Septic soil in Montgomery County, MD, by the numbers
Very limited
81.7% of Montgomery County, MD's surveyed soil comes back "Very limited" in USDA's own SSURGO data, the county's dominant condition for a septic absorption field. That reading means a conventional system is a poor bet here - 81.7% of Montgomery County lands in the strictest "very limited" bucket USDA uses. Montgomery County, MD is 493.142 square miles with a population of 1,082,273.
What Montgomery County's reading doesn't cover
Two lots a mile apart in Montgomery County can carry different soil, even though they share one countywide rating. A local percolation test, typically $750-$1,850 (around $1,300 typical), is the only way to know what one lot in Montgomery County will actually do.
What a system would cost here
With 81.7% of Montgomery County rating "Very limited", a standard gravity system is not a safe assumption here. The engineered-system range, $10,000-$40,000 (mound or aerobic treatment), is the more realistic number to plan around, against $3,000-$8,000 for a conventional system on the rare lot that passes.
How Montgomery County, MD compares
Nationally, Montgomery County, MD is the 1,948-most-constrained of 3,058 rated counties for septic soil. Within Maryland alone it ranks 10 of 23.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems