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Municipal Guide To Clean Water: Conducting Sanitary Surveys to Improve Coastal Water Quality

A dye test of a subsurface wastewater disposal (septic) system may be appropriate when:

  • The discharge of pollutants is observed but its source unknown.

  • High bacteria levels and/or optical brighteners are documented in surface waters.

  • Liquid is seen but it is not obvious whether or not it contains pollutants.

  • The malfunctioning status of the system is being questioned by the property owner.

  • The septic system is located in a very confined area (such as behind a retaining wall located adjacent to surface water).

  • The septic system is not found but there is surface water very close to the house.

Before beginning the dye test, verify the presence of all drains exiting the building. Introduce non-toxic dye to sinks, toilets, shop drains, and other plumbing fixtures and monitor the area near the septic leach field, nearby ditches, manholes, streams, etc. for the presence of dye. Depending on the situation, dye may be visible immedi- ately or may never appear on the surface.


  • Two people are best: one person to introduce the dye while the other watches for it in the field.

  • Use waterproof disposable gloves while conducting the dye test and when handling dye.

  • Green dye is often the easiest to see, depending on site conditions. Green dye is also the best for systems where the wastewater is likely to migrate through soil before discharging, because green dye binds less to soil particles than other colors. Red dye may be more visible especially when the water already has a greenish tint.

  • Use dye tablets for toilets and other household plumbing fixtures. They are easy to handle and quickly dissolve. Liquid concentrate also works well for any volume of flow but care must be taken not to spill it on carpets and furniture.

  • Use enough dye to detect a problem if there is one, and to avoid repeat testing. For the typical septic system, 50- 100 tablets should be adequate. For a straight pipe, 5-10 should be adequate.

  • Test a representative fixture (e.g., kitchen sink, bathroom toilet). Multiple drains may require multiple colors and/or testing several fixtures over a period of days.

  • Run the water for several minutes or flush the toilet several times to ensure that the dye moves through the system. Running too much water might overload or damage the system.

  • Dye may not be visible right away. Check the surrounding area within one to two days and continue to check for up to a week if necessary.

Keri Lindbergr

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