Location and Phone
120 S. Hays Street, Suite 200
Bel Air, MD  21014
410-877-2300

A soil percolation test, often called a “perc test,” is a procedure used to measure how quickly water drains through the soil at a proposed septic system location, essentially determining the soil’s ability to absorb and treat wastewater. Code of Maryland Regulations (COMAR) 26.04.02 specify a minimum of at least 4 feet of satisfactory soil material below the bottom of an on-site sewage disposal system to properly treat wastewater. For this reason, a percolation test is excavated to a depth 4 feet below the depth of the proposed system installation. The physical appearance of the soil is evaluated to determine if there is evidence of excessive rock, water, or characteristics indicative of seasonally high-water table. If the soil profile is acceptable, a percolation test is conducted in the proposed area to determine the absorptive rate of the soil. Based on the site evaluation and percolation test rate, a septic system can be designed.

Key points about a soil percolation test:

• Purpose:
  To assess the soil’s absorptive rate, soil profile, and determine that an adequate treatment zone is provided for onsite wastewater disposal.
• Process:
  Holes are dug at the proposed onsite wastewater disposal location, then filled with water and monitored to ascertain how long it takes for the water level to drop by a specified amount.
• Importance:
  A perc test is crucial before installing a septic system as poor soil drainage can lead to wastewater backup in the house and contamination of groundwater.
• Factors affecting results:
  Soil composition (sandy, loamy, clay), existing water table level, seasonally high water table, rock, slope, and hydrologic features are just a few of the numerous factors.

Wet Season Percolation Testing

A “Wet Season” perc test is a soil test performed during the wettest period of the year to assess the suitability of a property for an on-site sewage disposal system.

Why perform a “Wet Season” perc test?

• Assess groundwater levels
  “Wet Season” perc tests are performed when shallow groundwater levels are highest, usually in February, March, and April. This helps ensure that the onsite sewage disposal system will have the amount of treatment zone required by COMAR and that
  it will work properly in all seasons of the year.
• Identify restrictive soils
  Some soils may need to be tested during the “Wet Season” if they are known to be influences by shallow groundwater levels and flood plains.
• Avoid inaccurate assessments
  “Wet Season” perc tests help ensure that the onsite sewage disposal system will work properly in all seasons especially in time of high groundwater levels.

Feel free to contact the health department if you have additional questions about soil percolation testing at 410-877-2300.

Current Wet Season Memo
Soil Percolation Test Application Procedure
Soil Percolation Test Application
Innovative and Alternative On-Site Sewage Disposal System Consultants
Surveyors List 
Perc Test Update Application 
Soil Chart
Harford County GIS