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Impact of water table fluctuations on the concentration of borehole gas from NAPL sources in the vadose zone

Won-Tak Joun, Seong-Sun Lee, Youn-Eun Koh and Kang-Kun Lee
Impact of water table fluctuations on the concentration of borehole gas from NAPL sources in the vadose zone
Vadose Zone Journal (April 2016) 15 (4)


Dense non-aqueous-phase liquids (DNAPLs), such as trichloroethylene (TCE), adversely affect groundwater quality because they are difficult to remove from the subsurface environment. Trichloroethylene is volatile, and TCE gas can diffuse into a borehole to change the borehole gas concentration. Thus, borehole TCE gas monitoring can provide information on the presence of a TCE source. In this study, borehole gas extraction tests were conducted at a field site contaminated by TCE and its daughter products. Temporal changes in borehole gas concentrations appeared to be closely related to changes in the level of the water table. Laboratory experiments were performed to demonstrate that changes in gas concentrations were due to water table fluctuation at boreholes located near residual TCE point sources. Three source configurations for the borehole TCE gas migration, generated from either DNAPL, TCE, or dissolved TCE, were used to design two-dimensional sand tank experiments. Under fluctuating water table conditions, DNAPL TCE sources in the unsaturated zone produced slow response times but high and long-lasting gas concentrations. In contrast, dissolved TCE sources produced relatively weak gas concentrations characterized by a peak, quickly followed by decreasing gas concentration curves. The Subsurface Transport Over Multiple Phases (STOMP) simulator was used to simulate breakthrough curves under fluctuating water table conditions. The results of the sand tank experiments and numerical simulations both suggest that a fluctuating water table is a significant factor in determining borehole gas concentrations. The results also show that the combination of borehole gas and groundwater monitoring has an important role in field applications because the changing pattern of borehole gas concentrations can be used to understand characteristics of the presence of DNAPL sources, whether these remained only in the unsaturated zone or were also dissolved in groundwater.

ISSN: 1539-1663
Serial Title: Vadose Zone Journal
Serial Volume: 15
Serial Issue: 4
Title: Impact of water table fluctuations on the concentration of borehole gas from NAPL sources in the vadose zone
Affiliation: Seoul National University, School of Earth and Environmental Sciences, Seoul, South Korea
Published: 201604
Text Language: English
Publisher: Soil Science Society of America, Madison, WI, United States
Number of pages: 13
References: 55
Accession Number: 2016-058971
Categories: Environmental geologyHydrogeology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, sketch map
N37°19'60" - N37°22'00", E127°54'00" - E127°57'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Soil Science Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201628
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