Abstract

Soil matric and air pressures can significantly affect each other especially if the soil-gas phase becomes disconnected from the atmosphere. Such effects were presumed to be one possible explanation for observed preferential flow patterns in a mine soil. The objective of this study was to evaluate interactions between soil-air and water pressures in situ under field conditions. In a 2.5-m-wide mine soil pit, nine soil air pressure probes were horizontally installed 27 cm apart in a row at 80 cm depth and augmented by two rows of tensiometers, one row 10 cm below and one row 10 cm above the air probes. Field data were recorded for 3 wk in autumn 2001. At two of the nine measurement positions, deviations of soil air from atmospheric pressure were observed. Solely at these two positions, water-saturated conditions were found and drainage could be collected using a cell lysimeter, indicating that these air probes were disconnected from aboveground air. After accounting for soil temperature effects, the fluctuations in soil air pressure were found to correspond with those of the ambient soil water pressure during infiltration events. The gradual decrease of soil air pressure after the event might be induced by suction effects of draining soil water in the flow finger with the soil water pressures remaining above the air entry pressure of the soil. Results suggest that effects of flow on soil air pressure can occur locally. In this heterogeneous mine soil, the air–water pressure interactions were restricted to soil regions that coincided with preferential flow paths.

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