Where our well gets its water
January 2, 2011
By Jennifer Yauck
So where does that water come from, anyway? It’s not from Lake Michigan, though the big pond is just blocks away. Nor is it from Lake Superior, despite local stories that the well and the largest of the Great Lakes are connected.
The short answer is it comes from a regional aquifer, an underground layer of rock that holds water in its pores like a sponge. The long—and perhaps more interesting—answer requires understanding a bit of geology and Bay View history.
Two Aquifers Below Bay View
Imagine the ground below southeastern Wisconsin, including Bay View, as a giant cake. If you looked at a slice of that cake, you’d find it has several major layers (see diagram inside). The two uppermost layers are glacial sediments and dolomite, respectively—materials that water can permeate with relative ease. The next layer is Maquoketa shale, a low-permeability rock that restricts water flow.
When rain falls or snow melts in southeastern Wisconsin, water seeps downward and collects in the permeable layers above the Maquoketa shale. These water-holding layers are known as the shallow aquifer.
Go farther down, and there’s a second aquifer. Known as the deep aquifer, it occurs in a layer of permeable sandstone that’s sandwiched between the Maquoketa shale and a deeper layer of impermeable granite. Rain and snowmelt can only enter this aquifer by seeping through ground that lies beyond the shale layer’s edge. From Bay View, the closest such area is some 20 to 30 miles west, in Waukesha County.
A Tale of Two Wells
The original Pryor Avenue well was completed in 1883 and reached 1,500 feet into the ground, according to a Milwaukee Sentinel article from the time. At that depth, the well was definitely drawing water from the deep aquifer, said Doug Cherkauer, a retired hydrogeologist from UW-Milwaukee (UWM).
However, a subsequent article notes the well shaft was lined with pipe only to a depth of 270 feet. If that’s true, then the pipe ended somewhere above the Maquoketa shale, in the dolomite layer, Cherkauer said. As a result, at least a portion of the well shaft would have been open to the shallow aquifer—which suggests the well was drawing shallow water, too.
Cherkauer estimates the mixture was roughly 80 percent deep water and 20 percent shallow water, based on a regional aquifer model developed by hydrogeologists at the U.S. Geological Survey.
The same model suggests the deep aquifer was pressurized enough to keep the well artesian, or free flowing, until sometime around the 1930s or 1940s. After that, Cherkauer said, a pump would have been necessary to bring water to the surface. The drop in the deep aquifer’s pressure was likely due to Milwaukee’s heavy groundwater use in the preceding decades, said Tim Grundl, a UWM hydrogeologist.
In 1988, the original well was closed and a new one drilled nearby. Wisconsin Department of Natural Resources (DNR) records say the reason for the closure was that the 105-year-old well’s lining had deteriorated. But some anecdotal accounts say the closure occurred because construction of the Deep Tunnel in the shallow aquifer caused the area’s water level to decline. Cherkauer and Grundl think this is very unlikely, however. If any decline affected the well, they said, it was much more likely the decline in the deep aquifer—a result of the historic pumping in Milwaukee and newer pumping in Waukesha.
The new Pryor Avenue well—the one we see today—is drilled and lined to a depth of 118 feet, according to DNR records. That places it above the Maquoketa shale, and means it draws its water from the shallow aquifer.
Differences in Water Age, Quality
Water in the shallow aquifer is considerably younger (has spent less time underground) than water in the deep aquifer, due to the fact that it is not “trapped” by the Maquoketa shale the way deep-aquifer water is. Grundl recently analyzed a sample of the well’s water and found it to be about 3,600 years old; deep-aquifer water at this location is typically tens of thousands of years old.
Because it is closer to the surface and unprotected by the shale layer, the shallow aquifer is more susceptible than the deep aquifer to contaminants like road salt, lawn chemicals, or sewage. “There’s always a chance something from the surface could get in,” Grundl said.
However, when he recently analyzed the well water’s chloride content, it was within acceptable limits, suggesting road salt is not currently an issue for the well. In addition, as required by DNR, Milwaukee Water Works regularly monitors the well for nitrate, a compound from fertilizers and other sources, and coliform bacteria, which can indicate the presence of disease-causing organisms in the water system. The 2010 monitoring detected no bacteria and less than one part per million of nitrate, which is below the 10 parts per million maximum allowed in public drinking water by state and federal laws.
Jennifer Yauck is a science writer at UWM’s School of Freshwater Sciences and Great Lakes WATER Institute.
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