Using High‐Resolution Computed Tomography Analysis To Characterize Soil‐Surface Seals

Abstract

High‐resolution X‐ray computed tomography (HRCT) acquires three‐dimensional (3D) images of objects using rapid, nondestructive, and noninvasive procedures. Data from HRCT scanners may be useful for quantitative analyses of soil‐surface seals. Raindrop impact decreases porosity and increases density (ρ) through a process known as surface sealing. Few studies have attempted porosity measurements of intact seals. The objective of this study was to evaluate the ability of HRCT scans of surface seals from a silt loam soil to measure ≥ 15‐μm equivalent diameter (e.d.) pores within developing seals. Soil was either treated with anionic polyacrylamide (PAM) or untreated, and subjected to simulated rainfall durations of 7.5, 15, 30, and 60 min at a 55‐mm h−1 rainfall intensity. Seal analysis included: pore‐size distribution, cumulative porosity of the 0‐ to 2‐mm depth and cumulative porosity of the upper 0‐ to 1‐ and lower 1‐ to 2‐mm zones through seals. The difference in cumulative porosity between PAM‐treated vs. untreated soil significantly increased with rainfall duration. Pore frequency was greatly reduced in pores with pore sizes from 1 × 10−4 to 1 × 10−5 mm3 The modal pore‐size frequency with 1 × 10−5‐mm3 pores for the PAM‐treated soil was reduced from 992 to 812 pores between 7.5 and 60 min of rain, whereas pores in the untreated soil were reduced from 649 to 217 pores in the same conditions. The cumulative porosity of the 0‐ to 2‐mm sample after 7.5‐min of rainfall was 0.018 mm mm−1 with no difference between PAM‐treated and untreated soil. After 60 min of rainfall the cumulative porosity was reduced to 0.0156 mm mm−1 for PAM‐treated soil and 0.0004 mm mm−1 for untreated soil. The cumulative porosity for the 0‐ to 1‐mm depth zone for PAM‐treated and untreated soil after 60 min of rainfall was 0.011 mm mm−1 vs. < 0.001 mm mm−1, and 0.043 mm mm−1 vs. 0.008 mm mm−1 for the 0‐ to 1‐mm depth zone, documenting that PAM‐treated soil had significantly less reduction in porosity over the 60‐min rainfall duration. Results show that HRCT provides quantitative measurements of changes of ≥ 15‐μm pore characteristics over time.