In 2007, Wisconsin used dye to trace why 229 people got sick; septic-tank wastewater reached the drinking well in 6 days, while infiltration-field dye took 15

In June 2007, a brand-new restaurant in Door County, Wisconsin, provided the setting for perhaps the most extensively documented outbreak of waterborne disease of recent times in the USA, according to the CDC's Surveillance for Waterborne Disease Outbreaks Associated with Drinking Water, United States, 2007–2008, the MMWR summary covering 2007-2008 outbreaks. According to the same CDC report, 229 cases of acute gastroenteritis occurred among customers and staff at the restaurant not long after it opened, and six people were hospitalized. All this to address the single question of how human waste found its way into a well that complied with all the state building codes.The outbreak: 229 cases, one contaminated wellFirst, a case-control study by Borchardt and colleagues ruled out food poisoning and found that drinking the restaurant's well water was associated with illness (odds ratio = 3.2, a measure of how much more likely illness was in one group versus another). Second, testing detected norovirus genogroup I, a group of viral strains that cause acute gastroenteritis, in the restaurant's well water at around 50 genomic copies per liter. Third, genetic sequencing confirmed the same viral strain, genotype GI.2, in the patients' stool samples, the restaurant's septic tank, and the well water. Together, these findings identify the well as the primary source of infection in the outbreak, as documented by Borchardt and colleagues in the journal Ground Water. The well is 85 meters deep, and the septic system sits 188 meters away, separated from it by a 35-meter-thick soil layer called the vadose zone (the unsaturated ground above the water table).The red dye test revelations In order to determine how the contamination got to the well, two different dyes were injected into two parts of the septic system, the tanks and the infiltration field. Borchardt and colleagues reported that dye injected into the septic tanks reached the well within six days through a leaking fitting, while dye released into the infiltration field took about fifteen days to arrive. This is where the account of wastewater reaching the drinking water well in about a week began. Wastewater from both the tanks and the infiltration field ultimately reached the restaurant's well. This meant the septic effluent was carried by groundwater directly into the drinking water well.Why the Wisconsin Bedrock made this possibleDoor County is located on top of a fractured dolomite aquifer of Silurian age. This rock formation is made up of joints and bedding-plane channels that let water, along with whatever pollutants it carries, move rapidly with little natural filtration. As noted earlier, both the well and the septic system were built entirely in accordance with the state codes in place at the time. Adherence to those codes turned out not to be enough for this particular geology: the setback distances assumed groundwater would move slowly enough to be filtered naturally over weeks or months, but the dye tests showed it could travel through the fractured dolomite in a matter of days.A wider problem across northeastern WisconsinThis is not an isolated case. A related 2021 study by Burch and colleagues, Quantitative Microbial Risk Assessment for Contaminated Private Wells in the Fractured Dolomite Aquifer of Kewaunee County, Wisconsin, examined the same aquifer system in a nearby county, where roughly half the county's population, an estimated 10,000 to 12,000 people, relies on private wells. Using a year-long, countywide pathogen occurrence data set as the input to their quantitative microbial risk model, Burch and colleagues estimated, rather than directly observed, that contaminated private wells could be responsible for as many as 301 modeled cases of acute gastrointestinal illness per year in the county.Of the 301 modeled cases, the study attributed about 230 a year to cattle manure and 12 to human sources. The same study found that Cryptosporidium parvum alone accounted for an estimated 190 of those cases each year, the largest share among the eight pathogens modeled. This pathogen-level figure overlaps with, rather than adds to, the source-level breakdown above (230 bovine, 12 human), since the same modeled illnesses can be counted twice: once by fecal source and once by pathogen. Microbial source tracking allowed the researchers to distinguish bovine from human contamination, an important capability given that the Kewaunee County dolomite aquifer underlies many dairy farms and septic systems.What the findings changedTogether, the two cases have reshaped how experts think about the risks of groundwater contamination in fractured bedrock areas. Inspection of the water supply or proper septic-system design does not eliminate risk. The CDC's national waterborne disease outbreak surveillance report covered the Door County case directly, noting that pathogens reached the water through cracks in the dolomite that created a hydraulic connection between the septic system and the well. The same CDC report found that, among the 36 outbreaks linked to drinking water in 2007 and 2008, 13 of the 21 outbreaks tied to source-water, treatment, or distribution deficiencies were caused by untreated groundwater. The report also noted that EPA regulations do not apply to private wells, and that more than half of the drinking-water outbreaks it reviewed were linked to untreated or inadequately treated groundwater.Catch the latest World News and Live updates. Download the TOI app.