Haz-mat releases into sewers
Anthony M. Gaglierd, RO
Allegheny County (PA) Hazardous
Materials Unit
Red Team
I commend Gregory Noll for his excellent article “Handling Haz Mats: Subsurface Spills and Releases into a Sewer System” (April 1955). I would like to share some of my experiences in investigating similar incidents.
When discharged into sewers or the ground, liquids normally will follow the same path as the water. Vapors can travel in the opposite direction due to temperature differences, differences in elevation, pressure differentials, and the “chimney effect.” In soils, as Noll points out, vapors will take the path of least resistance, following water, sewer, gas, or utility lines due to the porous nature of the backfill used. This phenomenon causes some homes or buildings to be affected while others are not. It is not uncommon to find that the pathway for vapors into a building is through foundation walls or along water, gas, or utility lines. Sewer drains, if properly trapped and in good repair, normally do not act as pathways.
Noll makes another good point when he says, “Although combustible gas indicators (CGIs) are excellent tools for evaluating and assessing flammable atmospheres, such as those found with subsurface, surface, and sewer spills, often the amounts of vapors are below the LEL at which CGIs can detect them.” Also the spilled material may not be flammable and would not give a response with a CGI.
We use photoionization detectors (PIDs) with our CGIs. PIDs are very sensitive to organic compounds in the ppm (parts per million in air) range. Normally, most fire departments do not carry PIDs, but haz-mat teams, environmental agencies, and cleanup companies do. These units can be helpful in identifying the extent of the vapor spread in both sewer systems and the ground. They also can be used to assess the ambient air levels in homes and businesses, which, depending on the chemical involved, could create a nuisance condition or a health hazard.
As Noll points out, many chemicals can be detected by smell at very low levels. This situation causes concern among members of the public who smell the vapors in their homes and businesses.
Health departments and environmental agencies can assist in assessing the health and environmental effects of a spill and answer questions about health effects.
When using CGIs, it is a good idea to take two readings: one using a charcoal filter and one without. If you get a reading both times, you may have a natural gas leak and not be seeing the spilled product. If you get a reading only when not using the charcoal tube, you probably are seeing the spilled material. During a recent investigation of a leak from a gas station, we found two significant gas-line leaks in the area using this method. There was no mercaptan odor; it probably was filtered out by the soil.
Beware of the small spill, the leaking gas tank, the garage repairing a gas tank, or the homeowner who has old gasoline to get rid of and pours it on the ground. A little gasoline goes a long way. Just this year, we have had three incidents in residential areas traced back to someone who dumped a gallon or so of old gasoline on the ground.
Having been involved in investigating and remediating subsurface spills and releases into sewer systems, I feel the information contained in this article will be a valuable resource to those in the emergency services who find themselves investigating these types of incidents.
