COMPARATIVE BEHAVIOR OF DIELDRIN AND CARBOFURAN IN THE FIELD J. H. CARO, A. W. TAYLOR,and H. P. FREEMAN Agricultural Chemicals Management Laboratory Agricultural Environmental Quality Institute U.S. Department of Agriculture Beltsville, Maryland 20705
To measure the amounts of dieldrin and carbofuran lost to the environment, we incorporated them into soils in small (0.6-1.1 ha) watersheds in separate years. The disappearance of each was monitored by periodically measuring residues in the soil, runoff, maize plants, and overlying air (dieldrin only). Soil residues were nonuniformly distributed. Best estimate for the time for 95% disappearance of dieldrin from the soil was 12.8 years. Carbofuran disappearance conformed to a first-order reaction and gave 95% disappearance times ranging from 145 to 434 days, depending on soil pH, moisture, and temperature. Runoff losses of both pesticides were highest in rainfalls during the first month after application. Over the season, dieldrin losses ranged up to 2.3% of that applied and were concentrated in the solids. Carbofuran losses in runoff occurred largely in the water and comprised up to 1.9% of the application. More than twice as much carbofuran (and metabolites) as dieldrin was accumulated in the maize plants, mainly in the leaves. Volatilization was an important route of dieldrin loss, amounting in the first year to 4.5% of that applied. Volatility of carbofuran, which was only 1/18th that of dieldrin in a laboratory test, was not measured in the field. The data show that use of optimum management practices can substantially reduce the environmental impact of agricultural applications of these pesticides. This paper summarizes and compares the results of our field studies on residues of dieldrin (HEOD) ( 1,2,3,4,10,10-hexachloro-6,7-epoxy- 1,4,4a,5,6,7,8,8a-octahydro-l,4-endo-exo-5,8-dimethanonapthalene) and carbofuran (2,3-dihydro-2,2methyl-7-benzofuranyl methylcarbamate). The objective was to assess the environmental impact of agricultural applications of these pesticides by measuring their persistence in soil and their losses from the treated area by runoff, by plant uptake, and by volatilization. Dieldrin was applied in four successive years, 19661969, and carbofuran was applied in 1971 and 1972. Since all the experiments were conducted at the U S D A Research Station at Coshocton, Ohio, and all were done in the same general manner, the total program provided a unique opportunity to compare the behavior of a persistent organochlorine insecticide with that of a less persistent methylcarbamate insecticide. Results should be evaluated with a full realization that it was not possible to remove all variables between e x p e r i m e n t s - differences in weather and pesticide formulations are inherent in work of this nature. When a pesticide is applied to the soil, the ultimate impact o f the treatment on the environment is influenced by such factors as the ambient weather conditions; the Archives of Environmental Contamination and Toxicology Vol. 3, 437-447 (1975/76) 9 1976 by Springer-Verlag New York Inc.
J.H. Caro et al.
formulation, solubility, and volatility of the chemical; the extent and strength of adsorption on soil surfaces; and the rates of chemical, microbial, and photochemical degradation. As a class, the organochlorine insecticides are known to be less water soluble, more strongly adsorbed (and thus less leachable), and less subject to decomposition than the methylcarbamates. The properties of dieldrin and carbofuran pertinent to their potential environmental impact are shown in Table I. The results of our experiments illustrate the magnitude of the various pathways by which the two chemicals are lost after application and thus allow their differences to be quantitated. It is important to recognize that application losses, which are not treated here, can sometimes be as great as the postapplication dissipation processes that are discussed.
Description of experiments The experiments with the individual pesticides have been described in earlier publications (Caro and Taylor 1971; Caro et al. 1973). In brief, the pesticides were applied to 0.6- to 1.1-ha watersheds at rates of 3. I to 5.6 kg/ha of active ingredient and maize was planted on the same day. The soils in all the watersheds were silt loams of pH 5.2 to 6.4. The field slopes were between 7 and 14%. Dieldrin was applied in a spray of aqueous emulsion and was immediately disked into the soil to 7.5-cm depth. Carbofuran granules were either broadcast and incorporated to 7.5-cm depth or applied in the furrow along with the maize seed. For soil sampling purposes, the dieldrin-treated watersheds were arbitrarily divided into five or eight subplots and composited spade or core samples taken from each subplot were analyzed. Samples were taken within one day of dieldrin application and semiannually or annually thereafter. The soils in the carbofuran-treated plots were sampled on the day of pesticide application and at subsequent four- to eight-week intervals at 10, 16, or 24 points arranged in a regular pattern over the entire field. At each point, a 30-cm x 30-cm area of soil straddling the maize row was excavated deep enough to collect all the pesticide within the area and the excavated soil was uniformly subdivided to obtain an analytical sample.
Table I. Properties o f dieldrin and carbofuran pertinent to assessment o f environmental impact
Property Vapor pressure at 25~
2.6 x 10 - 6
8.3 x 10 - 6
Solubility in water at 25~ /.tg/1
250 x 103
Acute oral toxicity to rats, LDso, mg/kg
Behavior of Dieldrin and Carbofuran
After pesticide application, runoff from each watershed was sampled in a timed sequence as it occurred by using automatic equipment installed at the discharge flume at the foot of the sloping field. Suspended solids in the runoff were separated by continuous centrifugation and sediments remaining in the flume a f t e r each rainfall were sampled manually. All solids were frozen for future analysis. The maize crops were sampled by collecting 10 to 40 plants from each watershed, separating them into leaves, stalks, cobs, and kernels, and compositing the individual parts from groups of ten plants for analysis. Samples were taken at maturity in the dieldrin-treated plots and at both silage stage and harvest in the carbofuran treatments. Volatilization of dieldrin from the treated watershed was measured in 1969. The overlying air was sampled on seven days spaced throughout the crop season, starting with the second day after dieldrin application. On each day, the air at five heights ranging up to two m above the bare soil or above the maize canopy was sampled at two locations in the field for nine consecutive two-hr periods or four four-hr periods, with the pesticide traps being changed at the end of each period. Other measurements necessary for calculations of dieldrin vapor f l u x g w a t e r flux from the soil, moisture gradients in the air, net radiation, and profiles of wind speed and air temperaturegwere made concurrently with the pesticide samplings. Interpolation for periods between sampling days allowed estimation of the total dieldrin vapor flux for the season. Details of the measurement of pesticide concentrations in air have been described by Caro et al. (1972b) and the meterological treatment has been presented by Parmele et al. (1972). Soils, sediments, runoff waters, plant materials, and air were all analyzed for both dieldrin and carbofuran by electron-capture gas chromatography after extraction with a suitable solvent, as described in the publications cited earlier. Dieldrin was analyzed directly, whereas carbofuran was converted to its volatile 2,4dinitrophenyl ether derivative for gas chromatography.
Persistence in soil Irrespective of the sampling technique used, variability in pesticide content was high among soil samples taken at any given time from any of the treated fields. The differences resulted from the combined effects of irregular pesticide application, poor mixing during incorporation into the soil, nonuniform sub-sampling, and variable rates of dissipation in different areas of the fields (Taylor et al. 1971; Caro et al. 1973). Regression analysis showed that the best estimate of the rate of dieldrin disappearance from the treated field was" given by the linear relation D --- 2.72 0.200 ( - 0 . 0 9 0 ) T , where D is dieldrin concentration in /~g per g of dry soil, T is time in years, and the range of the regression coefficient corresponds to the 80% confidence interval (Freeman et al. 1974). Carbofuran disappearance was best described by a logarithmic rather than a linear function, indicating that the disappearance conformed to first-order reaction kinetics. Statistical analysis of the two
J . H . Caro et al.
carbofuran treatments of 1971, in which the pesticide was broadcast in one watershed (soil pH 6.4) and applied in the furrow in a second watershed (soil pH 5.2) gave the relations Log C = 2.528 - 0.0057 ( - 0.0008)T and Log C = 2.865 0.0030 ( • respectively, where C is carbofuran concentration in mg per m 2 of soil surface, T is time in days, and the 80% confidence limits of the coefficient are indicated. In 1972, carbofuran was applied in the furrows in the same field in which the material had been broadcast in the previous year. The later treatment yielded the relationship Log C = 2.892 - 0.0051 ( • The ~5% disappearance times calculated from the regression equations and the 80% confidence limits are shown in Table II. The measurements for carbofuran were more precise than those for dieldrin because carbofuran is less persistent and measurements were made over a larger portion of its life in the soil (Taylor et al. 1971). The 12.8 years indicated for 95% disappearance of dieldrin is about 50% longer than the average time found by other workers in a number of experiments in which dieldrin persistence was measured under normal cultural practices (Caro 1969). The longer persistence time probably reflects the infrequent cultivation of the soil, which was three times in the six-yr sampling period during the Coshocton experiment. In broad terms, Table II shows that dieldrin was 10 to 20 times more persistent than carbofuran in the Coshocton soils. Closer estimates are not possible because the persistence of carbofuran varied in the three treatments studied. Carbofuran applied in the furrow in 1971 was significantly longer lived than in the other two treatments, requiring 434 days for 95% disappearance. Clearly, if conditions are right, carbofuran could retain some biological effectiveness into the crop season following the one in which it was applied. Under other conditions, it may be essentially dissipated within one season, as in the 1972 watershed treatment. In a small area of this watershed where the soil was of higher pH and was slightly more moist than in the rest of the field, carbofuran disappeared with a half-life of only 33.5 days, corresponding to 145 days for 95% disappearance (Caro et al. 1973). Dieldrin also has been observed to disappear faster from wet areas than from dry ones (Freeman et al. 1974). The persistence of carbofuran is affected not only by soil moisture but also by soil acidity, soil temperature, and type of placement of the pesticide (Caro et al. 1973).
Losses in runoff Pesticide losses in the runoff from the three treatments in which sufficient runoff water and sediment were produced to permit detailed analysis are shown in Table III. An extensive examination of the runoff from the dieldrin- treated watersheds has been presented by Caro et al. (1972a) and from the carbofuran treatments by Caro et al. (1973). Runoff during the season contained about 1 or 2% of the amounts of dieldrin and carbofuran applied (Table III), but the two pesticides were distributed differently between water and sediments. Dieldrin was carried off the watershed primarily in the sediments because of its low water solubility and strong adsorptivity
113 118 113
Year of application
Time for 95% disappearance
Table II. Persistence o f dieldrin and carbofuran in acid silt loam soils
237 to 277 days
383 to 500 days
200 to 266 days
8.2 to 29.7 years
80% confidence limits