Heptachlor
PESTICIDE NAME: Heptachlor
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Trade name(s): Heptachlor
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Manufacturer(s): Velsicol Chemical Corp.
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I. Basic information
A. Molecular structure: C10H5Cl7
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B. Chemical name: 1,4,5,6,7,8,8-Heptachloro-3a,4,7,7a-tetra-
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hydro-4,7-endomethanoindene
C. Derivatives: heptachlor epoxide
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D. Molecular weight: 373 g/mole
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E. Solubility in water: practically insoluble
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F. Common physical appearance: No information available
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G. Oral LD50(rat): 90 mg/kg
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H. Pesticide classification: synthetic cyclic hydrocarbon
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insecticide
I. Restricted use list (N.Y.): yes
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EPA priority pesticide list: no
J. Crop use: No information available
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II. Text
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Heptachlor is a synthetic cyclic hydrocarbon insecticide which is
considered to be immobile and of long persistence in soil. It is
oxidized to heptachlor epoxide in the soil and gamma chlordane is the
principal impurity found in technical heptachlor. The scientific
literature contains substantialinformation on heptachlor and its fate
in the soil.
III. Soil information
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A. Degradation and transformation
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The degradation of heptachlor, which is first-order, follows the
pathway of rapid oxidation to heptachlor epoxide, dechlorination by
microorganisms to chlordene and then to chlordene epoxide(6,9). The
half-life is estimated to be 0.91yrs(5) with a 95% disappearance in
3-5yrs(5,6). A study of heptachlor degradation on red latosolic
oxisol, black mollisol and calcareous alluvial soils reported no
accumulation in field soil until 6mo after application of heptachlor.
Peak concentrations were reached at 9-10mo and the concentration
remained high until 1yr after initial application then declined and no
carry-over was noted from year to year(8). In a 9yr study with
heptachlor application rates of 10 and 100 lbs a.i./A, 70% of the
applied heptachlor was gone in 3yrs at the 10 lbs/A treatment with most
remaining as the epoxide. Six years after initial application there
was no detectable heptachlor and at 8yrs, 7% of the original
application remained as epoxide in non-cultivated soil with <1% in
cultivated soils. The 100 lb/A treatment gave similar results, i.e.,
3yrs - 75% gone, 7yrs -<10% heptachlor+epoxide and 8yrs - no heptachlor
with 4-5% epoxide(9).
The following tables present data concerning heptachlor
degradation in soils. The reference is given at the end of each title.
Ppm heptachlor remaining in 3 Hawaiian soils 7yrs after application of
recommended dose(2)
Coral Sandy Loam Clay
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Appl. Found Appl. Found Appl. Found
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358 1.44 474 3.88 503 5.23
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Percent recovery of initial heptachlor application in clay soil resting
on sand filter(4)
1 3 6 9 12 15 (days)
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82 77 76 68 64 52 (%)
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Percent heptachlor and epoxide recovered in sandy loam soil(1)
%recovered
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0 100 0
15 75.9 24.1
30 68.7 31.3
45 59.1 40.9
60 47.2 52.8
75 42.9 57.1
90 39.1 60.9
120 40.0 60.0
150 30.8 69.2
180 25.0 75.0
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Heptachlor concentration remaining in silt loam soil(5)
Time(years) PPM(mean 0-23cm)
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0 1.46
0.47 0.79
0.92 0.69
1.51 0.28
1.93 0.23
2.43 0.15
3.49 0.08
4.45 0.05
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B. Adsorption and transport
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Heptachlor is considered to be immobile although some absorption
into the plant from the soil has been reported; this occurs more
readily in sandy soil than in muck(6). The pesticide is more effective
in sandy soils than those with higher clay content although it can be
dislodged if rain occurs soon after application(6). More of the
epoxide than the original heptachlor has been found to leach into the
subsurface. Most residues are in the top 15cm.
The following tables present data concerning heptachlor adsorption
in soils. The reference i.M. Edwards. 1975. J.Ag.FoodChem.
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23. 1101-5.
6.Kahn, S.U. 1980. Pesticides in the Soil
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Environment.Amsterdam:Elsevier Press.
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*7.Stewart, D.K.R. and C.J.S. Fox. 1971. J.Econ.Ento. 64.367-71.
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*8.Talekar, N.S., H.T. Kao and J.S. Chen. 1983. J.Econ.Ento. 76.
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711-16.
*9.Wingo, C.W. 1966. Mo.Ag.Res.Sta.Bull. No.914.
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