Karst & water quality

In karst country, the surface and the groundwater are the same water.

Across Northwest Arkansas and northeastern Oklahoma, rain that lands on a field can reach a cave spring in a matter of hours, carrying whatever it picked up along the way. Much of the Illinois River watershed sits on limestone karst, so understanding that hidden plumbing is the first step to protecting it.

$Illinois River Watershed Partnership interpretive sign, “Cave Springs and the Karst Topography of our Region,” showing a 3-D cutaway of a karst landscape (stream, sinkhole, fracture, losing stream, infiltration, dissolved joint, cavern, subterranean river, and karst spring in limestone), a “What can you do?” list, and photos of the Ozark cavefish, gray bat, and lowland fragile fern.$

IRWP’s Watershed Sanctuary interpretive sign: the full “Cave Springs and the Karst Topography of our Region” panel.²

The bedrock under our feet

What “karst” actually means

Karst is the landscape that forms when water slowly dissolves soluble bedrock, usually limestone, into a network of cracks, fractures, sinkholes, caves, and underground channels called conduits. Instead of filtering slowly through sand and gravel, water in karst can move fast and far underground, often re-emerging miles away at a spring within hours.

Almost all of the land draining to Cave Springs Cave sits on the Boone Formation, a flat-lying Mississippian-age rock made of limestone and chert.¹ It hosts the Springfield Plateau Aquifer, the shallow part of the broader Ozark Plateaus aquifer system that Cave Springs Cave draws from. Where that limestone has dissolved, it behaves less like a sponge and more like a system of pipes.

Karst features like these (sinkholes, spring-fed streams, and caves) are common across the Illinois River watershed on both sides of the Arkansas and Oklahoma state line. Much of the basin has exposed and subsurface limestone, so its caves, sinkholes, springs, and losing streams act as direct conduits to the groundwater that feeds our drinking water and the species that keep it clean.²

Right here in the watershed

The spring at Cave Springs Cave flows into Partners Lake (formerly Lake Keith), the land IRWP is developing into a watershed sanctuary and education center.¹ It’s the example we know best, but it’s one of many: Logan Cave, a few miles north, shelters additional populations of Ozark cavefish and gray bats, and spring-fed streams thread the Ozark uplands on both sides of the state line, resurfacing in the Oklahoma Ozarks that feed the lower Illinois River and Lake Tenkiller. The karst story isn’t abstract; it surfaces in our own backyard. See IRWP’s karst study story and the NWARPC Cave Springs Karst Study.

Soil & epikarst

How karst works locally

Two very different routes carry water underground here, and the difference is everything for water quality.¹

Diffuse recharge: the slow, clean path

Most rainfall soaks slowly down through soil before reaching the epikarst, the weathered upper few feet of bedrock (commonly 30 feet thick or less). On this path, water spends time in contact with soil particles, which filter sediment and bind up many contaminants. Water held in the epikarst is also what keeps Cave Springs flowing during dry spells.¹

Discrete recharge: the fast, unfiltered path

Some water never gets filtered prior to entering karst systems. It pours straight into sinkholes and losing streams, stream reaches whose flow drains into the ground rather than running downstream. Losing streams are common: many channels stop flowing within days of a rainstorm because the water is sinking underground.¹ Flow at Cave Springs responds within hours of a big rain, proof of how quickly surface water reaches the cave with little filtering.¹ Scientists note a “groundwater trough” running roughly parallel to Highway 264 that channels contaminants straight toward the cave.¹

Soils matter, too. Some local soils filter runoff well; others barely at all, so the same activity can be far riskier on one parcel than the one next door.⁴

Why karst matters for water quality

Because karst moves water quickly and with little natural filtering, what happens on the surface shows up in the groundwater within minutes or hours. In fact, water quality inside Cave Springs Cave is treated as an indicator of regional water quality in the shallow aquifer.⁴ The cave holds the largest known population of the federally threatened Ozark cavefish (Amblyopsis rosae) in the world, which is entirely dependent on clean groundwater, as well as several bat species, one of them federally endangered.¹ The same holds across the region: Logan Cave shelters another population of these species, and everything that moves through these groundwater systems eventually reaches downstream neighbors, including the Osage River, the Illinois River (an Oklahoma Scenic River), and Lake Tenkiller. Other species found in these caves that benefit from clean groundwater include the Grotto Salamander, the Ozark Cave Amphipod, and several species of cave isopods.

80-95%

Share of a year’s pollutant load that can ride in just the first half-inch of stormwater runoff.¹

10×

How much higher urban-runoff oxygen demand (BOD) runs compared with forest and meadow runoff, with about 20× the nitrate.¹

12,515 ac

Acres draining to Cave Springs (19.5 sq mi) across Cave Springs, Rogers, Lowell & Springdale.⁴

The protection goals are specific: limit oxygen-depleting contaminants, turbidity and fine sediment, nutrients, and metals reaching the groundwater.⁴ Standard sediment controls fall short here: most BMPs capture only coarser sediment, and the only practices that reliably remove fine silt and clay are filtering ones like tall vegetated buffers.⁵

The contamination and recharge connection

Every recharge point is also a potential contamination point. A few documented examples from around the region make the link concrete:

Roads & spills

Arkansas Highway 264, which crosses the recharge area near the groundwater trough, was identified as the single-most severe threat to water quality in Cave Springs Cave.¹ And it isn’t only hazardous cargo; even spilled milk creates a high-BOD solution that strips oxygen from water and can suffocate aquatic life.¹ The danger is not hypothetical: in November 1981, a liquid-fertilizer pipeline break contaminated the groundwater feeding Maramec Spring in Missouri.⁶

Map of Benton County, Arkansas showing several karst recharge protection zones, with colored areas marking the recharge areas for Cave Springs, Logan Cave, Hewlitts Springs, Bear Hollow Cave, Old Pendergrass Cave, and Civil War Cave, and a legend. — Multiple karst recharge protection zones were mapped in the process of Benton County developing its drainage criteria manual. It identifies multiple areas that have direct connection to groundwater.

Sewage & septic

When sewer lines leak in karst, the liquid tends to migrate downward into the groundwater instead of surfacing where a leak would be noticed and fixed.¹ In Springfield, Missouri (the same geologic units as here), a single trunk-line break released an estimated 10 million gallons of raw sewage and opened a large sinkhole.¹ In another case, a power failure at a lift station with no backup caused a fish kill at Ozark Spring.¹ Learn more at Septic Tanks & Water Quality.

An excavator digging a trench in a backyard to replace a failing septic system, with a house, pool, and pecan trees in the background. — Replacing failing septic systems, particularly in karst areas where failing systems have a direct connection to groundwater, protects public health and the underground environment.

Stormwater ponds & dams

Even well-meant infrastructure can backfire. Conventional detention ponds in karst can become direct recharge points for the dirtiest first-flush water, sending it underground instead of treating it. Retrofitting detention ponds to infiltrate stormwater at an appropriate rate and bioremediate pollutants can be an opportunity to protect karst systems. Learn more at Detention Pond Retrofits.

Before and after of a stormwater detention pond retrofit: at left, a bare basin with standing water and litter; at right, the same basin filled with native grasses and red cardinal flowers that filter runoff and improve infiltration. — IRWP and community partners are improving ground and surface water quality through adding native vegetation to bioremediate contaminates and improve infiltration.

What this means for you

The science of our karst system is well established; the decisions each of us makes, makes the difference. Choose your view:

The stakes for your jurisdiction

The Northwest Arkansas Karst conservation area covers 550,266 acres and already ranks highest among comparable areas for dam density, pasture/hay cover (46%), and projected urbanization, with about 32% forecast to become urban by 2050.³ Decisions made now about zoning, infrastructure, and development standards set the trajectory for the cavefish, the cave crayfish, and the drinking-water aquifer.³ The same pressures cross the state line into the Oklahoma Ozark counties that drain to the Illinois River and Lake Tenkiller, where the river carries a state Scenic River designation, so what’s decided upstream in Arkansas shapes water quality downstream in Oklahoma.

A model ordinance, ready to be adapted

The City of Rogers’ Cave Springs karst chapter is one of the most detailed recharge-protection ordinances in the region, and its structure is straightforward for other Arkansas and Oklahoma jurisdictions to adapt:

Map the recharge area and manage the risk. The ordinance defines a mapped Direct Recharge Area and four graduated vulnerability zones, so rules apply exactly where recharge happens.⁴ The U.S. Fish and Wildlife Service already has mapped recharge areas for some cities; in other cases recharge area maps may need to be developed or improved.
Scale buffers to vulnerability: inner stream- and feature-buffers of 100 ft, 50 ft, and 25 ft by zone, with an outer buffer up to 300 ft in the most sensitive zone.⁴
Make new development soak in its own runoff: residential projects in the sensitive zones must disconnect at least 50% of on-lot impervious area rather than pipe it to a sinkhole.⁴
Keep the highest-risk uses out of the most vulnerable zones: gas stations, bulk fuel storage, and above- and below-ground fuel tanks are prohibited in the highest-vulnerability zones.⁴
Use soils as a siting tool: steer intensive development toward better-filtering soils and away from poor ones.⁴
Hold stormwater to the cave’s protection goals: limiting oxygen-demanding loads, fine sediment and turbidity, nutrients, and metals reaching the groundwater.⁴
Build karst-resilient sewers: require backup power at lift stations and extra care along losing-stream valleys.¹
Look beyond your own boundary: support uniform stormwater practices region-wide that protect our drinking and groundwater systems.

Local leadership can rely on partners to educate people on the science. Mapped karst recharge zones are included in IRWP's GIS Plaform Also learn more from The Nature Conservancy’s Ozark Karst Program and IRWP’s Watershed Management Plans.

If you own karst features, you own a direct line to the aquifer

A spring, seep, sinkhole, or losing stream on your property is a pathway straight into the groundwater. Whatever enters it skips the natural filtering that soil provides and can reach the cave and your neighbors’ wells within hours.¹

Practical steps that protect water (and your land)

Keep a buffer of undisturbed, deep-rooted vegetation around caves, sinkholes, springs, and losing streams, out to 300 feet where you can. Tall vegetated buffers are the one practice that reliably filters fine sediment.⁵
Don’t apply pesticides, herbicides, or fertilizer inside that buffer, and never dump or store anything in or near a karst feature.⁵
Manage manure, fuel, and chemical storage well away from any feature, and remember that even ordinary substances like milk are toxic to spring life.¹
Maintain your septic system; in karst, a failing system seeps unseen into the groundwater rather than surfacing.¹ More at Septic Tanks & Water Quality.
If you uncover a sinkhole, spring, or open void during digging or construction, stop work nearby and protect it; don’t fill it. Sediment controls need to be redundant around karst features.⁵
Reduce and slow runoff: keep ground covered, revegetate bare soil quickly, and minimize disturbance near drainages.⁵
Avoid or at least minimize fertilizer, herbicide, pesticide, and fungicide use across the whole property, not just near features. What you don’t apply can’t wash underground.²
Look for ways to let stormwater soak in instead of running off; rain gardens, vegetated swales, and planted low spots recharge the aquifer more cleanly.²
Mow only what you need to; let deeper-rooted vegetation stand near drainages, springs, and sinkholes.²

Your land is part of a watershed-scale plan. See how it fits in IRWP’s Watershed Management Plans, and explore the regional picture through TNC’s Ozark Karst Program.

We will continue to add sources....

Sources & citations

Aley, Thomas, and Shiloh L. Beeman. Vulnerability Assessment Summary for the Cave Springs Cave Recharge Area, Cave Springs, Arkansas: Final Report. Ozark Underground Laboratory, July 2015.
Cave Springs and the Karst Topography of Our Region. Illinois River Watershed Partnership, interpretive sign.
Northwest Karst: Arkansas River — Conservation Opportunity Area Report. Arkansas Wildlife Action Plan.
“Sec. 14-3. Cave Springs Area Karst Resource Conservation Regulations.” Code of Ordinances, City of Rogers, Arkansas, ordinance no. 15-98, 28 July 2015, amended by ordinance no. 16-103, 22 Nov. 2016.
Species Protective Measures for Benton County Cave Crayfish, Hell Creek Cave Crayfish, and Ozark Cavefish. U.S. Fish and Wildlife Service, Arkansas Ecological Services Field Office, 2022.
Vandike, James E. The Effects of the November 1981 Liquid-Fertilizer Pipeline Break on Groundwater in Phelps County, Missouri. Water Resources Report no. 75, Missouri Department of Natural Resources, Division of Geology and Land Survey, Mar. 1982.