Macropore transport of bromide as influenced by soil structure differences

Macropore transport of chemicals in soil often causes unexpected contamination of groundwater. The effect of soil structure on the functions of various sized macropores was assessed, investigating transport of nonreactive bromide (Br) under matric heads of 0, -2, -5 and -10 cm using undisturbed soil columns from A, Bw and E horizons of a Thatuna silt loam soil (fine-silty, mixed, mesic Xeric Argialbolls). The experimental breakthrough curves (BTC) for Br were described with a two-region physical nonequilibrium model. Greatest macroporosity occurred in the A horizon and lowest in the E horizon. The measured pore water velocity ν under saturated conditions ranged from 18.92 cm day-1 in the E horizon to 64.28 cm day-1 in the A horizon. While the greatest dispersivity λ occurred in the Bw horizon due to medium subangular blocky and prizmatic aggregates, the lowest dispersivity occurred in the E horizon due to its low macroporosity and massive structure. The fitted mobile water partitioning coefficient β ranged from 0.30 in the A horizon under 0 cm matric head to 0.93 in the E horizon under 0 cm matric head. The calculated values of rate of diffusive mass exchange α decreased with decreasing matric head in A and Bw horizons, and slightly increased and then decreased in the E horizon. The difference among each of the values of the parameters ν, β, α and λ for the A, Bw and E horizons was greatest under saturated conditions. However, gradually decreasing matric head until about -3 cm decreased the difference among the values for a particular parameter for different horizons, sharply. The difference remained fairly unchanged with further decreases in the matric head, suggesting that most of the variability in macropore transport of bromide for these horizons caused by pores with radii larger than about 0.5 mm. In A and Bw horizons, there was a sudden change in soil solution movement between -2 and -5 cm matric head, indicating that macropore flow generally occurred at matric heads greater than -5 cm in the A and Bw horizons. However, decreasing matric head had no effect on mobile water content of the columns from the E horizon. It was concluded that macropore transport of nonreactive solutes generated in the A and Bw horizons may be hampered in the E horizon. Therefore, the depth, thickness and position of the E horizon should be considered in studies targeted to modeling macropore transport of nonreactive chemicals in the soils of Thatuna Series.