St. Mary Parish, LA: the USDA septic-suitability picture
Very limited
USDA puts St. Mary Parish, LA at "Very limited" for septic absorption fields, the dominant reading across 82.8% of the county's soil. This is the reading that pushes a project toward an engineered system rather than a conventional one, and it covers 82.8% of St. Mary Parish's land. The county spans 558.528 square miles, home to 46,799 people.
What this number does and doesn't tell you
Two lots a mile apart in St. Mary Parish can carry different soil, even though they share one countywide rating. Budget $750-$1,850 for a real perc test on any specific parcel - that's the only figure a permit application accepts.
St. Mary Parish: conventional vs. engineered cost
82.8% of St. Mary 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.
St. Mary Parish's rank among Louisiana counties
Among the 3,058 rated counties on this site, St. Mary Parish, LA ranks 1,876 most-constrained nationally, and 42 of 61 within Louisiana.
Related guides
How this rating is built, and its real limits · Conventional vs. engineered septic systems