Scientists once thought that pathogens could not reach drinking water wells sunk into deep, protected groundwater aquifers. Nevertheless, over the past decade, researchers have identified diarrhea-causing viruses at a handful of deep bedrock well sites in the U.S. and Europe. Now, researchers in Madison, Wis., report where these pathogenic viruses may have originated. The viruses appear to seep from sewer pipes and then swiftly penetrate drinking water wells (Environ. Sci. Technol., DOI: 10.1021/es400509b).

The U.S. Environmental Protection Agency does not regulate the presence of viruses in drinking water, so many public water systems do not routinely test for them. In a 2007 study, a team of researchers, including Mark Borchardt, a microbiologist with the U.S. Department of Agriculture, and Kenneth Bradbury, a hydrogeologist at the University of Wisconsin-Extension, in Madison, investigated the integrity of aquifers confined beneath a thick layer of clay or shale (Environ. Sci. Technol., DOI: 10.1021/es071110+). Groundwater models predicted that surface contaminants would require tens to hundreds of years to reach wells in these aquifers, which typically sit more than 700 feet underground. Even if pathogens did find their way to the groundwater, they should be dead after that amount of time. “But we were shocked to find human-specific viruses—some of which were still infectious—in every well we sampled in Madison,” Bradbury says.

This group and others in the field suspected that leaking sewer pipes were the source, so the scientists launched a sampling program to determine how the pathogens got into the wells. Every two to four weeks, the team sampled sewage at the city’s waste treatment plant and water pumped from six wells prior to any disinfection. They identified viruses and measured their concentrations in the samples using real-time quantitative polymerase chain reaction.

Viruses appear and disappear in the human population on a monthly cycle, which allowed the researchers to track their presence in sewage and well water, Borchardt says. After the team detected viruses in the sewage, the same viruses appeared simultaneously in all six wells a few weeks later, usually after rain or snowfall melt. The scientists could correlate when the pathogens popped up in the well water with their appearance in the sewage system.

What’s more, the scientists cultured the viruses from the wells and showed that they were still infectious. Virus concentrations similar to those found in the study are sufficient to cause vomiting and diarrhea, Borchardt says. However, “because Madison chlorinates its water, no one has become sick,” Bradbury adds.

Bradbury thinks that the problem probably occurs in any city with wells located under sewage pipes. Over 147,000 public water systems supply drinking water from underground aquifers to 100 million people in the U.S., yet two-thirds of these systems don’t disinfect the water, Borchardt adds.