South Dakota / Bon Homme County
Score 25 · moderate · #23 of 60 in SD
Crawl space moisture in Bon Homme County, SD
Bon Homme County scores moderate: no single wet-soil signal dominates its mapped acres. Where a seasonal water table is reported — 42% of acres — it averages 1.2 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.2 ft
- 2Wet soils 23%
- 3Moisture score 25
- 4EPA radon zone 1
Reading the Bon Homme County soil survey
The score of 25 combines four parts, each from the USDA survey: wet drainage classes contribute 20.1 points of 100 on their own scale, group D soils 28.6, a shallow water table 34.2, and flooding 10.4; the weights are on the methodology page. The dominant hydrologic group is C (45% of acres): slow infiltration (clay layers).
A water table under two feet is the single most important number here. A crawl-space floor is usually 18 to 36 inches below the joists and often below outside grade, so in a wet season groundwater can stand at or above the crawl-space floor on the soils that report a water table — about 42% of Bon Homme County's acres. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Bon Homme County against the rest of South Dakota
Bon Homme County ranks 23 of 60 South Dakota counties, 2 points above the state median of 23. That places it in the middle range, nationally and for the state. Statewide, 9% of mapped soil is in the wet drainage classes; here it is 23%, half again the state figure or more. After well drained, the next most common class is somewhat poorly drained at 16%.
Flooding. 9.7% of acres with a flooding class are rated frequent; 0.4% of acres with a flooding class are rated occasional; 2.3% of acres with a flooding class are rated rare. Flood-prone soils are a minority here, concentrated along streams; the score mostly reflects what happens between floods. Ponding — water standing at the surface with nowhere to go — affects 10% of acres.
What it means for a crawl space here
In a moderate county the soil record is mixed rather than uniformly wet, so conditions vary from lot to lot. Where the lot drains, the model-code default — a vented crawl space with 1 sq ft of net vent per 150 sq ft of floor, or 1 per 1500 with a Class I vapor retarder on the ground — is a reasonable starting point; where the sections above point to a shallow water table or flood-prone soil on your lot, plan as for an elevated county. Either way, check the site first: grading that falls away from the foundation, gutters and downspouts that discharge well clear of it, and a ground cover over bare earth. A wet crawl space in a moderate county points to one of those first.
Radon. Bon Homme County is in EPA radon zone 1, the highest predicted potential. A sealed crawl space and a radon system share most of their parts — the liner, the sealed seams, a fan — so if you are encapsulating, test for radon first and design both together. EPA itself says the zone map is not a reason to skip testing in zones 2 and 3.
Near Bon Homme County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area SD009; drainage covers 97% of 373,734 mapped acres); EPA Map of Radon Zones, county table (June 2024 JSON); IRC R408.3 Unvented crawl space, as adopted in S.C. Code Regs. 8-1221 (LII). Checked 2026-10-23.