Chlorinated hydrocarbons are prevalent contaminants in soils and sediments due to improper disposal and accidental spillage. An important pathway for human exposure of these contaminants is the intrusion of their vapors into occupied buildings through the unsaturated vadose zone. Understanding the physical, chemical, and biological regulators of the vapor intrusion (VI) pathway is crucial to assess the health risks associated with chlorinated hydrocarbon contaminants (CHCs) at tens of thousands of pollution sites across the U.S. The overarching goal of the proposed research is to investigate an important, yet overlooked, chemical regulator of the chlorinated hydrocarbon vapor intrusion pathway, namely the nanoscale, heterogeneous reactions between vapor compounds and soil mineral surfaces. The proposed research takes an integrated approach combining experimental and modeling efforts. State-of-the-art analytical and surface-sensitive techniques, including gas chromatography mass spectrometry (GC/MS), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS), will be used to identify reaction products, quantify reaction kinetics, and elucidate reaction mechanisms. A multicomponent, multi-phase simulator, Michigan Soil Environment Remediation (MISER), will be modified to incorporate the new nanoscale chemistry into the assessment of vapor intrusion. In support of the goal of this project, three research objectives will be used to guide the formulation of hypotheses and the design and selection of experiments: (a) The first objective is to determine the prevalence of nanoscale surface reactions using representative chlorinated hydrocarbon compounds and soil minerals (including aged minerals). (b) The second objective is to investigate the effects of environmental parameters such as humidity and temperature on the reaction mechanisms, kinetics, and product stability. (c) The third objective is to mathematically evaluate the significance of vapor-mineral reactions as a chemical regulator of the vapor intrusion pathway.
The proposed project has five tasks. First, eleven representative CHC compounds, three classes of soil minerals, and reconstructed calcite will be screened for their potentials to react with one another in the vapor intrusion pathway. The CHC compounds are selected based on their prevalence at various contamination sites as well as structural diversity. The soil minerals are five carbonates, quartz, and two feldspars. Calcite reconstructed under high humid conditions is used to evaluate the reactivity of aged minerals. Second, the kinetics of CHC-mineral reactions will be quantified by monitoring both gas-phase and surface products in an AFM fluid cell. The fluid cell serves as a continuously stirred tank reactor. The evolution of the reactions will be quantified by GC/MS (for CHC vapor and gas-phase products) and AFM (for surface nanostructure growth). Third, the changes of reaction products and kinetics under varying humidity will be determined using the AFM fluid cell. The condensation of water monolayers from humid air can have complicated consequences for CHC-mineral reactions, including generating reactive hydroxyl groups, creating reactive mobile ions, and blocking reactive surface sites. The variation of humidity is a typical environmental condition that happens between seasons. Forth, the release of volatile compounds from CHC-induced nanostructures will be evaluated using batch reactors at elevated temperatures. Temperature change is another seasonal variation. The increase of temperature that occurs during the transition from a cold season to a warm one may destabilize the CHC-induced nanostructures and release toxic volatile compounds unexpectedly. Last, a numerical model will be developed to simulate the vapor intrusion pathway with the abiotic attenuation. The evaluation will be performed using site specific information acquired from the Indiana Department of Environmental Management. The main intellectual merit of the proposed research is to provide a knowledge base for more sophisticated and accurate modeling of chlorinated hydrocarbon vapor intrusion.
The research team also plans to make broader impacts in the proposed project by (1) training students from underrepresented groups on environmental nanogeochemistry research, (2) incorporating new knowledge obtained from the research frontier to the undergraduate-level courses for environmental science and engineering, and (3) providing local high-school students with research opportunities through outreach activities.
Chlorinated hydrocarbons are prevalent contaminants in soils and sediments due to improper disposal and accidental spillage. An important pathway for human exposure of these contaminants is the intrusion of their vapors into occupied buildings through the unsaturated vadose zone. Understanding the physical, chemical, and biological regulators of the vapor intrusion (VI) pathway is crucial to assess the health risks associated with chlorinated hydrocarbon contaminants (CHCs) at tens of thousands of contaminated sites across the U.S. This project is collaborated between the University of Notre Dame (UND) and the University of Nebraska-Lincoln (UNL). While UND focused on experimental investigation of the abiotic attenuation of chlorinated hydrocarbons, the UNL group devoted to mathematically simulating the effects of abiotic attenuation on VI. The main intellectual merit for the project is to provide a knowledge base for more sophisticated and accurate modeling of chlorinated hydrocarbon vapor intrusion. At UNL, a state of the art three-dimensional (3-D) numerical model was developed to simulate the reactive transport of chlorinated hydrocarbons vapor intrusion. The mathematical model was validated by comparing the modeling results with a field site measurement. Although multiple vapor intrusion models were developed in the literature, none of these models were validated. We have developed a unique approach to rigorously validate a 3-D VI model with field data. This approach allowed us to estimate crack area percentage, a critical parameter for VI which was typically assumed without any basis in most previous work. The comparison between our modeling results and field measurements went beyond typical reports, where only indoor air concentration was reported. In addition to indoor air concentration, we were able to compare (i) chlorinated carbon and oxygen concentration distribution under the building; (ii) diffusion flux, and (iii) advective flux. Those significant information provide much more reliable model validation. Because of our rigorous model validation, we identified several critical factors for VI simulation, which were not previously considered in most VI modeling papers. For the first time, the significance of soil anisotropy and second order reaction were quantified in a VI model. The project led to several notable broader impacts. The project fully supports one female Ph.D. student at UNL, and one undergraduate student majoring in computer science and mathematics. The PI and the graduate student participated in the Young Nebraska Scientist (YNS) Summer Camp program sponsored by Nebraska EPSCoR every summer of the project duration. PI’s group hosted a female high school student to conduct research related to this project, presented guest lectures to summer camps, and facilitated hand-on laboratory sessions for summer camps. Of the over 200 participants each year in YNS sponsored summer camps, 52 percent were female, 62 percent were underrepresented minorities, and 72 percent were low-income. Of the camps offered to students across the state, 40 percent of those participants were rural. The findings from this work were also distributed via multiple conference presentations in the professional conference and multiple peer-reviewed journal publications.
