Geologic atlases offer a peak at the location and quality of groundwater

At the edge of a gravel road in suburban Wright County, Howard Hobbs stands behind his four-wheel-drive pickup and guides a hydraulically powered auger slowly into the Earth.

Hobbs, a glacial geologist who works for the Minnesota Geological Survey, drills where the road was bulldozed through a ridge years ago. That allows him to begin drilling at a spot that already is 15 feet below the original surface of the land.

When the auger has penetrated 6 feet, Hobbs raises it and scrapes soil from its tip. The soil is moist and a bit gritty, not sticky as clay

Howard Hobbs of Minnesota Geological Survey
Howard Hobbs takes soil boring

would be, and it comes off the auger in fist-sized coils. It is primarily silt, washed there from somewhere else tens of thousands of years ago.

Hobbs, who has a doctorate in geology from the University of North Dakota, is confident he knows the composition of the sample. Nevertheless, he tests the soil with a 10 percent solution of hydrochloric acid. There is a slight fizz, confirming it contains calcium carbonate, basically ground-up limestone.

As Hobbs drills deeper, the color of his samples turns from brown to gray. Periodically, he compares the samples to color plates in a handbook. In a log book, he notes what he finds and what he observes at each step of the drilling. He puts some of the samples into bottles for later analysis in a lab.

That 2-inch-in-diameter, 25-foot boring by Hobbs last July with a drilling machine known as a Giddings Probe was one of about 50 similar borings he made in Wright County. Other geologists drilled about the same number of holes, a few much bigger and much deeper than the one Hobbs drilled. Over the next year and a half, a team of geologists and database and Geographic Information Systems technicians will correlate the new, detailed information from the borings and other types of sampling with much-less-precise information recorded in about 10,000 logs filed by well drillers over the last 30 years.

When the team’s work is done, the result will be an atlas, both printed and electronic, that will depict Wright County’s underground geology, from the surface of the Earth to the bedrock about 400 feet below it. A database containing the well drillers’ logs will be updated, expanded and made more precise.

Maps in the atlas will show the sand and gravel deposits that are likely to be water-saturated aquifers. They will depict the layers of rock and clay that separate the aquifers from each other and from the land above them. They will provide a key to areas on the land’s surface where rain and snow melt recharge the aquifers below. And they will show where groundwater from aquifers feeds rivers and lakes.

The county-by-county atlases produced by the Minnesota Geological Survey and a companion set of atlases produced by the state Department of Natural Resources are part of a $10 million-plus effort that has been under way in Minnesota since 1980. So far, the two sets of atlases have been completed for 17 of Minnesota’s 87 counties. Work on one or both parts of the atlas project is under way in 15 more counties.

Together, the two sets of atlases offer an imperfect-but the best-available-guide to the hidden world below the ground, to aquifers and the groundwater stored in and flowing through them.

Wells for homes can be drilled almost anywhere in most parts of the state and they will hit water. Drilling for high-volume wells, such as those that support city water supply systems or that provide water for large industries, is trickier. The atlases help drillers and planners know where to drill, and how deep to drill, to ensure those wells draw high-quality water and last for years.

Dale Setterholm supervises county geologic atlases
Minnesota Geological Survey’s Dale Setterholm


But, as important as finding water, is protecting it and ensuring it stays safe to drink. The mapping tells land-use planners where aquifers are most vulnerable to contamination from chemicals applied or spilled on the land.

“If people would quit putting stuff on the land surface that they wouldn’t want to drink, we probably wouldn’t need to do this work,” said Dale Setterholm, the Minnesota Geological Survey’s associate director who supervises the Survey’s atlas work.

“When you have the ability to direct your land use, that’s when you’re really going to use this,” said Setterholm. One example of such a decision, he said, might be putting a park-rather than, say, an industrial complex-on the recharge area for an aquifer providing a city’s water supply.

Terry Lee, the environmental resource services coordinator for Olmsted County, is a huge fan of the Geological Survey’s atlases. “Without the groundwater atlas, it would be the equivalent of a surgeon not knowing what they were going to find when they opened up the body,” he said.

In the 1980s, when Olmsted County officials were planning a new landfill near Rochester they looked for two attributes in a site: clay soils to prevent leakage, and lots of space between the land’s surface and the bedrock beneath it.

The Geological Survey’s atlas convinced planners the best place for the landfill was west of Rochester, and the atlas then helped convince county residents to accept the controversial siting decision, Lee said.


Counties where geologic atlases are complet or in progress
Status of County Geologic Atlases

Later, the atlas helped planners understand the flow of groundwater along the Decorah Edge, a mostly underground shale formation that is nearly surrounds Rochester. Nitrate-polluted groundwater in a close-to-the-surface aquifer is filtered and cleaned by biological action as it flows through the soil, vegetation and wetlands that lie over the shale and then into a lower bedrock aquifer from which Rochester draws its drinking water.

In 2006, both Rochester and Olmsted County enacted ordinances limiting development along the shale formation. The atlas helped define the area the ordinances protect.

The atlases produced by the Minnesota Geological Survey and the Department of Natural Resources are different, but complementary.

Geological Survey atlases focus on the groundwater plumbing system-the formations that can hold water, the connections between them and the barriers to water entering or leaving them.

The Department of Natural Resources atlases are prepared by a team led by Jan Falteisek, a hydrogeologist. The DNR atlases, based on measuring and sampling water from wells, focus on which formations actually contain water, what direction gravity forces the water to flow, the level of water in the aquifers and the naturally occurring contaminants, mainly arsenic and radium, in the water.

The DNR also collects samples of well water that later are age-dated through laboratory analysis. A key part of the analysis involves looking for tritium, a radioactive element released into the atmosphere in the testing of nuclear weapons in the 1950s.

Age-dating provides important information about water purity and safety. If there is tritium, the well draws from water that has entered the ground as rain or snow since the ’50s. That means the water may also contain thousands of other chemicals that have come into wide use since World War II.

“If you don’t see fallout residue in a public well, then you know that water probably predates the ’50s and is much less likely to be impacted by people,” said Bruce Olsen, a supervisor in the Minnesota Department of Health’s Source Water Protection division.

Olsen said the Health Department uses both sets of atlases when it draws maps, or approves cities’ maps, of wellhead protection areas around municipal wells. Cities must write plans for preventing contamination from entering groundwater beneath those areas.

The Health Department also has begun to use the atlases to produce Nitrate Probability Maps for counties, according to Olsen. Those maps show areas where aquifers are subject to contamination-from fertilizers and septic systems-because they are close to the land’s surface and not separated from the surface by rock or clay.