Matching Items (3)
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- All Subjects: Indoor air pollution
- Creators: Johnson, Paul C
Description
Vapor intrusion (VI), can pose health risks to building occupants. Assessment and mitigation at VI impacted sites have been guided by a site conceptual model (SCM) in which vapors originate from subsurface sources, diffuse through soil matrix and enter into a building by gas flow across foundation cracks. Alternative VI pathways and groundwater table fluctuations are not often considered.
Alternative VI pathways, involving vapor transport along sewer lines and other subsurface infrastructure, have recently been found to be significant contributors to VI impacts at some sites. This study evaluated approaches for identifying and characterizing the significance of alternative VI pathways and assessed the effectiveness of conventional mitigation at a site with an alternative VI pathway that can be manipulated to be on or off. The alternative pathway could not be identified using conventional pathway assessment procedures and can only be discovered under controlled pressure method (CPM) conditions. Measured emission rates were two orders of magnitude greater than screening model estimates and sub-foundation vertical soil gas profiles changed and were no longer consistent with the conventional VI conceptual model when the CPM test was conducted. The pipe flow VI pathway reduced the vacuum performance of the sub-slab depressurization (SSD) VI mitigation system, but the SSD system still provided sufficient protection to the house.
The relationship between groundwater table fluctuations and subsurface vapor emissions and transport is examined using multi-year data from the field site, and is studied in the laboratory. In addition, a broader range of conditions is examined through use of modeling validated with the experimental data. The results indicate that fluctuating groundwater tables will lead to amplified volatile organic chemical (VOC) emissions from groundwater to soil surface relative to steady water table elevation, however, the magnitude of this amplification is less concerned when long-term water fluctuation present. No clear correlations were found between VOC emissions and water table changes at the study site where annual water table fluctuations of about 0.3 m existed. Significant VOC emission amplifications by water table fluctuation would be expected under shallow groundwater conditions according to model analysis results.
Alternative VI pathways, involving vapor transport along sewer lines and other subsurface infrastructure, have recently been found to be significant contributors to VI impacts at some sites. This study evaluated approaches for identifying and characterizing the significance of alternative VI pathways and assessed the effectiveness of conventional mitigation at a site with an alternative VI pathway that can be manipulated to be on or off. The alternative pathway could not be identified using conventional pathway assessment procedures and can only be discovered under controlled pressure method (CPM) conditions. Measured emission rates were two orders of magnitude greater than screening model estimates and sub-foundation vertical soil gas profiles changed and were no longer consistent with the conventional VI conceptual model when the CPM test was conducted. The pipe flow VI pathway reduced the vacuum performance of the sub-slab depressurization (SSD) VI mitigation system, but the SSD system still provided sufficient protection to the house.
The relationship between groundwater table fluctuations and subsurface vapor emissions and transport is examined using multi-year data from the field site, and is studied in the laboratory. In addition, a broader range of conditions is examined through use of modeling validated with the experimental data. The results indicate that fluctuating groundwater tables will lead to amplified volatile organic chemical (VOC) emissions from groundwater to soil surface relative to steady water table elevation, however, the magnitude of this amplification is less concerned when long-term water fluctuation present. No clear correlations were found between VOC emissions and water table changes at the study site where annual water table fluctuations of about 0.3 m existed. Significant VOC emission amplifications by water table fluctuation would be expected under shallow groundwater conditions according to model analysis results.
ContributorsGuo, Yuanming (Author) / Johnson, Paul C (Thesis advisor) / Fraser, Matthew (Committee member) / Westerhoff, Paul (Committee member) / Arizona State University (Publisher)
Created2015
Description
Vapor intrusion (VI) pathway assessment often involves the collection and analysis of groundwater, soil gas, and indoor air data. There is temporal variability in these data, but little is understood about the characteristics of that variability and how it influences pathway assessment decision-making. This research included the first-ever collection of a long-term high-frequency indoor air data set at a house with VI impacts overlying a dilute chlorinated solvent groundwater plume. It also included periodic synoptic snapshots of groundwater and soil gas data and high-frequency monitoring of building conditions and environmental factors. Indoor air trichloroethylene (TCE) concentrations varied over three orders-of-magnitude under natural conditions, with the highest daily VI activity during fall, winter, and spring months. These data were used to simulate outcomes from common sampling strategies, with the result being that there was a high probability (up to 100%) of false-negative decisions and poor characterization of long-term exposure. Temporal and spatial variability in subsurface data were shown to increase as the sampling point moves from source depth to ground surface, with variability of an order-of-magnitude or more for sub-slab soil gas. It was observed that indoor vapor sources can cause subsurface vapor clouds and that it can take days to weeks for soil gas plumes created by indoor sources to dissipate following indoor source removal. A long-term controlled pressure method (CPM) test was conducted to assess its utility as an alternate approach for VI pathway assessment. Indoor air concentrations were similar to maximum concentrations under natural conditions (9.3 μg/m3 average vs. 13 μg/m3 for 24 h TCE data) with little temporal variability. A key outcome was that there were no occurrences of false-negative results. Results suggest that CPM tests can produce worst-case exposure conditions at any time of the year. The results of these studies highlight the limitations of current VI pathway assessment approaches and demonstrate the need for robust alternate diagnostic tools, such as CPM, that lead to greater confidence in data interpretation and decision-making.
ContributorsHolton, Chase Weston (Author) / Johnson, Paul C (Thesis advisor) / Fraser, Matthew (Committee member) / Forzani, Erica (Committee member) / Arizona State University (Publisher)
Created2015
Description
Up to 25 percent of the operating budget for contaminated site restoration projects is spent on site characterization, including long-term monitoring of contaminant concentrations. The sensitivity, selectivity, and reproducibility of analytical methods have improved to the point where sampling techniques bear the primary responsibility for the accuracy and precision of the data. Most samples represent discrete concentrations in time and space; with sampling points frequently limited in both dimensions, sparse data sets are heavily extrapolated and the quality of data further limited.
Methods are presented for characterizing contaminants in water (groundwater and surface waters) and indoor air. These techniques are integrative, providing information averaged over time and/or space, as opposed to instantaneous point measurements. Contaminants are concentrated from the environment, making these methods applicable to trace contaminants. These methods have the potential to complement existing techniques, providing the practitioner with opportunities to reduce costs and improve the quality of the data used in decision making.
A conceptual model for integrative sampling of environmental waters is developed and a literature review establishes an advantage in precision for active samplers. A programmable sampler was employed to measure the concentration of chromate in a shallow aquifer exhibiting time-dependent contaminant concentrations, providing a unique data set and sustainability benefits. The analysis of heat exchanger condensate, a waste stream generated by air conditioning, is demonstrated in a non-intrusive method for indoor air quality assessment. In sum, these studies present new opportunities for effective, sustainable environmental characterization.
Methods are presented for characterizing contaminants in water (groundwater and surface waters) and indoor air. These techniques are integrative, providing information averaged over time and/or space, as opposed to instantaneous point measurements. Contaminants are concentrated from the environment, making these methods applicable to trace contaminants. These methods have the potential to complement existing techniques, providing the practitioner with opportunities to reduce costs and improve the quality of the data used in decision making.
A conceptual model for integrative sampling of environmental waters is developed and a literature review establishes an advantage in precision for active samplers. A programmable sampler was employed to measure the concentration of chromate in a shallow aquifer exhibiting time-dependent contaminant concentrations, providing a unique data set and sustainability benefits. The analysis of heat exchanger condensate, a waste stream generated by air conditioning, is demonstrated in a non-intrusive method for indoor air quality assessment. In sum, these studies present new opportunities for effective, sustainable environmental characterization.
ContributorsRoll, Isaac B (Author) / Halden, Rolf U. (Thesis advisor) / Johnson, Paul C (Committee member) / Kavazanjian, Edward (Committee member) / Arizona State University (Publisher)
Created2015