By Kelly A. Reynolds, MSPH, PhD

Drinking water supplies are dynamic systems, impacted by changes in sources, treatment and even the weather. Just as the weather constantly varies, the quality of source water is also ever-changing. Increased rainfall in spring and summer months creates additional challenges for municipal water suppliers and private well owners as water moving over the land and through the soil accumulates added contaminants capable of causing human disease.

Heavy rainfall associated with waterborne disease
Surveys of extreme precipitation events indicate (rainfall more than two inches a day) and waterborne disease outbreaks (WBDO) in the US are strongly correlated. Retrospective comparison of 548 outbreaks documented by US EPA and precipitation data from the National Climatic Data Center from 1948 to 1994 showed that 68 percent of WBDOs were preceded by extreme precipitation events (i.e., above the 80th percentile or in the top 20 percent of events).1 Surface water was the most likely to be contaminated and result in an outbreak during the same month as the rainfall event but groundwater outbreaks lagged by about two months.

Twenty-three years ago in late March, the largest documented waterborne outbreak in US history occurred in Milwaukee, WI. Before identifying the problem, residents consumed contaminated water for over two weeks. Ultimately, more than 400,000 people were sickened with severe diarrhea and over 100 died. Cryptosporidium, a protozoan pathogen, caused the outbreak and may have been introduced due to increased precipitation and the presence of nearby cattle farms.

Crypto has been found in 64 percent of manure samples from a sampling of 50 livestock farms.2 Following rain and land runoff, Crypto from nearby farms is readily transported to surface supplies, where associated increases in turbidity further tax treatment works.

The Walkerton event
Another highly publicized waterborne outbreak in recent history occurred in May 2000 in Walkerton, Ontario. Approximately 2,300 people in the small community were sickened and seven died due to drinking tap water contaminated with Escherichia coli O157:H7 and Campylobacter jejuni bacteria. Long-term studies conducted for seven years after the outbreak identified a post-infectious irritable bowel syndrome (IBS) incidence increase of more than 26 percent in Walkerton residents who experienced acute gastrointestinal disease. Such chronic IBS can persist for at least eight years following initial illness.3

An investigation into the outbreak revealed a series of preventable errors, including improper and fraudulent practices at the public utility, a lack of proper water-quality testing and non-enforceable environmental and water-safety regulations.4 Less preventable, however, was the effect recent storm events had on the water quality. Rainfall in Walkerton hit a 60- to 100-year high several days prior to the well-known outbreak.5 Land runoff through nearby farms and shallow wells created the perfect storm for a tragic event.

Groundwater risks
Surface water risks are somewhat expected and municipalities have treatment tools, including the use of flocculants, filtration and disinfectants to settle out, filter and inactivate harmful microbes. While federal regulations mandate treatment of surface water, utilities accessing groundwater are not necessarily required to treat. Thus, less obvious and less controlled are groundwater contamination events. The greatest concern with seasonal groundwater contamination are human viruses. Viruses, unlike larger bacteria and protozoa, easily navigate the tortuous path from land surface to underground aquifers. Storms, however, can lead to sewer overflows and contamination of groundwater wells with a variety of microbial hazards.6

Recently Minnesota and Wisconsin state health departments announced evidence of disease-causing microbes in a high percentage of drinking-water wells. In Minnesota, eight percent of a collection of 478 samples and 37 percent of the 82 public water systems with a groundwater well supply tested positive for human viruses.7 Eleven percent were positive for Salmonella bacteria. Less is known about household well water supplies. An estimated 34 million households in the US are served by private wells. One Wisconsin study found that out of 50 wells from seven hydrogeologic districts, eight percent were positive for human viruses, including hepatitis A virus, rotavirus and noroviruses.8 With summer being Wisconsin’s rainy season, concern this time of year is especially heightened.

Most private and public groundwater supplies are not filtered or disinfected. Thus, without a POU water treatment device at the tap, there is no barrier between the unexpectedly introduced contaminant and the consumer. The presence of low levels of human virus genomes (including norovirus, enterovirus and echovirus) in groundwater is common and has been associated with a 30-percent increase in gastrointestinal illness.9) Up to 63 percent of gastrointestinal illnesses in children were attributed to these tap-waterborne viruses.

Predicting the future
Climate modelers predict increasingly warmer temperatures in the future, which increases regional precipitation and also contributes to the growth of bacteria already present in contaminated systems. The June-July-August temperature outlook for the US indicates above-average temperatures and rainfall for many regions.10

Climate and weather predictions should be considered as part of a complete water-quality management program. Each of us has likely experienced inaccuracies in weather predictions, proving first-hand how difficult such events are to track ahead of time. The lag period between heavy rainfall events and reduced water quality, however, tends to be days, allowing time for action in issuing boil-water notices or recommendations for POU treatment. POU devices offer a final barrier for unpredictable events in water quality management and can protect consumers against microbial exposures expected to increase in the summer rainy season.


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  2. Graczyk TK, Evans BM, Shiff CJ, Karreman HJ, Patz JA. Environmental and Geographical Factors Contributing to Watershed Contamination with Cryptosporidium parvum Oocysts. Environ Res. 2000;82(3):263-271. doi:10.1006/enrs.1999.4022.
  3. Marshall JK, Thabane M, Garg AX, et al. Eight year prognosis of post infectious irritable bowel syndrome following waterborne bacterial dysentery. Gut. 2010;59(5):605-611. doi:10.1136/gut.2009.202234.
  4. Salvadori MI, Sontrop JM, Garg AX, Moist LM, Suri RS, Clark WF. Factors that led to the Walkerton tragedy. Kidney Int. 2009;75(112):S33-S34. doi:10.1038/ki.2008.616.
  5. Auld H, MacIver D, Klaassen J. Heavy rainfall and waterborne disease outbreaks: the Walkerton example. J Toxicol Environ Health A. 2004;67(20-22):1879-1887. doi:10.1080/15287390490493475.
  6. Gotkowitz MB, Bradbury KR, Borchardt MA, Zhu J, Spencer SK. Effects of Climate and Sewer Condition on Virus Transport to Groundwater. Environ Sci Technol. 2016;50(16):8497-8504. doi:10.1021/acs.est.6b01422.
  7. Orrick D. Viruses, bacteria in MN drinking water wells? Maybe, but what does it mean?–Twin Cities. Twin Cities Pioneer Press. Accessed May 18, 2017.
  8. Borchardt MA, Bertz PD, Spencer SK, Battigelli DA. Incidence of enteric viruses in groundwater from household wells in Wisconsin. Appl Environ Microbiol. 2003;69(2):1172-1180. doi:10.1128/aem.69.2.1172-1180.2003.
  9. Borchardt MA, Spencer SK, Jr. BAK, Lambertini E, Loge FJ. Viruses in nondisinfected drinking water from municipal wells and community incidence of acute gastrointestinal illness. Environ Health Perspect. 2012;120(9):1272-1279.
  10. NOAA. Climate Prediction Center–Seasonal Outlook. Accessed May 18, 2017.

About the author
Dr. Kelly A. Reynolds is an Associate Professor at the University of Arizona College of Public Health. She holds a Master of Science Degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds is WC&P’s Public Health Editor and a former member of the Technical Review Committee. She can be reached via email at [email protected]


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