St. Tammany Parish's SSURGO septic rating
Very limited
USDA puts St. Tammany Parish, LA at "Very limited" for septic absorption fields, the dominant reading across 71.6% of the county's soil. Expect the ground itself to work against a plain gravity system; USDA's strictest bucket alone covers 71.6% of the county. The county spans 845.308 square miles, home to 277,615 people.
What this number does and doesn't tell you
USDA's survey covers the whole of St. Tammany Parish, LA at once, not any single address. Budget $750-$1,850 for a real perc test on any specific parcel - that's the only figure a permit application accepts.
St. Tammany Parish: conventional vs. engineered cost
71.6% of St. Tammany Parish carrying a "Very limited" reading changes the realistic budget here. Conventional systems run $3,000-$8,000 where they pass; where they don't, engineered alternatives (mound or aerobic treatment, whichever the health department requires) run $10,000-$40,000 - budget toward the higher figure until a perc test says otherwise.
Where this sits nationally
Among the 3,058 rated counties on this site, St. Tammany Parish, LA ranks 2,415 most-constrained nationally, and 55 of 61 within Louisiana.
Related guides
How this rating is built, and its real limits · Conventional vs. engineered septic systems