It is estimated that more than 7,000 Department of Defense (DoD) sites have contaminated groundwater, most of which will require investigation of the vapor intrusion (VI) pathway. Although screening criteria are available and accepted in many other types of investigation and regulatory contexts, the VI pathway is relatively new. As a result, screening criteria tend to be extremely conservative, “prioritizing” the majority of sites for further investigation.
The overall objective of this project was to develop and validate simple procedures to screen for VI and provide criteria for decision making, including no further need for site evaluation for the VI pathway. The procedures rely on easily obtainable site-specific information that can be applied to criteria that are protective but less conservative than the generic (i.e., Tier 1) screening concentrations provided in many U.S. Environmental Protection Agency and state VI guidance documents.
For this project, two methods were developed and evaluated: (1) Tier 2-level, site-specific evaluation and screening based on physical properties such as soil type and (2) building-specific Tier 3 field investigation of the VI pathway based on building characteristics. Performance objectives included collecting data from multiple sites in a consistent and comparable manner to allow evaluation of factors influencing volatile organic compound (VOC) migration and attenuation. Data was collected and analyzed at seven sites for the Tier 2 evaluation and six sites for the Tier 3 evaluation.
The Tier 2 and 3 procedures were based on the results obtained from ESTCP Project ER-200423. The demonstrations for Tier 2 and Tier 3 evaluation methods included:
- : (1) field and laboratory measurements of soil characteristics to determine the best method for identification of sites with fine-grained soils within 2 ft above the water table and (2) detailed VOC concentration measurements in groundwater and soil gas to evaluate vertical profiles and VOC attenuation. The demonstration program yielded groundwater to deep soil gas attenuation factors for seven sites covering a range of soil characteristics allowing validation of the hypothesis that VOC attenuation is higher at sites with fine-grained soils within 2 ft above the water table.
- : (1) control of building pressure to create negative and then positive building pressure conditions; (2) measurement of pressure gradients across the building envelope and building foundation; and (3) measurement of radon, indoor tracer gas, and VOC concentrations in indoor air and sub-slab soil gas under each pressure condition. The results of the demonstration program allowed an evaluation of (1) building foundation permeability based on measurement of cross foundation pressure gradients, (2) the effect of building pressure control on the movement of soil gas into buildings, and (3) the utility of sampling under controlled building pressure conditions to evaluate VI.
The field investigation program for Tier 2 evaluated the importance of factors such as soil permeability, hydraulic conductivity, soil moisture, and grain size in VOC attenuation. Based on the demonstration results, it is recommended that groundwater screening concentrations at fine-grained soil sites be increased by 100H over the default (i.e., Tier 1) screening values determined to be protective for all types of sites, subject to limitations described in the Implementation Issues section below. For identification of fine-grained soil sites, field-measured soil (intrinsic) permeability at depths within 2 ft above the water table was found to be the most accurate method. Visual inspection of soil cores also provided an accurate soil type classification at six of the seven demonstration sites.
The field investigation program for Tier 3 demonstrated that a building-specific investigation program utilizing sampling under controlled building pressure conditions provides an improved understanding of the potential for VI in the building. Controlled negative building pressure supported the flow of soil gas into the building, as documented by increased indoor concentrations of radon and VOCs originating from subsurface sources. Conversely, controlled positive building pressure suppressed the flow of soil gas into the building, as documented by radon concentrations in indoor air equal to the concentration in ambient air. The response of VOCs originating from the subsurface was similar to radon. In contrast, the indoor air concentration of VOCs originating from aboveground sources showed little difference between the induced pressure conditions. An expanded version of the Tier 3 demonstration program implemented in two buildings demonstrated reproducibility of the procedure.
In one building, VI was not evident during initial baseline sampling but was induced under negative pressure conditions. This finding is consistent with other studies, which have shown episodic VI in this building. The results from this building demonstrate that the Tier 3 investigation procedure reduces the uncertainty associated with temporal variability in VI.
The Tier 2 screening procedures will be useful only at certain sites (i.e., sites with fine-grained soil layers within 2 ft above the water table. Sites with exclusively sandy soils and sites in dry climates with low moisture content soils will not benefit). The Tier 2 screening procedure should not be applied to sites where the depth to groundwater is less than 5 ft below ground surface.
The Tier 3 procedure is not applicable to very large or very leaky buildings where the building pressure cannot be easily controlled. In addition, the pressure control method does not eliminate the spatial variability of VOC concentrations that is observed at many investigation sites. Results showing that VI occurs only under depressurized conditions will not be directly applicable to normal building operating conditions because the observed magnitude of VI under these conditions may be greater than under normal operating conditions. In these cases, the investigator may choose either preemptive mitigation or continued monitoring.