Soil residue dissipation data

The heptachlor, dieldrin, and toxaphene examples show that foliage and soil residue dissipation data can be used to estimate the amounts of residues volatilized when no significant degradation or runoff losses are incurred. For toxaphene, residue analyses indicated that 80% of the foliage residue and 51% of the top soil residue was lost by volatilization within ca 50 days. This is considerably more than the 24% vaporization loss reported for toxaphene within 90 days in a model chamber (8), but is comparable to foliage-applied heptachlor and dieldrin (75) both of which overlap in volatility with components of the toxaphene mixture. 

Reentry intervals are now established on the basis of (1) data on dermal absorption or dermal dose response (2) inhalation, dermal, and oral acute toxicity studies in animal models (3) foliar and soil residue dissipation data and, (4) available human exposure data. CDFA recommends several sources as useful guides for determining residues of pesticides on soil and leaf surfaces (dislodgeable residue) and conducting field reentry studies involving human volunteers (1-5). Human exposure studies may not be required if adequate animal data from (1) through (3) above are... 

Analyses of residues in the soils after incubation showed that the persistence of diazinon was considerably shorter in the previously treated soil than in the untreated soil. The half-life value for diazinon in previously treated soil was 1.7 days while in the untreated soil it was 9.9 days. Most of the insecticide added to the previously treated soil was lost within 10 days. Paddy water from the same fields were tested also for diazinon-degrading activity (17). Again water from a rice field treated previously with diazinon inactivated the insecticide more rapidly than did the water from an untreated field. In the water from the previously treated field the insecticide dissipated completely within 3-5 days of incubation after an initial lag of 1-2 days (17, 18). Table II summarizes the results of the study on the stability of diazinon in soil and paddy water. The data indicated clearly that a factor capable of degrading diazinon developed in rice fields of the Institute farm after insecticide applications. The diazinon-degrading factor, found in the diazinon-treated rice fields in the Institute farm, was noticed also in three other locations in the Philippines (19). 

As more sensitive analytical methods for pesticides are developed, greater care must be taken to avoid sample contamination and misidentification of residues. For example, in pesticide leaching or field dissipation studies, small amounts of surface soil coming in contact with soil core or soil pore water samples taken from further below the ground surface can sometimes lead to wildly inaccurate analytical results. This is probably the cause of isolated, high-level detections of pesticides in the lower part of the vadose zone or in groundwater in samples taken soon after application when other data (weather, soil permeability determinations and other pesticide or tracer analytical results) imply that such results are highly improbable. 

Figure 1 was taken from an unpublished report, DuPont Study No. AMR 4392-97, Dissipation of Dislodgeable Foliar and Soil Residues of Oxamyl Following Application of Vydate L Insecticide to Tomatoes in the USA - Season 1997-1998 . This study has been submitted to the EPA and the data were used to establish and verify re-entry intervals. Data from this study will be used to provide an example of the topics discussed throughout this article. 

These data were measured at or extrapolated to ambient temperature and pH values. The data are discussed in the text. NA = not available. b/ Kq = soil water distribution coefficient (K ) divided by the organic carbon content of the soil, cj Whenever possible, half-life for soil dissipation is derived from the field data half-lives described in the text rather than lab data. As such, it may not represent a true first-order process. Value has been estimated from the equation in ref. 20. e/ Hydrolysis of total residues (aldicarb + sulfoxide + sulfone). pK for p -phthalic acid is 3.5. The chlorine atoms of DCPA should lower the pK to about 2. Conditions optimized for soil metabolism. 

For the pristine chemistry studies which include studies such as hydrolysis, soil and water photolysis, soil dissipation, and rotational crop under environmental fate, metabolism studies, residue studies, and product chemistry studies, such as vapor pressure, octanol-water partition coefficient, and water solubility, the total study is audited. This includes the GUP issues, such as adherence to protocols, SOPs, and record accountability completeness of raw data the validation of data points and the overall scientific issues. 

The ability to perform quantitative assays on complex mixtures with little sample clean-up is perhaps the most attractive feature of immunoassays for application to agricultural chemistry. A large portion of the cost and labor involved in pesticide residue analysis is invested in sample extraction and clean-up steps to remove substances which may interfere with subsequent chemical analysis. Since most preparatory steps are not required prior to performing an immunoassay, samples can be analyzed much less expensively. This will permit the vast number of data points required for pesticide registration to be gathered in a more timely and cost-effective manner. Studies which were prohibitively expensive because they would have required large numbers of expensive assays can be completed using immunoassay procedures. Such studies may include analysis of pesticide movement from application areas and the rate of dissipation of pesticide from crop tissue, soils, and processed foods. 

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