SGM-SF_2017with 12/20 Revisions

Vapor Intrusion (VI)—Informational Guidance 64 ANSI/AARST SGM-SF 2017 (with 1220 revisions) Soil Gas Mitigation Standards for Existing Homes A-3 Monitoring Events and Stewardship A-3.1 Monitoring events Each post- mitigation monitoring event in Annex B should include: (1) measurement of indoor air; (2) PFE measurements; (3) system pressure measurements; and (4) review of fan and mechanical systems. A-3.2 Initial post-mitigation testing To provide an initial measure of mitigation effectiveness, the first post- mitigation monitoring event should be conducted no sooner than 2 weeks after mitigation unless specified differently by a regulatory authority (e.g., when acute risks are present). The 2-week delay period is warranted for reducing influences on indoor test results due to chemicals that may have adsorbed into building materials and finishings. This also allows chemical vapors and gasses to reach a state of equilibriumwith indoor air that is more representative of future expectations when mitigation systems are active. For acute risk situations, more frequent or multiple monitoring events should be considered prior to or in concurrence with seasonal post- mitigation monitoring. A-3.3 Initial seasonal verification To account for seasonal variations, monitoring events sufficient to evaluate effectiveness under each of the following conditions should, at a minimum, be considered: a) Heating Season Conditions. This is when the highest outdoor air temperatures are at least 10˚ F colder than indoor air—e.g., when outdoor temperatures are less than 65˚ F (18˚ C). If seeking to help characterize temporary worst-case pressure-driven entry of soil gas and stressed mitigation system conditions, at least one monitoring event should be conducted when outdoor temperatures are within 10˚ F (5`˚ C) or colder than the average for coldest local temperatures; b) Cooling Season Conditions. This is when outdoor air temperatures are at least 10˚ F warmer than indoor air for a significant portion of the daytime—e.g., when daytime outdoor temperatures are more than 85˚ F (29˚ C). This monitoring event helps evaluate if consistent activity of HVAC blowers temporarily enhance soil gas entry across this season; and c) Mild Weather Conditions. This is when the outdoor temperatures are similar to indoor temperatures of about 74˚ F (23˚ C). This monitoring event helps evaluate if HVAC blowers temporarily inhibit soil gas entry during other seasons. A-3.3.1 An additional monitoring event should be considered to evaluate effectiveness under other concerning condition(s) that can include: a) During Low Water Table (dry soil conditions) This monitoring event helps characterize if soil gas entry is temporarily enhanced due to a larger than normal volume of soil gas under a building. b) During High Water Table (wet soil conditions) Note that soil gas entry is typically inhibited during high water table conditions. However, other considerations may warrant this testing, such as when hazardous substances in liquid or aqueous form that are highly concentrated and reside in close proximity to the building. A-3.3.2 Seasonal Evaluations Any one of these seasonal conditions can result in enhancement or virtual cessation of soil gas entry for temporary or seasonal durations, depending upon building characteristics and mechanical systems. It is recommended to continue seasonal monitoring events until a baseline is established for yearlong effectiveness. Restarting the regimen of seasonal monitoring events is recommended if system alterations are found to be warranted for improving system effectiveness. A longer duration or more frequent monitoring may be warranted dependent upon: a) the degree to which measured concentrations exceed the action level, or b) the degree of observed variability above the action level, especially post- mitigation .