South Dakota / Union County
Score 31 · moderate · #9 of 60 in SD
Crawl space moisture in Union County, SD
Union County scores moderate: no single wet-soil signal dominates its mapped acres. Where a seasonal water table is reported — 43% of acres — it averages 1.7 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.7 ft
- 2Wet soils 30%
- 3Moisture score 31
- 4EPA radon zone 1
Reading the Union County soil survey
The score of 31 combines four parts, each from the USDA survey: wet drainage classes contribute 31 points of 100 on their own scale, group D soils 33.8, a shallow water table 32.1, and flooding 18; the weights are on the methodology page. The dominant hydrologic group is B (37% of acres): moderate infiltration.
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 43% of Union County's acres. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Union County against the rest of South Dakota
Union County ranks 9 of 60 South Dakota counties, 8 points above the state median of 23. That puts it in the wettest quarter of the state. Statewide, 9% of mapped soil is in the wet drainage classes; here it is 30%, half again the state figure or more. After well drained, the next most common class is poorly drained at 17%.
Flooding. 2% of acres with a flooding class are rated frequent; 18.4% of acres with a flooding class are rated occasional; 25% of acres with a flooding class are rated rare. That is a large flood-prone share; a crawl space in the floodplain needs flood vents under most local floodplain ordinances, which is a separate rule from R408 and can rule out a fully sealed design. Ponding — water standing at the surface with nowhere to go — affects 2.9% 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. Union 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 Union County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area SD127; drainage covers 99% of 299,111 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.