Submitted to: Geoderma
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/4/2001
Publication Date: 2/4/2002
Citation: Ersahin, S., Papendick, R.I., Smith, J.L., Keller, C.K., Manoranjan, V.S. 2002. Macropore transport of bromide as influenced by soil structure differences. Geoderma 108:207-223. Interpretive Summary: Knowledge of chemical movement through soils is important for enhancing crop growth and groundwater protection. As dryland agriculture moves into more conservation tillage and diverse crop production it is important that we understand movement of chemicals on the landscape. In most soils water movement through the profile can be different due to changing soil layers in the profile. Since soils in the Pacific Northwest region are highly layered we studied the flow of chemicals layer by layer in laboratory experiments. We leached a salt through the layers of soil at different water levels to study how the porosity of the soil affected the movement of the salt solution. Our results showed that chemicals that flow fairly quickly through the upper layers of the soil may be impeded in lower layers with less macropores. This suggests that water movement in these soils may move mostly laterally rather than leaching towards groundwater. Thus it will be important to monitor surface water in the Pacific Northwest area for chemical contamination.
Technical Abstract: 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, B_w 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 v under saturated conditions ranged from 18.92 cm d^-1 in the E horizon to 64.28 cm d^-1 in the A horizon. The fitted mobile water partitioning coefficient beta ranged form 0.30 in the A horizon under 0 cm matric head to 0.93 in the E 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 alpha decreased with decreasing matric head in A and B_w horizons, and slightly increased and then decreased in the E horizon. 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 B_w 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 B_w horizons. However, decreasing matric head had no affect 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 B_w 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.