Mapping Blood Lead Levels in China during 1980-2040 with Machine Learning.

Lead poisoning is globally concerning, yet limited testing hinders effective interventions in most countries. We aimed to create annual maps of county-specific blood lead levels in China from 1980 to 2040 using a machine learning model. Blood lead data from China were sourced from 1180 surveys published between 1980 and 2022. Additionally, regional statistical figures for 15 natural and socioeconomic variables were obtained or estimated as predictors. A machine learning model, using the random forest algorithm and 2973 generated samples, was created to predict county-specific blood lead levels in China from 1980 to 2040. Geometric mean blood lead levels in children (i.e., age 14 and under) decreased significantly from 104.4 µg/L in 1993 to an anticipated 40.3 µg/L by 2040. The number exceeding 100 µg/L declined dramatically, yet South Central China remains a hotspot. Lead exposure is similar among different groups, but overall adults and adolescents (i.e., age over 14), females, and rural residents exhibit slightly lower exposure compared to that of children, males, and urban residents, respectively. Our predictions indicated that despite the general reduction, one-fourth of Chinese counties rebounded during 2015-2020. This slower decline might be due to emerging lead sources like smelting and coal combustion; however, the primary factor driving the decline should be the reduction of a persistent source, legacy gasoline-derived lead. Our approach innovatively maps lead exposure without comprehensive surveys.

Using groundwater monitoring wells for rapid application of soil gas radon deficit technique to evaluate residual LNAPL.

The application of the 222Radon (Rn) deficit technique using subsurface soil gas probes for the identification and quantification of light non-aqueous phase liquids (LNAPL) has provided positive outcomes in recent years. This study presents an alternative method for applying this technique in the headspace of groundwater monitoring wells. The developed protocol, designed for groundwater monitoring wells with a portion of their screen in the vadose zone, is based on the use of portable equipment that allows rapid measurement of the Rn soil gas activity in the vadose zone close to the water table (i.e., smear zone) where LNAPL is typically expected. The paper first describes the step-by-step procedure to be followed for the application of this method. Then, a preliminary assessment of the potential of the method was carried out at two Italian sites characterized by accidental gasoline and diesel spills into the subsurface from underground storage tanks. Although the number of tests conducted does not allow for definitive conclusions, the results obtained suggest that, from a qualitative point of view, Rn monitoring in the headspace of monitoring wells is a promising, fast, and minimally invasive screening method that could also potentially reduce the costs associated with field data acquisition. This method proves to be suitable for detecting the presence of LNAPL in both the mobile and residual phases with results consistent with the other lines of evidence available at the sites, such as groundwater and soil gas monitoring. Future efforts should be directed toward evaluating the accuracy of this method for a quantitative assessment of residual LNAPL saturations.

Degradation of trichloroethylene vapors by micrometric zero-valent FeCu and FeNi bimetals under partially saturated conditions.

The degradation of trichloroethylene (TCE) vapors by zero-valent Iron-Copper (Fe-Cu) and Iron-Nickel (Fe-Ni) bimetals with 1%, 5% and 20% weight content (%wt) of Cu or Ni was tested in anaerobic batch vapor systems carried out at ambient room temperature (20 ± 2 °C) under partially saturated conditions. The concentrations of TCE and byproducts were determined at discrete reaction time intervals (4 h-7 days) by analyzing the headspace vapors. In all the experiments, up to 99.9% degradation of TCE in the gas phase was achieved after 2-4 days with zero-order TCE degradation kinetic constants in the range of 134-332 g mair-3d-1. Fe-Ni showed a higher reactivity towards TCE vapors compared to Fe-Cu, with up to 99.9% TCE dechlorination after 2 days of reaction, i.e., significantly higher than zero-valent iron alone that in previous studies was found to achieve comparable TCE degradation after minimum 2 weeks of reaction. The only detectable byproducts of the reactions were C3-C6 hydrocarbons. Neither vinyl chloride or dichloroethylene peaks were detected in the tested conditions above their method quantification limits that were in the order of 0.01 g mair-3. In view of using the tested bimetals in horizontal permeable reactive barriers (HPRBs) placed in the unsaturated zone to treat chlorinated solvent vapors emitted from contaminated groundwater, the experimental results obtained were integrated into a simple analytical model to simulate the reactive transport of vapors through the barrier. It was found that an HPRB of 20 cm could be potentially effective to ensure TCE vapors reduction.

Influence of advection on the soil gas radon deficit technique for the quantification of LNAPL.

The Radon (Rn) deficit technique is a rapid, low-cost, and non-invasive method to identify and quantify light non-aqueous phase liquids (LNAPL) in the soil. LNAPL saturation is typically estimated from Rn deficit using Rn partition coefficients, assuming equilibrium conditions. This work examines the applicability of this method in the presence of local advective fluxes that can be generated by groundwater fluctuations or biodegradation processes in the source zone. To this end, a one-dimensional analytical model was developed to simulate the steady-state diffusive-advective transport of soil gas Rn in the presence of LNAPL. The analytical solution was first validated against an existing numerical model adapted to include advection. Then a series of simulations to study the effect of advection on Rn profiles were carried out. It was found that in high-permeability soils (such as sandy soils), advective phenomena can significantly affect Rn deficit curves in the subsurface compared with those expected, assuming either equilibrium conditions or a diffusion-dominated transport. Namely, in the presence of pressure gradients generated by groundwater fluctuations, applying the traditional Rn deficit technique (assuming equilibrium conditions) can lead to an underestimation of LNAPL saturation. Furthermore, in the presence of methanogenesis processes (e.g., in the case of a fresh LNAPL of petroleum hydrocarbons), local advective fluxes can be expected above the source zone. In such cases, Rn concentrations above the source zone can be higher than those above background areas without advective phenomena, resulting in Rn deficits higher than 1 (i.e., Rn excess), and thus leading to a wrong interpretation regarding the presence of LNAPL in the subsurface if advection is not considered. Overall, the results obtained suggest that advection should be considered in the presence of pressure gradients in the subsurface to ensure an effective application of the soil gas Rn-deficit technique for quantitative estimation of LNAPL saturation.

In Situ Equilibrium Polyethylene Passive Sampling of Soil Gas VOC Concentrations: Modeling, Parameter Determinations, and Laboratory Testing.

The use of low-density polyethylene (PE) sheets as equilibrium passive soil gas samplers to quantify volatile organic compounds (VOCs) such as benzene, toluene, ethylbenzene, and xylenes, and chlorinated solvents (e.g., trichloroethene and tetrachloroethene) in unsaturated subsurface environments was evaluated via modeling and benchtop testing. Two methods were devised to quantify such VOCs in PE. Key chemical properties, including PE-water (KPEw) and PE-air (KPEa) partition coefficients and diffusivities in the PE (Dpe), were determined. These KPEw, KPEa, and Dpe values were consistent with extrapolations of data based on larger compounds. Using these parameter values, field equilibration times of less than 1 day were estimated for such VOCs when using 70-100 µm thick PE sheets. Further, benchtop batch tests carried out in jars filled with VOC-contaminated soils, after 1 or 2 days, showed concentrations in soil air deduced from PE that were consistent with concentrations deduced by analyzing either water or headspace gases recovered from the same tests. Thus, PE-based measurements may overcome inaccuracies from using total soil concentrations and equilibrium partitioning models that may overestimate vapor phase concentrations up to 2 orders of magnitude.

Modeling of soil gas radon as an in situ partitioning tracer for quantifying LNAPL contamination.

In the last decades radon (Rn) has been widely proposed as a naturally occurring tracer for non-aqueous phase liquids (NAPL) in the soil. This work examines the feasibility of using soil gas data collected at some distance from the source zone for the application of the Rn deficit technique for the identification and quantification of NAPL contamination. To this end, we used a steady-state 1-D analytical solution that is based on a 3-layer model that allows to simulate the transport and distribution of Rn in the source zone, capillary fringe and overlying unsaturated soil. The analytical solution was first validated against a more detailed numerical model available in the literature. Then, a series of simulations were carried out to evaluate the vertical concentration profiles of Rn in soil gas above the source zone and in background location not impacted by NAPL. Simulation results showed that the parameters that most influence the migration and distribution of Rn in the subsurface are the distance of the soil gas probe from the source zone and, to a lower extent, the type of contamination (e.g. diesel or gasoline) and the soil type. On the basis of these results, we developed some easy-to-use nomographs to estimate the residual NAPL phase based on the observed radon deficit in soil gas and on the probe to source distance and soil and NAPL characteristics. According to the obtained results, the radon deficit technique results a feasible method for a qualitative identification of residual NAPL when radon in soil gas is measured at distances lower than 2 m from the contaminated zone. However, for an accurate quantitative estimation of the NAPL phase content, soil gas probes should be preferably located at distances lower than 1 m from the source zone.

Refinement of the gradient method for the estimation of natural source zone depletion at petroleum contaminated sites.

Rates of natural source zone depletion (NSZD) are increasingly being used to aid remedial decision making and light non-aqueous phase liquid (LNAPL) longevity estimates at petroleum release sites. Current NSZD estimate methods, based on analyses of carbon dioxide (CO2) and oxygen (O2) soil-gas concentration gradients ("gradient method") assume linear concentration profiles with depth. This assumption can underestimate the concentration gradients especially above LNAPL sources that are typically characterized by curvilinear or semi-curvilinear O2 and CO2 concentration profiles. In this work, we proposed a new method that relies on calculating the O2 and CO2 concentration gradient using a first-order reaction model. The method requires an estimate of the diffusive reaction length that can be easily derived from soil-gas concentration data. A simple step-by-step guide for applying the new method is provided. Nomographs were also developed to facilitate method application. Application of the nomographs using field data from published literature showed that NSZD rates could be underestimated by nearly an order of magnitude if reactivity in the vadose zone is not accounted for. The new method helps refine NSZD rates estimation and improve risk-based decision making at certain petroleum contaminated sites.

Numerical study of building pressure cycling to generate sub-foundation aerobic barrier for mitigating petroleum vapor intrusion.

In this study, the role of building pressure cycling (BPC) method in generating a subslab aerobic barrier at petroleum contaminated sites was examined numerically. The numerical model was first validated with field observations and then used to simulate BPC applications in petroleum vapor intrusion scenarios. The results indicated that, after a long-term BPC operation (60 days), a subslab aerobic barrier could be generated with an adequate air injection rate (10 L/min in this study). The effects on hydrocarbon soil gas concentration profiles are expected to last for weeks even after the BPC system is turned off. Moreover, our investigations showed that the performances of the BPC application are virtually independent of hydrocarbon's reaction rate constant. The simulated sub-foundation aerobic conditions expected during BPC were comparable to those observed in a field study where a subsurface pipe system at the same air injection rate was used to create a subslab aerobic barrier. Thus, BPC application can represent an interesting alternative approach to the subsurface delivery systems as it is expected to achieve similar performance but with lower installation costs.

Total organic carbon as a proxy for metal release from biostabilized wastes.

In this study, we introduce a simple screening method to predict the metal release from biostabilized wastes as a function of the total organic carbon (TOC) content of the sample. The method relies on a model that simulates the release of dissolved organic carbon (DOC) as a function of the applied liquid to solid (L/S) ratio. The metal release is then estimated using generic empirical DOC to metal correlation coefficients (KDOC,Me) extrapolated from the literature. Specifically, the results of leaching tests carried out on different types of biowastes and biostabilized wastes were used to calculate the upper and lower KDOC,Me values that can be expected for common metals of concern (Al, Ba, Cr, Cu, Mo, Ni, Pb, V, and Zn). The statistical analysis of the estimated empirical coefficients highlighted that for most of the investigated metals, the adoption of generic DOC to metal correlation coefficients introduces relatively low uncertainties. The quartiles ratio (QR calculated as the ratio of third and first quartile) of the KDOC,Me coefficients extrapolated from the literature was indeed below 3 for Cu, Ni, and Zn and below 5 for Al, Ba, Cr, Pb, and V. Only for Mo, the QR was around 10 indicating that for this element, DOC can be a poor indicator of the release expected as a function of the applied L/S. Furthermore, by performing a sensitivity analysis, we found that the experimental conditions only slightly influence the metals release predicted by the model. Based on this evidence, simple nomograms that estimate the cumulative metal release in percolation column tests as a function of the applied L/S are provided. Furthermore, a simple equation that predicts the cumulative metal release expected at L/S of 10 L/kg is presented. The application of the latter to the results of percolation column tests carried out on 8 mechanical biological treatment (MBT) waste samples highlighted that the predicted metals release was close to the measured values with deviations within a factor of 5 for all the investigated metals.

Horizontal permeable reactive barriers with zero-valent iron for preventing upward diffusion of chlorinated solvent vapors in the unsaturated zone.

Chlorinated solvents are extensively used in many activities and hence in the past decades impacted a large number of sites. The presence of these contaminants in groundwater is challenging particularly for the management of the vapor intrusion pathway. In this work we examine the potential feasibility of using horizontal permeable reactive barriers (HPRBs) placed in the unsaturated zone to treat chlorinated solvent vapors emitted from groundwater. Zero-valent iron (ZVI) powders, partially saturated with water and characterized by different specific surface areas (SSA), were tested, alone or mixed with sand, in lab-scale batch reactors using TCE as model compound. Depending on the type of iron powder used, a reduction of TCE concentration in the vapor phase from approximately 35% up to 99% was observed after 3 weeks of treatment. The best performance in terms of TCE reduction was obtained using the ZVI characterized by the intermediated values of the specific surface area (SSA). This finding, which is in contrast with the results generally observed in in aqueous solutions, was tentatively attributed to a non-selective higher reactivity of the fine-grained iron samples with water and dissolved oxygen (with a consequent iron passivation) or to the occurrence of a diffusion-limited reaction kinetics. Based on the first-order kinetic degradation rate constants estimated from the experimental data, a horizontal barrier of 1 m containing ZVI or a mixture of ZVI and sand can potentially lead to an attenuation of TCE vapors over 99%.

Humic acids extracted from compost as amendments for Fenton treatment of diesel-contaminated soil.

In this study, we investigate the performance of a Fenton-like process carried out adding as amendments humic acids extracted from compost obtained from organic wastes. Namely, Fenton-like lab-scale tests with different dosages of the extracted humic acids and traditional stabilizing agent (KH2PO4) were performed on a diesel-contaminated soil collected in a former gasoline station. The performed tests showed a beneficial effect of the extracted humic acids on the hydrogen peroxide stability. Namely, the H2O2 lifetime in the tests carried out without the addition of any amendments proved to be quite limited, resulting equal to around 1 h. The adoption of the extracted humic acids alone entailed a limited increase of the hydrogen peroxide stability that anyhow was detected in solution for 24 h using 10 g/L of extracted HA. When the humic acids (10 g/L) were used in combination with KH2PO4 (8.2 g/L), the hydrogen peroxide lifetime increased up to around 150 h. A beneficial effect of the humic acids extracted from compost for a Fenton-like process was also observed in terms of diesel removal. Namely, without any amendment, a contaminant removal of around 55% was observed. Using KH2PO4 or HA alone, the contaminant removal raised up to around 75% while using the traditional stabilizer together with the humic acids extracted from compost, it was possible to remove up to 90% of the initial diesel content of the soil.

Dehalogenation of trichloroethylene vapors by partially saturated zero-valent iron.

The reduction of trichloroethylene (TCE) in gas phase by different types of granular zero-valent iron (Fe0) was examined in anaerobic batch vapor systems performed at room temperature. Concentrations of TCE and byproducts were determined at discrete time intervals by analysis of the headspace vapors. Depending on the type of iron used, reductions of TCE gas concentration from 35% up to 99% were observed for treatments of 6 weeks. In line with other experimental studies performed with aqueous solutions, the particle size was found to play a key role in the reactivity of the iron. Namely an increase of the TCE removal up to almost 3 times was observed using iron powders with particle size lower than 425 µm compared to iron powders with particle size lower than 850 µm. The manufacturing process of the iron powder was instead found to play only a limited role. Namely, no significant differences were observed in the TCE reduction by Fe0 obtained using an iron powder attained by water atomization and sieving compared to the removal achieved using an iron powder subjected to a further annealing processes to reduce the content of oxides. Conversely, the pretreatment of the iron powder with HCl was found to enhance the reactivity of the iron. In particular, by washing the iron powder of 425 µm with HCl acid 0.1 M the reduction of TCE after 6 weeks of treatment increase from approximately 80% for the as received material to >99% for the pretreated iron powder. We also performed tests at different humidity of the iron observing that not statistical differences were obtained using a water content of 10% or 50% by weight. In all the experiments, the only detectable byproducts of the reactions were C4-C6 alkenes and alkanes that can be attributed to a hydrogenation of the CCl bond.

Examining the use of USEPA's Generic Attenuation Factor in determining groundwater screening levels for vapor intrusion.

A value of 0.001 is recommended by the United States Environmental Protection Agency (USEPA) for its groundwater-to-indoor air Generic Attenuation Factor (GAFG), used in assessing potential vapor intrusion (VI) impacts to indoor air, given measured groundwater concentrations of volatile chemicals of concern (e.g., chlorinated solvents). The GAFG can, in turn, be used for developing groundwater screening levels for VI given target indoor air quality screening levels. In this study, we examine the validity and applicability of the GAFG both for predicting indoor air impacts and for determining groundwater screening levels. This is done using both analysis of published data and screening model calculations. Among the 774 total paired groundwater-indoor air measurements in the USEPA's VI database (which were used by that agency to generate the GAFG) we found that there are 427 pairs for which a single groundwater measurement or interpolated value was applied to multiple buildings. In one case, up to 73 buildings were associated with a single interpolated groundwater value and in another case up to 15 buildings were associated with a single groundwater measurement (i.e, that the indoor air contaminant concentrations in all of the associated buildings were influenced by the concentration determined at a single point). In more than 70% of the cases (390 of 536 paired measurements in which horizontal building-monitoring well distance was recorded) the monitoring wells were located more than 30 meters (and some up to over 200 meters) from the associated buildings. In a few cases, the measurements in the database even improbably implied that soil gas contaminant concentrations increased, rather than decreased, in an upward direction from a contaminant source to a foundation slab. Such observations indicate problematic source characterization within the dataset used to generate the GAFG, and some indicate the possibility of a significant influence of a preferential contaminant pathway. While the inherent value of the USEPA database itself is not being questioned here, the above facts raise the very real possibility that the recommended groundwater attenuation factors are being influenced by variables or conditions that have not thus far been fully accounted for. In addition, the predicted groundwater attenuation factors often fall far beyond the upper limits of predictions from mathematical models of VI, ranging from screening models to detailed computational fluid dynamic models. All these models are based on the same fundamental conceptual site model, involving a vadose zone vapor transport pathway starting at an underlying uniform groundwater source and leading to the foundation of a building of concern. According to the analysis presented here, we believe that for scenarios for which such a "traditional" VI pathway is appropriate, 10-4 is a more appropriately conservative generic groundwater to indoor air attenuation factor than is the EPA-recommended 10-3. This is based both on the statistical analysis of USEPA's VI database, as well as the traditional mathematical models of VI. This result has been validated by comparison with results from some well documented field studies.

Investigating the Role of Soil Texture in Petroleum Vapor Intrusion.

In this work we investigate the role of soil texture in petroleum vapor intrusion (PVI) by performing numerical modeling, analytical calculations, and statistical analysis of the USEPA's PVI database. Numerical simulations were conducted for three kinds of soil (sand, sandy loam, and clay), and the results indicate that the maximum attenuations of vapor concentrations from source to indoor air were observed when the clay soil is below the building. In the anaerobic zone, the normalized soil gas concentration profiles were observed to be similar and independent of soil type, whereas in the aerobic zone, a more significant attenuation was observed in finer grained soils. Such a finding is consistent with the statistical results of the USEPA's PVI database, which indicate that in the near-source zone, the soil gas concentration in coarse-grained soil tends to be lower than that in fine-grained soil, possibly caused by a weaker source due to mass loss by volatilization, whereas at a distance away from the source, the measured soil gas concentrations in fine-grained soils become much lower because of aerobic biodegradation with a shorter diffusive reaction length. Thus, 3 and 5 m are proposed as soil-type-dependent vertical screening distances for fine and coarse-grained soils, respectively. It should be noted that the validity of these screening distances is examined only for relatively homogeneous soils, and they may not be applicable for cases involving layered soil systems, where the availability of O in the subfoundation should be evaluated with subslab or multidepth samples to confirm the presence of aerobic biodegradation.

Examining the role of sub-foundation soil texture in chlorinated vapor intrusion from groundwater sources with a two-layer numerical model.

In this study we investigate the role of the sub-foundation soil texture in determining groundwater source-to-indoor air attenuation factors. A three-dimensional numerical model was used to simulate a series of two-layer scenarios, involving different sub-foundation and deep soil textures, foundation types and groundwater source depths. The results indicate that if the sub-foundation soil permeability is larger than 10-11 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.

Performance of passive sampling with low-density polyethylene membranes for the estimation of freely dissolved DDx concentrations in lake environments.

Laboratory and field studies were used to evaluate the performance of low-density polyethylene (PE) passive samplers for assessing the freely dissolved concentrations of DDT and its degradates (DDD and DDE, together referred to as DDx) in an Italian lake environment. We tested commercially available 25 µm thick PE sheets as well as specially synthesized, 10 µm thick PE films which equilibrated with their surroundings more quickly. We measured PE-water partitioning coefficients (Kpew) of the 10 µm thick PE films, finding good correspondence with previously reported values for thicker PE. Use of the 10 µm PE for ex situ sampling of a lake sediment containing DDx in laboratory tumbling experiments showed repeatability of ±15% (= standard deviation/mean). Next, we deployed replicate 10 µm and 25 µm PE samplers (N = 4 for 10 d and for 30 d) in the water and sediment of a lake located in northern Italy; the results showed dissolved DDx concentrations in the picogram/L range in porewater and the bottom water. Values deduced from 10 µm thick PE films compared well (95% of all comparison pairs matched within a factor of 5) with those obtained using PE films of 25 µm thickness when dissolved DDx concentrations were estimated using performance reference compound (PRC) corrections, whether left at the bed-water interface for 10 or 30 days. These results demonstrated the potential of this sampling method to provide estimation of the truly dissolved DDx concentrations, and thereby the mobile and bio-available fractions in both surface waters and sediment beds.

Leaching behaviour of incineration bottom ash in a reuse scenario: 12years-field data vs. lab test results.

Several types of standardized laboratory leaching tests have been developed during the past few decades to evaluate the leaching behaviour of waste materials as a function of different parameters, such as the pH of the eluate and the liquid to solid ratio. However, the link between the results of these tests and leaching data collected from the field (e.g. in disposal or reuse scenarios) is not always straightforward. In this work, we compare data obtained from an on-going large scale field trial, in which municipal solid waste incineration bottom ash is being tested as road sub-base material, with the results obtained from percolation column and pH-dependence laboratory leaching tests carried out on the bottom ash at the beginning of the test. The comparisons reported in this paper show that for soluble substances (e.g. Cl, K and SO4), percolation column tests can provide a good indication of the release expected in the field with deviations usually within a factor of 3. For metals characterized by a solubility-controlled release, i.e. that depends more on eluate pH than the liquid to solid ratio applied, the results of pH-dependence tests describe more accurately the eluate concentration trends observed in the field with deviations that in most cases (around 80%) are within one order of magnitude (see e.g. Al and Cd). The differences between field and lab-scale data might be in part ascribed to the occurrence in the field of weathering reactions (e.g. carbonation) but also to microbial decomposition of organic matter that modifying leachate pH affect the solubility of several constituents (e.g. Ca, Ba and Cr). Besides, weathering reactions can result in enhanced adsorption of fulvic acids to iron/aluminum (hydr)oxides, leading to a decrease in the leaching of fulvic acids and hence of elements such as Cu, Ni and Pb that strongly depend on DOC leaching. Overall, this comparison shows that percolation column tests and pH-dependence tests can represent a reliable screening tool to derive data that could be employed in risk-based analysis or life cycle assessment (LCA) frameworks for evaluating potential environmental impacts deriving from specific disposal/reuse options for waste materials.

Using dynamic flux chambers to estimate the natural attenuation rates in the subsurface at petroleum contaminated sites.

In this work, we introduce a screening method for the evaluation of the natural attenuation rates in the subsurface at sites contaminated by petroleum hydrocarbons. The method is based on the combination of the data obtained from standard source characterization with dynamic flux chambers measurements. The natural attenuation rates are calculated as difference between the flux of contaminants estimated with a non-reactive diffusive model starting from the concentrations of the contaminants detected in the source (soil and/or groundwater) and the effective emission rate of the contaminants measured using dynamic flux chambers installed at ground level. The reliability of this approach was tested in a contaminated site characterized by the presence of BTEX in soil and groundwater. Namely, the BTEX emission rates from the subsurface were measured in 4 seasonal campaigns using dynamic flux chambers installed in 14 sampling points. The comparison of measured fluxes with those predicted using a non-reactive diffusive model, starting from the source concentrations, showed that, in line with other recent studies, the modelling approach can overestimate the expected outdoor concentration of petroleum hydrocarbons even up to 4 orders of magnitude. On the other hand, by coupling the measured data with the fluxes estimated with the diffusive non-reactive model, it was possible to perform a mass balance to evaluate the natural attenuation loss rates of petroleum hydrocarbons during the migration from the source to ground level. Based on this comparison, the estimated BTEX loss rates in the test site were up to almost 0.5kg/year/m2. These rates are in line with the values reported in the recent literature for natural source zone depletion. In short, the method presented in this work can represent an easy-to-use and cost-effective option that can provide a further line of evidence of natural attenuation rates expected at contaminated sites.

A risk-based approach for assessing the recycling potential of an alkaline waste material as road sub-base filler material.

In this work we present an integrated risk-based approach that can be used to evaluate the recycling potential of an alkaline waste material such as incineration bottom ash (BA) as unbound material for road sub-base construction. This approach, which is aimed at assessing potential risks to the groundwater resource (in terms of drinking water quality) and human health associated to the leaching of contaminants from the BA, couples the results of leaching tests for the estimation of source concentrations with the fate and transport models usually adopted in risk assessment procedures. The effects of weathering and of the type of leaching test employed to evaluate eluate concentrations were assessed by carrying out different simulations using the results of laboratory leaching tests. Specifically, pH-dependence and column percolation leaching tests were performed on freshly collected and 1-year naturally weathered BA samples produced from a grate-fired incineration plant treating Refuse Derived Fuel (RDF). To evaluate a broad span of possible scenario conditions, a Monte Carlo analysis was performed running 5000 simulations, randomly varying the input parameters within the ranges expected in the field. In nearly all the simulated conditions, the concentrations of contaminants in the groundwater for the specific type of BA tested in this work were well below EU and WHO drinking water quality criteria. Nevertheless, some caution should be paid in the case of the establishment of acidic conditions in the field since in this case the concentration of some elements (i.e. Al, Pb and Zn) is expected to exceed threshold values. In terms of risks to human health, for the considered utilization scenario the probability of exceeding the acceptable reference dose for water ingestion was usually less than 1% (except for Cr and Pb for which the probability was lower than 3.5% and 7%, respectively).

A two-dimensional analytical model of vapor intrusion involving vertical heterogeneity.

In this work, we present an analytical chlorinated vapor intrusion (CVI) model that can estimate source-to-indoor air concentration attenuation by simulating two-dimensional (2-D) vapor concentration profile in vertically heterogeneous soils overlying a homogenous vapor source. The analytical solution describing the 2-D soil gas transport was obtained by applying a modified Schwarz-Christoffel mapping method. A partial field validation showed that the developed model provides results (especially in terms of indoor emission rates) in line with the measured data from a case involving a building overlying a layered soil. In further testing, it was found that the new analytical model can very closely replicate the results of three-dimensional (3-D) numerical models at steady state in scenarios involving layered soils overlying homogenous groundwater sources. By contrast, by adopting a two-layer approach (capillary fringe and vadose zone) as employed in the EPA implementation of the Johnson and Ettinger model, the spatially and temporally averaged indoor concentrations in the case of groundwater sources can be higher than the ones estimated by the numerical model up to two orders of magnitude. In short, the model proposed in this work can represent an easy-to-use tool that can simulate the subsurface soil gas concentration in layered soils overlying a homogenous vapor source while keeping the simplicity of an analytical approach that requires much less computational effort.