Vertical Screening Distance Criteria to Evaluate Vapor Intrusion Risk from 1,2‐Dichloroethane (1,2‐DCA)

Abstract

Vapor intrusion (VI) involves migration of volatile contaminants from subsurface through unsaturated soil into overlying buildings. In 2015, the US EPA recommended an approach for screening VI risks associated with gasoline releases from underground storage tank (UST) sites. Additional assessment of the VI risk from petroleum hydrocarbons was deemed unnecessary for buildings separated from vapor sources by more than recommended vertical screening distances. However, these vertical screening distances did not apply to potential VI risks associated with releases of former leaded gasoline containing 1,2‐dichloroethane (1,2‐DCA), because of a lack of empirical data on the attenuation of 1,2‐DCA in soil gas. This study empirically evaluated 144 paired measurements of 1,2‐DCA concentrations in soil gas and groundwater collected at 47 petroleum UST sites combined with BioVapor modeling. This included (1) assessing the frequency of 1,2‐DCA detections in soil gas below 10−6 risk‐based screening levels at different vertical separation distances and (2) comparing the US EPA recommended vertical screening distances with those predicted by BioVapor modeling. Vertical screening distances were predicted for different soil types using aerobic biodegradation rate constants estimated from the measured soil‐gas data combined with conservative estimates of source concentrations. The modeling indicates that the vertical screening distance of 6 feet (1.8 m) recommended for dissolved‐phase sources is applicable for 1,2‐DCA below certain threshold concentrations in groundwater, while 15 feet (4.6 m) recommended for light nonaqueous phase liquid (LNAPL) sources is applicable for sites with clay and loam soils in the vadose zone, but not sand, if 1,2‐DCA concentrations in groundwater exceed 150 μg/L. This dependence of the predicted vertical screening distances on soil type places added emphasis on proper soil characterization for VI screening at sites with 1,2‐DCA sources. The soil‐gas data suggests that a vertical screening distance of 15 feet (4.6 m) is necessary for both dissolved‐phase and LNAPL sources.

Evaluation of Vapor Intrusion Risk from Ethylene Dibromide Using the Vertical Screening Distance Approach

“…The conventional analytical methods used for soil-gas analysis (U.S. EPA TO-15 and U.S. EPA 8260B) in the available PVI investigation data at petroleum UST sites could not detect EDB at concentrations ≤0.16 μg/m 3 (EDB VISL in soil-gas at the 10 −6 risk level). As such, these soil-gas data were not suitable to estimate vertical screening distances for EDB (Kolhatkar et al 2019). This limitation of the existing analytical methods was primarily due to the interference from co-existing non-target compounds (i.e., BTEX and other gasoline range hydrocarbons) that required sample dilution to protect analytical instrumentation resulting in elevated detection limits above the EDB VISL in soil-gas.…”

“…The LNAPL indicator criterion of 1000 μg/L benzene in groundwater recommended by ITRC ( 2014) was used as the threshold to distinguish between dissolved-phase source and LNAPL source of EDB in the soil-gas. This threshold was empirically derived from groundwater data (Peargin and Kolhatkar 2011) and previously used for evaluating vertical screening distances for 1,2-DCA by Kolhatkar et al 2019.…”

“…These assumptions resulted in conservative estimates of the vertical screening distances for EDB, which were compared to the U.S. EPA recommended vertical screening distances (6 ft [1.8 m] for dissolved-phase and 15 ft [4.6 m] for LNAPL source) to assess if the U.S. EPA recommended distances were appropriate for EDB. Source concentrations for dissolved-phase and LNAPL sources for other constituents (benzene and TPHg as a sum of the aliphatic and aromatic fractions) ranged from detection limits to 95th percentile values in groundwater (Kolhatkar et al 2019). Benzene and TPHg (aliphatic and aromatic fractions) were considered as the constituents in the PVIScreen model to account for the oxygen demand associated with their aerobic biodegradation in the vadose zone.…”

“…They also recommended that the U.S. EPA incorporate the results of such investigations in making appropriate revisions to their PVI guidance in U.S. EPA (2015). Kolhatkar et al (2019) also attempted to evaluate the applicability of the U.S. EPA recommended vertical screening distances for EDB by analyzing soil-gas and groundwater data from petroleum UST sites. Unfortunately, all the EDB concentrations in soil-gas were non-detect at the available reporting limits (≫0.16 μg/ m 3 , EDB VISL in soil-gas at the 10 −6 risk level).…”

“…The first approach evaluated the percentage frequency of EDB detections in soil-gas ≤0.16 μg/m 3 and empirically assessed at several vertical separation distances above dissolved-phase and LNAPL sources. This approach was like the previous studies on petroleum hydrocarbons (U.S. EPA 2013;Lahvis et al 2013) and 1,2-DCA (Kolhatkar et al 2019). The second approach involved estimating first-order aerobic biodegradation rate constants for EDB associated with dissolved-phase and LNAPL sources by fitting the soil-gas concentration data to the predicted soil-gas profiles using the BioVapor model (DeVaull 2007;API 2010).…”