Sorption of PFOS in 114 Well-Characterized Tropical and Temperate Soils: Application of Multivariate and Artificial Neural Network Analyses.

The influence of soil properties on PFOS sorption are not fully understood, particularly for variable charge soils. PFOS batch sorption isotherms were conducted for 114 temperate and tropical soils from Australia and Fiji, that were well-characterized for their soil properties, including total organic carbon (TOC), anion exchange capacity, and surface charge. In most soils, PFOS sorption isotherms were nonlinear. PFOS sorption distribution coefficients (Kd) ranged from 5 to 229 mL/g (median: 28 mL/g), with 63% of the Fijian soils and 35% of the Australian soils showing Kd values that exceeded the observed median Kd. Multiple linear regression showed that TOC, amorphous aluminum and iron oxides contents, anion exchange capacity, pH, and silt content, jointly explained about 53% of the variance in PFOS Kd in soils. Variable charge soils with net positive surface charges, and moderate to elevated TOC content, generally displayed enhanced PFOS sorption than in temperate or tropical soils with TOC as the only sorbent phase, especially at acidic pH ranges. For the first time, two artificial neural networks were developed to predict the measured PFOS Kd (R2 = 0.80) in the soils. Overall, both TOC and surface charge characteristics of soils are important for describing PFOS sorption.

A review of electrokinetically enhanced bioremediation technologies for PHs.

Since the early 1980's there have been several different strategies designed and applied to the remediation of subsurface environment including physical, chemical and biological approaches. They have had varying degrees of success in remediating contaminants from subsurface soils and groundwater. The objective of this review is to examine the range of technologies for the remediation of contaminants, particularly petroleum hydrocarbons, in subsurfaces with a specific focus on bioremediation and electrokinetic remediation. Further, this review examines the efficiency of remediation carried out by combining bioremediation and electrokinetic remediation. Surfactants, which are slowly becoming the selected chemicals for mobilizing contaminants, are also considered in this review. The current knowledge gaps of these technologies and techniques identified which could lead to development of more efficient ways of utilizing these technologies or development of a completely new technology.

Development of a modular vapor intrusion model with variably saturated and non-isothermal vadose zone.

Human health risk assessment at hydrocarbon-contaminated sites requires a critical evaluation of the exposure pathways of volatile organic compounds including assessments of vapor exposure in indoor air. Although there are a number of vapor intrusion models (VIM) currently available, they rarely reproduce actual properties of soils in the vadose zone. At best, users of such models assume averaged parameters for the vadose zone based on information generated elsewhere. The objective of this study was to develop a one-dimensional steady-state VIM, indoorCARE™ model, that considers vertical spatial variations of the degree of saturation (or effective air-filled porosity) and temperature of the vadose zone. The indoorCARE™ model was developed using a quasi-analytical equation that (1) solves the coupled equations governing soil-water movement driven by pressure head and a soil heat transport module describing conduction of heat and (2) provides a VIM that accommodates various types of conceptual site model (CSM) scenarios. The indoorCARE™ model is applicable to both chlorinated hydrocarbon and petroleum hydrocarbon (PHC) contaminated sites. The model incorporates biodegradations of PHCs at a range of CSM scenarios. The results demonstrate that predictions of indoor vapor concentrations made with the indoorCARE™ model are close to those of the J&E and BioVapor models under homogeneous vadose zone conditions. The newly developed model under heterogeneous vadose zone conditions demonstrated improved predictions of indoor vapor concentrations. The research study presented a more accurate and more realistic way to evaluate potential human health risks associated with the soil-vapor-to-indoor-air pathways.

Two-dimensional chlorinated vapour intrusion model involving advective transport of vapours with a highly permeable granular layer in the vadose zone serving as the preferential pathway.

Vapour intrusion (VI) is the process through which volatile organic compounds migrate from the subsurface source to the soil predominantly by diffusion, entering the overlying buildings through joints, cracks or other openings. This activity poses potentially serious health hazards for the occupants. Because of these health risks, recommendations for site closure are often made by quantifying the VI risks using mathematical models known as 'vapour intrusion models' (VIM). Most of these VIMs seem to overlook the role of preferred pathways like utility lines, high conductivity zones of soil or rocks, etc., which act as the path of least resistance for vapour transport thereby increasing vapour intrusion risks. This study presents a two-dimensional (2-D) chlorinated vapour intrusion (CVI) model which seeks to estimate the source-to-indoor air concentration attenuation. It takes into account the effects of a highly permeable utility line embedment as a preferential pathway. The transport of 2-D soil gas is described using the finite difference method where advection serves as the dominant transport mechanism in the preferential pathway layer, while diffusion applies to the rest of the vadose zone. The model returned results comparable with other models for the same input parameters, and was found to closely replicate the results of 3-D models. The simulations indicate that the presence of highly permeable utility line embedment and backfill layers do trigger a higher indoor air concentration compared to a no preferential pathway scenario.

Monitored natural attenuation of a long-term petroleum hydrocarbon contaminated sites: a case study.

This study evaluated the potential of monitored natural attenuation (MNA) as a remedial option for groundwater at a long-term petroleum hydrocarbon contaminated site in Australia. Source characterization revealed that total petroleum hydrocarbons (TPH) as the major contaminant of concern in the smear zone and groundwater. Multiple lines of evidence involving the geochemical parameters, microbiological analysis, data modelling and compound-specific stable carbon isotope analysis all demonstrated natural attenuation of hydrocarbons occurring in the groundwater via intrinsic biodegradation. Groundwater monitoring data by Mann-Kendall trend analysis using properly designed and installed groundwater monitoring wells shows the plume is stable and neither expanding nor shrinking. The reason for stable plume is due to the presence of both active source and natural attenuation on the edge of the plume. Assuming no retardation and no degradation the contaminated plume would have travelled a distance of 1,096 m (best case) to 11,496 m (worst case) in 30 years. However, the plume was extended only up to about 170 m from its source. The results of these investigations provide strong scientific evidence for natural attenuation of TPH in this contaminated aquifer. Therefore, MNA can be applied as a defensible management option for this site following significant reduction of TPH in the source zone.

Uncertainty based optimal monitoring network design for a chlorinated hydrocarbon contaminated site.

An application of a newly developed optimal monitoring network for the delineation of contaminants in groundwater is demonstrated in this study. Designing a monitoring network in an optimal manner helps to delineate the contaminant plume with a minimum number of monitoring wells at optimal locations at a contaminated site. The basic principle used in this study is that the wells are installed where the measurement uncertainties are minimum at the potential monitoring locations. The development of the optimal monitoring network is based on the utilization of contaminant concentration data from an existing initial arbitrary monitoring network. The concentrations at the locations that were not sampled in the study area are estimated using geostatistical tools. The uncertainty in estimating the contaminant concentrations at such locations is used as design criteria for the optimal monitoring network. The uncertainty in the study area was quantified by using the concentration estimation variances at all the potential monitoring locations. The objective function for the monitoring network design minimizes the spatial concentration estimation variances at all potential monitoring well locations where a monitoring well is not to be installed as per the design criteria. In the proposed methodology, the optimal monitoring network is designed for the current management period and the contaminant concentration data estimated at the potential observation locations are then used as the input to the network design model. The optimal monitoring network is designed for the consideration of two different cases by assuming different initial arbitrary existing data. Three different scenarios depending on the limit of the maximum number of monitoring wells that can be allowed at any period are considered for each case. In order to estimate the efficiency of the developed optimal monitoring networks, mass estimation errors are compared for all the three different scenarios of the two different cases. The developed methodology is useful in coming up with an optimal number of monitoring wells within the budgetary limitations. The methodology also addresses the issue of redundancy, as it refines the existing monitoring network without losing much information of the network. The concept of uncertainty-based network design model is useful in various stages of a potentially contaminated site management such as delineation of contaminant plume and long-term monitoring of the remediation process.

Insights into vapour intrusion phenomena: Current outlook and preferential pathway scenario.

Vapour intrusion (VI) is the phenomenon by which volatile organic compounds (VOCs) migrate from the subsurface source through the soil and enter into the overlying buildings, affecting the indoor air quality and ultimately causing health hazards to the occupants. Health risk assessments associated with hydrocarbon contaminated sites and recommendations of site closure are often made by quantifying the VI risks using mathematical models known as 'vapour intrusion models' (VIM). In order to predict the health risk, various factors such as the lithological and geochemical conditions of the subsurface, environmental conditions, building operational conditions etc. are commonly evaluated using VIMs. Use of these models can overlook the role of preferential pathways like highly permeable subsurface layers and utility lines which act as the path of least resistance for vapour transport, which can increase the VI risks. The extensive networks of utility lines and sanitary sewer systems in urban areas can significantly exacerbate the uncertainty of VI investigations. The backfill materials like sand and gravel surrounding the utility lines can allow the vapours to easily pass through due to their high porosity as compared to natural formations. Hence, failure to understand the role of preferential pathways on the fate and transport of VOC in the vadose zone can result in more conservative predictions of indoor air vapour concentrations and wrong clean up approaches. This comprehensive review outlines the vapour transport mechanisms, factors influencing VI, VIMs and the role of preferential pathways in predicting indoor air vapour concentrations.

Application of infrared spectrum for rapid classification of dominant petroleum hydrocarbon fractions for contaminated site assessment.

In this study, the infrared spectrum (4000-400 cm-1) was applied to identify and classify the different alkanes based on carbon chain length (Cn). It was found there were two bands coherent to the doublet at location 2954 and 2872 cm-1, respectively can be applied to identify the fraction of carbon chains. From C20 to C37, by the increase of the Cn, the intensities of the two bands were reduced as demonstrated. There were another two doublets existed at the region from 1480 to 1450 cm-1 and the region at 750 and 730 cm-1. It was observed the intensity of one coherent band at each of these regions was increased following the increase on the Cn. The bands center at 1462 and 730 cm-1 were increased from C20 to C37. The intensity ratio of the coherent bands can be applied to identify the Cn. Successfully identify four different petroleum products with different fractions of carbon chains in soil samples, is evidence the theory can be applied to investigate the fraction of carbon chains in soil. Coupling with handheld FTIR, it is possible to rapidly estimate the dominant fraction of Cn in soil in field.

Influence of soil properties on vapor-phase sorption of trichloroethylene.

Current practices in health risk assessment from vapor intrusion (VI) using mathematical models are based on assumptions that the subsurface sorption equilibrium is attained. The time required for sorption to reach near-steady-state conditions at sites may take months or years to achieve. This study investigated the vapor phase attenuation of trichloroethylene (TCE) in five soils varying widely in clay and organic matter content using repacked columns. The primary indicators of TCE sorption were vapor retardation rate (Rt), the time required for the TCE vapor to pass through the soil column, and specific volume of retention (VR), and total volume of TCE retained in soil. Results show TCE vapor retardation is mainly due to the rapid partitioning of the compound to SOM. However, the specific volume of retention of clayey soils with secondary mineral particles was higher. Linear regression analyses of the SOM and clay fraction with VR show that a unit increase in clay fraction results in higher sorption of TCE (VR) than the SOM. However, partitioning of TCE vapor was not consistent with the samples' surface areas but was mainly a function of the type of secondary minerals present in soils.