Monitoring data have been collected from soil vapor
tubes strategically located on the Fruitland Coal Outcrop for over 4
years. Many locations showed methane saturation
of the soils at the onset of monitoring, while other locations showed no
measurable methane. Initial monitoring
was analyzed with an explosivity meter equipped with catalytic sensor
displaying concentration of methane gas in the soil as a percent of the lower
explosive limit of methane (LEL). A
one-percent LEL reading is equivalent to 500 parts per million (ppm) methane by
volume in air. Likewise a 100% LEL
reading would equate to 5 percent methane by volume
(50,000 ppm).
This is the lowest concentration of methane-in-air which would create an
explosive atmosphere. The upper
explosive limit for methane (UEL) is 15 percent methane by volume, above which
the methane–to-oxygen ratio would be too rich to ignite.
Soil vapor sites generally depicted seasonal patterns
that fluctuated in response to soil surface-sealing events such as
precipitation and frost, in contrast to dry, warm periods. Precipitation and frost tended to alter the
physical structure of the soil porespaces rendering the soil less permeable.
During soil surface-sealing events, the preferential escape route for soil gas
flow was through the unrestricted soil vapor tubes due to their penetration
through the surface seal. (Similar
responses were noted in protected crawl spaces beneath homes in the Pine River
Ranches Subdivision. Methane
concentrations were highest in the crawl spaces following rainstorms.) Annual cycles depict highest methane
concentrations around March with secondarily high concentrations around August,
with the rest of the year showing considerably lower concentrations (Appendix C: Chart 13). From an environmental perspective, the most
disconcerting changes were those noted at soil vapor tube locations which initially
harbored low-to-insignificant combustible gas concentrations, but later
exhibited escalating LEL values. These
identified sites were actively undergoing soil gas composition changes
reflective of environmental change, possibly induced by coalbed water
extraction. The following Chart 14a
shows three soil vapor tube responses to changes in soil
gas composition.
Also see Appendix C: Chart 14b
and 14c for historic trends of hydrogen sulfide in the Cinder Buttes area
and annual trends of methane concentration of soil gas at site #230, a mile
south of Valencia Canyon Gap.
CHART 14a