Studv On Influences Of Soil Texture And Indoor Pressure Fluctuation In Chlorinated Vapor Intrusion

In recent years,with the development of urbanization in China,more and more industrial enterprises have moved to suburbs or industrials parks,leaving a large number of contaminated sites.According to the Technical Guidelines for Risk Assessment of Polluted Sites promulgated by the Ministry of Ecology and Environment in 2014,the indoor inhalation risk of volatile pollutants in lower soil,namely vapor intrusion(Ⅵ),is the most critical factor,which often determines the repair depth and cost of the whole site.How to accurately predict the risk of vapor intrusion and take measures to prevent the threat of vapor intrusion to people’s health is extremely critical.Some studies showed that environmental factors may play an important role in VI process,and it is very important to identify and quantify environmental factors to accurately assess VI risk.Based on numerical model tools and field experimental data,this paper plans to study the role of shallow soil texture and indoor pressure fluctuation in vapor intrusion,taking typical chlorinated hydrocarbons—trichloroethylene as research objects.(1)In chlorinated contaminated sites,when the sub-foundation soil permeability is larger than10-12 m2,the convection dominates the soil gas transport into the building,and the indoor air concentration increases by half an order of magnitude with one order of magnitude increase of the sub-foundation soil permeability.Otherwise,diffusion plays a more important role and the sub-foundation soil texture does not cause significant variation of indoor air concentration.We found that,independently from the deep soil texture,the capillary fringe offers the main resistance to vapor transport.In these cases,the deep soil texture could induce at most half an order of magnitude of variation in total effective diffusion coefficient in deep soil as well as groundwater source-to-indoor air attenuation factors.Finally,we found that,as the thin capillary zone represents the higher resistance to upward soil gas flow,groundwater source depth has little influence in determining the chlorinated vapor intrusion risk(2)Even in the absence of communication between the subsurface soil gas and ambient air fluctuations of building indoor pressure can cause upward advection of contaminated soil gas from the subfoundation zone into a building.When both the building foundation and surrounding pavement block diffusional escape of the volatile soil gas contaminants to the atmosphere,high subfoundation soil gas contaminant concentrations can exist,and contaminant entry into the building through foundation breaches is enhanced beyond what would be expected from diffusion as the building undergoes normal pressure cycling.Upward advection into the building may be induced even when the indoor pressure appears,based on limited measurements,to be higher than that in the subslab,particularly when the indoor pressure in the building quickly fluctuates.This represents a limitation on VI mitigation approaches that rely on indoor pressurization,if those approaches cannot at the same time control significant fluctuation of indoor pressure.