Viruses Can Take Many Routes Into Wisconsin Groundwater
Leaky sewer pipes might be the source of viruses found in drinking water that Wisconsin municipalities draw from bedrock aquifers 800 feet below ground, research hydrogeologist Kenneth Bradbury said in an Aug. 1, 2012, talk at the Wednesday Nite @ the Lab science series on the University of Wisconsin-Madison campus.
Bradbury, who is a professor with the Wisconsin Geological and Natural History Survey, which is part of UW-Extension, described how he and colleagues are boring into the earth to track viruses that cause diarrhea and vomiting in humans, following up the surprise finding that these pathogens are found hundreds of feet below ground in drinking water. He and his colleagues studied wells used by the Madison Water Utility, but he said the same problem likely occurs in most urban areas with bedrock aquifers.
Groundwater moves through different types of rock formations into wells used by municipalities and individual property owners. Bradbury said that, before the research started, no one thought viruses could exist in the deep aquifer — a thick layer of sandstone that absorbs and retains water — that is a source for drinking water across much of Wisconsin.
Bradbury and his team took samples from multiple Madison wells, Lake Mendota and the city's sewage system in 2007, 2008 and 2009. Of the six wells tested, 46.6 percent of the samples were positive for viruses that can cause human illness. Samples from Lake Mendota contained viruses 82 percent of the time.
Bradbury and his colleagues also looked at weather patterns in the Madison area over these years, which included three periods with a lot of rain: in August 2007, and twice during the summer of 2008. These bouts of intense precipitation led to a lot of groundwater being recharged. This regeneration corresponded with their detection of many types of viruses in the samples from the various water sources. When wells tested positive for viruses, sewage had greater concentrations of viruses as well. The types of viruses found in each well also coincided with the kinds of viruses found in the sewage.
The geographic dispersal of the viruses found in Madison's water suggests the contamination was not due to an issue with a single well or a solo event like a spill, Bradbury said. Rather, the problem might stem from how older sewer systems and municipal wells interact with fractures in the aquitard, a rock layer of limited permeability that impedes the flow of groundwater down into the deep aquifer. Sewers are often old and they leak. Older, "cross-connecting" wells were drilled more than 700 feet into the deep aquifer to draw water, but their casings only extend to the shallow aquifer that is closer to the earth's surface. These older wells might facilitate the movement of viruses through the shallow aquifer and into deeper groundwater. Newer wells extend their casings into the deep aquifer below the aquitard, theoretically providing better protection from contamination of the water.
Fractures have been found in the shallow aquifer and the aquitard, Bradbury said. These paths through the bedrock might intersect with wells and allow viruses to reach the drinking water.
Bradbury's presentation was recorded for Wisconsin Public Television's University Place.
- An aquifer is a geologic formation that carries and absorbs useful quantities of water. Wisconsin has bedrock and glacial aquifers. Bedrock in northern Wisconsin is primarily made up of crystalline rocks like granite that don't contain a lot of water. Southern Wisconsin bedrock includes layers of sandstone and dolomite. The thick and porous nature of the sandstone makes it a great aquifer. Glacial deposits of sand and gravel and clay can form shallow aquifers.
- The 800-foot-thick layer of sandstone under Madison that makes up a deep aquifer in the region also extends throughout much of the Midwest. It is called the Mount Simon sandstone.
- At the time of Bradbury's 2012 talk, 60 municipal systems in Wisconsin did not disinfect their water at all to kill viruses and bacteria. (As of 2015, 57 systems don't disinfect.)
- A study of 14 Wisconsin communities using untreated water from shallower wells found viruses in every one of those systems. About 15 percent of the annual gastrointestinal illness (diarrhea, vomiting and discomfort) in those communities was due to people drinking this untreated water. That infection rate is very high, Bradbury said.
- Norovirus found in the well of a restaurant in Door County led to at least 229 patrons becoming ill and six people being hospitalized. The restaurant had to be shut down for a while. The viruses came from the building's septic system leaking into the restaurant's own well.
- Many sewers are as much as 100 years old, and some still use clay pipes. More recently built sewers are made with steel or PVC (or a similar material).
- After Bradbury and his colleagues reported their findings in 2009 to the Wisconsin Department of Natural Resources, the agency decided to require all public water utilities to disinfect their water. But in 2011, the Legislature rescinded a DNR rule requiring municipalities to require disinfection.
- On the quality of a Wisconsin aquifer: "So, this is Cambrian sandstone that's along the Wisconsin River in the Wisconsin Dells, and you can see that that formation covers about two-thirds of Wisconsin. It's a wonderful aquifer. Really, I will say one of the best in the world. It's because it's thick, it's porous, and it produces really great quality water."
- On finding viruses in a deep aquifer: "We thought our model said it should be tens to hundreds of years to get to these wells, and the virus lifetime or before they disintegrate were thought to be two years. And so it was a surprise because we found viruses in every well we sampled."
- On fractures helping viruses get into drinking water: "Hydrogeologists worry about fractures all the time but not usually in sandstone. ... Sandstone is thought to be a really great porous, uniform thing, and fractures don't really matter. We worry about fractures in granite or limestone, something that's a little less porous. But here we're starting to worry about fractures in sandstone."
- On groundwater not being protected from viruses: "Some of the take-home messages are that this groundwater from these deep confined bedrock aquifers which we usually assume to be protected from surface contamination is showing things up because we're seeing viruses in these deep wells."
- On the relationship between rain and snowmelt and viruses: "We think there's a link between virus detections and lots of precipitation events."
- On sewers as a source of viruses in drinking water: "But when you think of all the things that can happen to a sewer over a hundred years, tree roots growing through it, people digging into it, land, earth shifting and cracking it, we know that they leak quite a bit. … We know the sewage is loaded with viruses, so, again, it seems like that's a good potential source."
- On water testing: "I think a lot of people assume that when they go somewhere the water is treated. There's also no requirement they test for viruses. So you're kind of taking your chances. And we found a lot of the utility managers didn't understand the difference between viruses and coliform bacteria, for example."