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A Procedure to Calculate Biodegradation during Preferential Flow through Heterogeneous Soil Columns

Steenhuis, Dep. of Agricultural and Biological Engineering

Jason W. Kelsey

Inst. for Comparative and Environmental Toxicology

Martin Alexander

Dep. of Soil, Crop, and Atmospheric Sciences, Cornell Univ., Ithaca, NY, 14853

*Corresponding author (tssl{at}cornell.edu).

ABSTRACT

Pesticides are found in groundwater sooner than commonly is predicted with traditional management models. These models do not account for preferential flow and assume uniform biodegradation. For more accurate modeling, the interaction of preferential flow and nonuniform biodegradation needs to be addressed. In this paper, we tested techniques to measure the rate of biodegradation in preferential flow paths by continuously applying p-nitrophenol (PNP) to the surface of packed soil columns that contained two types of preferential flow paths. The first type was an open macropore in a uniform soil matrix (macropore column), and the second type was a channel of coarser soil in a finer soil matrix (channel columns). Breakthrough curves were obtained from separate small areas of the macropore column with fiberglass wicks to collect and separate macropore flow from soil matrix flow. Breakthrough curves were also obtained from the channel columns for comparison to curves from homogeneous columns. Average biodegradation rates were calculated from the breakthrough curves. The PNP biodegradation occurred in both matrix and macropore flow regions of the macropore column, but the time and rate of biodegradation differed in the two flow regions, as well as between the channel columns and the homogeneous columns. The PNP was degraded rapidly despite short travel times through the path of preferential flow, indicating that rapid biodegradation may occur in macropores and other preferential flow paths. The data provide a basis for improved modeling of fate and transport of chemicals through vadose soil to groundwater.

Received for publication August 23, 1993.

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