St. Mary Parish, LA Septic Rating
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, covering that same share of St. Mary Parish's land. The county spans 559 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