Impact of Water Table Fluctuations on the Concentration of Borehole Gas from NAPL Sources in the Vadose Zone

Core Ideas Field borehole TCE gas concentration responded to water table fluctuations. Laboratory and numerical tests were performed to analyze the response patterns. Borehole gas data can be used for identification of contamination sources. 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.