Ceramsite made from remediated soil: A risk assessment of its potential role serving as urban street cushion.

To promote the reuse of remediated soil (RS) and facilitate the cleanup of rainwater in sponge city, we investigated the effects of ceramsite made from RS serving as urban street cushion. Ceramsite was prepared by RS or pollution-free soil (PS) and showed no difference in physical properties. Compared with gravel, ceramsite had purification effects on effluents, reducing the content of chemical oxygen demand, total nitrogen, and ammoniacal nitrogen. However, the content of total phosphorus and the concentration of Cr(VI) and arsenic slightly increased in ceramsite groups, inferring potential risk. Microbial community analysis proved that ceramsite promoted microbial growth and increased microbial diversity. A long-term risk assessment indicated that ceramsite was good at fixing heavy metals during leaching process. Taken together, ceramsite prepared from RS could serve as excellent urban street cushion with little potential risk to surroundings.

Investigating the role of vadose zone breathing in vapor intrusion from contaminated groundwater.

The fluctuation of the groundwater table can cause upward and downward advection of soil gas within the vadose zone, just like breathing. In this study, we developed a three-dimensional numerical model and used it to investigate the role of vadose zone breathing in vapor intrusion, through which subsurface volatile chemicals migrate into the concerned building at contaminated sites. The developed model was first applied to a sand tank experiment, followed by examining the influences of fluctuation amplitude and period of water table, soil textures, and groundwater level depth. Our results suggest that the indoor pollutant concentration can be increased by about three times with the oscillatory water table of 0.4 m amplitude and 4 d period. Within one cycle of vadose zone breathing, the indoor pollutant concentration is found to vary by about 7 orders of magnitude. The results also show that, compared to the groundwater level depth, the soil texture plays a significant role in determining vapor intrusion risks. Specifically, when soil particles increase from 0.25 mm to 0.44 mm, the indoor pollutant concentration tends to increase and becomes more sensitive to groundwater table fluctuation.

Physically simulating the effect of lateral vapor source-building separation on soil vapor intrusion: Influences of surface pavements and soil heterogeneity.

The soil gas concentration attenuation in lateral diffusive transport determines the influence area of a contamination plume in soil vapor intrusion, a major exposure pathway of volatile chemicals at contaminated sites. In this study, we utilize both physical and mathematical models to investigate the roles of soil geology heterogeneities and impermeable surface pavements in determining the attenuation of contaminant soil gas concentration. The results indicate that the attenuation of soil gas concentration with lateral diffusion is observed to be the most significant if with a low-permeability soil layer at the bottom and a high-permeability layer on top, followed by the cases with the uniform soil properties, and the lateral attenuation is the least significant in cases with a high-permeability soil layer at the bottom and a low-permeability soil layer on top, regardless of the surface coverage. Compared to soil heterogeneity, the influences of surface conditions are less significant, and the capping effect of surface cover can only play a role in determining shallow soil gas concentration profiles. At last, the physical experimental results were used to examine a previously developed analytical vapor intrusion model including the influences of layering and surface conditions.

High-frequency fluctuations of indoor pressure: A potential driving force for vapor intrusion in urban areas.

In this study, we examine the impact of a building's indoor pressure fluctuations in drawing subsurface volatile contaminants into the building, and how the presence of an impervious pavement surrounding the building influences this. 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. For cases with the paved ground surface, the simulated volumetric soil gas entry rates are lower than steady-state cases with constant -5 indoor-outdoor pressure difference, by at least half an order of magnitude. When the indoor pressure fluctuation rate exceeds about 5 Pa/h (which corresponds a sinusoidal fluctuation with a period of 2 h), the predicted indoor air concentration of paved scenarios will be higher than the conventional case. 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.

Investigating two-dimensional soil gas transport of trichloroethylene in vapor intrusion scenarios involving surface pavements using a pilot-scale tank.

In this study, we investigate the soil gas concentration attenuation with diffusive transport in lateral and vertical transport in cases with surface pavements with a pilot-scale tank. Three scenarios were investigated, one with a completely open soil surface and the other two involving partially paved soil surface. The results, on the one hand, indicate that the soil gas concentration generally decreases linearly and exponentially in the vertical and horizontal transport directions, respectively, generally in accordance with available modeling studies. On the other hand, our experiment shows that low-permeability ground covers can increase shallow soil gas concentrations beneath the pavement by at most 3-4 times, inducing higher subslab concentration than that below open ground surface, even if the latter is obtained at a closer location to vapor source. For cases with uniform soil properties, our study suggests exterior soil gas sample should be taken at a depth below the building foundation by half of the building footprint size, if the vapor source is laterally extensive relative to the building footprint, or by 1 m and 2-3 m for slab-on-grade and basement scenarios, respectively, if the vapor source is located laterally away from the building.