Investigating The Roles Of Soil Structure And Surface Paving In Determining Soil Gas Concentration Attenuation Of VOCs In Two-dimensional Transport With A Scale Model

Since the reform and opening up,with the rapid adjustment of the industrial structure of cities in China,many cities have relocated and closed down some enterprises with "high pollution and high consumption",resulting in a large number of contaminated sites.In response,the State Department issued《The Soil Pollution Action Plan》in 2016,requiring remediation or risk control of contaminated sites before reuse.Whether remediation or risk management,the key is the human health risk assessment of contaminated sites.It is the process by which volatile compounds migrate from the underground soil into the indoor and are inhaled by the human body.At present,Johnson-Ettinger mathematical model(JEM)is recommended to evaluate the risk of vapor intrusion in the world(including China).However,in practice,due to the uneven distribution of pollution source.The volatile compounds often exist in two dimensions including vertical and horizontal transport.But JEM is a one-dimensional model,which only considers the transport of volatile compounds in the vertical direction,and can not reasonably assess the risk of vapor intrusion.In the past,the study of volatile compounds in two-dimensional transport and concentration attenuation is usually based on analytical or numerical model,and there is no verification of relevant sites and laboratory data.In this paper,We investigate the roles of soil structure and surface paving in determining soil gas concentration attenuation of VOCs in two-dimensional transport with a scale model.The experimental data is compared with results of the mathematical model.The main work is as follows:(1)The homogeneous soil with no surface paving experiment,using two kinds of soil,namely sand and sandy soil.The results show that the concentration distribution of pollutants in the soil is not related to the soil media,the experimental results are consistent with the concentration distribution predicted by the three-dimensional numerical model and analytical model:the concentration decays exponentially in the lateral direction and decays linearly in the vertical direction.(2)The homogeneous soil with surface paving experiment,compared with no paving,the surface paving will increase soil concentration.The surface paving includes continuous paving and non-continuous paving.At deep sampling points,continuous paving has greater impact on concentration than non-continuous paving,but the difference is not obvious.It shows that there is lateral distance between the source and building,the concentration of shallow pollutants under the paving is still larger than that under no paving.This shows that the US EPA uses the sampling points concentration below a building’s foundation and along the side of the building closet to the source to represent the concentration of pollutants under the foundation,which has limitations.In deep areas,the concentration decays exponentially in the lateral direction and decays linearly in the vertical direction,with little difference compared with the no paving.In shallow areas,the concentration has a "peak shape" distribution in the lateral direction due to the influence of the paving,and decays linearly in the vertical direction.The experimental results are basically consistent with the three-dimensional numerical model and analytical model.(3)The heterogeneous soil with no surface paving experiment,using three kinds of soil:sand、sandy soil and clay,and the diffusion coefficient decreases in turn.The positive or negative impacts on the concentration depend on the order of the media.The results show that compared with the homogeneous soil experiments,if the effective diffusion coefficient of soil decreases with the depth,the lateral attenuation of the concentration will increase,and that in the vertical direction will decrease;If the effective diffusion coefficient increases with depth,the lateral attenuation of the concentration will decrease,and that in the vertical direction will increase.The experimental results are consistent with the concentration distribution predicted by the three-dimensional numerical model and analytical model:the concentration decays exponentially in the lateral direction and decays in sections linearly in the vertical direction.(4)The heterogeneous soil with surface paving experiment,in deep areas,the concentration decays exponentially in the lateral direction and decays linearly in sections in the vertical direction,which shows little difference compared with no surface paving.In shallow areas,the concentration has a "peak shape" distribution in the lateral direction due to the influence of the paving,and decays linearly in the vertical direction.The experimental results are basically consistent with the three-dimensional numerical model and analytical model.