On a snowy November day, Martin Larsen parks in a hayfield, walks down a slope, unlocks a door to a small stone building set in a hillside, and begins an inspection of southeastern Minnesota's subterranean plumbing. That door is an entry point to Spring Valley Caverns—a five-mile-plus labyrinth of claustrophobic passages and crawl spaces, rooms with vaulted ceilings, and pits that drop to dark depths. As he makes his way through the caverns, Larsen's headlamp reveals leaks in the system. In spots, moisture from the hayfield 45 feet above literally rains from the limestone/dolostone ceiling. In other places, water makes its journey via steady dripping, that, through a chemical reaction with calcite, creates stalactites on the ceiling and stalagmites on the floor, as well as an eerily beautiful formation called flowstone, which has a scalloped, melted-wax effect.
Larsen explains that some of this moisture will eventually make its way back to the surface as drinking water. In fact, in southeastern Minnesota, some of the water coming out of faucets is sourced from the kind of groundwater that's flowing through these caverns. And that concerns Larsen, who, when he's not spending up to 500 hours a year exploring the region's underground, is raising crops on the surface and working with farmers who are trying to figure out how to better manage soil and runoff.
"For me, the science connects to water, and then it connects to farming, and then it connects to caving," he says.
That's why this caver isn't interested just in how water forms funky formations; he's also attuned to what people have done to modify it before it makes its way through the sinkholes, joints, cracks, and microscopic pores that characterize the karst geology underlying southeastern Minnesota's topsoil. Larsen and other cavers have had to wade through putrid mountains of foam created by manure and other pollutants, and they've traversed wet cave floors that he calls "demonic slip-and-slides" because of all the eroded soil that cakes them. He has also taken samples of cave drips that show nitrate levels well above drinking water standards. And extreme rainfall generated by climate change has altered the volume, and timing, of water moving through the system to the point where exploring caves, never a dull endeavor, can become life threatening.
Larsen's underground observations reinforce what scientific studies of the karst region have shown: levels of contaminants such as nitrates are increasing, and they are going deeper into the groundwater system. Larsen and cavers like him—he estimates around half a dozen people explore wild caverns in southeastern Minnesota weekly—provide an eyes-on-the-underground perspective that adds to the science showing groundwater's vulnerability.
But just as importantly, Larsen's day jobs on the surface world put him in a unique position to help improve the quality and quantity of water making its way underground. As a fifth-generation farmer, Larsen raises corn and soybeans on 700 acres near Byron. As a feedlot technician for the Olmsted County Soil and Water Conservation District, he works with livestock farmers to help them prevent or fix manure runoff issues. As a sustainability advocate, he is a member of a regional network that is bringing hundreds of farmers together around water-friendly farming practices. Larsen's ability to put science and his firsthand observations into action are invaluable, says Department of Natural Resources groundwater hydrologist Jeff Green, who studies the hydrogeology of karst country.
"I don't know how you describe Martin—a hydraulic renaissance man?" says Green. "He does caving, which is exploration, learning more. But he's also taking that information and getting people to make changes with it. Every county should have two or three of him."
The Thrill of Caving
Larsen began caving six years ago in order to follow the water. He was intrigued by the research of Green and other hydrologists who were dropping fluorescent dyes into sinkholes and disappearing streams to determine where and how quickly groundwater is moving (see "Mapping Subterranean Waters," March–April 2016). But from the time that dye enters the ground to when it exits in a spring or well—sometimes hours, days, months, or even a year later—little is known about its movements. Larsen wanted to see firsthand what was happening in the land's cellar and how farming was affecting groundwater.
He got in touch with veteran caver John Ackerman, who in 1989 created the Minnesota Cave Preserve, which provides access to 36 miles of wild cave passages in southeastern Minnesota and northeastern Iowa. Curiosity may have originally sent Larsen underground, but the pure adventure hooked him. He's been in caverns so shallow that the rumble of autos on a road could be heard overhead, and rappelled down pits that are dozens of feet deep with water pooled at the bottom. Exploring a new passage means leaving popsicle sticks or poker chips along the way to ensure a safe return to the surface.
"The thrill of caving is to find an unexplored new passage—when your light is the very first light, and your eyes are the very first eyes to ever see that passage," the compact, soft-spoken Larsen says as he clambers up a sloping, narrow joint in the bedrock of Spring Valley Caverns. "You're exposing yourself to a certain level of risk, doing things that we as humans aren't really meant to do. It's really an eye opener to a part of Minnesota that truly very few of us have seen."
Larsen and other cavers have worked with researchers to track water movements and bat populations, and they have collaborated with University of Minnesota geochemist Larry Edwards, who analyzes stalactites and stalagmites to gauge historical climate changes.
Larsen's thrill seeking, his ability to put up with discomfort, and his scientific curiosity often intersect, such as the time when he and some companions spent 16 hours in Tyson Spring Cave crawling on their stomachs through water in a passageway less than two feet high. They eventually came to a spot where the water was flowing in two directions, likely marking the first time in Minnesota someone has witnessed a groundwater divide, a place where underground water chooses different paths.
Green was particularly excited about that discovery since it confirmed what dye tracing had hinted at: Water can diverge underground and feed completely different watersheds. He encouraged Larsen to return to the spot and do a dye trace. At first, Larsen demurred to repeating the grueling journey—but now curiosity has gotten the best of him. "We will go back in there and dye trace it," he vows.
A Quiet Contaminant
The intimate connection between the land's human-dominated surface and its wild, bedrock-based roots is sometimes obvious. Larsen has photos of a well pipe emerging from a cave ceiling and "garbage trails" in passages that result when people use open sinkholes as dumps.
But the link between the surface and subsurface that most concerns Larsen and hydrologists is more nuanced. His sampling of cave drips has shown that when the land above is planted to row crops such as corn and soybeans, nitrogen fertilizer in the form of nitrate is routinely present in the water at levels well above the federal drinking water standard of 10 milligrams per liter.
While nitrate can occur naturally in water at levels below 3 mg/liter, higher levels indicate contamination from human activities, according to the Minnesota Department of Health (MDH). Drinking water above the federal 10 mg/liter standard is a significant health risk, especially for infants, pregnant women, and people with certain preexisting conditions. Using data from the Minnesota Department of Agriculture's Township Testing Program, MDH reports that "over 10 percent of the private wells sampled in some townships in southwestern, southeastern, central, and north-central Minnesota have nitrate levels above 10 mg/L." According to MDH, only a few community water systems have shown nitrate levels exceeding the federal standard in the past 20 years. The agency and communities have worked to address the issue, but some water systems remain vulnerable to nitrate contamination. MDH points out that "ongoing steps" are needed to prevent contamination and keep people safe. In addition to human health risks, nitrogen making its way into the Mississippi River has been pegged as a major cause of the Gulf of Mexico's "dead zone."
Corn requires lots of nitrogen—Minnesota farmers apply roughly 700,000 tons of nitrogen fertilizer annually—and even though soybeans create their own nitrogen, fertilizer from a previous growing season can leak from soybean fields. Manure and other pollutants washing straight into a sinkhole are clear groundwater threats. But nitrates quietly making their way into the karst is more insidious, says Green.
"Nitrogen in the groundwater is the number one water quality issue in this area," he says, "and most of it by far comes from row crop agriculture or manure application."
Even best farming practices meant to reduce nitrate pollution, such as calibrated fertilizer applications, aren't doing the job. "You could do the right rate at the right time in the right place, and you can still see nitrate levels that are above the drinking water levels," Larsen says. "We need to do something better than status quo."
Record rainfalls in Minnesota are sending more water racing through caverns and making it difficult for farmers to manage moisture and fertilizer. Crop plots at the Olmsted County Soil and Water Conservation District's Soil Health Farm show that as precipitation increased 42 percent from 2017 to 2019, groundwater nitrate concentrations jumped 44 percent.
If the caver in Larsen gets excited about discovering a new subterranean passage, the farmer in him gets just as animated about agronomic methods that regenerate the soil's ability to manage and clean up water. One promising method is cover cropping, where plants such as rye and brassicas are grown between the corn and soybean growing seasons. Studies show that cover cropping dramatically reduces erosion and surface runoff and, by increasing organic matter, builds the soil's ability to manage and store water and nutrients. Research that Larsen helped coordinate at the Soil Health Farm shows that water beneath soybean plots grown without cover crops had nitrate concentrations of 13.1 milligrams per liter. Cover cropping consistently reduced those levels below the safe drinking water standard.
A Direct Pipeline
Karst region farmer Ron Pagel is aware of the direct and indirect ways pollutants can enter the water. For example, Pagel, who raises dairy and beef cattle and grows row crops near Eyota, has long been mindful of a spring that is some 300 feet away from where he winters cattle. Feedlot runoff could contaminate the spring as well as a nearby stream that empties into the Root River. "It was no secret where heavy rains were going from that feedlot," says Pagel.
In 2014, he built a storage facility to keep waste out of the water and to allow him to store and use the manure as a soil-building fertilizer. Pagel credits Larsen for helping him through a complex process that basically took his farmyard to zero runoff. As a feedlot technician, Larsen helped to facilitate an 800,000-gallon manure storage facility and get government cost-share funds to help with construction. Such financial help is key, says Pagel, particularly when prices paid to farmers for commodities like milk are down.
Larsen has also helped Pagel deal with the less evident ways farming contributes to pollution. For the past half-dozen years, the farmer has been using cover crops to build soil health, reduce runoff, and provide forage for his cattle.
When Larsen offers advice, the fact that he's a farmer makes a difference, Pagel says. It doesn't hurt that Larsen has done his own dye tracing showing water from a neighbor's sinkhole took just minutes to travel to Pagel's spring a quarter-mile away.
"To be able to see those dye traces and those videos he takes of how those sinkholes are going straight into a cavern, boy, that is a direct pipeline," Pagel says.
Soil health community organizer Shona Snater says people like Larsen are a critical link in helping farmers transition from trying a new idea to making it a routine part of their operations. Snater helps coordinate the Land Stewardship Project's Soil Builders' Network, a group of 750 farmers and others who share information on soil health practices in southeastern Minnesota, northeastern Iowa, and southwestern Wisconsin. She says farmers get excited about a new soil health practice like cover cropping or no-till, but may drop it when they run into difficulties. Having someone like Larsen, who has successfully integrated such practices into his farming, affirms these methods can work. Larsen also hosts field days for the network.
"I appreciate that Martin has his feet in all these worlds, and a well-rounded perspective," says Snater, who has explored caves with Larsen. "He understands the potential of soil-building practices, and the dire consequences for the karst topography and our water quality and our land if we don't put these practices in place."
The dynamic relationship between land use, climate change, water movement, and karst geology sometimes creates scenarios that cut too close for comfort. Larsen recalls how he and his companions twice escaped Holy Grail Cave right before thunderstorms flooded it to the ceiling. Narrow escapes from the underground have become more common in recent years.
"It's a matter of, would you trust your life to a Minnesota weather forecast?" he quips while scrambling around in the relatively tame passages of Spring Valley Caverns.
Before exiting the mild 48-degree environs of the caverns into the bracing winter air of the farmland above, Larsen talks about how, like caving, building soil health through cover cropping, no-till planting, diverse crop rotations, and managed rotational grazing requires taking chances.
Both endeavors require ingenuity, teamwork, and, at times, a whole lot of groping around in the dark. "We can't protect or understand what we don't know exists, right? So you have to go discover it. Well, how do you discover it? You have to be determined, you have to take risks."