Some of the most valuable homes in the city also sit on the highest water tables.
During the debate on the teardown moratorium (now lifted), a map of “potential groundwater conflict” areas raised a few eyebrows at neighborhood meetings. The city map, which is based on data from the Minnehaha Creek Watershed District, shows a higher water table ranging 0-20 feet deep that can stretch several blocks around the lakes and Minnehaha Creek.
For homeowners, a shallow water table can lead to sump pumps, settling and costly construction. Some homeowners close to the lakes are repeatedly shaving doorways as homes shift. Others are installing helical piers below additions to prevent them from settling.
Much of Minneapolis was originally wetlands, so the issue crops up throughout the city. Near 22nd & Lyndale, where a pond once filled the area, Le Parisien Flats needed to use helical screw anchor piles to support a portion of the building. The developer of the nearby Theatre Garage property said he also planned to use piers that plunge deep into the ground. Developers at 1800 Lake built two floors of underground parking as much as 18 feet into the water table, and ended up installing high-powered sump pumps to handle a 170-gallon-per-minute water flow.
Tim Cowdery, a hydro geologist at the U.S. Geological Survey, explained that the chain of lakes and creek represent an old river valley about as big as the current Mississippi River Valley that runs through Downtown.
When glaciers came through, the valley was filled in with sand and gravel. Giant chunks of ice were buried in the valley to create the present-day chain of lakes and smaller lakes and ponds. Many of the lakes and wetlands have been filled in for development, Cowdery said.
“People in the 1800s started filling it in with anything they could find, even garbage — cinders from old coal furnaces,” Cowdery said. “It was literally a dump. People put a little bit of soil on top, and built a house on that.”
Particularly low-lying areas were eventually made into parks, such as Martin Luther King Park at 40th & Nicollet and Pearl Park at Portland & Diamond Lake Road — originally Pearl Lake.
“Almost everybody in Minneapolis to one degree has some fill, if you dig down far enough,” said Cowdery, contrasting Minneapolis’ flat yards with hilly suburban yards.
He estimated that the water table in Minneapolis ranges from 0-50 feet, depending on land height and distance from the lakes and river.
“It all has to do with how high you are above the closest water body,” he said. “If you’re anywhere near one of the old filled-in lakes, the water table is going to be right there.”
Lisa Cerney, director of Minneapolis’ Surface Water and Sewers, said Public Works staff reference the map of “potential groundwater conflicts” as part of their development review. The map shows areas with a higher water table that are known to impact standard residential construction.
Linden Hills resident Walter Pitt lives on the south side of Lake Calhoun on Upton Avenue, near Thomas Beach.
He said many homes in his area have sump pumps, which can lead to flooded basements in the spring if the power goes out and pumps don’t have a contingency in place.
“Without the pumps it would flood, because we’re down below the water table,” he said.
When Pitt built an addition on his home, he used 15 helical piers to reach stable soil underneath the sand and peat. He said a neighbor’s new fireplace was built with three or four helical piers, to prevent it from sinking over time.
“As the houses get bigger, we have to deal with it in some way,” Pitt said. “When you change the ecosystem, in terms of building, there are unintended consequences.”
Minneapolis park historian David Smith said the neighborhood along Xerxes south of Lake Calhoun was historically swampy and nearly became part of the lake itself. The Park Board voted at one time to dredge out a bay and extend Lake Calhoun almost to 43rd Street. Residents protested the expensive proposal, and the Park Board built Linden Hills Park instead at 43rd & Xerxes, draining most of the land in the corridor.
Brad Fagerstrom lives at 38th & Zenith, near the Southwest edge of Lake Calhoun. He said the primary impact of the water table is that most of the homes in his area are built on pilings. His house was built in the early 1960s, and has 28 treated timbers that reach 30-50 feet deep.
“It adds a lot to the construction cost of these homes,” he said.
Areas without underground pilings, like a garage or front steps, can settle away, he said.
“If things are done right, you really shouldn’t have issues over time,” he said.