Volume 5: Response activity for the VIAP
Appendix A.2 – 1.0 Introduction and conceptual site model

1.1 Introduction and overview

This section should briefly describe the purpose of this guidance.

1.2 Site setting and description of structures

This section should describe the site setting including the location, use, and current structures with an AVMS. This section should also include a description of the height, size, and general construction characteristics.

The results of a building survey should identify specific building characteristics, features, and configurations that affect the design and effectiveness of the AVMS. See Attachment 1 for an example of a building survey that should be performed immediately after the installation of the AVMS.

Note: Many buildings (particularly larger commercial/industrial buildings) have multiple foundation types and locations due to building additions over time; slab and sub-slab conditions often vary between building areas, and foundation walls or changes in floor elevation which may prevent airflow from one slab area to another. Other foundation features such as elevators, pits, sumps, floor drains, utilities, utility tunnels, and other structures located below the slab or floor level should be considered for their effect on AVMS designs.

Key supporting information

• Topographic Site Location Map (from USGS or site plans as available).
• General Site Feature Map that includes structures to scale with any property lines, right-of-way (ROW), and major utilities.
• Figure(s) in plan view of the structure to be mitigated and that identifies the room divisions, footings known and estimated, and other key features such as the location of pits, sumps, vents, basements.
• Cross-sectional view figures and drawings may be helpful for depicting other key features relative to the proposed mitigation.
• Appendix with building survey that includes a photo log (see Attachment 1).

1.3 Site history

This section describes the activities and operational history that occurred, any current use, as well as the hazardous substances or regulated substances were or are used.

Note: Describing the site history aids in identifying the presence or likely presence of hazardous or regulated substances in, on, or at a property due to any release to the environment or conditions indicative of a release to the environment and may provide some general information on the concentrations of hazardous substances that could be expected to be encountered.

Key supporting information

• Table (optional depending on the complexity of the site) that provides a list of known environmental activities or historical operations that may have led to a release of hazardous or regulated substances into the environment.
• Figure that identifies the location of each environmental activity or historical operation that may have or are known to have led to a release of a hazardous or regulated substance into the environment.

1.4 Geology/hydrogeology

This section should describe the site geology and hydrogeology information. Soils may be described using either the USCS, USDA, or other such as AASHTO/MDOT classifications, but the classification system should be identified. Supporting data including boring logs and any well construction logs. The lithology and presence or absence of groundwater, including seasonal groundwater or estimations in the expected variation, should be evaluated to at least five (5) feet below the lowest part of the structure unless the variation of the site groundwater is greater than 5 feet or would come into contact with it.

A cross section of the structure that is being mitigated should be provided with the site geology and hydrogeology information self-imposed on it. Structures that have various footing depths, such as basements, elevators, tunnels, or pits, and those that are larger than >5,000 square feet should include more than one cross-section to aid in the review and these types of features should be clearly depicted on them. The lowest part of the structure should include the depth of any footings, sumps, drains, pits, and elevator shafts.

If soils such as fill were brought on-site that is intended to either aid or be a component of the mitigation system, information on the type and characteristics, as well as the were it was placed should be identified, described, and supported through records.

Note: Soils within 12" of the structure should be described based on the available information and should include the subgrade, sub-base, and base-course soils with deeper or other information that is available that may be either influenced by the AVMS or connects the vapor source to the structure.

Key supporting information

• Figure identifying borings used to classify the geological and hydrogeological information.
• Cross-section(s) depicting the current or proposed structure, with the geological and hydrogeological information identified.
• Table(s) identifying any wells installed, depth of the wells, and the elevation of the groundwater.

1.5 Detailed building information

1.5.1 Below grade building features and foundations

This section should describe the foundation (i.e., footings, pier, columns, cast pilings, etc.) and building portions that are below grade (basements, crawlspaces, elevator sumps, pits, utility tunnels, storage areas, etc.) that are in contact with the soil. If there are any foundations that are unknown that may impact the function of the AVMS, they should be identified here and addressed in Section 2.2.

• All features above should be identified in plan view.
• Subsurface features identified should also be depicted on cross sections developed in Section 1.3.

1.5.2 Heating, ventilation, and air conditioning (HVAC)

This section describes the HVAC system including any air intakes or exhaust that are required for its operation.

• Figure(s) that depicts the location of any air intakes, vents, and includes components that are on the roof or side of the structure including air intakes or discharge points based on the information collected during the building survey.

1.5.3 Slab condition

This section describes and documents the current condition of the floor for an existing building.

• Figure(s) document the current location of any cracks with a width greater than 1/2-inch, areas that concrete is not present, or areas that the concrete is likely to need repair or replacement based on the information collected during the building survey.
• Appendix with building survey should contain a photolog with representative photos, especially in those areas that repairs to the slab or building may be warranted.

1.5.4 Utility penetrations and man-made preferential pathways

This section should describe and document the location of penetrations and sumps in which vapor may enter and directly vent into the structure. This is limited to those utilities that are in direct contact with a vapor source, or which vapor may directly enter into a structure.

• Figure(s) should identify the location of penetrations and sumps.

1.5.5 Historic buildings (if necessary)

This section should describe any design or layout limitations that are imposed on the structure due to a historic designation or other limitation.

1.6 Source areas and contaminant conditions

1.6.1 Applicable criterion

This section should identify the VIAP screening levels and/or criteria used to demonstrate that the response activity is necessary to protect the public health, safety, or welfare, or the environment, and identify the assumptions used in their development, the supporting information that the criteria are appropriate for use, and the values.

• Generic Criteria or RBSLs: If the use of the generic criteria is appropriate, supporting information should be described and presented.
• SSVIAC or SSTLs: When developed for the site or if VIAP Screening Levels (SLs) are used, the supporting documentation should be provided demonstrating the criteria are appropriate for use.

• Table that identifies the criteria used.
• Separate appendix that provides supporting documentation for use of the VIAP Criteria, including any previous approvals for the use of the site-specific criteria.

1.6.2 Vapor source

This section should identify the concentrations of the volatile substances that pose an unacceptable risk to the subject structures(s). The sampled media and location of samples collected should be depicted on a scaled figure relative to the structure to assist interpretation of the relationship between the vapor source and the structure for which the AVMS is installed. It also includes a discussion on how that data aligns with the volatile substance associated with the site history (Section 1.2), Representative soil, groundwater, and soil gas concentrations that include any samples collected above and near the vapor source, and where vapors may enter within the mitigation footprint of the structure should be identified and provided. Data that supports the design of the AVMS must be provided, especially in areas where an AVMS is not installed. Cross-sections should include below grade features but with the chemical data depicted.

• Tables that provide representative soil, groundwater, and vapor data that allows an evaluation of the maximum concentration that the AVMS will need to account for.
• Figure(s) that identify where the data was collected.

1.7 Man-made preferential pathways (if necessary)

This section should include an analysis of Section 1.5.4 above and if there is a need to design or propose a mitigation strategy for man-made preferential pathways or other preferential pathways that may not be addressed through the operation of the AVMS.

• Tables that provided the collected soil, groundwater, and vapor data.
• Figure that identifies where the data was collected.
• Appendix should include all analytical data sheets.