A procedure to calculate biodegradation during preferential flow through heterogeneous soil columns

Pesticides are found in groundwater sooner than commonly is predicted with traditional management models. These models do not account for preferential Bow and assume uniform biodegradation. For more accurate modeling, the interaction of preferential Bow and nonuniform biodegradation needs to be addressed. In this paper, we tested techniques to measure the rate of biodegradation in preferential Bow paths by continuously applying p-nitrophenol (PNP) to the surface of packed soil columns that contained two types of preferential Bow paths. The Brst 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 Bow. 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 Bow regions of the macropore column, but the time and rate of biodegradation differed in the two Bow 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 Bow, indicating that rapid biodegradation may occur in macropores and other preferential Bow paths. The data provide a basis for improved modeling of fate and transport of chemicals through vadose soil to groundwater.