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Ask the Expert / Darrell Hay

Q: I have a newer house in an older area on a fairly large lot. The house that was here before burned down long ago. My house was rebuilt on a different foundation. The older houses in the neighborhood originally had septic systems and were connected to the sewer system about 30 years ago.

My neighbor and I wonder if the septic tanks in our yards were not removed when the houses were converted to sewer, or that somehow mine slipped through the cracks because of the fire. The reason we started thinking about this is because we live in an area with a fairly high seasonal water table. What looks like an eight-foot-wide blob has been rising up out of the ground in the back yard. Could this be an old septic tank floating up? Are septic tanks supposed to be removed when connected to sewer?

A: While our collective attention and energies have been focused on decommissioning buried heating-oil tanks, a little-known septic-tank requirement has been in place. Washington Administrative Code (WAC) 246-272-18501 reads: “A person permanently removing a septic tank, seepage pit, cesspool, or other sewage container from service shall:

1) Have the septage removed by an approved pumper.

2) Remove or destroy the (tank) lid.

3) Fill the void with soil.

In my experience this filling does not always happen, particularly in older conversions, despite the long-standing requirement. A collapsible void in the yard, potentially with methane gas inside, is not a desirable thing. Even worse is the situation in which an addition or other structure is built on top of the tank unbeknownst (yes, this does happen).

Your tank may indeed be floating upward if it is partially empty, the water table in the soil is high and the bottom of each tank chamber was not punctured when abandoned. If the weight of the tank is less than water it displaces, then you’ve got yourself a boat. A pooper-tanker, if you will.
This “floating” phenomenon is a fairly common occurrence with empty swimming pools, but less common with septic tanks, as they usually contain several hundred gallons of, uh, material, when abandoned. In your case I would dig around the edges to see if indeed this is a septic tank. If so, lose the lid, pump and fill.

Q: Please help me sort fantasy from reality with regard to oil-eating bacteria I have heard about. Can they clean up leaky oil-storage-tank messes in the soil effectively?

A: LUST (leaking underground storage tank) problems can indeed be eliminated naturally through bioremediation.

Naturally occurring microbes are released onto a contaminated area and consume the hydrocarbons, converting them to carbon dioxide, water, trace carbon, bacterial cells and fatty acids. Microbes are found everywhere, and are the basic recyclers of organic materials. The average human carries 3 pounds of microbes in the body. The trick is choosing the correct microbes in the correct concentration to get a cleanup job done. You can’t just spit on the mess and expect clean dirt in a year.

Microbes are typically kept in an inert carrier until ready for usage. In order to work they need what all organisms need: oxygen, water, food (hydrocarbons), an acceptable temperature (32 F to 140 F) and correct pH (5.50 to 10.00). Once activated with water, the microbes get to work, the time necessary being dependent upon the concentration and viscosity of the mess. After chowing down, the microbes will die, return to natural concentration levels, or be eaten by other organisms.
Bioremediation may be acceptable in limited conditions, but physical removal of the soil may be preferable in most cases.

Phil Suetens, with Seattle Tank Service, tells me that bioremediation works only in those cases where the soil will readily accept liquid.
Soils with glacial till and clay won’t allow the microbes to effectively permeate. Bioremediation won’t work with extremely high levels of contamination, or with standing oils.

Drilling and ongoing testing increases the costs greatly. The bioremediation calendar can be very long, from 120 days to a year. Where bioremediation is appropriate and economically viable is in those areas machinery cannot access, on steep embankments, on very large jobs and under buildings.

In almost all cases it is cheaper to remove the contaminated soil.

Contaminated soil that is removed is thermally remediated (baked) and recycled for use as clean fill material.

Darrell Hay answers readers’ questions. Call 206-464-8514 to record your question, or e-mail Sorry, no personal replies.

Copyright © 2002 The Seattle Times Company
Northwest Life : Sunday, June 02, 2002

The Seattle Times