Drainfield/leach field failure vs. just needing a pump-out — how do I tell the difference?

A tank that simply needs pumping is diagnosed by measuring the sludge and scum layers: pump when "the bottom of the scum layer is within six inches of the bottom of the outlet," "the top of the sludge layer is within 12 inches of the outlet," or "more than 25 percent of the liquid depth is sludge and scum" (EPA, How to Care for Your Septic System). Drainfield failure is indicated instead by conditions in the soil: "soft, wet, or spongy soil (especially when there have been no significant rainfall events) around your drainfield is a good indication of a system failure," along with standing water, surfacing, unequal drainage between lines, and bright green spongy grass over the field (EPA, Resolving Septic System Malfunctions). A proper diagnosis opens all tanks, checks the distribution box for equal flow, may camera the lines "for blockages or collapsed piping," and may excavate part of the field to look for ponding (EPA); TCEQ adds that you should first rule out hydraulic overloading by comparing your metered daily water use against the design flow (TCEQ, Advice for an Owner of an OSSF).

Reviewed August 2026. Links go to the sources this answer was checked against.