Abstract: With the acceleration of industrialization, the problem of heavy metal pollution in soil is becoming increasingly serious. Soil heterogeneity, especially the difference in the size of secondary aggregates (soil clods), significantly affects the pore structure, permeability and hydrological characteristics of the soil, thereby changing the migration process of heavy metals. Recently, although some studies have focused on the effect of soil aggregates on heavy metal migration, the mechanism of the role of soil clods of different sizes in actual soil pollution control is still lacking in-depth exploration. In this paper, the Xiashu soil which is widely distributed in the middle and lower reaches of the Yangtze River was taken as the research object. The microscopic pore distribution under different soil clod sizes was analyzed by mercury intrusion injection test (MIP), and the effect of soil clod size on the migration characteristics of typical heavy metals (lead) in the region was explored by semi-dynamic leaching test and constant head saturated permeability test system. The results show that with the increase of soil clod size, the porosity and saturated permeability coefficient of the soil body decreased. The 2-4 mm soil clod had a high lead leaching concentration in the early leaching process, resulting in a large amount of migration and dissolution; the <1 mm soil clods had a relatively uniform lead migration rate due to their strong structural connectivity, and their migration was controlled by a diffusion-based mechanism; the 1-2 mm soil clods had a moderate lead leaching concentration in the early stage and a low dissolution amount in the middle and late stages, showing a relatively strong ability to inhibit lead migration and diffusion. There fore that the reasonable distribution of soil clod size plays a key role in inhibiting the migration of heavy metals. Optimizing the size of soil clods to form a suitable soil structure can effectively slow down the migration of lead, The results provide a scientific basis for the long-term control and remediation of heavy metal-contaminated soils.

Key words: secondary aggregates, compacted soil, lead, heavy metal migration, permeability