Soil structure changes of constructed soil in bioretention cell during three years

Constructed soils 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 is to elucidate changes in bioretention cell performance by studying the structural changes of soils at the microscale by invasive and noninvasive methods. Noninvasive visualization methods such as computed microtomography (CT), are an effective mean of soil structure assessment. X-ray CT is capable to investigate soil 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. 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 twice a year from each BC. The aluminum sampling cylinders had an internal diameter and height of 29 mm. Three batches of samples were taken during three years. The first set of 24 undisturbed samples was collected upon planting in June 2018, while the second set of 24 samples was taken after the end of the first vegetation period in November 2018. The second batch of 48 samples, were taken in the same period as in the previous year.  The last batch of 24 samples was taken in June 2020. Those collected samples were scanned by CT imaging.

Analyses of pore network morphologies were performed on the scanned samples. Macroporosity, pore thickness, pore connection probability, critical diameter and Euler-Poincare density were determined to understand pore space in the biofilter. Macroporosity in BC1 shows a decreasing trend in the first three periods, it can be a result of soil consolidation. In subsequent periods, macroporosity remains constant in BC1. The characteristic pore connection probability in BC1 also shows a decreasing trend in the first three periods, but compared to the macroporosity, the connectivity increases in the last two periods in BC1. This may be due to plants growth, which was most pronounced in 2019. The samples' most frequently represented pore thickness ranges from 80 to 330 µm in all periods in both BCs. The percentage of these pores was higher than 50% in both BCs.