South Dakota / Lyman County
Score 27 · moderate · #17 of 60 in SD
Crawl space moisture in Lyman County, SD
92% of Lyman County's soil falls in hydrologic group D or a dual D group: clays and high water tables that shed water slowly. Where a seasonal water table is reported — 9% of acres — it averages 3.1 ft below grade.
- 1Water table 3.1 ft
- 2Wet soils 3%
- 3Moisture score 27
- 4EPA radon zone 1
Reading the Lyman County soil survey
The score of 27 combines four parts, each from the USDA survey: wet drainage classes contribute 4.6 points of 100 on their own scale, group D soils 93.1, a shallow water table 5, and flooding 1.8; the weights are on the methodology page. The dominant hydrologic group is D (92% of acres): very slow infiltration, high runoff (clays, high water table).
At 3.1 ft the seasonal water table is usually below a crawl-space floor, but the 9% of acres that report one are by definition the wettest parts of the county; on those lots, a wet spring can bring water close to the footings.
Lyman County against the rest of South Dakota
Lyman County ranks 17 of 60 South Dakota counties, 4 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 3%, below the state figure — the score here comes from heavy group D soils rather than drainage class alone. After well drained, the next most common class is moderately well drained at 6%.
Flooding. 0.3% of acres with a flooding class are rated frequent; 2.1% of acres with a flooding class are rated occasional; 0.8% 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 5.4% 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. Lyman 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 Lyman County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area SD085; drainage covers 96% of 1,092,365 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.