Why this region runs high
Radon comes from uranium breaking down in rock and soil, so the map of radon follows the map of geology. Eastern South Dakota drew a strong hand of both. Glaciers spread crushed rock across this side of the state in deep, loose layers, and those deposits carry the trace uranium that keeps a slow, steady supply of radon moving up toward the surface. Loose soil lets the gas travel. Basements give it somewhere to collect.
That’s the regional picture. It is not a prediction for your address. Soil composition shifts lot by lot, and the cracks, sumps, and pressure balance of each house decide how much gas actually gets in and stays. Two neighbors sharing a fence line can test on opposite sides of the action level.
The authority worth knowing here is the South Dakota radon program, run by the state Department of Agriculture and Natural Resources (DANR). The program has tracked radon measurements across the state since the late 1980s, publishes guidance for homeowners, and periodically offers a limited number of free test kits to South Dakota residents. Everything specific on this page traces back to that program or to the EPA.
What the published numbers say
The EPA zone map puts this whole corner of the state in Zone 1, the highest of the three radon-potential categories, meaning the predicted average indoor level exceeds 4 picocuries per liter, the EPA action level. Minnehaha and Lincoln counties, which hold Sioux Falls and its suburbs, are both Zone 1, and so are the neighboring counties: Turner, McCook, Moody, Union. In fact, the entire eastern portion of South Dakota is Zone 1, the west is Zone 2, and no part of the state rates the low-potential Zone 3.
Statewide test results back the map up. Tallies of South Dakota tests routinely find half or more at or above the action level. An American Lung Association report on state test results put the share near six in ten. However you slice it, an elevated result here is unremarkable. It’s roughly a coin flip.
The data gets no finer than county grain. The state program doesn’t publish town-by-town or neighborhood numbers for places like Brandon or Harrisburg, and it says plainly that its maps and tables shouldn’t be used to decide whether a given home needs testing, because elevated homes turn up in every part of the state, so the advice is to test regardless of location.
One more researched fact worth knowing: South Dakota doesn’t license or regulate radon measurement and mitigation professionals. There’s no state list to check a contractor against. What fills that gap is process: written test results, a documented system scope, and a post-installation retest you can hold in your hand.
The practical reading of all of it: the odds say test, and only a test says anything about your house.
Wondering where your home sits? A test answers it
Turning the data into a decision
Living in a Zone 1 county doesn’t mean your basement is high. It means the question is worth answering, and the answer costs little.
If your home has never been tested, that’s the move. The state program’s free kits are a fine starting point when they’re available, a hardware-store kit works when the instructions are followed, and professional testing fits when a sale is involved or you want a monitored number. The testing guide sorts out which test answers which question.
If you’ve tested and the number came back at 4 picocuries per liter or higher, the region’s geology stops being trivia and becomes a to-do item. Radon mitigation is the fix: a system that pulls soil gas from under the home and vents it above the roof, verified afterward by a retest showing the level dropped below the action level.
And if a home purchase or sale put radon on your desk, the data above is context, not the decision. Deals run on deadlines, and the sequencing (who tests, who pays, what gets documented) is its own subject, covered in radon and home sales.
Half the houses in this state have a number worth knowing about. Find out which half yours is in.