via the air was determined separately. Although comparative long-term outdoor studies showed that an equilibrium is not reached within a short time period, plant concentration factors after 7 days could be correlated to some physicochemical and structural substance properties. Barley root concentration factors due to root uptake, expressed as concentration in roots divided by concentration in soil, gave a fairly good negative correlation to adsorption coefficients based on soil organic carbon. Barley root concentration factors, expressed as concentration in roots divided by concentration in soil liquid, gave a positive correlation to the n-octanol/water partition coefficients. Uptake of chemicals by barley leaves via air was strongly positively correlated to volatilization of chemicals from soil. Both root and foliar uptake by barley could be correlated well to the molecular weight of 14 chemicals. Uptake of chemicals by cress differed from that by barley, and correlations to physicochemical substance properties mostly were poor.

Fate of chemicals in plant-soil systems: Comparison of laboratory test data with results of open air long-term experiments

Three laboratory short-term test systems were used to determine volatilization rates, mineralization rates, and conversion rates of 14 14C-labeled chemicals in soils or soil-plant systems. The chemicals covered a wide range of water solubility, vapor pressure, chemical stability, and biodegradability. In order to compare the data obtained with the environmental behavior of the respective chemicals, data from an outdoor experimental setup which had been shown to give residue data within the range limits of field conditions were used. Volatilization rates, mineralization rates, and conversion rates obtained in the laboratory were correlated to those found under outdoor conditions. Based on these correlations, predictions of field residues were made. The predicted values for total field residues were comparable to those found experimentally with the exception of two chemicals. The prediction of residues of unchanged parent compounds in the field was less satisfactory. The reasons for differences observed are discussed.