Gateway to Mount Rushmore — and One of South Dakota's Highest Radon Risk Areas
Keystone is a tiny community of roughly 350 year-round residents tucked into a narrow valley in the central Black Hills of Pennington County, South Dakota. Best known as the gateway to Mount Rushmore National Memorial, Keystone draws over three million visitors each year who pass through its historic main street on the way to the iconic presidential carvings. But beneath the souvenir shops and scenic beauty, the same geological forces that created Mount Rushmore are producing dangerously elevated levels of radon gas — a colorless, odorless radioactive hazard that silently accumulates inside homes and buildings throughout the valley.
Keystone sits in a valley carved through Precambrian granite — the exact same Harney Peak granite formation that sculptor Gutzon Borglum selected for Mount Rushmore because of its density and fine grain. This ancient granite, estimated at 1.7 billion years old, is extremely rich in uranium. As uranium decays through the radium chain, radon gas is released from the rock and migrates upward through fractured bedrock, loose fill, and permeable soils into the foundations of residential structures. Pennington County carries an EPA Zone 1 classification, indicating that the average predicted indoor screening level exceeds 4 pCi/L — the federal action threshold. In Keystone, actual readings frequently surpass this figure by several multiples.
Historic Mining Town with Hidden Radon Pathways
Keystone was established in the 1880s as a mining town during the Black Hills Gold Rush. Tin, gold, feldspar, and mica were all extracted from the surrounding hills, and the remnants of that mining era are still present underground. Abandoned mine shafts, tunnels, and prospect holes riddle the terrain around town. These voids and fractured zones act as superhighways for radon gas, channeling it from deep geological sources directly toward the surface. Homes built near or above old mine workings can experience dramatically elevated radon concentrations that fluctuate with barometric pressure and seasonal groundwater changes.
Many residential properties in Keystone are built on steep, rocky terrain with foundations that sit directly on or against exposed bedrock. Unlike homes on flat alluvial plains where a thick soil column can partially buffer radon transport, Keystone homes often have minimal soil between the granite source rock and the foundation slab or crawlspace. Cracks in poured concrete, gaps around utility penetrations, sump pits, and unsealed floor-wall joints all provide direct entry points for radon to move from fractured granite into living spaces.
Vacation Rentals, Seasonal Cabins, and Sealed Buildings
A significant portion of Keystone's housing stock consists of vacation rentals, seasonal cabins, and short-term lodging properties that cater to the tourism economy. Many of these structures sit sealed and unoccupied for weeks or months at a time during the off-season. When a building is closed up with no ventilation or air exchange, radon accumulates to levels far higher than what would be measured in a continuously occupied home. Property owners who reopen cabins each spring may be unknowingly exposing themselves, their families, and their guests to extreme radon concentrations during the first days and weeks of occupancy.
At an elevation of over 4,300 feet, Keystone experiences harsh Black Hills winters with extended periods of sub-zero temperatures. Residents and rental operators seal windows, doors, and any gaps to conserve heat, which eliminates the natural air exchange that would dilute indoor radon. The thermal stack effect — warm air rising inside the home and drawing cold soil gas in through the foundation — is amplified in Keystone's climate, pulling radon-laden air from the uranium-rich granite directly into heated living spaces throughout the heating season.
Radon Services We Provide in Keystone
We deliver complete radon testing and mitigation services to Keystone and the surrounding central Black Hills. Our technicians understand the unique challenges of installing systems on steep terrain, in historic structures, and on properties with exposed bedrock foundations. Every system we design is customized to the specific conditions of your property.
Protect Your Keystone Property from Radon
Whether you own a full-time residence in Keystone, manage vacation rental cabins, or are purchasing property in the Mount Rushmore corridor, radon testing should be your first priority. A professionally installed sub-slab depressurization system is quiet, energy-efficient, and reduces dangerous radon levels by up to 99 percent in most structures. We offer free on-site estimates, same-day appointments when available, and guaranteed results below the EPA action level. Protect your investment and the health of everyone who walks through your door. Call us today at (605) 640-5642 to schedule your radon test in Keystone.