October 7, 2004 — LA CROSSE, Wis. – 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
"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
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.
Source : Marshfield Clinic