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In a Pickle: The Mystery of the North Shore's Salty Well Water

Image of van and man working on North Shore well.

Digging wells along the North Shore of Lake Superior can be tricky business because of the salt water that lurks underground.

We have running water,” jokes Michael Monroe of Hovland, MN. “I run and get it.”

With Lake Superior and ten percent of the world’s fresh surface water just yards away, it seems farcical that Monroe and Deb Mueller fetch water with plastic containers and muscle. They and many of their North Shore neighbors are in something of a pickle, living in a place where the ground water can be saltier than the sea.

“Hyper-saline water is no shock at the bottom of a copper mine a kilometer underground,” said John Swenson, assistant professor of geological sciences at the University of Minnesota Duluth. “What’s surprising is to find water with such chemistry coming out of a household spigot.”

This salty predicament is sporadic around the fringes of Lake Superior’s western lobe; drinkable water can come from a well 100 feet away from one that might have a chloride concentration greater than 40,000 parts per million (35,000 ppm is typical for ocean water).

It’s ironic that potable water is not a sure bet around Western Lake Superior. It’s intriguing that nobody really knows why. Several scientists and well drillers are willing to speculate and a Sea Grant staffer offers suggestions for afflicted well-owners.

Under Pressure

Geologists think the pockets of extremely salty water contain Canadian Shield brine. Canadian Shield brine is older than the hills, dating back to Precambrian seas covering the planet two billion years ago; before the supercontinent Pangaea formed; before dinosaurs. Although this water has ties to a distant marine environment, its flavor is born of bedrock's calcium chloride not oceanic sodium chloride. Hyper-saline water typically forms when ordinary water has a chance to sit for a good, long while in bedrock, which is the case around Western Lake Superior.

Swenson suggests that the hefty Laurentide Ice Sheet, during its million-and-a-half year dance, produced a complex distribution of salinity that is most conspicuous near the modern lakeshore.

Beyond Salt

The calcium chloride bite of some North Shore well water makes it tough to drink, but it’s the elevated levels of fluoride and boron leached from the bedrock that could affect your health.

Fluoride: too much or too little can damage teeth and bones.

Duluth public water supplies are fluoridated to 1.2 parts per million (ppm).
Groundwater along the North Shore can range from 0.4 to 14 ppm, sometimes within several hundred feet. About half of the wells in this region appear to fall within an optimal range (0.7-2.0 ppm).

Boron: high doses damaged reproduction and fetuses in study animals.

The Minnesota Department of Health suggests drinking other water when concentrations exceed 0.6 ppm (level may be raised to 1 ppm within the next year).

Groundwater along the North Shore can range from virtually zero to 5.4 ppm. Most wells in this region appear to be within recommended guidelines.

If you want your well water tested, contact a certified laboratory.

“During the last glaciation, a glacial lobe parked in the Lake Superior Basin,” said Swenson. “The pressure possibly forced melt water into the bedrock, flushing the underlying deep brine to the edge of the ice sheet. At the edge, briny water gushed to the surface through cracks in the basement basalt. The edge…of the glacier, is approximately where Lake Superior’s modern shoreline falls.”

Swenson, considering Earth in geologic time, calls the high-salinity waters along the edge of Lake Superior a “transient phenomenon” and has worked with colleagues to develop a model that might explain it. Following deglaciation, he suggests a mixing zone developed where precipitation, glacial melt water, and backflushing brine interact to this day. Lake Superior’s surface, which sets a base for flow, defines one edge of this mixing zone. Under the lake and beneath the Sawtooth Mountains, the brine mix is beyond ordinary reach, but within a few miles of the shore it can trickle from artesian wells and taint water found less than eight feet deep.

“It would be good to do some testing of the theory,” said Swenson. “Right now, the data is consistent with the model but there are no smoking guns.”

Hydrologists recognize two geochemically distinct ground waters in the North Shore Volcanic Group rocks. The most widespread has a high pH (10.26) and elevated boron (in concentrations around 5.4 parts per million). The second is the calcium chloride brine, which Bob Tipping, a hydrogeologist from the Minnesota Geological Survey calls “highly evolved,” implying that it has stewed in the rocks for so long it is saturated with salts.

Worn Down and Exposed

Tipping suggests that something much older than the ice sheet – the Midcontinent Rift System – can alone account for the salty water rimming Western Lake Superior.

The Midcontinent Rift System is a billion-year old tear that nearly severed North America and made a new ocean. It stretches through the Lake Superior Basin to Southern Kansas and is possibly related to a fracture in the Earth’s crust extending across Michigan and into Ohio. The basaltic lavas and reddish sedimentary rocks exposed along Superior’s shore from Canada through Duluth and around Michigan’s Keweenaw Peninsula are products of the Midcontinent Rift.

Cross-section diagram of north shore and 'lens' of freshwater above saline with mixing zone inbetween.

“When you poke a hole through the basalt, you encounter the thick reddish interflow layer that dips off under the lake,” said Tipping. “The water in this layer of clay, sandstone and siltstone has been out of contact with the modern world. Towards the top of the interflow, you're more apt to find good water since the denser salt water sinks.”

“It’s like a lens of fresh water floats on top of the salt water,” agrees Swenson, who thinks the brine harkens back to the rift system but believes a fluid-impelling mechanism, like the pressure from a glacier, is necessary to drive those deep dense brines skywards. He suggests the fresh water lens is thinnest near the lakeshore.

Bathtub Made of Basalt

Bob Rasmussen, a well driller from Two Harbors, worked on approximately 100 wells this year and estimates he found brine in six. “Up the shore is worse,” he said.

Up the shore, Roger (Bill) McKeever drills wells from Split Rock to the Canadian Border and says along the lake is saltiest. “After you go about four miles inland the water seems fine but you know, we don’t do much drilling inland so data are scarce. Forty years ago, I’d say one out of five wells were good. Now, working with geological maps and a lifetime of experience, we can generally find drinkable water.”

How can the sweet water of Lake Superior sit next to and presumably on top of hyper-saline water? Blame it on basalt. Basalt is essentially impermeable except for its vein-like fractures; it keeps Lake Superior in as much as it keeps ground water out.

According to Swenson, if a well is drilled into fractures that associate with surface water then you’re in luck. Near the Canadian border, one property owner had seven brushes with brine before fresh water came from an eighth well. Those who have given up on finding potable ground water haul it from community pumping stations and other places; a handful stubbornly drink their brine.

Barb Liukkonen, water resources education coordinator for Minnesota Sea Grant, advises caution when investing in systems to treat North Shore water.
“The capacity of most treatment systems can be quickly overwhelmed by the load of minerals,” she said. “Filters may need to be changed impractically often and water conditioners may not be effective with such high mineral levels.”

As mentioned above, homeowners are advised to carry in their water from elsewhere or keep drilling.

Liukkonen remembers the diversity of well water quality found during water testing clinics sponsored by Sea Grant and University of Minnesota Extension in 1988. Many wells on the North Shore had elevated chloride, sulfate, sodium, and calcium forcing owners to overcome hard water as well as saltiness. Some wells tapped ground water that mixed with surface water, a circumstance that became most evident in spring when meltwater caused turbidity (cloudiness).

“Shallow ground water is more susceptible to surface contamination and should be tested regularly to ensure it is safe from bacterial contamination,” said Liukkonen. “On the other hand, it's not salty.”

By Sharon Moen
April 2005

Return to April 2005 Seiche

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