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. 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. Mercaptans generated in the environment can be carried by rain or irrigation water through the soil. 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.