Grimes County, TX: the USDA septic-suitability picture
Very limited
Grimes County, TX's SSURGO dominant class comes back "Very limited", covering 95.3% of the county. A standard gravity system routinely fails on ground like this; the same share of the county falls in USDA's worst absorption-field bucket outright. For scale: 787.47 square miles and 33,448 people call Grimes County home.
Before you rely on this for one lot
Nothing about Grimes County, TX's SSURGO figure applies to a specific address - it's a county-wide average. The real answer for one lot is still a site-specific perc test, running about $750-$1,850 (typically near $1,300).
Budgeting for a system in Grimes County, TX
Grimes County's soil reading pushes the likely outcome toward an engineered system rather than a conventional one. 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, Grimes County, TX ranks 632 most-constrained nationally, and 40 of 253 within Texas.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems