A percolation test measures how fast water soaks into your soil. Required for new septic permits; determines drain field size and design. Cincinnati-area perc rates vary widely by county and lot.
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A perc test (percolation test) excavates 4-6 test holes at the proposed drain-field location, fills them with water, allows them to soak for 4-24 hours to reach soil saturation, then measures the rate of water-level drop in inches per minute. The result is expressed in minutes-per-inch (mpi), with most Ohio jurisdictions accepting drain fields where perc falls between 10 and 60 mpi. Faster soils (under 10 mpi) drain effluent too quickly for adequate treatment and require modifications. Slower soils (over 60 mpi) require larger absorption areas or alternative system types (mound, ATU, or fill systems). Greater Cincinnati perc rates vary by location: Madeira and Mason fall mostly in the favorable 15-30 mpi range; Indian Hill and Anderson Township have spots with heavy clay running 60+ mpi requiring alternative designs; near-river lots (Loveland, Milford) can have water-table issues that affect testing. The county health department or licensed installer conducts the test as part of the permit process. Cincinnati soil-condition tendencies: Hamilton County clay-heavy soils tend toward slower perc rates (30-50 mpi) requiring larger drain fields; Warren County and Butler County silty-loam soils typically perc at the more favorable 15-30 mpi range; Clermont County varies widely depending on lot. The perc test result is the single most important factor in determining what type of drain-field design a lot can support.