Union County, South Dakota

A shallow water table limits basements on 42% of Union County's mapped land, a higher share than 60% of US counties. Shrink-swell clay limits houses without basements on 33%.

1.2% of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list 24 flood losses here since 2016, 55% of them in buildings with basements. 41% of homes were built before 1980, against 50% nationally; the median home dates from 1993.

Data as of October 13, 2026 · sources

33%

expansive clay: shrink-swell limits slab homes

42%

high water table: limits basements

in drought, October 13, 2026

41%

of homes built before 1980

Soil under Union County

Expansive clay (limits slab homes)	32.7%
High water table (limits basements)	41.8%
Septic fields very limited	74.4%
Clay swell (LEP, by area)	6.8%
High-swell land (LEP 6%+)	27.2%
Mapped soil	299,111 acres

Soil series	Share here	Series nationally: high water table
Crofton	12%	n/a
Alcester	10%	n/a
Moody	6%	0%
Nora	6%	n/a
Forney	6%	n/a
Wentworth	5%	n/a

Union County (heavy outline) and nearby counties, shaded by high water table share.under 10%10-25%25-40%40-60%60%+

How we count · survey areas SD127 (2025-09-10)

What the numbers mean here

Clay swell averages a linear extensibility of 6.8% by land area here, and 27% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 33% of the mapped land.

Clay shrinks as it dries: 1.2% of the county is in drought on the latest map, and 33% of Union County's mapped land has shrink-swell clay.

On 42% of Union County's mapped land, soil stays saturated within about 6 feet of the surface for part of the year, which is basement depth.

Rain over the 12 months to August 2026: 18.72 in, 9.83 in below the 1991-2020 average year of 28.55 in.

USDA rates 74% of the land very limited for septic absorption fields.

Drought

1.2% of the county is in drought (D1 or worse) on the October 13, 2026 map. Part of the county was in drought in 52 of the last 52 weeks; the most was 100% of its area, in 14 weeks, most recently on the September 1, 2026 map.

October 21, 2025Share of area in drought, weeklyOctober 13, 2026

Category, October 13, 2026	Share of county
D0 abnormally dry or worse	37.1%
D1 moderate drought or worse	1.2%
D2 severe or worse	0%
D3 extreme or worse	0%
D4 exceptional	0%

Flood losses since 2016

24 flood losses since 2016, $2.5 million paid. The worst year was 2024, with 14. Foundations of the buildings, where recorded: 55% basements, 30% slabs, 15% elevated.

Year	Flood losses
2016	0
2017	0
2018	1
2019	8
2020	0
2021	0
2022	0
2023	1
2024	14
2025	0
2026 to date	0

Rain

Average year, 1991-2020	28.55 in
12 months to August 2026	18.72 in (-9.83)
2025	28.04 in (-0.51)
Wettest year since 1895	2018: 39.32 in
Driest year since 1895	1976: 13.55 in
Days with 1 inch or more, normal at VERMILLION 2 SE (14 mi away)	6.5

Homes

Housing units	7,588
Median year built	1993
Built before 1980	41%
Built 2000 or later	37%
Owner-occupied	73%
Median value, owner-occupied	$301,100
Single-family detached	72%

What repairs cost

Published ranges from cost guides, not quotes. More sources, and where they disagree.

Work	Range	Source
Basement waterproofing, US	$3,000 to $10,000	Fixr, February 6, 2026
Sump pump installation, US	$800 to $2,000	Fixr, January 31, 2025
Crawl space encapsulation, US	$2,000 to $8,000	Fixr, August 29, 2025

Nearby counties

Place	Distance	Expansive clay	High water table
Clay County, SD	18 mi	29%	53%
Plymouth County, IA	23 mi	8%	27%
Dixon County, NE	26 mi	2%	9%
Sioux County, IA	30 mi	26%	35%
Dakota County, NE	31 mi	13%	22%
Lincoln County, SD	31 mi	28%	51%
Cedar County, NE	34 mi	2%	7%
Yankton County, SD	39 mi	21%	30%

Sources: USDA SSURGO (September 10, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status