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SOLID WASTE & HAZARDOUS MATERIALS

FAQGas Odor FAQs

What's that gas odor smell that I can occasionally detect?

Please don't be alarmed. The gas odor is the result of a natural process which occurs as a result of our local geology and biology. The gas has been identified by the US EPA as methyl mercaptan. This is the same chemical that is used to odorize the normally odorless propane.

Methl Mercaptan

Methyl mercaptan is a powerful odorant as is its cousin, ethyl mercaptan, which is found in the chemical substances found in a skunk's repellent. Humans also generate methyl mercaptan when they ingest chicken eggs. Methyl mercaptan is also generated in the decaying roots of Raphanus sativus.

So how do you know that the smell isn't from the propane tanks in the area?

The propane tanks in this area have been checked for leaks on a number occasions. No leaks have been found, nor has propane been detected in the storm drains.

Why are you concerned about storm drains in relation to the mercaptans?

Mercaptans generated in the environment can be carried by rain or irrigation water through the soil. Large amounts of water associated with storm events "wash" the water soluble mercaptans out of the soil. Once the water enters larger portions of the drainage system, however, the volatility of the mercaptans overtakes water solubility, and they escape into the gaseous phase; often leaking out of open storm drains. The condition is frequently observed in El Dorado County in areas such as the Fair Grounds on Placerville Drive, the Gold Country Retirement Center on Missouri Flat Road, near 550 Main Street in Placerville, and near the east end of Green Valley Road.

Is there a problem?

The EMD has periodically measured the concentrations of methyl mercaptan at storm drains for over seven years. The Immediately Dangerous to Life or Health (IDLH) level set by OSHA for this compound is 400 parts per million (ppm). The allowable worker exposure level, normalized for a lifetime of eight-hour workdays, is 0.5 ppm. Results of the storm drain monitoring have all been below these levels. The problem is that the human nose can detect methyl mercaptan at concentrations as low as 0.002 ppm.