Viruses from human sources occur in the La Crosse, Wisconsin municipal drinking water supply prior to its chlorination, according to a study published today in the scientific journal Applied and Environmental Microbiology.
Although the city's treated water meets or exceeds state and federal standards for drinking water, researchers and public health officials agree that more study is needed to pinpoint sources of the viruses and to determine if some viruses are surviving chlorination and making people sick with diarrhea and vomiting.
The objective of the study, by Marshfield Clinic Research Foundation (MCRF) and the U.S. Geological Survey (USGS), was to determine whether the amount of Mississippi River water moving through the riverbank and infiltrating city wells was related to the frequency of virus detection in the wells.
"There is a misconception that groundwater is pure because it is filtered simply by the process of passing through the soil, but that isn't always the case," said Mark Borchardt, Ph.D., the study's lead investigator and a scientist in MCRF's National Farm Medicine Center. "Understanding viruses in groundwater is a research priority among drinking water professionals. La Crosse is leading the way and has the most extensive dataset of any community in the U.S. that uses groundwater as its drinking water source." Researchers tested 48 untreated water samples from six of the city's 15 wells to evaluate whether viruses were present in the raw water. Half of the water samples contained at least one type of gastrointestinal virus and 23 percent of the samples contained two or more virus types. The study found enteroviruses, rotavirus, hepatitis A virus and noroviruses. Besides noting the presence of these viruses with molecular testing methods, researchers attempted to culture the enteroviruses and hepatitis A viruses to see if they were infectious. None of the enteroviruses sampled were infectious, although three samples were positive for infectious hepatitis A viruses. During the study period, however, no cases of hepatitis A linked to drinking water were reported.
"The situation in La Crosse is not unique," Borchardt said. "Approximately one-third of the groundwater pumped in the U.S. is from shallow sand and gravel aquifers, like La Crosse. If we were to test the water in communities that use these aquifers we would probably find the same level of viruses."
In setting up the research project, Borchardt and Randy Hunt, Ph.D, co-author of the study and hydrogeologist at the USGS in Middleton, reasoned that if they could predict virus occurrence in wells based on Mississippi River water infiltration, this would help water managers tailor disinfection treatments and pumping schedules specific to each well and prevent viruses from entering the drinking water supply. "We know from our previous studies on groundwater flow in La Crosse that some city wells have high levels of Mississippi River infiltration, other wells have intermediate levels and the remaining wells have little river water in them," Hunt said. "We also know that the Mississippi River is affected by upstream sewage treatment plants and is a source for viruses."
Contrary to expectations, researchers detected viruses in all wells, even those with low contributions of Mississippi River water. This suggested that other unidentified sources, in addition to river water, were responsible for contamination.
"The only other human fecal source near the wells is sanitary sewer lines," Borchardt said. "Sanitary sewers can leak and it is possible the leakage is large enough to reach the wells."
La Crosse's source of water is an aquifer consisting of a deposit of glacial outwash sand and gravel approximately 170 feet deep, bounded on the east by the bluffs and on the west by the Mississippi River. Sand and gravel aquifers are among the most vulnerable to fecal contamination.
The La Crosse Water Utility includes 15 wells and 212 miles of water main. In 2003, the water utility delivered an average of 12 million gallons of water per day to its customers.