Q. What types of soil vapors are of concern?

 

A.  Explosive and toxic soil vapors are of concern. Methane is the soil gas which is most likely to be found in explosive concentrations, and it is often naturally occurring. Toxic vapors fall largely under the category of volatile organic compounds (voc’s).  Many voc’s are manmade chemicals, and some are toxic in minute concentrations.

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Q. Where do soil vapors come from?

 

A.  Methane gas in the soil may result from ancient or modern buried organic material, leaking utility gas pipes, swamps, bogs, glacial till geologic areas, silty soil, and engineered dirt fill. Methane and other hydrocarbon gases may also be present in the soil above and around natural gas and oilfields geologic formations, and sometimes around hydrocarbon well borings. VOC’s in the soil environment may result from leaking storage tanks and various other industrial or commercial chemical spills. Groundwater can spread chemicals away from the original point of contamination, and onto offsite properties.

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Q. How do I know if there are vapors of concern in the soil at my property?

 

A. Sites can be tested for methane or voc’s. Test probes or wells may be constructed by a variety of techniques, from simple bar holes pounded into the shallow soil by hand, to deeper test holes made with push-probe rigs, to more substantial test wells drilled with truck mounted augers. Testing may be done using field equipment, or samples may be taken to a laboratory for more sophisticated analyses. Methane investigations commonly focus on soil vapor, while voc investigations may address vapor, soil and groundwater matrices.

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Q. When are soil vapors dangerous?

 

A. Methane is widespread in the soil, from a variety of sources, and is not hazardous unless it is found in extremely high concentrations and under pressure. These conditions are rare. On the other hand even minor amounts of voc’s seeping into an overlying habitable structure may pose a health risk.

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Q. What must be done to protect from hazardous concentrations of soil vapors?

 

A.  Remediation and mitigation are common options for dealing with environmental pollution.  Remediation  refers to source clean-up or removal. Mitigation refers to protection against soil vapors which will remain.

 

Methane soil gas may be impossible to remediate if the source is offsite or deep. For this reason methane is usually mitigated rather than remediated. VOC onsite sources may be remediated by a variety of techniques. But mitigation of voc’s is becoming a primary option, with remediation done concurrently or later.

 

For new construction, subslab passive barrier and venting systems are perhaps the most common method of mitigating habitable space against soil vapors. Intrinsically safe building design (i.e. ventilated crawl space or podium) is also an option.

 

For existing structures, the use of electronic gas detectors connected to alarms and ventilation fans can mitigate methane intrusion. For methane and voc control, soil gas venting wells and trenches may be installed around existing structures, along with positive pressure interior ventilation, and pathway plugging. Or slabs may be retrofitted with barrier membrane and venting. Indoor air treatment is an option for voc’s.

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Q. What must be done after a building is completed with a gas mitigation system?

 

A.  Monitoring of passive probes and vent systems above and below membranes, and monitoring of building air space, are common after occupancy. Electronic or mechanical systems require operation and maintenance. Future modifications to mitigated buildings must not compromise the mitigation systems. Completed buildings are identified with a warning sign to help prevent accidental future damage to the subslab mitigation systems. Any work which affects these systems must be done with local building or fire authority permits.

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Application for Permit

To obtain a permit, the applicant shall file an application on forms furnished by local agency. Once the agency determines that the information on the application meets its requirements, a permit is issued.

Building Permit

A permit is required to build, remodel, repair, demolish, remove or move any building or structure. A separate permit may be required for each separate building or structure for methane or soil vapor mitigation barrier work.

Plumbing Permit

A plumbing permit is required to add, alter, construct, install, move, relocate, remove, reconstruct, repair, or replace any plumbing piping. Soil vapor mitigation system piping commonly requires a plumbing permit.

 

Electrical Permit

An electrical permit is required for electrical wiring or equipment installed for meeting methane or soil vapor monitoring and mitigation requirements. Systems may include electronic monitoring and alarms; and electrical systems for mitigation blowers and other devices.

Mechanical (HVAC) Permit

An HVAC permit is normally required to install, alter, repair, relocate, replace or add any heating, ventilating, or air-conditioning equipment. A permit shall be obtained for all heating, ventilating, air-conditioning equipment installed for meeting soil vapor control or methane code requirements