Septic field sketch (C) Daniel FriedmanDiagnosing and Curing Sewer Gas Smells and Septic Tank Odors Caused by Site Problems

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This article describes how to diagnose, find, and cure odors in buildings including septic or sewage or sewer gas smells or "gas odors" in buildings with a focus on homes with a private onsite septic tank but including tips for owners whose home is connected to a sewer system as well. What makes the smell in sewer gas? Sewer gases are more than an obnoxious odor.

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Site Conditions, Weather Conditions, or Failing Neighboring Septic Systems or Plumbing Can Produce Local Sewer Gas Odors

Flood waters rising at Wappingers Creek (C) Daniel Friedman

Watch out: Because sewer gas contains methane gas (CH4) there is a risk of an explosion hazard or even fatal asphyxiation. (METHANE GAS SOURCES)

[Click to enlarge any image]

Sewer gases also will typically contain HYDROGEN SULFIDE GAS (H2S). In addition some writers opine that there are possible health hazards from sewer gas exposure, such as a bacterial infection of the sinuses (which can occur due to any sinus irritation).

Depending on the sewer gas source and other factors such as humidity and building and weather conditions, mold spores may also be present in sewer gases.

Also see Wet Weather or Cold Weather Septic Odors or Sewage Odor Diagnosis & Repair Guide for additional odor tracing and cure advice for odors occurring during wet or cold weather.

  • Check for sources of ground water or surface water that are flooding your septic tank or drainfield. Runoff from roof drainage or surface or subsurface water can easily enter and flood a septic tank and drainfield, including at surprising locations such as an opening at the septic tank top, a crack in the septic tank, or an opening in a drain line entering or leaving the septic tank. Our photo (above left) shows Wappingers Creek in flood conditions (Dutchess County, New York) - nearby septic drainfields are also flooded and saturated.
    • Excavate and the soil around the drainfield: a few test holes, combined with a septic loading and dye test can tell us if the water flooding a septic field is coming from the building's septic tank and wastewater or from invading surface runoff.

      Septic loading and dye test procedures are given in detail at this website. See Dye Tests: how to perform a Septic Loading and Dye Test - the complete procedure for septic loading & dye testing, a septic function test.
    • Inspect the septic tank for evidence of flooding from invading ground water. See TANK INSPECTION PROCEDURE
    • Use an intercept drain: Probably the most common problem, however, is ground water or surface water flooding the leach field, causing it to fail or back up. It may be possible to install intercept drainage around the septic field to salvage the system.
  • Check neighboring properties and drains: before digging up your yard and septic tank, walk around the neighborhood. Is the smell getting stronger as you walk away from your property? Perhaps a neighbor has a failing septic system or is having work performed on their septic system.
  • Check nearby storm drains for odor sources.
  • Check for unusual site and wind conditions: I've encountered buildings where all of the plumbing vent installations appear to be to code and complete, but where unusual terrain shape (house at the bottom of a large hill) and prevailing winds conspired (in some weather conditions) to blow septic gases back down from above the roof to a bedroom window or even to ground level.

    Depending on the building roof shape, orientation, and prevailing or even uncommon wind direction, wind blowing at the building can cause downdrafts around a plumbing vent stack, sending normal sewer gases and odors back closer to the ground or even into the building.

    If your sewer gas odors seem to correlate to windy conditions I'd check this out further. Extending the plumbing vents higher or installing a wind block at the vent top might help.
  • Weather: how rain and flooding can cause sewage odors at a building or site:
    • Rain or stormwater flooding a local sewer/storm drain system: can cause sewer gases to back up through a building's drain/waste/vent piping and into the building.
    • Flooding the private septic system drainfield causes sewage effluent to reach the surface
    • Failing septic drainfields: Even a septic drainfield that is not flooded may be smelly if it is improperly constructed or has lost its ability to treat septic effluent, but these conditions may be expected to be worse in wet weather
    • Flooding the septic tank, by invasive ground water, surface runoff, or from actual flood conditions is likely to also leave the system not functional and possibly releasing septic gases See SEPTIC SYSTEM FLOOD DAMAGE REPAIR
  • Other Sources of Smells that are less like "sewer gas" odors (in my opinion) include the odor of burning electrical components. If you trace odors to an appliance or fixture or switch, shut off electricity to that device (or un-plug it if it's an appliance) and have the system or appliance checked by a licensed electrician. Burning electrical components and insulation, and overheating florescent light ballasts can make quite an odor but that's the just a warning sign of an unsafe condition that needs prompt attention.
  • Submissions are invited: Contact Us if you have other examples of tracking down septic or sewer gas smells to their source. Credit and link-exchanges given.

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