Heath Consultants Optical Methane Detector (OM) - Manual (Page 46)

46
A. Moisture Content of the Soil
The higher the moisture content, the more difficult it
becomes for gas to vent and thus to be picked up by the
instrument. During heavy rains, or immediately after, it
may be necessary to stop the survey, or move to an area
where venting conditions are satisfactory due to a change
in soil type or elevated gas pressure. Whenever venting
conditions are questionable, re-survey one or more
known leak locations that were low volume when first
detected to see if venting is adequate.
B. Soil Type
Heavy clay type soils and soils with hardpans tend to
limit and restrict the venting process. Heavy moisture
in this type of soil tends to further restrict venting.
C. Effect of Paving
Continuous paving certainly reduces venting of gas from
the soil. However, most paving includes expansion joints
and most asphalt paving has cracks, depending on its
age, and gas will vent at these locations. Freshly paved
concrete and asphalt areas provide very poor and lim-
ited venting points as opposed to older streets. Should
the main be, for example, ten feet into the street from
the curb, with the street having a new concrete cover,
the curb line would provide the most desirable sampling
point. A very compact soil with a gravel type cover, for
example, or a heavily traveled alley, will also restrict
venting.
A
PPENDIX
A