Environmental Testing Confirms Soil Vapor Levels Acceptable for Residential Construction
The City of Flagstaff has had a fourth and final independent set of tests, completed by the environmental consulting firm of Ninyo & Moore, which confirms soil vapor levels at the Sawmill property are acceptable for residential construction without mitigating measures and do not pose a significant threat to human health. The three sets of test results performed after initial testing do not support initial results which indicated elevated soil vapor levels.
The Ninyo & Moore report is available to the public on the City website in summary form and will also be made available in its entirety to prospective bidders on the Sawmill property.
The report does not recommend additional remediation of the site nor property use restrictions. Ninyo & Moore offer some foundation design mitigation recommendations for developers to consider in order to minimize the potential for any soil vapor intrusion into buildings. These recommendations are not requirements to obtain a building permit, nor would they generally add significantly to the cost of construction.
The City is currently preparing to re-open bidding on the southern portion of the Sawmill property.
CONCLUSIONS AND RECOMMENDATIONS
The analysis of the data from a number of different perspectives has led Ninyo & Moore to submit a number of conclusions and recommendations. Based on the soil gas data, there does not appear to be a significant threat to human health when compared to the various soil gas screening standards cited above and the EPA Johnson-Ettinger model estimates for subsurface soil vapor intrusion into future buildings. There are remnant soil gas concentrations in site soils.
As development of the various lots proceed, Ninyo & Moore would recommend that future structures be designed in a manner that would minimize the potential for vapor intrusion into the buildings. Design considerations might include: • Slab on grade construction rather than basement or crawl space type foundations
• Post tensioning of slabs to reduce later cracking of the floors, preventing intrusion pathways from developing • Using vapor barriers beneath all site foundations and slabs • Seal all utility penetrations through slabs/floors to ensure vapor tightness against possible soil gas migration into the structure.
Future building design may also include the installation of a vapor removal system, below the building slab’s vapor barrier. This precaution, when included in the initial design is a cost effective method for controlling soil gases that would tend to build up under the building’s slab, and eventually migrate into the building’s air space. Once installed this piping could be available for connection to either passive or mechanical ventilation systems to draw soil gases from beneath the building as needed based on indoor air quality testing results. Including some form of sub-slab ventilation during the initial design and build phases of construction can reduce the cost of post construction sub-slab ventilation installation by an order of three to ten times, should indoor air remediation be required at a later date.
The recommendations stated in this report are specific for soil gas considerations, and Ninyo & Moore does not believe that any design considerations for these lots should trigger use restrictions for the property.
Seasonal soil temperatures are relatively stable at depths of five feet or greater, where soil gas samples were collected. The design frost line in Flagstaff is 32 inches, below which, soil temperatures do not vary enough to affect water lines and other structures. Taking these factors into account, Ninyo & Moore does not anticipate a significant change in soil gas concentrations during warmer weather when the source of the contaminants would be the most volatile and in turn produce the highest possible movement of gases through the site soils.
Ninyo & Moore has conducted this soil gas monitoring event and subsequent data analysis in a manner consistent with good engineering practice. The report is intended to be used in its entirety and unauthorized use of component statements will invalidate the resultant conclusions described herein.