Mapping a fragile geology's underground water routes

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Jeff Green
DNR hydrologist Jeff Green pours fluorescent dye into a sinkhole near Harmony, Minn. on March 12, 2010. Later, he will return to springs a mile away to retrieve the carbon "bugs" that will absorb the dyes.
MPR Photo/Stephanie Hemphill

When snow is melting in the woods and fields of southeastern Minnesota, Jeff Green wants to know where it's going.

For Green, a karst hydrologist for the state Department of Natural Resources, the running waters of spring offer a chance to work on maps of underground pathways that water takes in the fragile geology of southeastern Minnesota. His maps will help fire departments, land-use planners, farmers, and people who want to protect trout streams.

Finding underground pathways is important, because Minnesota's driftless area wasn't scraped by the last glacier. The area's honeycombed limestone bedrock, called karst, makes it highly vulnerable to pollution caused by chemical spills, development or poor farming practices.

The first step in making groundwater maps is dye tracing, a process in which scientists pour dye in melting snow and track where it leads.

Near the small town of Harmony, Green and his colleague Jeremy Rivord climbed a fence and splashed through a stream that's overflowing its shallow banks and flooding a pasture. Bordering the creek, a natural rock wall riddled with cracks dripped water. The limestone is like a superhighway for water, Rivord said.

As melting snow seeps into limestone, it runs down vertical cracks to horizontal openings. Green called the bigger ones conduits, as the water picks up speed when it reaches them.

In a pasture near Harmony, Minn. spring #A398 pours groundwater to the surface. In the karst country of southeastern Minnesota, the water flows readily between surface and underground. DNR hydrologist Jeff Green placed a charcoal sampler, or "bug," in this spring before pouring dye in a sinkhole a mile away.
MPR Photo/Stephanie Hemphill

"This thing is two or three inches wide, [and] you can imagine a pipe that big, water would move very fast," Green said. "So these conduits are what we're dye tracing. Water is moving through those -- miles per day -- and coming out of these springs."

Green and Rivord traipsed into the wet pasture to put what they call a "bug" in the spring. It's a small mesh bag about the size of a cell phone, packed with charcoal. The charcoal will capture a dye that they'll pour into melting snow in a sinkhole a few miles away.

Their goal is to determine where the water comes from that feeds the spring. That helps Green make what he calls spring-shed maps, that show how the water flows underground.

Farmers are usually happy to let him walk on their fields, and he gets help from other locals too, especially later in the year when snow no longer melts.

Map: Minnesota karst lands
Karst topography is a fragile landscape shaped by layers of bedrock being dissolved over time.
Minnesota Department of Natural Resources

"The Harmony Fire Department is fabulous; they bring me water in their fire truck," Green said. "They know that if there's a spill and something nasty gets into a sinkhole, they're part of the response team and they want to know where it's going to come out. And by doing this work, we can tell them that."

After the scientists placed "bugs" in several springs, they drove about a mile away to pour their dyes into sinkholes.

A muddy field was dotted with small groves of trees. The trees were growing around miniature canyons, about 20 feet deep. The sinkholes made it clear how the honeycombed water highway works.

"This is a place where there was a conduit, an opening in the limestone, a fracture that got dissolved," Green said. "And as water moved through the soil, it carried soil into that opening, and made an air-filled void in the soil at the bedrock surface. Eventually the soil collapsed, and you have a sinkhole."

After finding some running water, Green donned rubber gloves and poured a cup or so of fluorescent dye into the snow.

Different dyes
Hydrologist Jeff Green uses three different kinds of fluorescent dyes. That allows him to trace the flow from three different sinkholes at the same time.
MPR Photo/Stephanie Hemphill

Rivord jotted down the exact time, and on a GPS unit, pinpointed the spot where surface and groundwater meet -- a sure sign that what happens on the land directly affects the quality of groundwater, the scientists said.

"In this case, it's pretty good, you've got conservation tillage, lots of corn stalks left to keep the soil from eroding, and then you've got grass, permanent cover, around the sinkholes," Green said.

Southeast Minnesota is known for its trout streams. The map Green is making will help protect those streams by pinpointing the source of water that feeds them.

Green will later check the "bugs" he put in the springs to determine where the dye from the sinkhole went.

He usually finds water traveling 1-3 miles underground before it surfaces in a spring.

When the spring-shed map is finished this spring, Green will share it with local governments, farmers and people who want to protect the region's water.