Applicability of Soil Concentration for VOC-Contaminated Site Assessments Explored Using Field Data from the Beijing-Tianjin-Hebei Urban Agglomeration

Understanding petroleum vapor fate and transport through high resolution analysis of two distinct vapor plumes

No field study has provided a detailed characterization of the molecular composition and spatial distribution of a vadose zone plume of petroleum volatile organic compounds (VOCs), which is critical to improve the current understanding of petroleum VOC transport and fate. This is study reports a high-resolution analysis of two distinct vapor plumes emanating from two different light non-aqueous phase liquid (LNAPL) sources (an aliphatic-rich LNAPL for Zone #1vs an aromatic-rich LNAPL for Zone #2) at a large petrochemical site. Although deep soil vapor signatures were similar to the source zone LNAPL signatures, the composition of the shallow soil vapors reflected preferential attenuation of certain hydrocarbons over others during upward transport in the vadose zone. Between deeper and shallower soil gas samples, attenuation of aromatics was observed under all conditions, but important differences were observed in attenuation to aliphatic compound classes. Attenuation of all aliphatic compounds was observed under aerobic conditions but little attenuation of any aliphatics was observed under anoxic conditions without methane. In contrast, under methanogenic conditions, paraffins attenuated more than isoparaffins and naphthenes. These results suggest that isoparafins and naphthenes may present more of a vapor intrusion risk than benzene or other aromatic hydrocarbons commonly considered to be petroleum vapor intrusion risk drivers. While the overall vapor composition changed significantly within the vadose zone, diagnostic ratios of relatively recalcitrant alkylcyclopentanes were preserved in shallow soil vapor samples. These alkylcyclopentanes may be useful for distinguishing between petroleum vapor intrusion and other sources of petroleum VOCs detected in indoor air.

Determining the soil odor control area: A case study of an abandoned organophosphorus pesticide factory in China

Currently, soil odor-active substance screening and evaluation methods for contaminated sites are underdeveloped, with unclear treatment objectives and areas. Consequently, some sites suffer from odor issues during and even after remediation. This study focused on an organophosphorus pesticide factory site in Guangdong Province, China. It established a method of determining the odorant control area using a comprehensive approach combining instrumental and olfactory soil sample analyses. The main odor-active substances identified were ethylbenzene, phenol, m, p-xylene, styrene, toluene, and o-xylene, with odorant control values (the limit of odor-active substance contents) of 35.2, 28.1, 8.0, 11.3, 40.2 and 89.3 mg/kg respectively. Instrumental analysis of soil samples revealed 11 sampling points where the main odor-causing substances exceeded standard levels. Among the substances, ethylbenzene (1.48E+04 mg/kg) had the highest content, exceeding the limit up to 421-fold. Olfactory analysis indicated 14 sampling points with odor intensity surpassing the standard (OI > 2). Based on the instrumental analysis results and the odorant control value, the initial estimated odor control area (area with the risk of odor nuisance) was 5.64E+03 m2. Incorporating the olfactory analysis findings, the control area was adjusted by 1.25E+03 m2, leading to a final calculated soil odor control area of 6.89E+03 m2 for the study site. The comprehensive approach to analyzing soil samples for odor control can help evaluate the extent of soil odor pollution in contaminated sites and provide a scientific basis for effectively removing and managing odor-causing substances in soil.

Contaminant characterization of odor in soil of typical pesticide-contaminated site with shallow groundwater

Odor emission from the soil of pesticide-contaminated sites is a prominent environmental problem in China, but there are very few researches about the component and spatial distribution of odorous substances in the soil of contaminated sites. In this paper, to investigate the odor pollution condition of an organophosphorus pesticide production site in a city of South China, the odor pollutants in the soil and soil gas were analyzed and the key odor-contributing substances were identified. Besides, the correlation between the concentrations of odorous substances in soil and soil gas was analyzed, and the measured results were compared with the predicted results by the linear model and DED model. An off-line soil gas sampling device was designed to collect the gas emitted from soil because the groundwater level in the site was too shallow to build a soil gas well. The key odor substances were screened from the detection results of soil gas via odor activity value (OAV) analysis, which revealed that the key odorous substances included benzene, ethylbenzene, ammonia, toluene, m,p-xylene, methyl sulfide, dimethyl disulfide, and formaldehyde. Furthermore, the spatial distribution of the odor substances in the soil of the pesticide-contaminated site was closely related to the layout and geologic structure of the site. The odor pollutants in soil were mainly distributed near the phosmet production workshop and the drainage ditch network. As for the deep distribution, the odorous substances were mainly enriched in the silty clay or clay layer (5.6-11 m), followed by the sludge layer (1-3.6 m). Finally, the predicted model (linear model and DED model) analysis suggested that the linear model was more suitable for predicting the concentration of odorous substances in the soil gas with the detection data of soil in this pesticide-contaminated site.

The passive sampler assisted human exposure risk characterization for tetrachloroethene soil vapor intrusion scenario

The potential human health risks associated with soil vapor intrusion and volatile organic compounds (VOCs) exposure were characterized at an industrialized site by the quantification of gaseous VOCs in soil pores using a passive sampling technique. The gaseous tetrachloroethene (PCE) in soil pores varied between 12 and 5,400 μg m-3 showing 3 orders of magnitude variation with dependence on groundwater PCE concentrations. Though the PCE concentration in the air only varied between 0.45 and 1.5 μg m-3 showing negligible variations compared to the variation observed in soil pores. The PCE concentration in the air varied between 0.45 and 1.5 μg m-3. The calculation of fugacity suggested that the PCE in the test site originated from groundwater. Measured PCE in groundwater ranged from 14 to 2,400 times higher than PCE in soil gas. This indicates that conducting a vapor intrusion risk assessment using passive soil gas sampling is critical for accurate risk characterization and assessment. Estimated PCE inhalation cancer risks for street cleaners and indoor residents varied between 10-6 and 10-4 with a low plausible hazard, and between 10-3 and 10-2 with a high risk, respectively. The results of this study demonstrate that passive sampling offers a significantly lower cost and labor-intensive approach compared to traditional methods for assessing pollution distribution in contaminated sites and characterizing risks. This highlights the potential for wider application of passive sampling techniques in environmental studies.

The rapid increase of urban contaminated sites along China's urbanization during the last 30 years

Contaminated sites pose serious threats to the soil environment and human health. However, the location and temporal changes of urban contaminated sites across China remain unknown due to data scarcity. Here, we developed a machine-learning model to identify the contaminated sites using public data. Results show that the trained model with 2,005 surveyed site samples and six variables can achieve a model performance evaluation value of 0.86. 43,676 contaminated sites were identified from 83,498 polluting enterprise plots in China. However, these contaminated sites have significant spatiotemporal heterogeneity, mainly located in economically developed provinces, urban agglomerations, and core urban areas. Moreover, the contaminated sites increased by 325% along with urban expansion from 1990 to 2018. The abandoned contaminated sites increased rapidly, but the contaminated sites in production decreased continuously. This methodological framework and our findings contribute to the precise management of contaminated sites and provide insights into urban sustainable development.

Combined assessment of health hazard and odour impact of soils at a contaminated site: a case study on a defunct pharmaceuticals factory in China

Surveys and assessments of contaminated sites primarily focus on hazardous pollutants in the soil with less attention paid to odorants. This makes the management of contaminated sites difficult. In this study, hazardous and odorous pollutants in the soil were assessed for a large site that was previously used for production of pharmaceuticals to determine the degree and characteristics of soil contamination at pharmaceutical production sites, for undertaking rational remediation measures. The main hazardous pollutants at the study site were triethylamine, n-butyric acid, benzo(a)pyrene (BaP), N-nitrosodimethylamine (NDMA), dibenzo(a,h)anthracene (DBA), total petroleum hydrocarbons (C10-C40) (TPH), and 1,2-dichloroethane; TEA, BA, and isovaleric acid (IC) were the main odorants. As the type and distribution of hazardous and odorous pollutants differ, it is necessary to separately assess the impact of these pollutants at a contaminated site. Soils in the surface layer pose significant non-carcinogenic (HI = 68.30) and carcinogenic risks (RT = 3.56E-5), whereas those in the lower layer only pose non-carcinogenic risks (HI > 7.43). Odorants were found at considerable concentrations both in the surface and lower layers, with the maximum concentrations being 29,309.91 and 41.27, respectively. The findings of this study should improve our understanding of soil contamination at former pharmaceutical production sites and should inform the assessment of the risks posed by contaminated sites, with problems associated with odour, and possible remediation strategies.

Development of an in situ equilibrium polydimethylsiloxane passive sampler for measuring volatile organic compounds in soil vapor

An equilibrium passive sampler made of polydimethylsiloxane (PDMS) fiber was developed to measure volatile organic compounds (VOCs) in soil vapor. Expanded polytetrafluoroethylene (ePTFE) was used to protect PDMS from pollution and direct contact with soil components. For all tested VOCs, equilibrium was reached after 7 days at 5 °C. The equilibrium partition coefficients of VOCs between PDMS, gas, and water were measured at three different temperatures. The analyte concentrations in PDMS exposed to gas and water separately were almost the same, which suggests that C_gas and C_water in soil pores can be accurately deduced from C_PDMS after equilibrium at various temperatures. To evaluate the passive sampler, active sampling measurements were performed simultaneously. Concentrations of VOCs deduced from the passive sampler were consistent with the concentrations measured by active sampling near the 1:1 line. Tests with artificial soils were conducted to observe the effects of soil components on passive sampling. The results suggest that the effect of water saturation can be ignored; in other words, the developed passive sampler can be applied in the vadose zone, which has fluctuating water saturation. With a holder for the sampler made of stainless steel, the developed in situ passive sampler can measure VOCs in contaminated soil vapor. The developed passive sampler was proven to be an alternative for measuring VOCs in soil vapor, which can be helpful for soil risk assessment and for observing the diffusion of VOCs in contaminated sites.

Three-dimensional distribution characteristics of multiple pollutants in the soil at a steelworks mega-site based on multi-source information

Soil pollution at steelworks mega-sites has become a severe environmental issue worldwide. However, due to the complex production processes and hydrogeology, the soil pollution distribution at steelworks is still unclear. This study scientifically cognized the distribution characteristics of polycyclic aromatic hydrocarbons (PAHs), volatile organic compounds (VOCs), and heavy metals (HMs) at a steelworks mega-site based on multi-source information. Specifically, firstly, 3D distribution and spatial autocorrelation of pollutants were obtained by interpolation model and local indicators of spatial associations (LISA), respectively. Secondly, the characteristics of horizontal distribution, vertical distribution, and spatial autocorrelations of pollutants were identified by combining multi-source information such as production processes, soil layers, and properties of pollutants. Horizontal distribution showed that soil pollution in steelworks mainly occurred in the front end of the steel process chain. Over 47% of PAHs and VOCs pollution area were distributed in coking plants and over 69% of HMs in stockyards. Vertical distribution indicated that HMs, PAHs, and VOCs were enriched in the fill, silt, and clay layers, respectively. Spatial autocorrelation of pollutants was positively correlated with their mobility. This study clarified the soil pollution characteristics at steelworks mega-sites, which can support the investigation and remediation of steelworks mega-sites.

Exploring the nonlinear partitioning mechanism of volatile organic contaminants between soil and soil vapor using machine learning

Traditional phase equilibrium models usually depend on simplified assumptions and empirical parameters, which are difficult to obtain during regular site investigations. As a result, they often under- or over-estimate soil vapor concentrations for assessing the risks of volatile organic compound (VOC)-contaminated sites. In this study, we develop several machine learning models to predict soil vapor concentrations using 2225 soil-soil vapor data pairs collected from seven contaminated sites in northern China. Compared to the classic dual equilibrium desorption model, the random forest (RF) model can provide more accurate predictions of soil vapor concentrations by at least 1-2 orders of magnitude. Among the employed covariates, soil concentration and organic carbon-water partition coefficient are two of the most significant explanatory covariates affecting soil vapor concentrations. Further examination of the developed RF model reveals the phase equilibrium behavior of VOCs in soil is that: the soil vapor concentration increases with soil concentration at different rates in the first two intervals but remains almost unchanged in the last interval; the solid-vapor partitioning interface may still exist at up to 15% mass water content in our simulations. These findings can help site investigators perform more accurate risk assessments at VOC-contaminated sites.

Effect of vapour-solid interfacial adsorption on benzene multiphase partition and its implication to vapour exposure assessment of contaminated soil in arid area

The partitioning of volatile organic compounds (VOCs) in soil multiphase system is a critical process for vapour intrusion, however, the importance of vapour-solid interface adsorption doesn't receive the due attention, which causes the exposure assessment too conservative particularly in arid conditions. This paper proposed a multiphase partitioning equilibrium (MPE) model establishing the quantitative relationship between VOCs and its various partitioning phases in soil, including solid-liquid interface adsorption phase, vapour phase and dissolved phase and vapour-solid interface adsorption phase. Taking benzene as the targeted pollutant, the model was found in good agreement with the experimental data while the errors were within one magnitude basically. The role of vapour-solid interface adsorption under different soil moisture conditions was also investigated by the model. The results reveals that a) soil moisture is the conspicuous controlling factor that affects the benzene partitioning in soil; b) vapour-solid interface adsorption dominates benzene uptake when soil relative saturation (RS) is under 20% among three typical soils; c) as adsorption by soil minerals (vapour-solid interface adsorption) is reduced by increasing amounts of humidity (RS > 20%), uptake by partitioning into the soil organic matter (OM) increasingly becomes a controlling factor; d) the common sense that vapour concentration of benzene is particularly high with low level of RS may not occur since the vapour-solid interface adsorption dominates benzene uptake in arid environment. The MPE model is suitable for prediction of VOCs partitioning and vapour exposure risk assessment of contaminated soil in arid area.

Does the Geographic Difference of Soil Properties Matter for Setting Up the Soil Screening Levels in Large Countries Like China?

China issued the unified national soil screening levels (NSSLs) in 2018 to assist the regulation of contaminated sites, but the applicability of NSSLs was not thoroughly evaluated. Datasets from the National Qinghai-Tibet Plateau Scientific Data Center indicated great variability of soil organic matter (0.8-173 g/kg), soil water content (0.05-0.6), soil porosity (0.4-0.6), and soil bulk density (1.11-1.59 kg/m³). We analyzed the effects of soil properties on the derivation of SSLs by using Monte Carlo simulations. The soil factors mainly affected the inhalation exposure pathway of volatile organic compounds (VOCs). They had an effect of more than two orders of magnitude on SSLs for most selected VOCs, particularly with the parameters 0.35 > Henry's law constant > 0.1 and carbon-water distribution coefficient of >100. We compared NSSLs with the recommended SSLs assuming fifth percentile by using Monte Carlo simulations. In general, NSSLs were not sufficient to identify contaminated sites that require additional investigation in the south, central, and northwest regions but were too conservative in screening sites out that required no further action in the east and northeast regions. Our framework and findings may contribute to more scientific and effective soil quality management in other large countries.

An integrated geospatial correlation analysis and human health risk assessment approach for investigating abandoned industrial sites

An integrated geospatial correlation analysis (GCA)-human health risk assessment (HHRA) approach was developed to investigate abandoned industrial sites featured by heterogeneous contamination data. Critical areas of high health risk concerns can be prioritized for remediation using the integrated approach. An abandoned chemical complex site in Hubei, China was investigated as a case study. GCA and HHRA were performed using soil and groundwater sampling data collected in 2016 and 2019. Benzene, chlorobenzene, dichlorobenzenes, 2-nitrochlorobenzene, and α-hexachlorocyclohexane were determined to be critical contaminants in soil. The 2019 sampling data revealed new contaminated locations that were not found in the 2016 sampling campaign. High concentrations (89.81-386.55 mg/L) of vinyl chloride were also found in groundwater samples. Several critical location clusters of high concentrations of dichlorobenzenes, chlorobenzene, and α-hexachlorocyclohexane were found within the site according to the GCA outcomes. These contaminants could pose significant cancer and non-cancer risks to onsite workers. The critical areas were ranked according to cancer and non-cancer risks estimated by HHRA, respectively, for informed remediation planning. Among the critical contaminants, α-hexachlorocyclohexane, 2-nitrochlorobenzene, and 1,4-dichlorobenzene in soil, as well as vinyl chloride in groundwater, contributed a predominant part to the total health risk. The integrated approach can be used to assess the contamination of other similar abandoned industrial complex sites.

The inhibitory mechanism of natural soil colloids on the biodegradation of polychlorinated biphenyls by a degrading bacterium

In spite of extensive studies of soil model components, the role of natural soil colloids in the biodegradation of organic pollutants remain poorly understood. Accordingly, the present study selected Mollisol colloids (MCs) and Ultisol colloids (UCs) to investigate their effects on the biodegradation of 3, 3', 4, 4'-tetrachlorobiphenyl (PCB77) by Bradyrhizobium diazoefficiens USDA 110. Results demonstrated that both natural soil colloids significantly decreased the biodegradation of PCB77, which partly resulted from the significant decrease in the bioaccessibility of PCB77. Furthermore, the activity of Bradyrhizobium diazoefficiens USDA 110 was remarkably inhibited under the exposure to the two types of soil colloids, which was mainly ascribed to the inhibition of cell reproduction but not the lethal effect of reactive oxygen species. The calculated results from Ex-DLVO theory further indicated that the repulsion between UCs and biodegrading bacteria retarded the effective contact of cells with adsorbed PCB77 from UCs, resulting in the decline of the rate of cell reproduction. In general, the inhibition of MCs was limited to PCB77 bioaccessibility, whereas the negative effect of UCs was controlled by PCB77 bioaccessibility and the effective contact of cells with colloids. This study could provide implication for the enhancement of microbial remediation in contaminated soil.

Vapor Intrusion Investigations and Decision-Making: A Critical Review

At sites impacted by volatile organic compounds (VOCs), vapor intrusion (VI) is the pathway with the greatest potential to result in actual human exposure. Since sites with VI were first widely publicized in late 1990s, the scientific understanding of VI has evolved considerably. The VI conceptual model has been extended beyond relatively simple scenarios to include nuances, such as biological and hydrogeological factors that may limit the potential for VI and alternative pathways, such as preferential pathways and direct building contact/infiltration that may enhance VI in some cases. Regulatory guidance documents typically recommend initial concentration- or distance-based screening to evaluate whether VI may be a concern, followed by a multiple-lines-of-evidence (MLE) investigation approach for sites that do not screen out. These recommendations for detailed evaluation of VI currently focus on monitoring of VOC concentrations in groundwater, soil gas, and indoor air and can be supplemented by other lines of evidence. In this Critical Review, we summarize key elements important to VI site characterization, provide the status and current understanding, and highlight data interpretation challenges, as well as innovative tools developed to help overcome the challenges. Although there have been significant advances in the understanding of VI in the past 20 years, limitations and knowledge gaps in screening, investigation methods, and modeling approaches still exist. Potential areas for further research include improved initial screening methods that account for the site-specific role of barriers, improved understanding of preferential pathways, and systematic study of buildings and infrastructure other than single-family residences.