Sioux Falls is the biggest city in South Dakota — roughly 200,000 people and climbing — and it sits right along the Big Sioux River, which serves as one of its primary drinking water sources. The city also draws from a network of wells and the Lewis & Clark Regional Water System, a massive pipeline project that brings Missouri River water to communities across the region. Between the agricultural landscape, a military airfield with PFAS history, and rapid population growth, Sioux Falls has a more complex water quality story than most people realize.
The Big Sioux River: Beautiful but Burdened
The Big Sioux River runs about 420 miles from its headwaters near Watertown, South Dakota to its confluence with the Missouri River at Sioux City, Iowa. Along the way, it passes through some of the most heavily farmed land in the Great Plains — and it collects the runoff to prove it.
Nitrates from fertilizer application, bacteria from livestock operations, and sediment from eroding cropland all end up in the Big Sioux. The river has appeared on South Dakota’s 303(d) list of impaired waters for bacteria (E. coli) and total suspended solids in various segments over the years. During spring runoff and heavy rain events, the river’s raw water quality can deteriorate significantly.
Sioux Falls’ water purification plant treats this water through a multi-step process including lime softening, sedimentation, filtration, and chloramine disinfection. The treated water consistently meets EPA drinking water standards, but the plant works harder for it than utilities drawing from cleaner sources.
The city also operates several well fields that tap the Big Sioux Aquifer, providing a supplemental groundwater source. These wells generally produce water with lower turbidity and bacteria than the river, but they’re not immune to contamination from surface activities — especially in a region dominated by agriculture.
PFAS at Joe Foss Field
Joe Foss Field — Sioux Falls’ regional airport and home to the South Dakota Air National Guard — is at the center of the city’s PFAS concerns. Military and civilian firefighting operations at the airport used AFFF (aqueous film-forming foam) for decades, and those foams contained PFAS compounds that have since been detected in groundwater near the facility.
The Department of Defense has been investigating PFAS contamination at National Guard and military facilities nationwide, and Joe Foss Field is among the sites under scrutiny. Groundwater sampling near the airport has shown detectable levels of PFOA, PFOS, and other PFAS compounds [NEEDS VERIFICATION].
The critical question is whether this contamination has migrated to the city’s municipal well fields or the Big Sioux River. Sioux Falls has been testing its finished drinking water for PFAS, and the city’s water utility has been working to assess the scope of the issue as the EPA’s new PFAS maximum contaminant levels (4 ppt for PFOA, 4 ppt for PFOS) take effect.
The Lewis & Clark Regional Water System
One of the most ambitious water infrastructure projects in the Upper Midwest, the Lewis & Clark Regional Water System is a pipeline that delivers treated Missouri River water to communities in South Dakota, Iowa, and Minnesota. Sioux Falls is one of the largest member communities.
The system was authorized by Congress in 2000 and has been built in phases over the past two decades. When fully operational, it provides Sioux Falls with a supplemental water source that’s less vulnerable to the agricultural contamination issues that affect the Big Sioux River. The Missouri River water is treated at a plant near Vermillion, South Dakota before distribution.
This diversification of supply is a significant advantage. Cities that rely on a single water source — especially a surface water source in an agricultural watershed — are more vulnerable to contamination events and drought. The Lewis & Clark system gives Sioux Falls a backup that many similar-sized cities lack.
Agricultural Runoff: The Ongoing Challenge
South Dakota ranks among the top states for corn, soybean, and livestock production, and the Big Sioux watershed sits squarely in that agricultural heartland. The consequences for water quality are predictable: elevated nitrates during the growing season, bacteria spikes from livestock operations, and pesticide detections that track with application schedules.
Sioux Falls’ water treatment plant monitors for over 80 regulated contaminants as required by the Safe Drinking Water Act. Nitrate levels in the treated water have historically stayed below the EPA’s 10 mg/L maximum contaminant level, but raw water levels from the Big Sioux can run higher during peak runoff periods.
Atrazine and other agricultural herbicides are also detected in the Big Sioux River seasonally. While treated water levels typically remain well below EPA limits, the presence of these chemicals in the source water underscores the connection between land use and drinking water quality.
What Sioux Falls Residents Can Do
Read the annual water quality report. The city publishes a Consumer Confidence Report every year. It’s your best source for actual measured contaminant levels in your tap water.
Test if you’re on a private well. If you’re outside the municipal system — or if you’ve got a well on your property — test for nitrates, bacteria, and PFAS. The South Dakota Department of Agriculture and Natural Resources can direct you to certified testing labs.
Know your plumbing. Older homes in Sioux Falls may have lead service lines or lead solder. Run your tap for 30 seconds to two minutes first thing in the morning to flush standing water from pipes.
Consider filtration. Activated carbon filters (including pitcher filters certified to NSF 53) can reduce many contaminants. For PFAS specifically, granular activated carbon or reverse osmosis systems are the most effective home treatment options available.
If you’re concerned about your water quality, a certified water treatment professional can test your water and advise on solutions.