Influence of Fluctuating Groundwater Table on Volatile Organic Chemical Emission Flux at a Dissolved Chlorinated‐Solvent Plume Site

Abstract

Groundwater elevation fluctuation has been recognized as one mechanism causing temporal indoor air volatile organic chemical (VOC) impacts in vapor intrusion risk assessment guidance. For dissolved VOC sources, groundwater table fluctuation shortens/lengthens the transport pathway, and delivers dissolved contaminants to soils that are alternating between water saturated and variably saturated conditions, thereby enhancing volatilization potential. To date, this mechanism has not been assessed with field data, but enhanced VOC emission flux has been observed in lab‐scale and modeling studies. This work evaluates the impact of groundwater elevation changes on VOC emission flux from a dissolved VOC plume into a house, supplemented with modeling results for cyclic groundwater elevation changes. Indoor air concentrations, air exchange rates, and depth to groundwater (DTW) were collected at the study house during an 86‐d constant building underpressurization test. These data were used to calculate changes in trichloroethylene (TCE) emission flux to indoor air, during a period when DTW varied daily and seasonally from about 3.1 to 3.4 m below the building foundation (BF). Overall, TCE flux to indoor air varied by about 50% of the average, without any clear correlation to changes in DTW or its change rate. To complement the field study, TCE surface emission fluxes were simulated using a one‐dimensional model (HYDRUS 1D) for conditions similar to the field site. Simulation results showed time‐averaged surface TCE fluxes for cyclic water‐table elevations were greater than for stationary water‐table conditions at an equivalent time‐averaged water‐table position. The magnitudes of temporal TCE emission flux changes were generally less than 50% of the time‐averaged flux, consistent with the field site observations. Simulation results also suggested that TCE emission flux changes due to groundwater fluctuation are likely to be significant at sites with shallow groundwater (e.g., < 0.5 m BF) and permeable soil types (e.g., sand).

A Review of Volatile Organic Compound Contamination in Post-Industrial Urban Centers: Reproductive Health Implications Using a Detroit Lens

“…Shallow groundwater has a high risk of contamination from surface spills and buried waste due to the relatively short vertical distance to the water table and small hydraulic retention time [ 63 , 64 , 65 ]. Areas with shallow groundwater, including coastal southeast MI and Detroit are impacted by additional environmental problems which include direct flooding of houses during storm events and VOC vapor intrusion [ 53 , 66 ]. In fact, VOCs are common subsurface contaminants found in urban groundwater systems across the USA [ 33 , 67 ].…”

“…The amount of water transferring through this shallow system is not well quantified since most of the water resource studies are focused on deep groundwater systems [ 93 , 94 , 95 ]. Guo et al [ 66 ] noted that fluctuating groundwater tables present potential exposure routes via flooding and increasing proximity to groundwater. Evaluating vertical subsurface water movement can contribute to understanding this exposure route.…”

A Review of Recent Vapor Intrusion Modeling Work

“…Later, Shen et al (2013) applied a 1D analytical solution to investigate soil moisture profiles’ effects on the soil gas vapor concentration away from any surface capping by buildings or pavement. Guo et al (2019) also used a 1D numerical model (HYDRUS 1D) to evaluate the effects of groundwater elevation fluctuation on VI and found that emission flux changes due to groundwater fluctuation are likely to be significant at sites with shallow groundwater and permeable soil types. Similar results were also reported by Qi et al (2020).…”

Evaluation of Passive Diffusive‐Adsorptive Samplers for Use in Assessing Time‐Varying Indoor Air Impacts Resulting from Vapor Intrusion

“…The residential building used for testing has been previously described as it has been used for other VI studies (Holton et al 2013;Guo et al 2015;Holton et al 2015;Holton et al 2018;Guo et al 2019;Guo et al 2020). The house overlain a dissolved trichloroethylene (TCE) groundwater plume with dissolved TCE groundwater concentrations ranging from 10 to 50 μg/L-H 2 O.…”