Seasonal high water table is the highest level groundwater is expected to reach at a site during a typical year, not the level it happens to be at on the day of testing. It is one of the main limits on conventional septic design, because a drainfield needs unsaturated soil beneath it for effluent to be treated properly, and a shallow water table can force a raised or alternative system instead.
A soil evaluator identifies the seasonal high water mark by reading soil color patterns (mottling caused by cycles of saturation) rather than by digging on a day when the table happens to be visibly high or low. This is why the evaluation looks for evidence in the soil itself instead of just checking current groundwater depth, which can be misleadingly low outside the wet season.
Where the water table sits too close to the surface for a conventional trench system to maintain the required separation, the usual solutions are raising the drainfield above natural grade (a mound system), reducing loading rate with a more efficient dispersal method, or in some cases pre-treating effluent to a higher standard before it reaches the soil (as an aerobic treatment unit does), which can allow a shallower or smaller field than a conventional system would need.
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The highest level groundwater is expected to reach at a site during a typical year, identified from soil evidence rather than the water level on the day of testing.
The drainfield needs unsaturated soil beneath it to properly treat effluent, and a shallow water table reduces or eliminates that separation for a conventional system, often requiring a raised or alternative design instead.
Soil evaluators look for mottling, color patterns in the soil caused by repeated cycles of saturation, which shows where the water table reaches even outside the wet season.
Guide last reviewed 2026-08-31.