Evaluating Large Buildings and Assessing the Feasibility of Applying Active Soil Depressurization as a Remedial Solution for Vapor Intrusion

Transcription

1 Evaluating Large Buildings and Assessing the Feasibility of Applying Active Soil Depressurization as a Remedial Solution for Vapor Intrusion ABSTRACT Thomas E. Hatton Clean Vapor, LLC 3 Maple Street Andover, NJ Tel: (973) Fax: (973) thatton@cleanvapor.com Strip malls, department stores, industrial manufacturing sites and schools that are impacted by chlorinated compounds present a unique set of challenges when designing vapor intrusion mitigation systems that rely on Active Soil Depressurization as a remedial solution. Achieving sufficient under-slab vacuum in residential homes or buildings can often be achieved just by applying robust suction to the soil beneath the slab. Large buildings require an understanding of the source, entrainment pathways, evaluation of the HVAC system and precision under-slab permeability mapping. Correct permeability mapping of the under-slab soil yields data that enables the system designer to select the correct suction point locations, pipe diameter, blower specifications and project a radius of influence. This paper shares diagnostic methodology and data collected from a shopping center and demonstrates the techniques of soil permeability mapping, specifying blowers and verifying the performance of the installed system. INTRODUCTION Commercial buildings are very different from residential structures. Designing and installing vapor intrusion mitigation systems presents a unique set of considerations that are not encountered in residential remediation. Commercial buildings that are impacted by vapors from volatile organic compounds (VOC s) are typically older buildings that are built over compacted indigenous soils. They have stem walls that are often made of block that extend several feet below grade. These block walls often have several below grade penetrations and incomplete mortar joints that allow the hollow cores to act as conduits for soil gas delivery. Most buildings have multiple additions with a variety of construction styles. It is not uncommon to discover that these additions have been built over parking lots and sidewalks. There are floor wall joints that may appear inconsequential. However, a 1/8 th inch floor wall joint that is two hundred feet long will have an open surface area of more than two square feet. Segmented concrete slabs that have

2 open expansion joints can serve the function of a bellows as they are driven over by swift moving forklifts. Each section of the building will usually have separate heating and cooling systems that are maintained as needed. Exhaust bowers that are used in manufacturing, dry cleaning operations and restaurants create multiple pressure zones that can manipulate the flow of soil gas and need to be accounted for. The loading doors, roof geometry, ceiling height and condition of the windows all contribute to how a building will respond to wind loading. How a building is heated and the temperatures that are maintained will contribute to stack effect. The slab thickness, types of soil, presence of groundwater, under-slab utilities, the building s use, availability of interior columns to run vapor transfer tubes along side of, roof types and construction warrantees all need to be considered. What is the impact of the HVAC system? Are employees able to adjust the air handling equipment? Will the negative pressure of the HVAC system overpower the mitigation system or can it be used to assist in building pressurization and be a beneficial component of the mitigation system? All these items need to be addressed during the investigation process. One cannot forget the people factor. There is almost always a mix of stakeholders, often with competing interests, intertwined in vapor intrusion projects. There are property owners, tenants, a party responsible for the contamination, a bank and usually a few attorneys. Once the mitigation system designer has figured out how to incorporate all of the parties into a project solution, it is time to start understanding the soil conditions and designing a mitigation system. Although vapor intrusion systems share the same mechanical principles as radon systems they are very different. When radon is the contaminant, in most radon cases the indoor concentration in a commercial building is only marginally elevated and a precisely engineered approach to the solution is not considered to be as critical. For example, if the subject building is in New Jersey where the current nonresidential screening value for indoor trichloroethene is 3. ug/m 3 and the measured indoor air concentration is 370. ug/m 3 then the designers responsible for developing the plan have a far greater challenge. Principles of Vapor Intrusion Pressure differentials between the soil and the building are the driving force that draws soil borne pollutants into buildings. The primary method for reducing soil borne contaminants is Active Soil Depressurization (ASD). An ASD system will reverse or mitigate the entrainment process by creating a negative pressure beneath the slab. An ASD system draws contaminants from beneath the slab, through piping, to the exterior of the building where they are vented above the roof line and quickly diluted with ambient air. The ASD system also removes moisture that can enter the building which improves the overall indoor air quality of the building. Determining the potential for soil depressurization is the most critical component of determining a solution. Soil borne contaminants enter a building as a result of three primary variables:

3 (1) source strength of the contaminant in the soil; (2) entry routes; and (3) pressure differentials that draw contaminants from the soil into the building. Quantifying and understanding the relationship between these three variables and the effect they have on the final contaminant concentration is the key to developing an effective remedial plan. A thorough investigation will provide the necessary information to manipulate the influencing factors and correct the problem. Forces That Contribute to Pressure Differentials Depending on the buildings use there can be exhaust blowers that mechanically induce significant negative pressure loads on the interior of the building. Exhaust blowers often accelerate the rate at which soil gases are drawn into the building. Buildings that were used for manufacturing, laundromats and restaurants typically exhaust large volumes of air creating negative pressures throughout the building envelope. A strip mall with a drycleaner, multiple restaurants and a beauty salon will have multiple negative pressure zones manipulating the under-slab soil gas plume. Temperature differentials will also affect the pressure between inside and outside of the building. When warm air inside the building rises, the building undergoes what is known as stack effect. This will induce a negative load on the building interior that is applied to the surface of the slab. Stack effect will be greater during the heating season. Temperature differences alone have been known to contribute to as much as inches water column (w.c.) of pressure differential. Wind induced negative pressures are transferred into the structure resulting in the uptake of soil gas. Wind creates a complex pressure field around a building. It can create a positive pressure on the windward side and a negative pressure on the leeward side. For this reason exterior doors should remain closed while pressure field measurements are being conducted. Vacuum field testing should not be conducted on gusty days. Establishing Relationships and Planning the Building Investigation Establishing personal relationships and points of contact early on in the project is an essential first step. The free flow of information between all the parties will make everyone s job much easier. It is a good idea to list all the project participants, their function and chain of command before getting started. If the potential for litigation is high try to use a third party Public Relations Consultant to facilitate the flow of information. The designer should request as much information about the building as possible. Understanding the history of the building will provide crucial information about what to expect during the investigation process. For example if the building was used for manufacturing, multiple concrete machine pads which are several feet thick may be located just beneath the slab and present a variety of design challenges. There will probably be a combination of active and abandon utility lines. Ask for an under-slab utility mark out. Later this will help in selecting areas that are preferable for drilling. Try to obtain the entire history of blueprints. Learn as much about additions as possible. Sections of a building that have

4 been constructed over a former parking lot or materials staging yard may have extremely compacted soil beneath the slab. If possible, find a maintenance person to interview, preferably someone who has been working there for their entire career. This person will often have a wealth of valuable information. The more facts that can be incorporated from properly asked questions on the front end of the project the more streamlined the entire project will flow. The walk through phase is an important part of the investigation. Note the physical characteristics and use considerations that will need to be prioritized in the planning of the field investigation. The construction features need to be recorded. Changes in building materials or construction styles often indicate an addition to the building. Sometimes the material changes that indicate additions can be as subtle as a slightly different pattern in the metal roof decking or a small change in a roof truss design. Each addition represents a segmented building foundation. The soil conditions from one foundation area to another can be significantly different. The investigators will need to indicate on the floor plan different building sections and structural items such as stem walls. The next step is to carefully catalog all potential contaminant entry points such as slab penetrations, conduit openings, expansion joint openings, floor wall joint openings, plumbing fixtures, block wall openings, open block tops, or anything that would be considered a soil gas entry point. Using colored pencils to represent different structural areas and soil gas entry points will be useful in recalling information during the report writing phase. Many buildings have a history multiple uses and often have exhaust equipment that is no longer in use. All exhaust blowers should be cataloged and noted whether they are functioning or not. The CFM of operating blowers should be recorded. The supply ventilation should be examined, the operation status noted. This will become important information when selecting contaminant exhaust blower locations. Re-entrainment of soil vapors must be avoided. Developing a Vapor Intrusion Remediation Plan The first step in developing a vapor intrusion remediation plan is to understand as many of the variables as possible. It is important to have the vapor problem completely characterized. Key questions that need to be answered during the initial are the ones that define the contaminant plume. What are the contaminants? What are the concentrations in the groundwater, in the soil gas beneath the slab, in the indoor air and outside of the building? Is methane present? Sufficient sampling needs to occur to define the extent of the under-slab soil gas plume. Often building owners do not want to know that they have a problem and when they do acknowledge a problem exists they do not want to allocate appropriate funds for sampling. It is important that the consultant present a good case for a sampling budget that allows the zone and source of contamination to be understood. The designer needs to have a clear understanding of what portion(s) of the under-slab or if the entire building under-slab is impacted by the subject contaminants.

5 It is recommended that a sample be taken on the roof top, preferably collected in front of the fresh air intake that is thought to be closest to a potential source of contamination. Background air samples should be collected out in the parking lot and behind the building. Careful attention should be given to recording wind speed and direction. Outdoor air sampling is not recommended if the wind speeds are consistently greater than 12 mph. Going back to our example building in New Jersey, achieving 3. ug/m 3 TCE in the indoor space will not occur if there is 50.0 ug/m 3 on the roof top and 8.5 ug/m 3 in the parking lot as a result of the neighboring dry cleaning operation. This data will potentially tell the designer what portion of the indoor air problem can be attributed to vapor intrusion. Before any drilling occurs it is important to have an access agreement in place that defines the intrusive components of the investigation. This task should be allocated to the attorney and PR individual. If methane or other potentially explosive compounds are the contaminants, safety is critical. It is recommended that continuous air monitoring and explosion proof equipment be integrated into the safety and work plans. Once the plume of under-slab soil gas that is contributing to the vapor intrusion problem is delineated, the physical features of the building need to be defined. Commercial buildings have complicated HVAC systems that can create different pressure zones in different areas of the building that have contact with the soil. HVAC systems can also have different settings to perform different functions based on the time of day, day of the week or outside temperature. Some buildings will have vestibules that will serve the function of regulating rapid changes in air pressure. The designer will need to understand the pressure differences between the inside and outside of the building, the pressure differences between the inside of the building and the under-slab and the effects of building use on these pressures. The ventilation equipment needs to be functional during the building investigation. It is important that the owner s representative who is responsible for operating the air handling and exhaust equipment be on site during the investigation so pressure measurements can be recorded under normal and severe exhaust load conditions. A vapor intrusion mitigation system should be designed to function under a worst case scenario. Once the structural components of the building are understood and there is a general idea of what variables affect air pressures within a building it is time to start the sub slab soil permeability mapping phase. The purpose of soil permeability testing is to simulate the sphere of influence from a single suction point. Once the data is obtained, understanding the data collected during this phase of the investigation is the most critical component of the design because it determines the remedial design and scope of work. This is the part that eliminates the ad hoc guess work and provides a defined plan. It also eliminates installing something that is not effective in reducing soil borne contaminants. Once a plan is in place, definite costs can be determined.

6 Thousands of mitigation dollars and the success of the vapor mitigation system will hinge on correctly interpreting this data in the thousands of an inch of water column range. The vacuum field extensions are determined by permeability mapping the soil beneath the individual slabs. Locations that are selected should be those which are best for future suction points. In most cases, primary holes will be drilled just off interior column pads. The soil in the interior of the building typically has the lowest permeability because it receives the most equipment traffic which compacts the soil during construction. Conversely, the soil along perimeter walls is usually loose and will produce higher airflows with limited impact on the more compacted soil toward the interior of the building. Also soil permeability testing near the outside of a building can produce skewed results because there are often large floor wall joints that allow building air to enter the sub slab and reduce the pressure field. Pressure Field Testing Procedures The test suction holes are made by core cutting a hole through the floor slab and then auguring out soil to a depth of approximately 18 to 20 inches. Ground fault shut off equipment should be used while drilling to minimize potential damage to sub-slab utilities. The physical characteristics of the sub-slab material are recorded on a data sheet. Permeability tests are conducted to determine vacuum extensions for the purpose of soil depressurization. This is done by applying a known quantity of vacuum ranging from approximately 2.7 to 50 water column (w.c.) to the primary suction hole. Where there is compacted indigenous soils, a shop vacuum that creates 50 w.c. is selected because it simulates the vacuum characteristics of a high vacuum low flow blower. These blowers are frequently specified in a design when there is compacted indigenous soil. In situations where there is course, washed crushed stone a low vacuum high flow blower such as the Fantech HP-220 blower can be used to simulate the vacuum field. Test holes are drilled through the slab in the theorized radius of influence from the suction hole to test the pressure field extension. The recommended plan should be to drill 3/8 test holes on an x and y axis at metered intervals from the suction hole. The location of all holes should be accurately recorded on a scaled floor plan. A micromanometer capable of reading down to w.c. should be used to make pressure differential measurements. Prior to applying test vacuum to the under-slab, baseline pressure differential measurements should be made to record the pressure difference between the interior space and the soil beneath the slab. Baseline measurements that fluctuate at a test hole can indicate highly permeable soil or a reservoir of soil gas beneath the slab. A pitot tube or vain anemometer is inserted in the air stream of the vacuum exhaust and air flow measurements made. These measurements are then interpolated or extrapolated to project an expected radius of influence. If the building is occupied the exhaust air should be ducted to the outside of the building. The CFM reduction as a result of the applied drag from the exhaust hose needs to be included as a correction to your exhaust air flow data. Air flow data is critical to selecting the proper blower. The specific objective of this mapping is to specify a blower and suction point

7 configuration that will provide vacuum coverage from.016 w.c. to.008 w.c. with a minimum coverage of w.c. pressure differential between the indoor space and sub-slab material with the slab itself being the defining barrier. This simulated vacuum grid procedure should be repeated as many times as necessary in a building and at least once for each segmented foundation that exists as a result of an addition. Once the data has been evaluated, the types of suction blowers and location of suction points can be determined. It is important that areas such as floor joints that open to the sub-slab are not left open for building air to be drawn in during the vacuum test. This will basically short circuit the pressure field test. For this reason a building investigator should be equipped with duct tape, plumbers putty, and expandable foam. Case Study, Shopping Center Site Description The building selected for the case study from which the data has been collected is a big box electronics store down gradient of a dry cleaning operation that spilled trichloroethylene (TCE) through sloppy recapture procedures over a twenty year period. Soil gas, groundwater and background air sampling have been completed. The utility mark out was done and an access agreement is in place. The store has been vacated and is in fit-out transition in preparation for a new tenant. The total space is 29,000 square feet. The original building is about 21,000 square feet with an 8,000 square foot addition. The sub-slab material beneath the original building is compacted silted clay and the addition has crushed stone. The perimeter foundation walls are block. The interior supports are steel vertical columns. The ceiling is a twenty foot high with steel deck and truss system. The roof is constructed of built-up asphalt material. Suction hole and vacuum test hole transects were set up as shown on the Suction Point and Test Hole diagram. The data collected is graphed on the Pre Remediation Depressurization verses Distance Data Table. Vacuum and Magnehelic Micromanometer

8 RESULTS OF UNDER-SLAB PRESSURE FIELD TESTS

9 SYSTEM DESIGN The system designed for this building has fifteen suction points and seven blowers. The blowers selected were a combination of RadonAway HS 2000 Blowers and AMG Force blowers. Tighter soils will require high vacuum low flow blowers and more permeable material such as crushed stone will require high flow low vacuum blowers. The design was later amended to include some trenching and lateral drilling to create a negative pressure field beneath the adjacent dollar store. See pictures and technical drawing of trenching and lateral drilling. Blower and Suction Point Design

10 POST REMEDIATION UNDER-SLAB PRESSURE FIELD TESTS RESULTS

11 Measuring Air Flow Measuring Exhaust Air Flow from a HS2000 Blower AMG Force Blower RECOMMENDATIONS It is recommended that a shop vacuum with a maximum static vacuum of 50 inches water column be selected to measure vacuum field extensions where there are low permeable soils beneath the slab because performance range of the shop vacuum is similar to a RadonAway HS 5000 blower. The performance range of the remediation blower selected can be matched to the static vacuum and airflow ranges of blowers rated for less static vacuum and greater air flow. Recommendations for selecting pipe diameter once airflow in cubic feet per minute (cfm) has been projected: 5 to 20 cfm 2 Pipe 20 to 60 cfm 3 pipe 60 to 240 cfm 4 Pipe 240 to 400 cfm 6 pipe

12 CONCLUSIONS Designing vapor intrusion systems for large commercial buildings is far more complicated than designing systems for small buildings. Integrating skills and responsibilities with a team approach will add to the success of the project. Measuring sub-slab vacuum field extension is essential for designing any commercial soil depressurization system. Interpolating diagnostic under-slab pressure field measurements can produce sufficient data to design a soil depressurization system that will be effective in mitigating the entrainment of soil gas into a building. It is difficult to precisely project the extension of a negative pressure field when extending vacuum through low permeability fill unless the material is sand with a similar grain size. Under-slab vacuum fields can be projected with greater accuracy when the under-slab material is crushed stone. Sealing slab openings such as floor wall joints is a critical component of extending a negative pressure field. REFERENCES 1. Brodhead, Bill. 2002, 12 th Annual International Radon Symposium, Reno, Nevada. Designing Commercial Sub-Slab Depressurizations Systems. WPB Enterprises, Inc Slifer Valley Rd., Riegelsville, PA. 2. New Jersey Department of Environmental Protection. 2005, Vapor Intrusion Guidance, October United States Environmental Protection Agency. 1993, Radon Reduction Techniques for Existing Detached Houses: Technical Guidance (Third Edition) for Active Soil Depressurization Systems, EPA 625/R , October New York State Department of Heath, Bureau of Environmental Exposure Investigation. 2006a, Guidance for Evaluating Soil Vapor Intrusion in the State of New York, October, 2006.

13 KEY WORDS Vapor Intrusion Vapor Intrusion Remediation System Design Building Investigation Soil Depressurization Low Permeable Soils Pressure Field Extension ACKNOWLEGEMENTS Bill Brodhead, WPB Enterprises, Inc. Flint and Mewin Kinkade, Viridian Environmental, Inc. David Folkes, EnviroGroup, Ltd. Kristin Hatton, Clean Vapor, LLC Michael Labate