UPDATE, 1:26 p.m.: A City of Eureka press release sent just now has a bit more information:

The City of Eureka will be performing smoke testing of the sanitary sewer system at the locations shown in the included map in Eureka beginning at 8:00am, on Wednesday, September 4th and will continue for through December 4th. This testing involves introducing a non-toxic smoke into sewer manholes where it is forced through the sewer mains and laterals to locate defects in the system. Smoke will be visible in the area, and may travel to nearby sewer lines as well. There is no need to contact the Police or Fire Department, they have already been notified.

Traffic will remain open during testing; however, smoke may be visible exiting manholes. Please use caution while driving in the area. In addition to manholes, smoke will be visible coming from building vents and external cleanouts. This smoke is non-toxic, will not leave a residue, and will dissipate shortly after the test. Smoke should not enter buildings unless there is defective plumbing or dry drain traps. It is recommended that you flush all toilets and run water down all sinks, tubs, laundry drains, and floor drains prior to testing. Door hangers will be distributed before the testing occurs, notifying of the testing. This work will be performed between the hours of 8:00am and 5:00pm, WEATHER PERMITTING.

The City of Eureka’s Contractor, Total Flow, will be on site with information, should there be any questions.

The patience and cooperation of residents and business owners in the vicinity of the testing zone is greatly appreciated. Repairing and maintaining our sewer collection systems is vital to the health and safety of our residents.

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Via the  Humboldt Bay Fire socials:

Smoke testing on sanitary sewer lines in the City of Eureka has started and will continue through October.

Smoke tests are done to find damaged pipes, improper connections, and areas where unwanted water may enter the sanitary sewer collection system.

During testing, the smoke may be seen coming from roof vents, building foundations, catch basins, clean-outs, downspouts, sewer laterals, and manhole covers. The smoke is clean and non-toxic to humans, pets, and plants.