HERBICIDE RESIDUE DISTRIBUTIONS IN RELATION TO SOIL MORPHOLOGY IN TWO CALIFORNIA VERTISOLS

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Abstract
The distributions of herbicide residues in two California Vertisols, one uncultivated and one cultivated, were studied to determine residue relationships to soil morphology, particularly cracking patterns. Soils were sampled from backhoe trench walls in a grid pattern related to crack features. In the uncultivated Vertisol, herbicide residues (trifluralin, 7 to 3750 ppb, and 2,4-D, 8 to 143 ppb) were largely confined to surface samples. In contrast, herbicide residues (atrazine, 2 to 39 ppb) in the cultivated Vertisol were detected not only at the surface, but in crack walls and directly below open cracks to a depth of 125 cm. No residues were detected within the soil matrix between open cracks in the cultivated Vertisol, suggesting that herbicides have moved preferentially via cracks. Probable mechanisms of herbicide redistribution include being carried with surficial material that falls into cracks, and moving through cracks either in solution or in association with suspended colloids. Cultivation practices, including plowing and irrigation, apparently promote herbicide translocation, whereas redistribution is much slower in uncultivated Vertisols. Rapid transport of herbicides to the base of Vertisol cracks may be a process that can contribute to ground water contamination, particularly where Vertisols occupy basins with seasonally high water tables.

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