X-ray study of soil structure changes in Constructed Technosol of the layered system of bioretention cells

Constructed Technosols play an important role in urban hydrology e.g. in the functioning of green roofs and stormwater bioretention cells. Water infiltration, colloid transport, and heat transport are affected by changes in pore system geometry particularly due to the development of macropores and clogging by particles. The aim of the study is to relate changes in bioretention cell performance to the structural changes of soils at the microscale by invasive and noninvasive methods. Noninvasive visualization method of X-ray microtomography was used to investigate soil of the biofilter in terms of structure development, pore-clogging and pore geometry deformations.

Two identical bioretention cells were established in December 2017. The first bioretention cell (BC1) collects the stormwater from the roof of the nearby experimental building (roof area 38 m2). The second bioretention cell BC2 is supplied from a tank using a controlled pump system for simulating artificial rainfall. Each BC is 2.4 m wide and 4.0 m long. Subsurface of the bioretention cell is formed by biofilter (Constructed Technosol), sand filter and a drainage layer. The 30 cm thick biofilter soil mixture is composed of 50% sand, 30% compost, and 20% topsoil. Bioretention cells are isolated from the surrounding soil by a waterproof membrane. The regular soil sampling program was initiated in 2018 in order to visualize and quantify the soil structure and internal pore geometry of samples. Undistributed samples were collected from the surface of the filter layer from each BC. The aluminum sampling cylinders had an internal diameter and height of 29 mm. Batches of 12 samples were collected on June 5, 2018 (7 months after establishment), November 1, 2018 (12 months after establishment), May 5, 2019 (18 months after establishment), June 29, 2019 (22 months after establishment) and the last batch of samples on June 18, 2020 (31 months after establishment) from each BCs. Those collected samples were scanned by CT imaging at the water content equilibrated at -330 hPa.

Analyses of pore network morphology were performed on the segmented 3D images of samples. Macroporosity, pore thickness, pore connection probability, critical diameter and Euler-Poincare density were determined to understand pore space in the biofilter. Furthermore, porosity, dry sample bulk density and volumetric water content at pressure head representing a field capacity of -330 hPa were measured on all samples.

During the first year, the macroporosity decreased in both BCs due to soil consolidation. A significant correlation was found between macroporosity and connection probability, as well as between macroporosity and critical diameter. Pore thickness analysis revealed that the most represented pore fraction during the three years was 80-310 μm in size. Results of the study show that short term consolidation was followed by gradual development of macropore system in Constructed Technosol of bioretention cell. The biofilter exhibited optimal conditions for plant growth, particularly in BC1 with natural water inflow. There was no significant drying in the biofilter layer in BC1 and the volumetric water content ranged from 0.2 to 0.4.