Study on Water Diffusion Characteristics in the <i>In-situ</i> Soil of Exfiltration Type Bioretention

Abstract

Exfiltration type bioretention can collect rainwater runoff to recharge groundwater, but the water diffusion in the in-situ soil can have an impact on the foundation of adjacent structures (such as roads). Generally, the existing studies are primarily focused on the exfiltration of bioretention as an index of runoff control or the water balance. However, research is lacking on the diffusion characteristics of soil water content in different in-situ soils. Therefore, in this study, the VADOSE/W model was used to simulate the water transport process of bioretention ponds and in-situ soils, under long-term rainfall. The water diffusion characteristics of four in-situ soils were studied: silt loam (SL), loam (L), sandy clay loam (SCL), and sandy loam (SaL). The results showed that under 12 rainfall events, with a monthly maximum rainfall of 268 mm in the study area, for four in-situ soil types, the bioretention pond's bottom exfiltration volume per unit area reached 3.93-7.91 times that of the lateral. The order of bottom exfiltration volume was SaL>SCL>L>SL. Over time, the in-situ soil water content fluctuated with rainfall events. The order water content was usually SL>L>SCL>SaL, and the water diffused into the in-situ soil was distributed in a symmetrical arc along the horizontal direction. After rainfall events, at depths of 1, 3 and 5 m, for SL, L, SCL, and SaL soils, the lateral water diffusion ranges were ~1. 25-1.47 m, ~1.23-1.45 m, ~1.22-1.77 m, and ~1.46-1.60 m, respectively. With a continuous supply of water, the horizontal diffusion distance of each in-situ soil tended to be the same, although the water diffusion range of SCL was relatively larger. Therefore, when an exfiltration type bioretention area is designed, the distance between the bioretention edge and the adjacent structures should be